Synthesis and Characterization of Ring-Opening
Metathesis Polymers with. Pendant Carborane
Groups
by
Alice Man
S.B., Massachusetts Institute of Technology, 1993
Submitted to the Department of Materials Science and Engineering
in Dartial fulfillment of the reauirements for the degree of
a
VES
MASSACHUSETTS
INSTITUTE
OFTECHNOLOGY
Doctor of Philosophy in Polymers
at the
MAR 0 2 Z00
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
LIBRARIES
September 1999
©)Massachusetts Institute of Technology 1999. All rights reserved.
Author
....................
. .....................................
Department of Materi1al Science and Engineering
_/August
6, 1999
Certified by ..........................
Professor Robert E. Cohen
St. Laurent Professor of Ch, mical Engineering
Thesis Supervisor
Readby...................................................
....
Professor Linn W. Hobbs
John F. Elliott Professor of Materials
Thesis Reader
Acceptedby..................
...............................
Professor Linn W. Hobbs
John F. Elliott Professor of Materials
Chairman, Department Committee on Graduate Students
Synthesis and Characterization of Ring-Opening Metathesis
Polymers with Pendant Carborane Groups
by
Alice Man
Submitted to the Department of Materials Science and Engineering
on August 6, 1999, in partial fulfillment of the requirements
for the degree of Doctor of Philosophy in Polymers
Abstract
A series of ring-opening metathesis (ROMP) polymers functionalized with
pendant carborane (C2B1 0 HI 2 ) groups was synthesized. Norbomene-based polymers with
the bulky, pseudoaromatic functionality were found to have glass transition temperatures
(Tg's) between 29 °C and 290 °C. This variation in Tg can be attributed to structural
differences in how the functional group is attached to the polymer backbone. Block
copolymerization was used to improve the material processibility. The block copolymers
microphase separate into boron-rich and boron-free microdomains.
Carborane-functionalized
monomers and polymers were used as precursors and
models for the synthesis of polyanions with pendant cobalt dicarbollide (Co(C2B 9H)2 - l)
and carborane anion (CB1 H12-) groups via ROMP. These polyelectrolytes were found
Polymers
to work as hydrophilic ion-exchangers in aqueous acidic solutions.
functionalized with either cobalt dicarbollide or carborane anions demonstrated selective
binding of cesium over sodium. Copolymerization and cross-linking were used to reduce
the hydrophilicity and solubility of the materials in a stripping solution of strong (8M)
nitric acid.
Thesis Supervisor: Robert E. Cohen
Title: St. Laurent Professor of Chemical Engineering
Thesis Reader: Linn W. Hobbs
Title: John F. Elliott Professor of Materials
Acknowledgments
Many thanks to Professor Cohen, who has been an encouraging and supportive advisor
with many great ideas. His unwavering enthusiasm and kindness have been invaluable.
Thanks also to the helpful members of the Cohen group, past and present. A special
thanks to B.H., Colleen, and Jin-Kyu for getting me started down the road to chemical
synthesis. I am particularly grateful to Jason Gratt for his chemistry advice and
conversation which he has generously dispensed 24/7 throughout the last ten years.
I am grateful for the advice and analytical expertise kindly provided by Tony
Modestino, Barry Grant, Tim McClure, and Libby Shaw. Without their help, I'd
never have been able to tell one compound from another. And for assistance with
innumberable neccessities over the years, I'd like to thank Steve Wetzel, Liz Webb,
Arline Benford, and Gloria Landahl.
Lots of love and thanks to my husband, David, who has done a lot of cooking,
cleaning, chauffering, editing, formatting, and snuggling to get me through the writing
of this thesis.
This thesis is dedicated to all the people who hate nuclear waste but love icosahedrons.
3
Contents
1
Background
1.1
14
Carboranes
. . . . . . . . . . .
.
. . . . . . . . . . . . ......
1.1.1
General
1.1.2
Applications
1.1.3
Synthesis of Carborane Polymers
Properties
. . . . . . . . . . . . . . .
. . . . . . .
1.2 Cobalt Dicarbollide ..........
.
14
. .....
14
. . . . . . . . . . . . ......
16
.
...............
20
.................
.
1.2.1
Nuclear Weapons, Nuclear Power, Nuclear Waste
1.2.2
A Summary of Cesium Separation Techniques .........
1.2.3
Cobalt Dicarbollide in Solvent Extraction
1.2.4
Cobalt Dicarbollide in Polymeric Solid Phase Separation
.
.....
.
22
23
24
......
.
.. .
26
27
1.3 Ring-Opening Metathesis Polymerization
(ROMP) ..................................
27
1.3.1
Catalyst Systems
28
1.3.2
Kinetic Effects
.........................
.
.........................
30
2 Synthesis of Carborane Functionalized Norbornene Monomers
2.1
32
Nor-Si-Carb: A Silicon Bridged Carborane-Norbornene
.......
34
2.2 Nor-C-Carb: A Carbon Bridged Carborane-Norbornene
........
35
2.2.1
Effect of Reagent Stoichiometry on Yield ............
44
2.2.2
Unusual Tosyl Carborane Reaction ..............
49
2.3 Nor-CC-Carb: A Monomer with Twice-Bound Carborane ......
2.3.1
The Proton Transfer Mechanism .................
4
50
53
2.4
Nor-6-Carb: A Norbornene Monomer with Pendant Carborane Attached through a Spacer Chain
3
.....................
57
2.5
Nor-Si-Anion: A Norbornene Monomer with a Carborane Anion Group 60
2.6
Experimental Details .........
60
.............. . .60
2.6.1
Materials
2.6.2
Equipment .
. . . . . . . . . . . . . . .
63
2.6.3
Nor-Si-Carb ..........
. . ..
.
63
2.6.4
Nor-C-Carb ..........
. . . . . . . . . . . . . . .
64
2.6.5
p-Toluenesulfonyl Carborane .
. . . . . . . . . . . . . . .
64
2.6.6
Nor-CC-Carb
. . . . . . . . . . . . . . .
65
2.6.7
Nor-6-Carb
. . ..
66
2.6.8
Nor-Si-Anion. Cesium Salt
Synthesis
...........
.........
..........
3.2
. . . . . . . . . . . . . . .
3.1.1
Solvent
3.1.2
Kinetics and Polydispersity
3.1.3
Spectroscopic Characterization
Systems
. . . . . . . . . . .
.
Block Copolymers .
3.3.1 Glass Transition Temperature ....
3.5
. ..
. .
. . . . . . . . . . . ..
. .
.67
Thermal Gravimetric Analvsis .
.
Magnetic, Optical, and Dielectric Properties
3.4.1
Index of Refraction.
3.4.2
Dielectric Constant ..........
3.4.3
MIagnetic Response ..........
Experimental Details.
3.5.1
Equipment .
3.5.2
Processing
......... ..68
.. ....... . .69
.. ....... . .70
. . . . . . . . . .
.........
3.3 Structure/Thermal Property Relationships
3.4
. . . . ..
....
68
Polymerization
3.3.2
. ...
and Characterization of o-Carborane Funtionalized Poly-
mers
3.1
..
72
. .. 81
.. ....... . .84
.. ....... . .84
.. ....... . .85
......... . .87
......... . l87
......... . .87
......... ..88
.. ....... . .91
......... . .91
..... 92
..
5
3.5.3
Films for UV/IR
3.5.4
Polymerization
93
......
...................
93
.......
...................
4 Cobalt Dicarbollide and Carborane Anion Polymers: Materials Syn97
thesis
4.1
Poly-Nor-C-Carb-Co: Attachment, Polymerization, Sandwiching
4.2
Poly-Nor-C-CDC: Attachment, Sandwiching, Polymerization
4.2.1
Sandwiching
4.2.2
Polymerizing
. . . . . . . . . . . . . .
.
99
.
102
.
102
. . . . . . . . . . .
107
..........................
4.3
Poly-Nor-11-CDC: Sandwiching, Attachment, Polymerization .
113
4.4
Poly-CDC-siloxane: Polymerization, Sandwiching, Attachment
115
4.5
Poly-Nor-Si-Anion: Polymerizing the Carborane Anion Monomer
117,
4.6
Experimental Details.
118
4.6.1
Degrading o-Carborane
. . . . . .
. . . . . . . . . . . .
118
4.6.2
Poly-Nor-C-Carb-Co
. . . . . .
. . . . . . . . . . . .
123
4.6.3
...
Poly-Nor-C-CDC
. . . . . .
. . . . . . . . . . . .
128
4.6.4
Poly-Nor-11-CDC
. . . . . .
. . . . . . . . . . . .
133
4.6.5
Poly-CDC-Siloxane
. . . . . .
. . . . . . . . . . . .
143
4.6.6
Poly-Nor-Si-Anion
. . . . . . . . . . . .
147
5 Cobalt Dicarbollide and Carborane Anion Polymers: Potential Ap152
plications
5.1
5.1.1
Cobalt Dicarbollide and Carborane Anion Polymers for Cesium
Binding
5.2
152
Cesium Ion-Exchange ...........................
153
..............................
.....................
.
.
159
5.1.2
Solubility in Nitric Acid.
5.1.3
Cesium Stripping .........................
161
5.1.4
Cesium and Sodium Binding ...................
163
5.1.5
Conclusions ............................
166
Solid Electrolytes
5.2.1
. . . . . . . . . . . . . .... . . . . .
Ionic Conductivity with the Small Molecule Lithium Salts
6
167
. . 168
5.3
5.2.2
A Polyelectrolyte
5.2.3
Potential Pathway to an Unusual Comb Copolymer ......
Lithium Salt .................
].70
170
Experimental Details ...........................
172
5.3.1? Copolymerization ..
172
..................
5.3.2
Testing
5.3.3
Sodium and Cesium Analysis
5.3.4
Converting
5.3.5
Reaction of Propylene Oxide with Norbornene Carborane Car-
a Potential
Crosslinking
to Lithium
Salts
Agent
. . . . . . . . . ....
..................
. . . . . . . . . . . . . . .....
175
175
178
banions
.. .......... . ......... . 179
7
List of Figures
1-1
Carborane
Isomers
. . . . . . . . . . . . . . . .
.
. .
.
15
1-2 Partial Charges on the o-Carborane Molecule .............
15
1-3 The Carborane Anion
16
..........................
1-4 A DEXSIL Polymer for High Temperature Use .............
17
1-5 The Use of a Carborane Framework for the Attachment of a Catalyst
19
1-6 Examples of Polymers with Carborane Groups .............
20
1-7 Cobalt Dicarbollide ............................
23
1-8
Some Materials Used in Cesium Separations
..............
25
1-9 ROMP of Norbornene-Based Monomers ................
28
1-10 Two Organometallic Catalysts for ROMP
30
..............
2-1 Carborane Functionalized Norbornene-Based Monomers ........
33
2-2 Butyl Lithium Deprotonation of o-Carborane ............
2-3 Unsuccessful Route to a Dicarborane Monomer (1)
.
..........
2-4 HNMR of 5-(Chlorodimethylsilyl)-Norbornene in D-Chloroform
2-5
Synthesis of Nor-Si-Carb
.........................
2-6 HNMR of Nor-Si-Carb in D-Acetone
13 CNMR
of Nor-Si-Carb in D-Acetone
2-9
13 CNMR
of o-Carborane in D-Chloroform
2-10 1BNMR of Nor-Si-Carb in D-Acetone
2-11 llBNMR of o-Carborane in D-Chloroform
2-12 Synthesis of Nor-C-Carb
.........................
8
34
. . .
36
37
.
2-7 HNMR of o-Carborane in D-Chloroform
2-8
33
.
.
.
.
.
.................
38
...............
39
................
40
..............
41
................
42
..............
43
44
2-13 HNMR of Nor-Tosylate
in D-Chloroformn ................
45
2-14 HNMR of Nor-C-Carb in D-Chloroform .................
46
2-15
47
3CNMR of Nor-C-Carb
in D-Chloroform
. . . . . . . . . . . .....
2-16 "lBNMR of Nor-C-Carb in D-Chloroform ................
48
2-17 Synthesis of Tosyl Carborane
50
......................
2-18 HNMR of p-Toluenesulfonyl Carborane in D-Chloroform
2-19 Unsuccessful Route to a Dicarborane Monomer (2)
2-20 Synthesis of Nor-CC-Carb
51
..........
52
........................
53
2-21 HNMR of Trans-5,6-bis(methyltosylate)norbornene
ride
.......
in D-Methylene Chlo-
.....................................
54
2-22 HNMR of Nor-CC-Carb in D-Chloroform ................
55
2-23 Proposed Proton Transfer Mechanism in the Synthesis of Nor-CC-Carb
56
2-24 Synthesis of Nor-6-Carb
58
.........................
2-25 HNMR of Nor-6-Carb in D-Chloroform .................
2-26 Synthesis
of Nor-Si-Anion
.
. . . . .
.
.
.
59
.
. . . .....
61
2-27 HNMR of Nor-Si-Anion in D-Methanol .................
62
3-1 Polymerization Rate for Nor-CC-Carb
71
.................
3-2 HNMR of Polv-Nor-Si-Carb in D-Acetone
...............
74
3-3 HNMR of Poly-Nor-C-Carb in D-Methylene Chloride .........
3-4
13 CNMR
3-5
13CNMR of Poly-Nor-C-Carb
3-6
"BNMR of Poly-Nor-C-Carb in D-THF
of Poly-Nor-Si-Carb in D-Acetone
in D-THF
75
..............
76
................
77
................
78
3-7 IR Transmittance of Carborane Functionalized Polymers .......
79
3-8 Near IR Transmittance of Carborane Functionalize Polymers .....
80
3-9 TEM of Poly-Nor-CC-Carb/Poly-Nor-OTMS
Block Copolymer
82
3-10 TEM of Poly-Nor-OTMS/Poly-Nor-Si-Carb
Block Coplymer .....
3-11 Weight Loss as a Function of Temperature
for Poly-Nor-CC-Carb
Poly-Nor-C-Carb .. ...........................
3-12 Dielectric Constant Measurement for Poly-Nor-Si-Carb
9
....
83
vs
86
. .......
89
.
3-13 Magnetic Response of Poly-Nor-C-Carb ...............
4-1
4-2 Synthesis of Poly-Nor-C-Carb-Co
98
..........
"Sandwiching" to Synthesize Cobalt Dicarbollide
90
100
....................
4-3 Poly-Nor-C-Carb-Co: A Cross-Linked Resin ..............
101
4-4 Synthesis of Nor-C-CDC .........................
103
4-5 HNMR of Degraded Nor-C-Carb, Trimethylammonium Salt, in D-
4-6
104
Acetone
..................................
13 CNMR
of Degraded Nor-C-Carb, Trimethylammonium Salt, in D-
.
Acetone .................................
105
4-7 llBNMR of Degraded Nor-C-Carb, Trimethylammonium Salt, in DAcetone
106
..................................
108
4-8 HNMR of Bis(Nor-C)-CDC in D-Acetone ................
4-9 HNMR of Nor-C-CDC in D-Acetone
109
..................
of Cobalt Dicarbollide, Cesium Salt, in D-Acetone
4-10 HNNMIR
......
110
4-11 '3 CNMR. of Cobalt Dicarbollide, Cesium Salt, in D-Acetone
.....
111
4-12 "BNMR of Cobalt Dicarbollide, Cesium Salt, in D-Acetone
.....
112
113
4-13 Synthesis of Bromobutyl Cobalt Dicarbollide .............
4-14 Mixed Elimination and Substitution Products Resulting from the Reaction of Norbornene Methanol and Bromobutyl Cobalt Dicarbollide
4-16 Synthesis
of Nor-11-CDC
. . . . . . . . . . . . . . . .
.
4-17 HNMR of Polv-Si-Anion in D-Methanol
115
.......
.
4-15 Synthesis of an Ester Linked Norbornene Alcohol
.
114
116
......
119
..........
4-18 HNMR of Degraded o-Carborane, Trimethylammonium Salt, in D-
Acetone.
120
........
.......
4-19 ' 3 CNMR of Degraded o-Carborane, Trimethylammonium Salt, in D-
4-20
. . . . . . . . . . . . . .
.
. . . . . . . . . . . . . ..
Acetone
..
11 BNMR
of Degraded o-Carborane, Trimethylammonium Salt, in D-
Acetone
..................................
121
122
4-21 HNMR of Degraded Poly-Nor-C-Carb in D-THF .......
10
....
124
1 3CNMR
of Degraded Poly-Nor-C-Carb in D-THF ..........
.
125
4-23 "BNMR of Degraded Poly-Nor-C-Carb in D-THF ..........
.
126
4-22
4-24 IR Spectra of Polynorborne, Poly-Nor-C-CDC-Co, and Cobalt Dicarbollide
. . . . . . . . . .
. . . . . . . . . . . . . . . . . .
. . . . .
4-25 HNMR of Poly-Nor-C-CDC in D-Acetone ................
4-26 HNMR of Norbornene Butanol Ester in D-Acetone
129
132
134
..........
4-27 HNMR of Bromobutyl Cobalt Dicarbollide, Cesium Salt, in D-Acetone 136
4-28 HNMR of Nor-11-CDC in D-Acetone
.
.................
139
4-29 HNMR of Nor-11-CDC after Treatment with a Schrock Catalyst in
D-Acetone .................................
140
141
4-30 HNMR of Poly-Nor-11-CDC in D-Acetone ...............
4-31 HNMR of Elimination (Butene Cobalt Dicarbollide) and Substitution
(Methoxynorborne Cobalt Dicarbollide) Products ill D-Acetone
. . . 144
146
in D-Chloroform
4-32 HNMR of Polydimethylsilane/Polymethylhydrosilane
Products Reacted with a
4-33 HNMR of Mixed Elimination/Substitution
148
Polyhydrosilane in D-Acetone ......................
4-34 HNMNRof First Hydrosilation Reaction Products Reacted with More
Polyhydrosilane in D-Acetone ..
5-1 HNMR of Polynorbornene(90)/Poly-Nor-Si-Anion(30)
mer in D-THF
149
...........
.........
. . . . . . . . . . . . . . . .
.
Block Copoly154
. . . . . . . . ...
Block Copolymer in D-
5-2 HNMR of Polynorbornene/Poly-Nor-C-CDC
Toluene ..................................
156
5-3 HNMR of Polynorbornene in D-Toluene
.
157
...............
5-4 Conductivity as a Function of Temperature for the Lithium Salts of
Cobalt Dicarbollide and the Carborane Anion in POENI ......
5-5 Proposed Synthesis of Poly-Nor-C-Carb/PEO
.
169
Comb Copolymer . . .
171
5-6 HNMR of the Reaction of Deprotonated Nor-Si-Carb with Propylene
Oxide in D-Acetone ............................
11
173
5-7 HNMR of the Reaction of Deprotonated Poly-Nor-Si-Carb with Propylene Oxdie in D-Acetone
5-8
. . . . . . . . . . . . . . . .
.
......
HNMR of a Polymerization of Mixed Nor-C-CDC and Bis(Nor-C)CDC in D-Acetone ............................
5-9
174
176
HNMR of a Polymerization of Nor-C-CDC with Bis(Nor-C)-CDC Added
Later
in D-Acetone
. . . . . . . . . . . . . . . .
12
.
.........
177
List of Tables
84
.........
3.1
Comparison of o-Carborane Functionalized Materials
3.2
Refractive
4.1
Elemental Composition of Poly-Nor-C-Carb-Co
4.2
Viscometry
5.1
Weight Loss After 40 wt% Nitric Acid Treatment
5.2
Cesium Removal by Strong Nitric Acid .................
5.3
Cesium and Sodium Binding by Carborane Anion and Cobalt Dicar-
Indices
Results
of Carborane
Polymers
. . . . . . . . . . .
...
.......
. ..
. . . . . . . . . . . . . . . .
bollide Functionalized Polymers ...................
5.4
.
87
.
127
151
.........
160
...........
162
..
164
Cesium Binding Efficiency of Carborane Anion and Cobalt Dicarbollide
....................
Functional Groups in Polymers.
13
166
Chapter
1
Background
In this research, we have used ring-opening methathesis polymerization (ROMP) of
norborenene based monomers to synthesize polymers with pendant carborane groups.
Carborane functionalized polymers are interesting materials because the carborane
group may be expected to result in unusual to physical properties. These polymers
may also be synthetic precursors for cobalt dicarbollide materials with selective cesium
ion binding properties.
Because carborane and cobalt dicarbollide are somewhat
obscure molecules, their properties, applications, and polymer history are reviewed
in Chapter I. We also review the aspects of ROMP that make it especially suitable
for the synthesis of these materials.
1.1
Carboranes
1.1.1
General Properties
"Carborane" is the generic term for a family of polyhedral cage molecules containing
boron, carbon, and hydrogen. The boron and carbon atoms in these molecules are not
connected by normal covalent bonds, although each is covalently bonded to a hydrogen
atom.
Instead, carboranes have a three-dimensional pseudo-aromatic structure in
which the electrons are delocalized and each skeletal atom is hypercoordinated [37].
The most common carborane is the ortho carbon isomer of the icosahedral C2B10 H1 2
14
ortho-carborane
meta-carborane
para-carborane
Figure 1-1: Carborane Isomers
+
s
U
-
Figure 1-2: Partial Charges on the o-Carborane Molecule
molecule, 1,2-dicarbadodecaborane (see Figure 1-1). Typically known as o-carborane
or just carborane, this compound exhibits the high chemical, thermal, and biological
stability typical of carbon-boron clusters. Thermally, o-carborane is stable up to to
400 °C. o-Carborane rearranges into the meta isomer at around 425 °C, and converts
to the para isomer at higher temperatures [38, 91]. The biological inertness of ocarborane is demonstrated by its unusually high LD5 0 of 9 g/kg (compared to 1 g/kg
for calcium chloride) [54, 55]. Chemically, the skeleton of o-carborane is stable enough
to tolerate a range of reactions to attach or modify substituent groups [38]. Carborane
is not degraded by acids, but can react with some bases and reagents such as butyl
lithium [92].
:
Other interesting properties also result from o-carborane's unusual structure. The
molecule has a strong permanent dipole moment of 4.45 D [93] with a slight positive
charge between the carbons (see Figure 1-2). This makes the hydrogens bonded to
carbons more acidic than those bonded to borons [52], and also makes the borons
15
-1
CB 11H1 2 - I
Figure 1-3: The Carborane Anion
adjacent to both carbons removable [39]. Upon removal of one B-H group, the icosahedral cluster becomes an open-faced anion [49, 137] that can form organometallic
sandwich complexes [43, 46, 134, 48].
Similar in structure to o-carborane is the carborane anion CB 11 H2 (see Figure 13). Carborane anions are unusual because the negative charge is delocalized over
the entire molecule [62, 61, 112, 18]. This anion has an extremely low charge density and a low nucleophilicity [112]. The conjugate acid of this anion is exceedingly
strong [18]. Salts of this carborane anion completely dissociate in non-aqueous solvents [106]. Many of the anion properties of CB 1lIH2 are shared by carborane based
organometallic sandwich complexes such as cobalt dicarbollide [106], described in
Section 1.2.
Carboranes are a high boron density material, and thus have some characteristics due to elemental boron properties. The ' 0 B isotope of boron, which has a 20%
natural abundance, is the lightest element that reacts with slow neutrons [45]. Carboranes enriched in
10 B can
be synthesized, maximizing the material's ability to capture
neutrons.
1.1.2
Applications
High Energy Fuel
Boron cluster compounds were initially developed as high energy-density fuels for
rockets and jets [45, 106, 30]. The high burning rate and very high heat of combustion
16
i
I
i,
- I '01
Si
Li
I
J
n
Figure 1-4: A DEXSIL Polymer for High Temperature
Use
of these boranes make them an interesting alternative to traditional hydrocarbon
fuels. However, the solid boron oxide combustion products add unacceptable amounts
of wear and tear to turbines and engines [45]. Although polyhedral boranes have not
found successful commercial application as fuels, the research effort in this field did
show that these compounds can be synthesized on a large scale [45].
High Temperature Rubber
The most successful commercialization of carborane materials has been the DEXSIL
polymers developed by Olin corp [40]. These siloxane polymers incorporate mcarborane units into their main chains (see Figure 1-4). The carborane units dramatically improve the high temperature performance of the polymer; gaskets and o-rings
made from DEXSIL products (containing fillers, crosslinking, antioxidants) can be
used up to 900 F [118, 40]. This increased high temperature performance is due to
a combination of steric and chemical mechanisms [40]. The steric bulk of the carborane functionality (about 4 A) inhibits polymer crystalization and impedes molecular
motion [118]. The carborane group withdraws electrons from adjacent atoms an it
is hypothesized that they provide resonance stabilized energy sinks [118].
Cancer Therapy
Recently, 10B enriched carborane compounds have been investigated as agents in
boron neutron capture therapy (BNCT) for the treatment of brain cancers. BNCT
has also been considered for lung and prostate cancers [45, 22]. In BNCT, boron
17
compounds are injected into the blood stream and accumulate in tumors. When
the area near the tumor is irradiated with slow neutrons, high energy byproducts of
the boron capture reaction destroy nearby cells. Only cells with significant boron
concentration are damaged by the neutrons.
Carborane compounds are good candidates for use in BNCT because they have low
toxicity and are quickly excreted from the body. These compounds can be modified
to selectively accumulate to the necessary concentrations in cancerous cells [45].
Ceramic Precursor
Polymers containing carboranes or boron cluster compounds have been considered as
easily processed precursors to boron carbide or boron nitride ceramics [11, 122, 121].
Polymers can be drawn into fibers, coated onto surfaces, filled into pores, or used
as binders for other ceramic precursors. Because these cluster compounds are much
more expensive sources of boron than naturally occuring boron cmpounds,
such
polymers would only be practical as ceramic precursors for applications that demand
specialized shapes not acheivable through standard ceramic shaping methods.
Optics
The permanent dipole of o-carborane can be modified and strengthened by attaching electron withdrawing or donating groups at the appropriate sites [93]. Modified
carboranes have been patented as optical switching and optical modulation materials
[69, 68]. A number of modified carborane compounds have been studied as potential
materials for second-harmonic generation [93, 94]. While some of these compounds
did show some second-harmonic generation activity, the magnitude of the effect was
small because these compounds crystallize in centro-symmetic space groups [93, 94].
It is possible that polymers incorporating such compounds might show more significant non-linear optical behavior after poling, since carborane polymers are usually
non-crystalline.
18
roll
1,n
3
I
Figure 1-5: The Use of a Carborane Framework for the Attachment of a Catalyst
Non-aqueous Electrolytes
Borane and carborane anion salts dissociate completely in many non-aqueous solvents
[106]. This strong ionic dissociation, in addition thermal and chemical stability, low
charge density, and non-nucleophilicity, makes polyhedral boron compounds potentially useful for non-aqueous electrolyte applications. Several borane cluster lithium
salts were found to be promising electrolytes in lithium-titanium disulfide battery cells
[63, 64]. More recently, there has been an attempt to incorporate pendant boron cluster anions into polymers to make polyelectrolytes [21]. Such polyelectrolytes might be
useful as ion-conductive membranes or electrolytes for rechargable lithium batteries.
Ligands for oranometallic catalysis
One challenge in engineering catalytic materials is how to firmly attach catalytic
small molecules onto solid substrates without an accompanying loss of activity. The
open-faced carboranes formed by the removal of a single boron vertex from C2 B31
0H 12
isomers have been used to complex with organometallic rhodium, forming catalytically active C 2 B9 HllRhH(PPh 3) 2 species. These catalysts can isomerize and hydrogenate alkenes in solution [44]. Carboranes can be grafted onto chloromethylated
polystyrene beads through skeletal carbon or boron before attaching the rhodium
complex [15, 129] (see Figure 1-5). Alternatively, carborane containing monomers
can be polymerized and then functionalized with the rhodium complex. In either
case, the catalytic species remains attached to the substrate and catalytic activity
is maintained through multiple turnovers [29, 129]. The uncrosslinked functional19
o
[--OH 2CH CC
2
C CCH2 H2 0 2CCH2 C-CCH
CCH20CH
0
2 0CH 2
V
C
CCH2 C--
= o-carborane
COOCH 9
I
L
I
J
Figure 1-6: Examples of Polymers with Carborane Groups
ized polymer was found to be more catalytically active than the material made from
crosslinked polystyrene beads, possibly because of a difference in catalyst site accessibility [29].
1.1.3
Synthesis of Carborane Polymers
The most commercially promising carborane-based materials have been polymers [40].
Carboranes have been incorporated into polymers both as part of the main backbone
and as pendant groups, despite some synthetic challenges. Figure 1-6 shows some examples of such polymers. Primarily, difficulties in polymerization reactions are cause
by the large steric bulk and electron withdrawing nature of the carborane group
[36, 34, 35]. Additionally, difunctional o-carborane monomers sometimes have a tendency to form small cyclic oligomers instead of linear polymers during condensations
reactions
[118, 104].
Main Chain Carborane Polymers
A series of inorganic main chain carborane polymers has been synthesized with repeat
units consisting of single atoms of silicon, germanium, lead, tin, carbon, phosphorus,
20
sulflur or mercury bonded to the skeletal carbon atoms of m-carborane or p-carborane.
In the case of silicon, germaniium, lead, or tin, the condensation reaction is between
Li2 (C2 B10oH
1 o) and R2 MC12, where M is any of the listed elements [40]. The sulfur and
mercury bridged carborane polymers are unusual in that there are no substituents
other than carborane bonded to the bridge atoms [118]. In the case of phosphorus
bridge atoms, each phosphorus is also bonded to a chlorine. Carbon bridged carborane
polymers have been synthesized with carbonyl carbons used as bridging atoms [118].
The meta- or para- isomers of carborane can form single atom bridged carborne
polymers, but the ortho- isomer has too much of a tendency to cyclize. Such polymers
have fairly low molecular weights, with at most 30 or so repeat units [118]. In some
cases, random copolymers can be made fom mixtures of meta- and para- carborane
or mixtures of tin and germanium [118].
Many of the typical condensation polymerization methods can be used to make
main chain carborane polymers.
o-Carborane diols, diacids, and diesters can be
reacted with each other or with other difunctional monomers to form polyesters with
molecular weights from 2000-20,000 [40, 34]. Condensation reactions were found to be
inhibited when the carborane group was close to the functional ends of the monomer;
this effect was attributed
to both steric and electronic factors [34]. Polyamides,
polyurethanes [138] and polyformals [34] have also been synthesized.
The commercially successful series of m-carborane siloxane polymers is described
earlier in the Applications section 1.1.2. These DEXSIL polymers were synthesized
from the ferric chloride catalyzed condensation reaction of difunctional methoxysilyl
m-carborane monomers and chlorosilanes. Molecular weights of up to 16,000 have
been measured but higher molecular weight materials were too insoluble for analysis [104]. Hydrolysis of difunctional bis(chlorosilyl) carborane monomers did not
result in polymers, possibly due to the electron withdrawing effect of the carborane.
Monomers made from the ortho- isomer were found to form cyclic oligomers [104].
Carborane siloxanes with fewer molecules between the carborane groups along the
main chain backbone are crystalline. while those with a greater number of (Si-O)
groups separating the carboranes are amorphous [104].
21
Polymers with Pendant Carborane
A wide variety of synthetic strategies have been used to make siloxane and vinyl
polymers with pendant carborane groups.
Both condensation and addition poly-
merizations have been employed, as well as grafting reactions [40]. Generally, the
presence of the carborane cage tends to increase the thermal transition temperature
of the polymers [35] and inhibit thermal degradation [40], although to a lesser extent than with main chain carborane polymers. The pendant carborane groups are
generally the ortho- isomer.
Vinyl carborane polymers with molecular weights up to 140,000 have been synthesized by phenyl lithium catalyzed free radical initiation [30]. Similarly, methyl vinyl
carborane and methyl isopropenyl carborane have been used. These monomers have
also been copolymerized with butadiene [138]. Acrylate and methacrylate carboranes
have also been homopolymerized and copolymerized free radically [40, 35]. Copolymerization has sometimes been necessary for monomers such as isopropenylcarborane,
which do not homopolymerize [35]. The lowered activity of carborane monomers
towards radical polymerization is attributed to both steric inhibition and electron
withdrawing effects [35, 29]. The inductive effect can be avoided by converting the
carborane into the open-faced, anionic form before polymerization[29].
A series of polysiloxanes with pendant carborane groups was synthesized by both
hydrolysis-condensation and ring-opening polymerization of cyclic carborane siloxanes
[40]. The first method has been used to make polymers up to 14,000 molecular weight,
with large amounts of cyclic side product. A grafting reaction to attach isopropyl
carborane to poly(hydrosiloxanes) has also been reported [40].
1.2
Cobalt Dicarbollide
Cobalt dicarbollide is an organo-metallic compound analogous to ferrocene (see Figure 1-7). Described as "peanut shaped" [106], it is constructed from two modified
o-carborane fragments and a cobalt center [46, 48]. With a -1 charge delocalized over
both carborane fragments, cobalt dicarbollide has an extremely low charge density.
22
-1
-1
[(7-C 2 B 9 H 1 1 )2Co(I II)]
Figure 1-7: Cobalt Dicarbollide
The conjugate acid is one of the strongest known acids hucause there are no free electrons to bind a proton [106, 80]. Cobalt dicarbollide has been shown to be an effective
and selective phase transfer agent for the solvent extraction of cesium from aqueous
solutions [119, 106]. A practical method for separating cesium from solution could
be very useful because the United States (and other countries) have large amounts of
aqueous nuclear waste containing the radioisotope
137 Cs
[7, 103].
Nuclear Weapons, Nuclear Power, Nuclear Waste
1.2.1
Typically, nuclear power plants are fueled by uranium rods containing 3.3% 235 U and
96.7%
2 3 5 U,
238 U
[95]. After a usable life of about 3 year, spent fuel rods contain 0.81%
94.3% 2 3s8 U, small amounts of plutonium,
and 3.5% other fission products [95].
Thus, "spent" fuel rods are still extremely radiactive. Such spent fuel rods can be
processed to separate uranium and plutonium from fission products such as cesium137 and strontium-90. Because purified plutonium can be used for nuclear weapons,
the reprocessing of fuel rods is controversial in the United States [96]. However,
reprocessing to recover uranium is commonly practiced in France and the United
Kingdom
[56, 57].
When fuel rods are reprocessed to remove uranium and plutonium, the remaining
23
substance is left dissolved in strong acid (> 0.5M nitric) [119]. In the course of
building nuclear weapons, the U.S. produced a large amount of this radioactive waste
solution [119, 97, 103]. Currently stored in tanks at the Hanford and Savannah River
sites, this solution is unacceptable for permanent disposal because it is difficult to
reliably contain a liquid [103, 4]. A number of tanks have already leaked radioactive
waste into the environment. Cesium-137 and strontium-90 are the most plentiful and
dangerous radioisotopes in this waste [103, 7, 19]. Cost estimates for remediating
these wastes have been steadily increasing, one estimate is $100 billion [89].
Reprocessing waste may be directly vitrified for disposal, or it may be separated
into a small volume of highly radioactive material and a large volume of low radioactivity [103, 7]. Currently, the location and design of a permanent storage facility
for highly radioactive materials is still being debated. However, it is projected that
if cesium and strontium were separated from reprocessing waste, the total disposal
costs would be much lower because there would be a much smaller volume of highly
radioactive material to store [119, 7].
Thus, a method of separating cesium from aqueous solution would be useful in
dealing with existing reprocessing waste. If cesium and strontium were separated from
the waste solutions, they might become a resource instead of a liability. Recovered
137Cs
can be used to sterilize food or medical supplies [119, 98, 19]. With a growing
demand for food irradiation in the U.S., there will be a growing market for
13 7 Cs.
Strontium-90 can be used for long life, no maintainance thermoelectric generators for
remote locations [119, 98, 19].
1.2.2
A Summary of Cesium Separation Techniques
Although reprocessing wastes are acidic, they are often made alkaline for storage to
minimize container corrosion [103]. Some of the past storage and treatment processes
have also produced waste sludges which will have to be dissolved in acidic solutions
for element separations [7]. As the mechanisms of most ion binding processes are
pH dependant, different separation techniques are needed to treat acidic and alkaline
wastes.
24
fl"o-h
Na+
modified crown ether
calixarene
tetraphenyl borate
Figure 1-8: Some Materials Used in Cesium Separations
Alkaline Wastes
Tetraphenylborate
(see Figure 1-8) and metal ferrocyanides or ferricyanides can be
used to precipitate cesium from alkaline solutions [87, 86, 78, 42, 119, 20]. Such precipitation processes have the disadvantage of producing a radioactive sludge. Additionally, the tetraphenyl borate releases large amounts of benzene byproduct [65]. Solvent
extraction processes using phase transfer agents such as dipicrylamine, tetraphenylborate, polyhalides, and specialized phenols in a variety of solvents have also been
studied [119]. However, the currently favored separation processes use solid phase
extractants such as phenolic or resorcinol based ion-exchange resins, or inorganic
zeolites
[73, 7].
Acidic Wastes
Techniques for removing cesium from acidic solutions are less well developed than
the alkaline side treatments. Phosphotungstic acid has been used as a precipitating
agent. Crown ethers [83], calixarenes
[105, 3, 72, 84, 60] (see Figure 1-8), specialized
phenols [13], organo-phosphoric or sulfunic acids, and cobalt dicarbollide (see Section 1.2.3) have been studied as phase transfer agents in solvent extraction processes.
Solid, inorganic ion-exchange materials such as titanium phosphate or ferrocyanide
molybdate
have also been examined [126, 140, 6, 132, 123, 88, 85]. Additionally, these
inorganic materials have been combined with various supports to develop a material
25
more suitable for use as a column packing [131, 51, 135, 136].
Modified crown ethers and the cobalt dicarbollide molecule demonstrate promising cesium binding functionality, but separation processes using these reagents have
not been fully developed [119]. Questions remain about how to effectively strip cesium from crown ether solutions and whether an acceptable solvent can be found
for the cobalt dicarbollide solvent extraction process. Various researchers have finctionalized polymers or other support with crown ethers [142, 66, 50, 10] or cobalt
dicarbollide (see Section 1.2.4) to try and combine the selective binding activity of
the functional group with the process advantages of a solid material. We believe that
this approach is promising. Since the mechanisms of binding, selectivity, and stripping are more straightforward for the cobalt dicarbollide molecule than the crown
ethers, we are concentrating on using cobalt dicarbollide to functionalize polymers
for cesium binding.
1.2.3
Cobalt Dicarbollide in Solvent Extraction
Although there has been some investigation of cobalt dicarbollide as a precipitating agent [24, 25], the majority of studies have examined its properties as a phase
transfer
catalyst for cesium in water/organic
systems [5, 80, 110, 1, 74, 109]. Gen-
erally, the organic phase is a polar solvent like nitrobenzene or ether [106]. With
the addition of polyethylene glycols, strontium and barium can also be extracted
[106, 109, 75, 5, 80, 1]. Other additives can shift selectivity to trivalent metals like
europium and americum [110]. Thus, a cobalt dicarbollide based process might be
useful in separating other radionuclides as well as cesium from nuclear waste solutions.
Several partially chlorinated versions of cobalt dicarbollide, with two to six of
the boron hydrogens replaced by chlorine, have been synthesized [53]. Halogenation
improves the molecule's stability in strong acid without affecting its cesium binding
properties [106, 53]. In Russia, chlorinated cobalt dicarbollide in nitrobenzene has
been used in a pilot scale solvent extraction process to selectively recover cesium from
high level nuclear waste [119].
Because nitrobenzene is a relatively dangerous solvent, efforts have been made
26
to find alternative solvents for the extraction process[5]. Unfortunately, many polar
solvents such as t-butanol seem to stabilized H+ in the organic phase and inhibit Cs+
extraction while non-polar solvents do not sufficiently dissolve cobalt dicarbollide
[90]. One promising approach is to modify the solubility of cobalt dicarbollide by
alkyl substitution at the carbons so that non-polar solvents can be used [90].
1.2.4
Cobalt Dicarbollide in Polymeric Solid Phase Separation
Separations processes which use solid phase ion-exchange materials have several advantages over solvent extraction processes. The primary advantage is the elimination
of organic solvents, which may be carcinogenic, toxic, volitile, or flammable. Solid
phase materials are much less likely to be lost into the aqueous phase than the organic
solvents and phase transfer catalysts used in solvent extraction processes. In the particular application of radioactive waste treatment, a solid phase material could also
be used as a relatively safe form in which to transport radioisotopes. For example, a
filter with cesium bound to it could be moved from the waste generation location to
the waste storage area without the danger of spilling radioactive liquids.
Researchers at Los Alamos have grafted cobalt dicarbollide directly onto crosslinked resin beads [133]. They found that cobalt dicarbollide functionalized polystyrene
beads did not remove cesium from aqueous solutions, and attributed this to the hydrophobic nature of the polystyrene.
Partial sulfunation of the cobalt dicarbollide
functionalized bead increased hydrophilicity and cesium uptake, but probably adversely affected selectivity since sulfunate groups non-selectively bind cations.
1.3
Ring-Opening Metathesis Polymerization
(ROMP)
There are two routes to produce a functionalized polymer. Either a monomer containing the desired functional group can be polymerized, or the functional group can
27
ROMP
Y
X
Figure 1-9: ROMP of Norbornene-Based Monomers
be grafted onto an existing polymer. Generally, the first method can produce more
well defined polymers. Unfortunately, some functional groups tend to interfere with
many methods of polymerization. The carborane group is already known to inhibit
various methods of polymerization and cobalt dicarbollide may have similar effects.
One polymerization system which tolerates a variety of functional substituents is the
ring-opening metathesis polymerization of norbornenes (see Figure 1-9). Thus, the
ROMP system has been identified as a promising route to well-defined carborane and
cobalt dicarbollide containing polymers. This system also has the advantage that
polymerization tends to go to completion with good yield of polymer. Additionally,
it can be used to make copolymers, combining the desirable properties of multiple
homopolymers.
1.3.1
Catalyst Systems
Functionalized norbornene monomers can be polymerized by three varieties of commercially available catalysts. The first catalysts to be used were transition metal chlorides [102]. Norbornene-type monomers with carboxylic acid, ester, anhydride, ether,
and methyl chloride substituents have been polymerized with RuCl 3 , MoCl 5/EtcAl,
WC1 6 /Me 4 Sn, ReC15 , IrCl 3 /cycloocta-1,5-diene
[79, 59, 58]. These catalyst systems
are easy to use because they work at temperatures between 20-70 °C and in solvents
such as water/ethanol.
The main drawback of these catalysts systems is that they
sometimes need long induction times (hours or days) before the metal chlorides un-
28
dergo reaction to form the actual initiating species [102]. The resulting polymers have
molecular weight polydispersities of about
[79].
The later varieties of catalysts are both capable of initiating "living" polymerization.
Polymers made with these organometallic catalysts call have very narrow
molecular weight distributions; well-defined block copolymers can also be produced.
The Schrock catalysts, which contain molybdenum or tungsten, are extremely sensitive to deactivation from oxygen or acidic protons.
These catalysts have been
used to polymerize norbornene monomers containing anhydridles, esters, amides,
ethers, thioethers, pyridine, tertiary amines, buckeyballs, and organometalic complexes [17, 116, 8, 82, 141, 2] but are deactivated by alcohols, carboxylic acids, or
thiols. Generally moderate temperatures are used, and polymerization takes minutes
or hours. Monomers and solvents used must be rigorously dried and purified.
The Schrock ROMP system has even been used to make resin particles for study as
ion exchange materials. These particles have a "filr-like" structure, with many linear
arms attached to a crosslinked core. The arms are first polymerized from a solution
of metal binding monomer. Then a crosslining agent is added and the growing chain
ends become part of the core [12, 125].
The Grubbs catalysts, containing ruthenium, are much easier to use because they
can tolerate water or other protic chemicals and are only slowly deactivated by oxygen [102]. However, monomers with alcohol or carboxylic acid groups near the olefin
bond will still deactivate these catalysts by intramolecular interaction with the propagating metal center. If these groups are attached to the monomer through a spacer
chain, they do not poison the catalyst. Strongly donating groups such as nitriles or
pyridines also prevent polymerization [12]. These catalysts are considered somewhat
less active than the Schrock catalysts [12]. Norbornene monomers with ester, ether,
anhydride, imide, and "distant" alcohol or nitro groups have been polymerized with
these catalysts [67, 100, 101, 139, 28]. Our research primarily used a Grubbs catalyst because preliminary efforts with a Schrock system were unsuccessful, probably
due to insufficient purification of monomers. Figure 1-10 shows the structures of the
catalysts used in this work.
29
0O
IA- r
H3 )2
Figure 1-10: Two Organometallic Catalysts for ROMP
1.3.2
Kinetic Effects
Polymerization Solvent
The main considerations in choosing a polymerization solvent are the solubilities of
the monomer, catalyst, and resulting polymer. Generally, ROMP reactions will not go
to completion in a living manner unless all three components are completely dissolved.
The catalysts are soluble in common solvents, such as methylene chloride, toluene, and
THF. Most norbornene based monomers are also soluble in these solvents. However,
many of the substituted polynorbornenes which were synthesized in this project were
found to precipitate from toluene or methylene chloride.
A secondary consideration for polymerization solvent choice is the effect of the
solvent on the catalyst. Although the Grubbs catalysts have been found to have comparable activity in solvents of varying polarity or coordinating ability, polymerization
is faster in non-coordinating solvents such as methylene chloride [101].
Substituents
The functionalized monomers used in this research were all a mixture of endo and
exo isomers. Due to a combination of steric and intramolecular coordinating effects,
30
the endo isomer is usually less reactive than the exo isomer [67]. However, unless the
substituent actually poisons the catalyst, this reactivity difference will probably not
be noticible in the mixed isomer monomers.
Unsubstituted norbornene polymerizes more readily and more quickly than functionalized norbornene monomers. If propagation is slow, there is more time for side
reactions like chain transfer or catalyst deactivation to occur. Thus, slower polymerizations tend to produce broader molecular weight distributions.
As molecular
weight can have some effect on polymer properties, more monodisperse samples are
preferable for characterization.
However, a narrow molecular weight distribution is
not necessary for applications such high temperature use materials or ion-exchange
resins.
31
Chapter 2
Synthesis of Carborane
Functionalized Norbornene
Monomers
This chapter discusses the synthesis of four o-carborane functionalized norbornene
monomers and a carborane anion norbornene monomer (see Figure 2-1). The synthetic routes described in this chapter demonstrate that norbornene can be readily
functionalized with o-carborane through a range of precursor monomers. The resulting carborane monomers may be used as intermediates in the synthesis of cobalt
dicarbollide functionalized monomers or polymers. Additionally, the variety of carborane monomers made it possible to synthesize a series of polymers all containing the
same bulky, pseudo-aromatic group attached in different ways and to compare the
effects of specific means of attachment
on polymer properties.
These monomers all have the carborane groups attached through a carborane
skeletal carbon (as opposed to boron). This type of bond is readily made by first
deprotonating the carborane with butyl lithium to form a carborane carbanion, as
illustrated in Figure 2-2. Since the carbon protons are more acidic than the boron
protons, skeletal carbon is selectively deprotonated.
The resulting carbanion can
displace leaving groups, forming a Si-C or C-C bond [38, 52]. Purification can be
acheived through solvent extraction, recrystallization, and column chromatography.
32
(I) nor-Si-carb
(11) nor-C-carb
5-(methylcarborane)-norbornene
5-(dimethylsilylcarborane)-norbornene
-1
I
(11
5, 6-(dimethylcarborane)-norbornene
5-(dimethylsilylcarborane anion)-norbornene
(IV) nor-6-carb
5-(methoxybutylcarborane)-norbornene
Figure 2-1: Carborane Functionalized Norbornene-Based Monomers
-
Li+
+
BuLi
o
Figure 2-2: Butyl Lithium Deprotonation of o-Carborane
33
k
/
Li+~
I
aI
Figure 2-3: Unsuccessful Route to a Dicarborane Monomer (1)
In this research, the structure of monomer compounds was verified mainly through
proton nuclear magnetic resonance (HNNMR).Some carbon-13 NMR, boron-10 NMR,
and elemental analysis results were also used. Elemental analysis of some boron cage
compounds underreports the amount of boron because the molecules are unusually
stable and may be incompletely digestioned [133].
2.1
Nor-Si-Carb: A Silicon Bridged Carborane-
Norbornene
Carbanions generally react extremely readily with Si-Cl bonds. Carborane carbanions
are no exception, and there is a long history of using this route to attach groups to
carboranes [38, 52]. Because a C-Si bond is more flexible than a C-C bond, functional
groups attached to a polymer backbone through a silicon bridge may also be relatively
mobile.
Although two carborane groups are reported to have been attached to the single
silicon center of dimethyldichlorosilane [52], our attempts to attach two carborane
groups to the commercially available 5-(dichloromethylsilyl)-norborene were unsuccessful. Figure 2-3 shows the attempted synthesis scheme. The resulting product
mixture was found through chromatography to have approximately one equivalent
of unreacted carborane, but was not further characterized. Possibly the steric effect
of the norbornene and first carborane group prevents reaction of the second Si-Cl
34
bond. Alternatively, a proton transfer of the type describe in Section 2.3.1 may have
occured.
A monofunctional norbornene chlorosilane is not commercially available, but can
be synthesized by a Diels-Alder leaction between chlorodimethylvinylsilane and d!
cyclopentadiene [27]. The resulting 5-(chlorodimethylsilyl)-norbornene (see Figure 24) readily reacts with butyl lithium deprotonated o-carborane in ether to form 5(dimethylsilylcarborane)-rrorbornene in good yield (see Figure 2-5). Nor-Si-carb is a
crystalline monomer that melts at 85-87 °C.
NMR supports the structure of this compound shown in Figure 2-5. Proton NMR
(see Figures 2-6, 2-7) confirms the ratio of one carborane carbon-hydrogen to two
olefin hydrogens to six dimethylsilyl hydrogens. As with all boron cage compounds,
the boron hydrogen peaks are broad and overlapping. This problem can be eliminated
by using "B decoupling, but this option was not available at our facility. 13CNMR
(see Figure 2-8) confirms that the two carborane carbons are non-equivalent, one
with the weaker signal typical of quaternary carbons, and shows that both peaks are
shifted downfield compared to the single carbon peak of o-carborane (see Figure 29). Also visible are the olefin carbon peaks (4, due to endo/exo splitting), 5 carbon
peaks corresponding to the non-olefin carbons of the norbornene group, and a group
of methylsilyl carbons peaks. t1 BNMR (see Figures 2-10, 2-11) shows six main peaks
consistent with the expected 2:2:2:2:1:1 ratio for this compound, along with minor
peaks resulting from the endo/exo isomerism of the dimethylchlorosilyl-norbornene
starting material.
2.2
Nor-C-Carb: A Carbon Bridged Carborane-
Norbornene
Since Si-C bonds in which the carbon is part of an aromatic group tend to be less
robust than C-C bonds, a carbon bridged carborane norbornene was expected to
be more stable than the silicon bridged version. One way to form a C-C bond is
35
2
r
O
!
ltO
iuu/
It
nI
Figure 2-4: HNMR of 5-(Chlorodimethylsilyl)-Norbornene
36
in D-Chloroform
r",
A> / +Li+
Si
0.
Ci
Si
Figure 2-5: Synthesis of Nor-Si-Carb
the reaction of a carbanion species with a compound containing the tosylate leaving
group (see Figure 2-12). Although tosylate leaving groups have not been used widely
in carborane compound synthesis [21], we have found that the carborane carbaniorl
displaces tosylate to form a C-C bond.
Nor-C-carb was synthesized from 5-(methyltosylate)-norbornene
(see Figure 2-
13), which can be synthesized from the commercially available endo/exo norbornene
methanol. The reaction between deprotonated o-carborane and the tosylate has a
lower yield than the reaction with the chlorosilane, but is still reasonable. Nor-Ccarb has a crystalline melting point of 82-87 °C.
HNMR confirms a 2:1 ratio between olefin hydrogens and carborane carbon hydrogens (see Figure 2-14). As with other boron cage compounds, the boron hydrogens
give broad, overlapping peaks which do not have clear integration ratios.
13 CNMR
(see Figure 2-15) shows one carborane carbon to be shifted downfield compared to
o-carborane, while the other carborane carbon peak is too weak to be visible. Additionally, the expected endo/exo split olefin carbon peaks and the other six aliphatic
carbon peaks (five of which show endo/exo splitting) are present.
expected six boron peaks with 1:1:2:2:2:2 ratio (see Figure 2-16).
37
BNMR shows the
rZ
Co
-
LO
c
7>
-C'J
-
}j
-
V.:
I
Figure 2-6: HNMR of Nor-Si-Carb in D-Acetone
38
C2 B1 oH; 2
J
I
I-
-(I
I
I
I
4
I
~~~~I'I
I
I
3
1 I
2
~...
'
'I
'-
1I
1
1
B-H
Figure 2-7: HNMR of o-Carborane in D-Chloroform
39
I .
I
ppm
C ax
!
C"
OTS · 0--/
L90' Z
LO
OZ..P.'
Oaa 6Z"a
ZL. I GE
cl
o
cu
gg'
999'
LE
09T
o
oJ
Figure 2-8: 13CNMR of Nor-Si-Carb in D-Acetone
40
IC
1
w a
"
N c
N oto to
10
NtOCOJ~
'402W1L
C 2 B oH 12
120
100
80__
60
.
40
1
Figure 2-9:
3 CNMR
of o-Carborane in D-Chloroform
41
20
Si,c.
/C
y~~1v
5
I,)
ax
0
Cu
m
1
02
1
4
5,6
-III
I
-2
-0
-2
-4
-6
-8
-10
III
I
II
-12
III
III
II I
-14
Figure 2-10: llBNMR of Nor-Si-Carb in D-Acetone
42
-TIIM I III II f 1-1II III rT
-16
ppm
c
ID ~
4
C2 BoH
1 1
I0
D
W
nr
2
to
'4
S,
m
i
4Q
Figure 2-11:
1 1BNMR
-. Lft
of o-Carborane in D-Chloroform
43
-l
ppm
S//
-6 'reel
O'
pyridine
%a
Li+
-I
.
0 ~1
Figure 2-12: Synthesis of Nor-C-Carb
2.2.1
Effect of Reagent Stoichiometry on Yield
Unreacted norbornene tosylate can be recovered from the nor-C-carb synthesis reaction crude product. Since the butyl lithium deprotonation of o-carborane is known
to proceed to completion, this suggests that it is the reaction between the carborane
carbanion and norbornene tosylate which is hindered. One possibility is that the equilibrium for this reaction might not strongly favor the products. Another possibility
relates to the proton transfer mechanism explained in Section 2.3.1.
According to the proton transfer mechanism, a carborane carbanion can abstract
a proton from the carbon of a carborane moity which is already attached to a norbornene monomer framework. The resulting carborane is unreactive, since it is no
longer deprotonated, and the carborane carbanion monomer is deactivated by steric
hinderance from the norbornene group. This carbanion monomer later reacts with
ambient water to revert to nor-C-carb.
Both of these explainations sugggest that increasing the carborane to norbornene
44
906W,'.6PEV' 069B~t' 0
;ial9'
t
_
.08'0
.
-
M.j,' ;/iE'
-N
It'1""
T-
ao,
o
LO;B'
LE7EB'
060'
a
06LO0P
..I
9EL9V'
E951
cre;i
rH
LL[S'
L665'
L96L'
L2Z;8
r)
L-
?66L9' l03LB9'1
.
-·
g0O90'9
9
9VL9o'g
1 -\
·-/
l
-
0000'
_/
'co
0 o
C
-
Figure 2-13: HNMR of Nor-Tosylate in D-Chloroform
45
90'0
'
0000'Z
leJalui
0
cl
cl
.
II
I
I
-.
.E
a
a
Figure 2-14: HNMR of Nor-C-Carb in D-Chloroform
46
E
EL
0
0CU
o0
K
0o
Il
s
c. f3
oss E
0o
0
89T
00
7%.
cu
To
0
'.4
Figure 2-15: l3 CNMR of Nor-C-Carb in D-Chloroform
47
,
I
Iu
I
cJ
a
I
ADD
N
I
(D
I
(D
(0
ut
/
If
I
l
03
'-- s
LUl
Lo
for-C-Carb in D-Chloroform
48
tosylate ratio would increase the percentage of norbornene tosylate reacting. However,
experimental results from changing the reagent ratio do not support either hypothesis
as an explaination of incomplete reaction. A batch of nor-C-carb synthesized using
a 2:1 carborane to norbornene tosylate ratio actually had a significantly lower yield
calculated based on norbornene tosylate starting material.
This unexpected result could be explained by some difficulties encountered during
the column chromatography purification of the crude product. Alternatively, there
might be an impurity or side reaction that increases with increasing amounts of ocarborane or butyl lithium.
A more detailed analysis of the reaction by-products
would be necessary to understand the mechanisms by which reagent stoichiometry
affects yield
2.2.2
Unusual Tosyl Carborane Reaction
During another batch reaction to test the effect of reagent stoichiometry on the yield
of nor-C-carb, old diethyl ether was inadvertantly used as the solvent. Although
this ether had been stored in a nitrogen glove box, it may have developed peroxides
or free radicals that catalyzed an unusual reaction to make the alternate product,
p-toluenesulfonyl carborane (see Figure 2-17). It is unusual but not unknown for a
nucleophile to react with a tosylate group in this manner [81].
p-Toluenesulfonyl carborane is a white, crystalline solid melting between 125 and
135 C. HNMR of this compound (see Figure 2-18) is very similar to o-carborane,
with the addition of two aromotic peaks and one aliphatic peak which can be assigned
to the toluenyl protons. The aromatic peaks are the same size and each is twice the
size of the carborane carbon proton peak.
49
Li+
_L
+
7
0
/~
Figure 2-17: Synthesis of Tosyl Carborane
2.3
Nor-CC-Carb: A Monomer with Twice-Bound
Carborane
In this section we describe how an attempt to synthesize one interesting monomer led
to an unexpected, also interesting, alternative monomer.
All these carborane functionalized monomers are potential intermediates to a
cobalt dicarbollide functionalized polymer. Since cobalt dicarbollide is made from
two carborane derivatives, it might be synthetically advantageous if two carborane
groups could be attached to a monomer. If the two carborane groups were attached
so that the distance between them was approximately equal to the distance between
the two carboranes derivatives in a cobalt dicarbollide molecule, it might be possible to convert the monomer with two carboranes into a cobalt dicarbollide monomer
without any additional carborane.
As described in Section 2.1, an attempt to react two carboranes with a dichlorosilane was unsuccessful, possibly due to steric hinderance around the single silicon
center. Trans-5,6-bis(methyltosylate)norbornene
50
has two reactive sites which are far
99LL2
02vOL
99890'
-cu
-'
ZEE'
00vZa'
OS9LE
L£E6L
9LEEg'
-M
S;EE9'
GP;69'
10698'
E89L§
ZPEBE'
--10
ifl
_LD
E6;IO'
B§E8'
-r-.
99'
ZEE6L'
69L5L'
LOOSB'
Ei6;B'
tI
t8L6
wd
_ / _leJ6alul
Figure 2-18: HNMR of p-Toluenesulfonyl Carborane in D-Chloroform
51
E
0.
0.
+
Li+
00
2
Figure 2-19: Unsuccessful Route to a Dicarborane Monomer (2)
enough apart that substitution at one site may not hinder reaction at the second
site. These sites would also provide appropriate spacing for two carborane groups
to become incorporated into the same cobalt dicarbollide molecule. This ditosylate
can be made from the trans dicarboxylic acid norbornene which is synthesized from
cyclopentadiene and fumaric acid.
Upon reaction with deprotonated o-carborane, the ditosylate did not convert into
a dicarborane monomer (as pictured in Figure 2-19). Instead, the resulting monomer
contained single carborane group attached through carbon at both the 5 and 6 sites
(see Figure 2-20). Thus, nor-CC-carb is pseudo-tetracyclic; a tricyclic structure fused
to the pseudocyclic carborane group. Nor-CC-carb was somewhat more difficult to
purify than nor-C-carb, and was recovered in slightly lower yield. The monomer is a
white crystal that melts between 150 and 170 °C and is soluble in THF, chlorform,
and acetone. None of the originally expected dicarborane monomer was recovered.
Unlike most of the precursor monomers used in this study, trans-5,6-bis(methyltosylate)norbornene does not have different endo/exo isomers, since each molecule has
52
Li+
+
2
1-10/'1
I
Figure 2-20: Synthesis of Nor-CC-Carb
one endo and one exo substituent. This is reflected in the HNMR, where there is no
endo/exo peak splitting (see Figure 2-21). Similarly, HNMR spectrum of nor-CCcarb has no endo/exo splitting (see Figure 2-22). The expected broad, overlapping
boron hydrogen peaks are visible and there is no remaining carborane carbon hydrogen peak. The expected norbornene framework hydrogen peaks are also present.
2.3.1
The Proton Transfer Mechanism
In order for both carborane carbons to form bonds to the ditosylate norbornene
monomer, both skeletal carbons would have to be deprotonated since the protonated
carborane carbon is not nucleophilic. It has been reported that if two equivalents of
butyl lithium are used so that both carborane carbons are deprotonated, then the
dicarbanion can react at both carbons [38, 52]. However, only 1.2 equivalents of
butyl lithium were used in our synthesis and the nominal excess of 20% was used to
compensate for partial deactivation of the butyl lithium.
We propose that during the course of the reaction, first some 5-(methylcarborane),6(methyltosylate)-norbornene is formed (see Figure 2-23). Then, the remaining carborane carbon proton of the monomer is removed by a deprotonated carborane in solution. This type of proton interchange between carboranes and carborane carbanions
has been previously reported [38]. An intramolecular reaction between the tosylate
arm and the deprotonated carborane arm of the monomer completes the synthesis.
53
9669r'
562V
E990E
i
y_- 090' 1
6LEEE'
mv,
.N
S66'
~6BO'
E0P9
_
OLEEa
OEEO';
-m
2269S'
Z90PL
C\4
r
56091'
1996B'
95025
-,,
-u
ZLOZ
I
C
I
06ZEO
9LOO'
92LSO0
.in
E6pE'
E9LSE
VE09E'
BL LL
968LL
S098L
61E6L'§
.I
-
6100'T
_
266-0'9;
,,.,.
WOOS W
0000'
wr
9LEOP'L
SESOP'L
666 'L
cJ
8SEc L
BSZcP'L
aE99SL'L
90
8BLL'L
I'~
2P66L'L
dd
'
6E66'E
E
teJ6alul
Figure 2-21: HNMR of Trans-5,6-bis(methyltosylate)norbornene
Chloride
54
in D-Methylene
[
0.
-
i
.CU
{
L
(D
N
0)
CM
E
LL'O
8
v
0
0
000'
g
Is
8
wdd
leJ6alui
Figure 2-22: HNMR of Nor-CC-Carb in D-Chloroform
55
iI1
LI+
I
+
v
C
e0
Figure 2-23: Proposed Proton Transfer Mechanism in the Synthesis of Nor-CC-Carb
Possibly, the attachment of the first carborane group slightly deactivates the second tosylate due to steric hinderance, and then the intramolecular reaction occurs in
preference to the addition of a second carborane group.
The proton transfer between deprotonated carborane and a carborane already
bound to a norbornene group could be studied by combining nor-C-carb with deprotonated carborane in solution, and then adding an unhindered chlorosilane, tosylate,
or halide. For example, if ethyl iodide was used, the product would be examined for
the relative amounts of 5-(methylethylcarborane)-norbornene
and ethyl carborane.
It would also be interesting to repeat the synthesis starting from the commercially available cis-dicarboxylic acid norbornene and going through the cis-ditosylate
norbornene. Both the substituents of the commercially available cis monomer are in
the endo position, so the distance between and relative orientation of the two reaction sites would be different than they were for the trans monomer. If this spatial
arrangement favored intermolecular instead of intramolecular reaction, use of the cisditosylate might result in a dicarborane norbornene, which would be a good candidate
for a cobalt dicarbollide norbornene precursor.
56
2.4
Nor-6-Carb: A Norbornene Monomer with Pen-
dant Carborane Attached through a Spacer
Chain
Polymers with pendant carborane may find applications as ion-exchange resins or
polymer electrolytes. For both applications, the accessibility or mobility of the functional group is important.
Thus, a molecular architecture which uses spacer chains
to separate the functional groupsfrom the polymer backbone might improve properties.
Although these applications are envisioned for polymers with ether cobalt
dicarbollide or carborane anion groups and not for polymers functionalized with ocarborane, we have synthesized a carborane monomer with a spacer chain because
carborane monomers can be precursors to cobalt dicarbolllide polymers. Additionally, the reaction chemistry of carborane and the carborane anion are quite similar,
so the analogous carborane anion monomer synthesis should be possible.
A spacer chain length of six atoms was chosen because spacer effects usually start
to appear when the chain length reaches 4-6 atoms.
The scheme used here (see
Figure 2-24) can be easily extended to make spacer chains of various lengths by using
different dibromoalkanes.
The functionalized norbornene precursor monomer has a bromide leaving group,
which is less reactive than the tosylate group. However, deprotonated carborane has
been successfully reacted with alkyl bromides [52]. We found that the deprotonated
carborane is able to displace bromide, but that the product is recovered in relatively
low yield. Nor-6-carb is a clear oil at room temperature.
HNMR shows the characteristic carborane carbon proton peak in 1:2 ratio with the
olefin hydrogen peaks (see Figure 2-25). The broad carborane boron hydrogen peaks
and the peaks corresponding to the hydrogens nearest the oxygen can be identified.
Other peaks corresponding to the other spacer chain protons and the norbornene
framework are also visible.
57
+
Br
Br
NaOH, TBAH
H2 0/hexane
Li+
Br
-t
/
Figure 2-24: Synthesis of Nor-6-Carb
58
THF
OLS2'
,I\
899£'
568V5'
;269g
960L'
V;9LG'
L m
F
80OE9'
I
;6L8'
860E8'
ZZtrZ
r
>M
-,J
m:
2 692'
82982'
aLz8'
98E68'
80P66
-m
069t0'
096E0'
090O'
A
5ELZO'
.>
22EO
cV
o0000o
ZS9E-'
6£ZE'
ELLOP'
MO'
890g'
896F6'
P9926
LEE6'
CM
ZL80'
g6EV'
g909;
wdd.
TeJ6alu!
Figure 2-25: HNMR of Nor-6-Carb in D-Chloroform
59
.LO
E
2.5
Nor-Si-Anion: A Norbornene Monomer with
a Carborane Anion Group
The chemistry of the carborane anion, CB11HI2 , has not been as extensively studied as
those of o-carborane or cobalt dicarbollide. However, this anion has been recognized
as sharing some of the interesting properties of the cobalt dicarbollide ion which result
from an extremely low charge density [106, 112, 62, 18]. A polymer functionalized
with carborane anion groups might be useful as a flexible solid electrolyte for batteries
or as a selective ion exchange material.
Since the chemistry of the carborane anion is generally considered to be similar
to that of the o-carborane [70, 71, 62, 107, 61], we have used a synthesis analogous
to the one described in section 2.1 to make a carborane anion norbornene monomer
(see Figure 2-26). However, the anion's different solubility properties [61] make it
necessary to adjust some parts of the synthesis and purification procedure.
Because our experience making o-carborane norbornene monomers showed that
the best yield was obtained using the 5-(chlorodimethylsilyi)-norbornene precursor,
we used the same chlorosilane precursor. The reaction produced a white, solid product that melted at 200-210 °C. The HNMR of nor-Si-anion shows the broad boron
hydrogen peaks and the expected olefin and methylsilyl hydrogen peaks in the correct
ratio (see Figure 2-27).
2.6
Experimental Details
Although these types of reactions can be done in a chemical hood using air-free
apparatus, we carried out all butyl lithium reactions in a nitrogen glove box.
2.6.1
Materials
Tosyl chloride was purchased from Aldrich and purified by dissolving in the minimum amount of chloriform, adding 5 times by volume petroleum ether, filtering, and
evaporating the filtrate to recover crystals. n-Butyl lithium in hexanes (either 2.5M
60
-1
-1
Li+
+
BuLi
THF
I
I
-1
i--CI
+| i
/
Si
THF
I
0
-1
Figure 2-26: Synthesis of Nor-Si-Anion
61
6;'
50'
0OQ
to.
20'
5VV2'92~
-o
60'
I'
2;'
a'
L.
62'
OE'
LO
CV)
,I
tE'
EE'
SE'
98'
98'
-ru
L8'
88'
68'
68'
06'
16'
'E'
08''
96L6'4
ZE'l
EE'
1*
I *s
96ZO-Z
I *
EE'
PE'
E9'1
06'1
56't
-Lf
99'_
E9''
88't
68'
68'(
06'G
t6'
0666';
-w7
L '9
L '9
ZVS
[eA;§uI
Figure 2-27: HNMR of Nor-Si-Anion in D-Methanol
62
cl
or 10.OM), anhydrous solvents, and silica gel were purchased from Aldrich and used
wit!ott
purification. o-Carborane was purchased from Consumer Health Research
Inc, which became E. Kantor Ent and is now out of business, and used without
purification. The trirnethylammonium salt of carborane anion was purchased from
Katchem Ltd in the Czech Republic.
2.6.2
Equipment
All '1 BNMR and some HNMR and '3 CNMR were done at 3M by Dr. James Hill,
with interpretation by Dr. Richard Newmark. A 500 MHz UNITY sepctrometer was
used. The rest of the
1
3 CNMR
and HNMR was taken on a Bruker 400 MHz FTNMR.
Melting points were determined visually using a Mettler FP82 hot stage and Nikon
optical microscope with a heating rate of 20 C/minute.
2.6.3
Nor-Si-Carb
An endo/exo mixture of chlorodimethylsilyl norbornene was synthesized by Dr. Randall Saunders according to the Finkelshtein procedure [27] and donated for our research. Nor-Si-carb was synthesized in batches ranging from 1 to 5 grams of starting
o-carborane. In a typical reaction, 1 g of o-carborane was dissolved in 10 ml of anhydrous diethyl ether. A solution of 2.77 ml of 2.5 M n-butyl lithium in 10 ml ether
(one equivalent) was slowly added to the stirred carborane solution. After 1 hour,
1.295 g (one equivalant) of the liquid chlorodimethylsilyl norbornene was added. The
solution became cloudy with precipitate and boiled. After stirring overnight at room
temperature, the solution was shaken with 20 ml distilled water and 20 ml ether in
a separatory funnel. The ether phase was dried and the solid recrystallized from
boiling methanol. Typically, first batch yields were 60-70%. The monomer is soluble
in ethanol, methylene chloride, THF, and acetone. Elemental analysis by Galbraith
gave mass percents: carbon 44% (calc. 45%), hydrogen 8% (calc. 9%), boron 37%
(calc. 37%), silicon 10% (calc. 10%).
63
2.6.4
Nor-C-Carb
An endo/exo mixture of 5-(methyltosylate)-norbornene was synthesized by the literature procedure [17, 116]. Nor-C-carb was synthesized in batches ranging from 1 to 5
grams of starting o-carborane. In a typical reaction, 1 g of o-carborane was dissolved
in 10 ml of anhydrous diethyl ether. A solution of 2.77 ml of 2.5 n-butyl lithium (one
equivalent) in 10 ml ether was slowly added to the stirred carborane solution. After
30 minutes, a solution of 1.93 g of norbornene tosylate (one equivalent) in 3 ml ether
was added to the stirred solution. After 2 hours, off-white precipitate was observed.
The solution was stirred overnight at room temperature and then added to 20 ml
ether and 40 ml water in a separatory funnel. The ether phase dried to an peach colored wax. The crude product was fractionated using column chromatography, with
a 5:1 hexane:ethyl acetate eleutant and silica gel adsorbant. The product is the first
eleuting compound. After chromotographic purification, the yield is typically 40-50%.
Approximately 35% of the crude product mass never eleutes from the column, even if
a very polar solvent like acetone is used at the end. Nor-C-carb is soluble in acetone,
THF, and chloroform.
In the batch of 5-(methylcarborane)-norbornene
synthesized with a 2:1 ratio of
carborane to norbornene tosylate starting material, the procedure was identical except
that half as much norbornene tosylate was used. The yield after purification was 16%
based on the amount of norbornene tosylate starting material.
2.6.5
p-Toluenesulfonyl Carborane
The anhydrous ether used as solvent had been stored in a nitrogen glove box at room
temperature for at least a year. A solution of 3.736 g o-carborane in 40 ml ether was
stirred. A solution of 3.118 ml of 1OM n-butyl lithium in hexanes (1.2 equivalents)
in 40 ml ether was slowly added.
The butyl lithium solution had recently been
titrated and determined to be partially deactivated so that a 20% nominal excess was
needed. After 30 minutes, a solution of 3.616 g 5,6-bis(methyltosylate)-norbornene
(0.5 equivalents) in 12 ml ether was dripped into the stirred carborane solution.
64
After stirring overnight, the solution was evaporated to a thick liquid and rinsed
with 20 ml hexane.
The liquid was diluted with 20 ml ther and shaken with 3
times 20 ml distilled water. Upon addition of the water, gas was evolved from the
solution. The ether fraction dried to a white crystalline solid melting between 125 and
135 C. The product could be purified by either recrystallization from ether allowed
to evaporate, or column chromatography using silica gel and a 1:2 mixture of ethyl
acetate/hexane.
2.6.6
The product is the first eleuting fraction.
Nor-CC-Carb
The trans-5,6-bis(methyltosylate)-norbornene
was synthesized by Vishak Sankaran
during his graduate research [116]. It was stored for approximately 5 years in a clear
glass jar at room temperature and used without further purification. To 25 ml of
anhydrous diethyl ether was added 2.5 g o-carborane. To 25 ml anhydrous ether was
added 2.00 ml of 10M n-butyl lithium in hexanes (1.2 equivalents). The butyl lithium
solution was slowly dripped into the stirred carborane solution. After 30 minutes, 4
g of ditosylate norbornene (0.5 equivalents) in 10 ml anhydrous tetrahydrofuran was
dropped into the stirring solution. The solution boiled and became cloudy. Although
the ditosylate norbornene is not soluble in ether, it will mostly stay in solution if first
dissolved in THF.
The solution was stirred overnight and filtered. The filtrate was extracted twice
with 100 ml distilled water. The ether phase was dried to an orange solid. This
crude product was purified using column chromatography with chloroform eluent
and silica gel adsorbant.
The product is the first fraction eleuted. After column
chromatography, the product is a white crystal melting between 112 and 170 °C. The
product was further purified by recrystallization from boiling ethanol to give a white
crystal in 32% yield, calculated on a ditosylate norbornene basis. The recrystallized
product melted at 150-170 °C.
65
2.6.7
Nor-6-Carb
The synthesis of this monomer was done in anhydrous THF solution instead of diethyl
ether, for no particular reason.
Generally, diethyl ether is used in these kind of
reactions because the resulting lithium salts (lithium chloride, lithium tosylate, or
lithium bromide) have a low solubility in ether and precipitate, thus keeping the
concentration of these coproducts low.
5-(metosybutylbromide)-norbornene
was synthesized by Dr. Jin Kyu Lee using a
method analogous to that of the literature [77].
A solution of 0.56 grams o-carborane in 15 ml THF was stirred while a solution of
0.466 ml 10 M n-butyl lithium in hexanes (1.2 equivalents)
added.
in 5 ml THF was slowly
After 15 minutes, this solution was dripped into a stirred solution of 1.0
g 5-(methoxybutylbromide)-norbornene
(1 equivalent) in 20 ml THF. The solution
turned yellow and was stirred overnight. There was no precipitate.
Because there was concern that carborane carbanion might not readily displace
the bromide leaving group at room temperature,
a 15 ml aliquot of the reaction
solution was then removed and heated to 55-60 °C for 4.5 hours. Both the unheated
and heated solutions were drid to a thick liquid, and then 50 ml ether was added
to each. Each was then extracted with three times 50 ml distilled water. The ether
phases were dried to yellowish liquids.
The crude product from the unheated portion was purified by column chromotography using silica gel and a 1:8 mixture of ethyl acetate and petroleum ether. This
particular chromatography system tended to have a lot of column cracking.
The crude product from the heated portion was purified by column chromatography using silical gel and a 1:1 mixture of hexane and chloroform. TLC tests comparing
the two different solvent mixtures suggest that the ethyl acetate/petroleum
ether mix-
ture eleutes the fractions with less tailing. However, the chloroform/hexane system is
preferred to minimize column cracking. In either case, the product is the second fraction eleuted. Because of the different purification schemes, the yields for the heated
and unheated reactions could not be dirctly compared. The product fractions from
66
each batch were combined. The total yield was 23%.
2.6.8
Nor-Si-Anion, Cesium Salt
Upon deprotonation with butyl lithium, the carborane anion becomes a dianionic
species containing one carbanion. The solubility of this dianionic species depends on
its counterions. Since deprotonation is accomplished with butyl lithium, one of the
cations must be lithium. However, the choice of the original cation for the carborane
anion is critical [61]. The preferred counterion is trimethylammonium, because this
ion reacts with butyl lithium, evolving the gas trimethylamine and leaving behind
lithium as a replacement counterion. The dilithiurn salt of the deprotonated dianion
is sufficiently soluble in THF to permit further controlled reaction [61].
A solution of 0.5 g of the carborane anion trimethylamine salt in 15ml THF was
stirred and a solution of 0.542 ml 10 M n-butyl lithium in hexanes (2.2 equivalents) in
5 ml THF was slowly added. About halfway through the addition, the stirred solution
became light yellow. This color change is associated with the deprotonation of the
carborane anion. After 45 minutes, a solution of 0.506 g 5-(dichloromethylsilyl)norbornene (1.1 equivalents) in 5 ml THF was added.
The solution was stirred
overnight, then dried to a paste. The paste was dissolved in 10 ml distilled water
and aqueous cesium chloride was added to precipitate an off white solid. The solid
was rinsed with 3 times 10 ml water and 3 times 10 ml hexane. The solids were dissolved in an acetone/water mixture and the solution allowed to evaporate until solids
precipitated.
The first fraction collected gave a 33% yield. The remaining liquid
was evaporated to collect a fraction of approximately 33% yield of less pure material.
These fractions were rinsed with hexane. The cesium salt of nor-Si-anion is soluble
in methanol, THF and acetone, and slightly soluble in toluene.
67
Chapter 3
Synthesis and Characterization of
o-Carborane Funtionalized
Polymers
We have synthesized a series of four norbornene-based polymers with pendant ocarborane groups through ROMP of monomers described in Chapter 2. Additionally,
block copolymers containing a glassy carborane functionalized block and a rubbery
block were synthesized. In each case, a commercially available Grubbs ruthenium
catalyst was used as the initiator.
Some thermal and optical properties of these polymers were measured. Comparison of the polymers demonstrates how changes in molecular structure affect macroscopic properties. The diblock copolymers synthesized are the first reported polymers
showing microphase separation between a carborane containing block and a boron-free
block. Characterization of these copolymers also demonstrates that copolymerization
can improve the processibility of materials synthesized by ROMP.
3.1
Polymerization
The Grubbs ruthenium catalyst system was selected because of its ease of use and
ability to tolerate functional groups. Although the o-carborane group is not acidic,
68
nucleophilic, or otherwise chemically reactive, its electron withdrawing effect and
steric bulk have inhibited some types of polymerization reactions [34, 35, 36]. We
found that the o-carborane groups do slow polymerization compared to unsubstituted
norbornene, but that the rate of reaction is still reasonably high.
ROMP is commonly used to make polymers with up to 1000 repeat units. Since
there was no need for high molecular weights, we chose polymer lengths of approximately 100 repeat units for our homopolymers. Both block of the diblock copolymers
were also approximately 100 repeat units.
Although the Grubbs catalysts are not deactivated by water and are only slowly
deactivated by oxygen [102], all polymerizations were carried out in a nitrogen glove
box because the catalysts were stored there. All polymerization was done at room
temperature.
3.1.1
Solvent Systems
Initially, methylene chloride was used as the polymerization solvent for our syntheses.
Non-coordinating solvents such as methylene chloride result in the highest rates of
polymerization for the Grubbs catalyst [101]. Although all the monomers are quite
soluble in methylene chloride, some of the polymers have limited solubility. These
polymers do not precipitate out of methylene chloride, but do form a separate polymer
rich liquid phase. Such phase separation is undesirable because it can result in incomplete polymerization of the monomer. Phase separation is more pronounced with
the two carbon bridged carborane polymers than with the silicon bridged carborane
polymer.
The carborane functionalized polymers are not soluble in toluene, but are very soluble in THF. Thus, later polymerizations of the carborane monomers were all carried
out in THF. The reaction remains homogenous in THF and the rate of polymerization
is still acceptably high. THF solutions also have the advantage of precipitating more
cleanly in methanol that methylene chloride solutions do. Although these polymers
are at least somewhat soluble in acetone and ethanol, the use of these polar solvents
was not investigated because satisfactory result were obtained with THF.
69
3.1.2
Kinetics and Polydispersity
The Grubbs ruthenium catalysts can produce polymers with very narrow molecular
weight distributions [67]. Although a narrow molecular weight distribution is not
critical to most polymer applications, measurement of the polydispersity index (PDI)
can provide information about the polymerization process. If a sample has a PDI only
slightly over 1, it indicates that initiation is fast compared to propagation and that no
chain transfer or termination occurs. Termination, chain transfer or coupling can be
caused by impurities in the monomer or solvent. Although the Grubbs catalysts are
not deactivated by protic solvents, they can be poisoned by monomers with protic
substituents in some positions [102]. Also, oxygen causes slow deactivation of the
catalyst.
In order to obtain the minimal PDI, polymerization should be terminated by the
addition of an agent such as ethyl vinyl ether. This cleaves the ruthenium species
from the end of the polymer. We have found that low polydispersity samples will
undergo chain transfer or coupling with time if not deliberately terminated. Samples
left overnight may go from a PDI of 1.1 to 1.2 and evidence a high molecular peak
"shoulder" in GPC data. This type of shoulder is probably due to the same type of
chain coupling that is observed with the Schrock molybdenum catalyst system.
We have not conducted a rigorous study of polymerization kinetics, but have
made some preliminary investigations. It is important to know approximately how
long polymerization takes in order to allow sufficient reaction time per block when
making block copolymers. If the second monomer is added before the first monomer
is depleted, a statistical or "random" copolymer may be produced. Block copolymers
and random copolymers may have very different physical properties, so it is important
to be able to control copolymerization.
Norbornene polymerizes so quickly in methylene chloride with the Grubbs catalyst
that the heat of reaction can cause the solvent to boil. Functionalized monomers react
more slowly, and coordinating solvents such as THF also slow the reaction. However,
the polymerization of carborane functionalized norbornene monomers still reaches
70
Mt
I-
0.9
50000
iii
y = -0.019951 + 0.015714x
R= 0.P9933
0.8
c
0.7
'.
0.6
E
0.5
0.4
0.3
0.2
-r
1(
o
.
0.,
. . .
.
.
20
.
.
.
.
.
.
.
30
.
.
.
.
40
.
., I. ....'''
.
.
.
.
50
.
.
.
60
70
time (min)
Figure 3-1: Polymerization Rate for Nor-CC-Carb
90% or higher conversion within hours.
Batch process living polymerizations are modeled by the equation
In([/o,]/[M])= Kp[I]t
where [Mo]]is the original monomer concentration, [M] the instantaneous monomer
concentration, Kp the polymerization rate constant, [I] the initiator concentration,
and t is time. To calculate rate constants from molecular weight data, we used the
relationship
[M]/[M]= Mnf /(Mnf - MIn)
where Mnf is the final number average molecular weight of the polymer and Mn is
the instantaneous number average molecular weight of the polymer. Technically, the
true final weight is only reached at infinite time, but can be estimated from finite
time data. Figure 3.1.2 demonstrates a plot used to calculate Kp.
Our carborane monomers have polymerization rate constants of approximately
71
30 l/mol.minute.
For the initiator concentrations typically used, this means that it
takes from one to three hours for 90% or more of the monomer to be polymerized.
This is comparable to the rate of other substituted norbornenes such as norbornene
methoxy trimethylsilane, and consistent with other reported rates of metathesis by
Grubbs catalysts in THF [100].
An upper limit on reagent concentration is imnposedby the viscocity of the resulting polymer solution. It is undesirable to produce a very viscous solution, as such
solutions are more difficult to keep well-stirred. We have generally used between 0.05
and 0.2 g of monomer in 5 ml of solvent.
3.1.3
Spectroscopic Characterization
NMR
In general, the NMR spectra of polymeric materials have broad peaks due to slight
variations in the environments of repeat units at different points along the polymer
backbone. This means that it is much easier to analyze monomer chemical structure
than polymer structure with NMR. As the ruthenium catalyzed ROMP process produces polymer structures which are predictable from the structure of the monomers,
we have primarily analyzed the monomers and only used NMR on the polymers to
confirm the expected reaction.
As expected, the carborane functionalized polymers have HNMR spectra similar
to those of the monomers but with broader peaks. The primary difference between
monomer and polymer spectra is the shift in position of the olefin hydrogen peaks
from approximately 6ppm in the monomers to approximately 5ppm in the polymers.
This shift reflects the change from a strained cyclic structure to a linear polymer
structure (see Figure 1-9). The carborane carbon hydrogen peak and the broad
carborane boron hydrogen peaks are evident at much the same positions as in the
monomer spectra. As expected, the peaks due to the backbone norbornene structure
are broadened the most upon polymerization, while the peaks due to the pendant
carborane functionality are less affected (see Figures 3-2, 3-3). Similarly, l3 CNMR
72
of the polymers resembles that of the monomers, with significant broadening of the
norbornene carbon peaks (see Figures 3-4, 3-5). l1BNMR peaks are also broadened
(see Figure 3-6).
UV/IR
Carborane compounds have a very strong characteristic mid IR absorption band at
about 2600 cm-1 which is attributed to the carborane boron-hydrogen bonds [34, 117,
104, 26, 1, 108]. This peak can be seen in the spectra of our carborane functionalized
polymers (see Figure 3-7). o-Carborane also absorbs strongly in a number of peaks
between 2850 and 2950 cm-', but these peaks are less useful for identification because
aliphatic hydrocarbons have similar absorptions. The carborane peak near 700 cm-'
is also not useful for identification purposes.
Another characteristic adsorption peak for carborane compounds is near 3070
cm- 1 [108, 26]. This band is due to the pseudoaromatic carbon-hydrogen bonds.
Thus, the band is absent in carborane main-chain polymers where both carborane
carbons lack attached hydrogen [104]. This peak is absent in the spectrum of poly-norCC-carb because the pendant carborane groups are bonded through both carbons to
the norbornene structure and thus lack carborane carbon-hydrogen bonds. However,
this peak is visible in the spectrum of polynorSicarb, reflecting the presence of the
carbon-hydrogen bond in each pendant carborane group.
Carborane compounds also have a weak near IR adsorption peak around 1920
nm, an overtone of the strong boron-hydrogen absorption at 2600 cm- 1 [1]. This
characteristic peak is present in the spectra of our polymers (see Figure 3-8). The
other near IR adsorption peaks in our polymer spectra can be attributed to bonds
within the norbornene framework. Carborane does not have other visible, UV, or
near-IR adsorption peaks.
73
90c0' 0-
0)
0
oo
LTE 0O
5SBE' 0
LEO'
Esaz,
GGLP'T
E08'I
6906';
L090'
2990'
LLO0'Z
ILLO'2
E92'Z
9ZZ-Z
BLZZZ'
Z008'
'rO
86,BZ'
Z006'2
09g; 0
S9E'E
BE9 E
o
6ESL'E
6lB'O
66L'
0
_-n
960'5
C
0000oooo'a
d98d
*
TeJ6aluI
o .
wdd
Figure 3-2: HNMR of Poly-Nor-Si-Carb in D-Acetone
74
OC
U
tm
11
o]
cu
a1
w
C.I
ft
er}
M
tl
0
mv
v
m
mll"
n . N
o 0M M
O ' D ILv
1
U
-
.;- .t .~
4 .~ . .
7N I
rm
UO
M1 cu -
wO
4
ao-
-. q. I.u- o.: o0 *6
- . f*~. uNt
KEN\\ NM I~Y
I;,
2
7
6
8
*
9
B-H,3-8
0
a
7
6
5
-N
N
4
3
2
1
Figure 3-3: HNMR of Poly-Nor-C-Carb in D-Methylene Chloride
75
0
a
5
.O
-I
-o
-cu
-'
.l
-o
-O
CU
: s
Figure 3-4: 13CNMR of Poly-Nor-Si-Carb in D-Acetone
76
E
aQ
o
1$
o
}O}
a0
oo
CD
o
0
cu
o
qq
Figure 3-5: ' 3 CNMR of Poly-Nor-C-Carb in D-THF
77
C3
(0
IIb
*-
* I
-a
-C
Figure 3-6: 1lBNMR of Poly-Nor-C-Carb in D-THF
78
3500
3000
2500
3500
Wavenumbers (cm-l)
3000'
Wavenumbers (cm-1)
Figure 3-7: IR Transmittance of Carborane Functionalized Polymers
79
2500
!
poly(nor-CC-carb)
B-H
norbomene framework
28.0 e68e
.eB8
1ie.e8.is
14e.I
1888 .88
2288. 88
nM
Figure 3-8: Near IR Transmittance of Carborane Functionalize Polymers
80
3.2
Block Copolymers
Copolymerization is often used to produce materials which combine the desirable
properties of each homopolymer. Solubility, thermal behavior, and mechanical properties can be tailored through copolymerization. Block copolymerization can produce
materials with microphlase separation of the two component polymers. Such nanoscale
morphology (lamellae, cylinders, or spheres) may enhance physical properties and can
provide a way to segregate functional groups into small domains.
As describe in the following section, most of the carborane finctionalized norbornenebased homopolymers that we synthesized have high glass transition temperatures
(Tg). Such polymers are difficult to process into thick films, which are used in mechanical testing, because thermal decomposition may occur before the material begins
to flow. Additionally, the materials are too brittle for normal handling. The Tg of
carborane functionalized polymers can be lowered by attaching the functional group
to the norbornene backbone via a long spacer chain. Alternatively, high Tg carborane polymers can be incorporated into block copolymers with a companion low Tg
norbornene-based polymer. We have used poly(trimethylsilylmethoxynorbornene).
The resulting material can be thermally processed at lower temperatures.
Whilestatistical copolymersgenerally have thermal transition temperatures intermediate between those of the two homopolymers, block copolymers usually demonstrate two distinct transitions. The temperatures of these two transitions are nearly
identical to the the transition temperatures of the homopolymers. Thus, tests on
block copolymers can be substituted for tests on the component homopolymers. Our
block copolymer materials were used to measure the Tg of the homopolymers through
dynamic mechanical analysis (DMA).
Transmission electron microscopy was used to image the microphase separation
of these block copolymers. Both lamellar and cylindrical morphology were observed
(see Figures 3-9, 3-10). These materials are unusual because they contain phases with
high boron density (37-43 wt%) and phases with no boron content. Pyrolisis of such
polymers might produce nanoscale boron ceramic particles.
81
--I000 A,
I .,,
t.
.
t:
···
?.
·
·,·
r
r
.-·'·S'
s.
:··. ,L"
-·
1:.
,r.
·::
:
-. · ·
,.· ·
fr:1
·· ·
··
::`
lr-
:?.
:.·
;
·.,`lr·
'J
·'
''
'·
2;4f)
:B
-·-
:· '
,·
Z
''
rr
=r'
A,
54v-:L-,
W'i
..
---
,,
Figure 3-9: TENMof Poly-Nor-CC-Carb/Poly-Nor-OTMS
82
Block Copolymer
'
.+
aau(t
1
%
e
t
*
}
z
h
is
Z
H;H
.
4i
Figure 3-10: TEM of Poly-Nor-OTMS/Poly-Nor-Si-Carb Block Coplymer
83
Table 3.1: Comparison of o-Carborane Functionalized Materials
Monomer
Tm(°C)
Tg(°C)
wt%boron
nor-Si-carb
nor-C-carb
nor-CC-carb
nor-6-carb
norbornene
87
87
170
<20
46
187
200
291
29
32
37
43
41
34
0
3.3
Structure/Thermal Property Relationships
3.3.1
Glass Transition Temperature
It was anticipated that structural differences in how the carborane functional group
was attached to the polymer backbone would affect thermal properties. Generally, the
addition of bulky side groups raises a polymer's Tg because chain motion is sterically
hindered. Tightly bound groups near the backbone raise Tg significantly, while more
distant groups have less effect. These trends were observed in our series of carborane
polymers. Table 3.1 lists the monomer Tin, polymer Tg, and wt%boron for these
materials.
Polynorbornene has a fairly low Tg of 32 C. The addition of a pendant carborane
group via a silicon bridge dramatically increases the Tg to 187 °C, as measured by
DMA. The dimethyl silicon, although more bulky than a carbon bridge, is also more
flexible because the Si-C bond is longer than the C-C bond. This flexibility difference
is reflected in the higher Tg of the carbon bridged nor-C-carb, 200 C. The twicebound pendant carborane has much less freedom of motion than the singly-bound
carborane. The measured Tg for the polymer with twice-bound pendant carborane
is 291 C, significantly higher than either of the singly-bound carborane polymers.
Thus, the presence of the carborane group near the polymer backbone raises the
polymer Tg by 150-260 C, depending on the bridging structure.
Our last carbo-
rane polymer was synthesized with a six atom long flexible spacer chain betweeen the
norbornene structure and the pendant carborane. It was expected that this length
84
would weaken the effect of the carborane group. The Tg of the carborane polymer
with spacer chain is 29 °C, essentially tile same as that of unfunctionalized polynorbornene.
Since a spacer chain length of six atoms is enough for the backbone chain motion to
be unrestricted by the presence of the carborane pendant group, we can hypothesize
that any deactivation of functional group properties due to attachment to a polymer
backbone would also be ameliorated by a short spacer chain. This is important because our carborane functionalized monomers and polymers have been synthesized
with the goal of using them as intermediates to cobalt dicarbollide functionalized
polymers, and the main application of such polymers would be as ion-binding materials. It is common for the effectiveness of binding groups attached to a polymer
backbone to be hindered by their proximity to the backbone. If the binding properties of cobalt dicarbollide are adversely affected by the norbornene backbone, spacer
chains could be used to decouple any interaction.
3.3.2
Thermal Gravimetric Analysis
The weight loss of poly-nor-C-carb and poly-nor-CC-carb samples during heating
to 900 C at 200 C/min was measured. Both samples lost most of their mass during
pyrolysis. However, the poly-nor-CC-carb polymer was stable to a higher temperature
and retained more mass after pyrolysis. Weight loss began after the polymer glass
transition temperatures were reached, but the majority of the weight was lost between
400°C and 500°C. Figure 3-11 shows the polymers's weight loss curves.
These result demonstrate that pendant carborane groups which raise a polymer's
Tg also raise its decomposition temperature. The more tightly bound the carborane
is to the polymer backbone, the greater its effect. Although we did not measure the
thermal decomposition behavior of the other carborane functionalized polymers, it is
anticipated that the behavior of poly-nor-Si-carb would be similar to that of poly-norC-carb, while poly-nor-6-carb would show weight loss at much lower temperatures.
85
60.1
40.4
20.00
poly(nor-CC-carb)
poly(nor-C-carb)
200.0
600.0
400.0
800.0
Temp. C
Figure 3-11: Weight Loss as a Function of Temperature for Poly-Nor-CC-Carb vs
Poly-Nor-C-Carb
86
Table 3.2: Refractive Indices of Carborane Polymers
3.4
Polymer
Refractive Index
nor-Si-carb
1.568
nor-C-carb
nor-CC-carb
1.558
1.556
Magnetic, Optical, and Dielectric Properties
Polymers with low dielectric constants are used in integrated circuits. High dielectric
constant materials can be used in capacitors. Most polymers are relatively low dielectric constant materials. The o-carborane group has a high dipole moment and polarizable electrons. This may result in interesting dielectric or optical properties, particularly when modified with electron donating or withdrawing substituents [93, 94]. Although our polymers are functionalized with only the unmodified o-carborane group,
we have made some basic dielectric and refractive measurements.
3.4.1
Index of Refraction
The index of refraction for poly-nor-Si-carb, poly-nor-C-carb, and poly-nor-CC-carb
were measured on thin films spin coated onto silicon wafers. The measurements were
taken at a wavelength of 633 nm at room temperature using an ellipsometer. These
polymer films are believed to be non-crystalline and unoriented.
Polymeric materials have refractive indices ranging from 1.3-1.7 [120]. The refractive indices of these norbornene-carborane polymers lies ill the average range, between
those of amorphous polyethylene and amorphous polystyrene (see Table 3.2) [120].
The indices of the two carbon-bridged carborane polymers are essentially identical
while the refractive index of the silicon-bridged carborane polymer is slightly higher.
3.4.2
Dielectric Constant
A non-magnetic, non-absorbing material's dielectric constant and index of refraction
are related at a given wavelength by the equation n2 =E'; where n is the index of refrac87
tion and
e' the
dielectric constant. Often, a polymer with a high dielectric constant
or index of refraction has polarizable electrons or polar groups which are able to be
aligned by an electric field. As the frequency of measurement increases, the dielectric
constant usually drops, indicating chat the frequency of the mneaurement is faster than
the molecular motion of polar groups. This drop in dielectric constant is analogous
to the drop in mechanical storage modulus at the glass transition temperature.
Our dielectric measurements were taken at room temperature at frequencies ranging from 20 to 106 hz. This is much slower than the frequency of light used for the
index of refraction measurements. The corresponding dielectic constant at a wavelenth of 633 nm is 2.4, a lower bound for our materials.
Figure 3-12 shows the
changes in dielectric constant with frequency for poly-nor-Si-carb. The variation between samples can be attributed
to local variation in film thickness.
A dielectric
constant of approximately 3.4 in the 100-105 hz range is comparable to that of PVC
[16] and a silver loaded norbornene polymer [127]. However, it is much lower than
that of poly(vinylidene fluoride) [130]. The beginning of a drop in dielectric constant
between 105 and 106 hz is probably and artifact of the aluminum electrodes.
3.4.3
Magnetic Response
Although there is no particular reason to believe that carborane polymers would have
unusual magnetic properties, we checked the magnetic response of poly-nor-C-carb.
This polymer shows a very weak paramagnetic response (see Figure 3-13). This might
be due to the presence of impurities with some permanent magnetic dipole moment,
or could indicate that the carborane group has some magnetic behavior. The most
likely contaminant is the polymerization catalyst or spent catalyst byproducts.
A
control study of poly-norbornene synthesized by the same methods could indicate
whether catalyst contamination was responsible for the observed magnetic behavior.
88
1
J.0
_
3.6
I
3.4
A'
II
4)
3.2
3
es
4
2.8
· · · ·· · ·· _· · · ···· ··
_._1
100
,,o 10
_ · · · 11···
1000
·
· · ·· · · ··
U.0
·
·
··· ·· ·
o
104
-
I
10'
I I
;
0.7
0.6
Wi
0.5
:)
,A
!AL.
c"
0.4
~
~
~~
~
A
A
~
-
I
0.3
0.2
2
0.1
a
gal
00~~
~~ W~~~
0
.
10
........
,........
100
..........
.
...
.El.
.
1000
.......
..........
..
.....
106
Frequency (Hz)
Figure 3-12: Dielectric Constant Measurement for Poly-Nor-Si-Carb
89
emulcm^3
Applied Field [Oe]
UpwardPart
| Downward pan I
Average
HysteresisLoop
Hc Oe
IParameter'definion'
Hysteresis
Parameters
87.470
-78.191
82.831
CoerciveField:Fieldat whichM/H changes
sign
Ms emulcm3
1.569E-2
-1.751E-2
1 66QE-2
Saturation Magnetization:maximum M measured
M at H maxemulc
Mr emuJcm^3
S
S'
Hs Oe
SFD
0.015
-6.784E-4
0.043
0.883
6514.025
0.069
-0.017
8.38E-4
0.048
0.860
-7568.274
0.016
7584E-4
0 046
0.861
7041.150
M at themaximumfield
Remanent
Magnetization:
M at H0
Squareness:Mr/Ms
1-(MrlHc)(l/slope
at Hc)
Saturabonfield,field a whichM reaches0.95Ms
Switching
field distribution:
dHIHc,whereHd is thefield betwee
|
Figure 3-13: Magnetic Response of Poly-Nor-C-Carb
90
3.5
3.5.1
Experimental Details
Equipment
Molecular weight measurments were taken using a GPC equiped with three Waters
Styragel HR4 columns and a Waters 410 differential refractometer. Polystyrene calibration standards were used, All measurements were taken at a flow rate of 1 ml/min.
DMA measurements were taken with a Seiko DMS200 with nitrogen purge. TG/DTA
measurements were taken on a Seiko TG/DTA 320 with nitrogen purge. Optical observation of Tg and Tm was done with a Nikon optical microscope and Mettler FP82
hot-stage. TEM was done on a JOEL 200CX operating at 200 k.
Samples were
microtomed with a MT5000 Ultra Microtome at room temperature using glass knives
made on a Sorvall LKB type 7801A glass knife maker.
IR spectra were taken on a Nicolet Magna-IR 860. UV-vis spectra were taken with
a Cary 5E UV-Uis-NIR spectrophotometer. HNMR spectra were taken with a Bruker
400 MHz FTNMR at MIT. Carbon and boron spectra were taken at 3M Corporation by Dr. James Hill and assistance with interpretation provided by Dr. Richard
Newmark. A UNITY 500 MHz spectrometer was used, with proton decoupling using
Waltz broad-band decoupling.
Dielectric measurements were taken by Dr. Amlan Pal from Professor Michael
Rubner's group. The polymer films were spin-coated between aluminum electrodes
which were vacuum evaporated onto a glass slide by Jason Gratt.
attached to the device with silver paste.
Gold leads were
Magnetic measurements were taken by
Douglas Twisselmann from Professor Caroline Ross's group. Index of refraction was
measured with a Gaertner ellipsometer at 633 nm. A Tencor P-10 profilometer was
used to measure the thickness of spincoated films.
91
3.5.2
Processing
Hot-Pressing
DMA samples were prepared by hot-pressing polymer in a channel die with a width of
approximately 6 mm. Generally, sample lengths ranged from 10 to 20 mm, with thicknesses of less than 1 mm. Pressures of approximately 10 Kpsi were used. Temperature
were kept under 150 °C, to prevent polymer degradation. Poly-nor-C-carb and polynor-CC-carb could not be processed as homopolymers because higher temperatures
were needed to induce material flow. However, copolymers of these carborane polymers with poly-nor-OTMS could be hot-pressed at approximately 120 °C. A lower
temperature
(50 C) was used to hot-press poly-nor-6-carb.
For each polymer, a
high enough temperature was used so that the sample became uniformly translucent,
indicating complete softening.
It is important to dry polymers under vacuum before hot-pressing.
Polymers
which are allowed to dry in air may retain enough solvent to cause foaming in the
hot-press.
Samples which were hot-pressed
at temperatures
above 150 °C showed yellowing
and were no longer completely soluble in THF. These samples were probably crosslinked through reaction of the olefin backbone bonds. Such heat-altered sample were
not used for further testing.
Spin Coating
Solutions were spin coated onto either silicon wafer or untreated glass slides. Polynor-Si-carb samples were dissolved at a concentration of 20 ul/ml in chlorobenzene
or THF. Spin rates of 1000 and 2000 rpm were used. A profilometer was used to
measure film thickness on coated glass slides. Generally, the chlorobenzene solutions
resulted in smoother films (smoothness is important for ellipsometry). Spin coating
from chlorobenzene solution at 1000 rpm gave approximately 1000 A thick films. A
spin speed of 2000 rpm resulted in films approximately 800 A thick. THF solutions
spin coated at 1000 rpm resulted in films approximately 1700 A thick.
92
3.5.3
Films for UV/IR
Both hornopolymer and copolymer films were cast from solution for spectroscopic
analysis.
Some films were cast directly onto KBr pellets for IR analysis.
Other
films were cast onto Teflon-coated aluminum foil or glass slides and then peeled off
to become free-s;anding films. Much thinner films could be cast onto glass than
Teflon-coated foil because the polymer solutions all wet glass.
THF was used as the casting solvent, and casting was done in solvent saturated,
enclosed spaces. Films cast onto glass had to be peeled off while still plasticized with
solvent; most films became brittle upon complete drying. Some of the hot-pressed
samples were also used for UV analysis.
3.5.4
Polymerization
All polymerization solvents were Aldrich anhydrous grade. Unless otherwise specified,
the polymerization solvent was also used in the catalyst solutions. Polymerization
was carred out in a nitrogen glove box at room temperature.
Poly-Nor-Si-Carb
This monomer was polymerized many times using a Grubbs ruthenium catalyst (see
Figure 1-10) in methylene chloride or THF, with batch sizes of 0.1 to 1.0 g monomer.
Polymerization was sometimes terminated by the addition of ethyl vinyl ether; other
batches were allowed to terminate slowly in air.
In a typical polymerization, 0.122 g of monomer was dissolved in 4 ml methylene
chloride. To the stirred solution, 1.139 ml of a 15 mg in 5 ml solution of Grubbs
catalyst was added for a target length of 100 repeat units. Upon addition of the
purple catalyst solution, the colorless monomer solution turned peach colored. After
30 minutes, the solution was pinkish gold. After 24 hours, the solution was golden.
The polymerization was terminated with 10 ttl of ethyl vinyl ether and precipitated
in 60 ml of methanol.
The solids were filtered from methanol for a yield of 0.1 g
(82%). The incomplete yield was caused by some polymer getting through the filter,
93
not incomplete polymerization. Precipitation is supposed to remove catalyst residue,
but often the precipitated polymer retains a gray color indicative of catalyst residue.
Poly-Nor-C-Carb
Poly-nor-C-carb was synthesized in batches ranging from 0.1 to 1.5 g of monomer.
Both methylene chloride and THF were used as polymerization solvents, though THF
is preferred to eliminate phase separation of the polymer.
In one batch using mthylene
chloride, 1.5 g of nor-C-carb was dissolved in 40
ml methylene chlcride and 49 mg Grubbs catalyst in 5 l! methylene chloride added
for a target length of 100 repeat units. After 4 hours, the reaction was terminated
with 50 SLlethyl vinyl ether. A viscous blob of polymer separated from the solution.
The blob was removed, and the rest of the methylene chloride solution precipiated
in 200 ml methanol. Solids were filtered from the methanol and combined with the
phase separated
solid. Approximately
1.2 g (80% yield) was recovered.
Because of
the phase separation during reaction, polymerization may have been incomplete and
the molecular weight lower than target.
Poly-Nor-CC-Carb
In one batch, 0.5 g of monomer was dissolved in 8 ml methylene chloride. For a target
length of 105 repeat units, 1.5 ml of a 20 mg in 2 ml solution of the Grubbs catalyst
was added. Upon addition of the purple catalyst solution, the colorless monomer
solution became rose colored and then yellow and cloudy. Purple blobs separated from
solution. It is likely that this polymer did not achieve the target molecular weight due
to phase separation. THF can be used as the polymerization solvent, eliminating the
phase separation problem. After stirring overnight, the viscous solution was thinned
out by the addition of 2 ml THF. The solution was precipiated in 100 ml methanol
and the solids recovered by filtration.
94
Poly-Nor-6-Carb
A solution of 0.15 g nor-6-carb
in 5 ml THF was stirred,
and 0.464 ml of a 33 mg
in 4 ml Grubbs catalyst solution added for a target length of 100 repeat units. The
solution became peachy-yellow after minutes. The reaction was terminatEl' after 3
hours with 10 1lof ethyl vinyl ether and precipitated in 20 ml methanol. The polymer
was recovered as a sticky tar by decanting the methanol.
Block Copolymers
The carborane monomers were copolymerized with trimethylsilylmethoxynorbornene
(nor-OTMS). Poly-nor-OTMS was chosen because it has a low Tg (13 C) and is
easily stained for TEM by diethyl zinc vapor. Additionally, the ether units cause polynor-OTMS to strongly microphase separate from the less polar carborane polymers.
Nor-OTMS monomer was synthesized by Jason Gratt according to the literature
procedure, stored in a nitrogen glove box, and used without further purification.
To make a nor-CC-carb/nor-OTMS
block copolymer, a solution of 0.065 g nor-
CC-carb monomer in 4.75 ml THF was polymerized to a target length of 100 repeat
units with 0.25 ml of 33 mg in 4 ml Grubbs catalyst
solution.
At 15, 30, and 60
minutes, 0.05 ml aliquots were withdrawn and terminated with 0.5 p1 ethyl vinyl
ether. After 1 hour, a solution of 0.049 g nor-OTMS in 0.5 ml THF was added, for a
target block length of 100 repeat units. At 15, 30, and 60 additional minutes, similar
aliquots were taken. At the end of the second hour, the entire solution was terminated
with 0.05 ml of ethyl vinyl ether. This polymerization was later repeated in reverse
order and 3 hours alloted for the polymerization of each block. The solution was
precipiated in 50 ml methanol and the solid recovered by filtration.
To make a nor-OTMS/nor-Si-carb
block copolymer,
0.088 g of nor-OTMS
was
dissolved in 4.75 ml THF. For a target of 90 repeat units, 0.5 ml of a 33 mg in 4
ml solution of the Grubbs catalyst was added. Aliquots of 0.1 ml were taken at 1,
2, and 3 hours and terminated with 2 lI ethyl vinyl ether. After 3 hours, 0.126 g of
nor-Si-carb in 0.6 ml THF was added for a target block length of 90 repeat units.
95
After 15, 45, 105, and 180 additional minutes, aliquots were similarly taken. After
the second 3 hours, the entire solution was terminated with 40 l of ethyl vinyl ether.
The solution was precipitated in 50 ml methanol and the solid recovered by filtration.
To make a nor-OTMS/nor-C-carb
block copolymer, 0.078 g of nor-OTMS was
dissolved in 3 ml THF and 0.308 ml of a solution of 32 mg in 3 ml Grubbs catalyst
added for a target block length of 100 repeat nits. After 2 hours and 40 minutes,
0.1 g of nor-C-carb in 0.7 ml THF was added for a target block length of 100 repeat
units. After an additional 3 hours, the reaction was terminated with 40 Pl of ethyl
vinyl ether. The solution was precipiated in 20 ml methanol and the solid recovered
by filtration.
96
Chapter 4
Cobalt Dicarbollide and Carborane
Anion Polymers: Materials
Synthesis
A cobalt dicarbollide or carborane anion polymer might have application as an ionexchange resin for removing cesium from solution, or as a polymer electrolyte in
lithium batteries. In this chapter, we describe and evaluate some synthetic routes to
produce such materials.
Polymers functionalized with organometallic sandwich compounds such as cobalt
dicarbollide are the result of three synthetic steps which will be referred to as sandwiching, attachment, and polymerization. Because these steps can be taken in any
order, there are many possible approaches to the synthesis of a cobalt dicarbollide
functionalized polymer.
I use the term "sandwiching" to describe the process of forming the organometallic
complex. In the case of cobalt dicarbollide, carborane must first be degraded to the
open-faced C2B 9H10 dianion which then reacts with cobalt (see Figure 4-1). This
degradation process requires basic conditions. Thus, if sandwiching happens after
attachment, any group previously attached to the carborane moity must be stable to
base.
Since it is not practical to make a polymer which contains only cobalt and carbo97
-1
KOH
+
EtOH
B(OCH 2CH 3 )3
7
C 2 B1oH12
C 2 B9 H1 2
-1
NH(CH 3 ) 3 +
F
C 2 B9 H1 2
-2
2Na+
C
i
NaH T
THF
2 H2
I
+
C 2 B 9 H11
N(CH 3 ) 3
]
-2
2Na+I
3/2 CoC!2
2
-1
Na
1/2 Co
THF
+
((-;2B9H 1 1)2Co
C 2 B 9 H 11
Figure 4-1: "Sandwiching" to
Synthesize Cobalt Dicarbollide
98
3 NaCI
rane fragments [43], "attachment" to some other compound is necessary. Carborane
or cobalt dicarbollide may be attached to polymers or monomers. It may be easier to
attach carborane, because the cobalt center of the sandwich compound can undergo
side reactions.
Polymerization can take place after attachment, or before. We believe that ROMP
is a good way to polymerize monomers functionalized with carborane or cobalt dicarbollide. Alternatively, a pre-made polymer can be functionalized by grafting on
these groups onto the polymer backbone. While the use of a pre-made polymer can
reduce the amount of chemical synthesis necessary, functionalization of such pre-made
polymers may not go to completion. Additionally, there are few pre-made polymers
to which cage compounds can be easily attached.
W. P. Steckle and others at Los Alamos National Laboratory have investigated
attaching cobalt dicarbollide onto pre-made polymer resins in addition to attaching
carborane onto resins and then sandwiching. Their studies have emphasized the need
to control polymer hydrophobicity so that ions in solution can access the functional
groups. Our approach has primarily been attachment before polymerization.
This
approach takes advantage of the synthetic flexibility of ROMP and norbornene-based
monomers to produce deliberately tailored materials.
4.1
Poly-Nor-C-Carb-Co: Attachment, Polymerization, Sandwiching
This approach consists of attaching a carborane group to a norbornene monomer,
polymerizing to form a polymer with pendant carborane groups, and then building
the cobalt dicarbollide functionality from pendant carborane groups on the polymer
and additional carborane and cobalt (see Figure 4-2). The advantage of this route is
that the synthesis and polymerization of the carborane norbornene monomer is fairly
straightforward. The disadvantage is that reactions on polymer functional groups are
difficult to drive to completion, thus there may be incomplete conversion of pendant
99
KOH
NaH
EtOH/THF
THF
2Na+
,
CoC12
-~poly-nor-C-Car
THF
Co
CCI2
Figure 4-2: Synthesis of Poly-Nor-C-Carb-Co
carborane into cobalt dicarbollide.
Additionally, pendant carborane groups from
different polymer chains can sandwich the same cobalt center and create crosslinking.
Although a crosslinked resin is actually the desired form for real world cesium cleanup applications, a strictly soluble model polymer would be much easier to characterize
and study.
Nor-C-carb was used as the carborane polymer. A procedure modelled on the
established methods for degrading carborane to the open-faced C2 B 9H2 anion, deprotonating, and then reacting with cobalt chloride to form cobalt dicarbollide was
applied to the polymer.
The degradation step was modified by using tetrahydrofuran with a small amount
of ethanol instead of pure ethanol as solvent because the carborane polymer is not soluble in ethanol before degradation. The polymer salt was then left in potassium form,
instead of converting to trimethylammonium form. Changing to the trimethylammonium form is helpful for recrystallization of the C2 B 9 H-2 species, but the polymer
form did not need to be recrystallized for recovery.
Once the polymer's pendant carborane groups were converted into the C 2 BgH2
9
100
-1
Figure 4-3: Poly-Nor-C-Carb-Co: A Cross-Linked Resin
form, they were deprotonated and combined with cobalt dicarbollide and additional
C2 B9 Hi-2 . During this reaction, the product precipitated out of THF'solution as a
greenish powder. The solid product is believed to be a crosslinked, anionic resin (see
Figure 4-3) with sodium and potassium counterions.
Elemental analysis confirms
the presence of cobalt, but was inconclusive in determining the complete material
composition with a large percentage of the total mass was unaccounted for by any
element checked. Possibly this result may be explained by the unusual stability
of cobalt dicarbollide compounds, which can result in underreporting of elemental
composition, especially for boron. IR spectroscopy is not useful for distinguishing
between various carborane and carborane organometallic sandwich species.
This product, poly-nor-C-carb-Co, is in the form of a fine powder. These particles
might be too small in diameter for use as a solid column packing for water treatment.
However, the degree of crosslinking and particle size could probably be controlled by
101
altering the reaction conditions. An attachment-polymerization-sandwiching
route is
promising for producing a cobalt dicarbollide functionalized resin, but other routes
are favored for producing soluble materials which can be better characterized.
4.2
Poly-Nor- C-CDC: Attachment, Sandwiching,
Polymerization
This approach uses a carborane functionalized norbornene monomer as an intermediate to a cobalt dicarbollide functionalized norbornene monomer, which is then polymerized. The advantage of this approach is that it produces a linear, soluble polymer
that can be characterized. This product can then be crosslinked by reacting the unsaturated bonds in the polymer backbone to form an insoluble resin. Alternatively,
the cobalt dicarbollide monomer can be copolymerized with a crosslinking agent. The
disadvantage of this scheme is that the carborane functionalized monomer must be
stable to the basic conditional necessary for sandwiching. This requirement imposes
limits on the possible structure of the carborane monomer.
4.2.1
Sandwiching
Preliminary experiments showed that the bond between silicon and the carborane carbon in nor-Si-carb is susceptible to base cleavage. This was true both in KOH/ethanol
and piperidine/toluene.
Thus, further efforts to convert a carborane functionalized
norbornene monomer into a cobalt dicarbollide monomer focused on nor-C-carb. The
resulting synthetic scheme is shown in Figure 4-4. Nor-C-carb can be treated with
KOH/ethanol to degrade the carborane group and purified as the trimethyl ammonium salt. Although the twice-bound carborane monomer, nor-CC-carb, was not used
in these experiments, it is also expected to be stable to base.
The structure of this intermediate monomer was confirmed by H,
3 C,
and
11B
NMR. Proton NMR shows the expected upfield shift in the position of the carborane carbon hydrogen. Also, the expected 2:9 ratio of olefin to trimethylammonium
102
N
KOH
EtoH
/
-1
7
+
-/001
THF
0
-1
-
J
_
_
I
C1
L
NaH, CoC12
-1
-1
-1
CI'
+
C
I
Figure 4-4: Synthesis of Nor-C-CDC
hydrogens is found (see Figure 4-5).
13 C
NMR shows that the carborane carbon
peaks have shifted and the trimethylammonium carbon is visible (see Figure 4-6).
The
11B
NMIR spectrum shows the complicated splitting pattern expected, with nine
major peaks of approximately the same area, some of which are split (see Figure 4-7).
The 1" B spectrum is complicated because in addition to the endo/exo isomerism, the
monomer now has additional isomerism relating to the orientation of the open face
of the carborane. The carborane carbon bonded to the norbornene methyl carbon is
now chiral, and the product contains both isomers.
The monomer functionalized with degraded carborane can then be deprotonated
with sodium hydride and combined with additional degraded, deprotonated carborane plus cobalt chloride. A mixture of three products was expected: cobalt dicarbollide, norbornene methyl cobalt dicarbollide (nor-C-CDC), and bis (norbornenemethyl) cobalt dicarbollide.
However, because the norbornene carborane is less
reactive than the unhindered carborane, no measurable amount of the third product
is formed. Both cobalt dicarbollide and norbornene methyl cobalt dicarbollide can
103
oSZP 0
80Et
£LP'O 0
LLPr, 0
-o
gL21E';
61EE'l
WeE I
C:)
E PE';
BBLP'l
8BL9'
E2BL'
-cnJ
Z06Li
9008* 'I
OOB' I
j
90I8
EEB
9990'
g02' 2
L6EE '
£E6'B
:)
EL99'2
E61LZ
1
E6L8Z
5600 E
§6
E
SB99E'E
80BS'E
E86S'E
J
''
I
I
r
O
I-
"'
-Lr
6666'E
-,n
ELS'G
g0L6B'
8OL6'G
8LL6'S
cO
,CD
6620'9
PIE0 9
N
l2S0'9
E;00'a
-I-./'
6G0' 9
E
-I'l
,
L990' 9
IJ
eJluI
wd
l
Figure 4-5: HNMR of Degraded Nor-C-Carb-,Trimethy ammonium Salt, in D-Acetone
104
a.
-
CL
: a
_}.e a
- O
0
-
: O0
-0
re
-C
' o
_
I
_
-
,
V-
Figure 4-6: 13CNMR of Degraded Nor-C-Carb, Trimethylammonium Salt, in DAcetone
105
v.
J'
-1
2
-5
-15
-25
-20
ppm
peak assignmentstentative
Figure
4-7:
l1 BNMR of Degraded Nor-C-Carb, Trimethylammonium
Acetone
106
Salt, in D-
be recovered from this reaction. Bis(norbornene methyl) cobalt dicarbollide can be
formed if the degraded, deprotonated carborane methyl norbornene is mixed with
cobalt chloride without additional carborane, but this reaction is slower than the
others. (See Figure 4-8 for the HNMR spectrum of bis(nor-C)-CDC.)
Nor-C-CDC can be mostly separated from the cobalt dicarbollide side product
based on solubility differences. Since the presence of cobalt dicarbollide does not
affect the polymerization catalysts used, rigorous purification was unnecessary. The
cesium salt of nor-C-CDC (mixed with some residual cobalt dicarbollide) was analyzed
using HNMR (see Figure 4-9). Four distinctive cobalt dicarbollide carbon hydrogen
peaks are visible. One corresponds to the residual cobalt dicarbollide, whose carbon
hydrogens are all identical. The other three correspond to the three non-equivalent
carbon hydrogens of the cobalt dicarbollide attached to norbornene. The broad boron
hydrogen peaks are also visible. Integration ratios between the olefin hydrogens and
the pendant cobalt dicarbollide carbon hydrogens are approximately correct. This
monomer is a glassy, orange solid. (Figures 4-10, 4-11, 4-12 show the NMR spectra
of cobalt dicarbollide for comparison.)
4.2.2
Polymerizing
Both Grubbs and Schrock type catalysts were investigated for use as initiators in the
ROMP of the cobalt dicarbollide functionalized monomer. Both types of catalysts
initiated polymerization, but the Grubbs system was chosen for further investigation
because of ease of use. Homopolymers were analyzed by GPC. The polymers did
not eleute from the polystyragel GPC columns using THF, so a 50 mM solution of
LiBr in THF was used as eleuant. However, this did not enable the homopolymers to
eleute, even though a poly-C-CDC/poly-norbornene
copolymer was eleuted. These
polymers are brittle, orange solids which are soluble in THF and acetone.
107
-
GE
0
'9E; 0
099P' 0
80'
C
V,60Z C
989.8
EL62E
9PE'?
'
LSSO
90
o
L990
teggo
9ZLO Z
9LLO
'
620; '2
j
6L,; 'Z
-'V
gE DIE'8,
5LE 'Z
0oLt9'
'Z
E00
gOO
'E
9aES
Z988 '
89L6
'
LZ86 5
Lr66 5
;666 5
00O '9
GL90 9
V90 9
6L0'9
6;60'9
0660'9
L9g0'9
£;;
9
wad
Figure
4-8
HNMR of BisNor-C)-CDC in D-Acetone
Figure 4-8: HNMR of Bis(Nor-C)-CDC in D-Acetone
108
09060'
9P66E
I
2299'
gBE9'
6660
2Zgta
LPLt-
I
PS090'
EO990
L
PI
flu
EOLZO
9560
0606
OP91;'
6L9?Z
-C,,
20[E2
9050
Z2gLB
?L668
C
0OLE6
LEOLE'
Z606E'
gwO19
zgPL,
BLIL6'
00296'
06g95'
v6E90,
Oz[60'
IL860
Cnl
-Lt
C6LpO'
E619i
OE691
E98'
E
-c
Z;996'
waOO
Figure 4-9: HNMR of Nor-C-CDC in D-Acetone
109
en
Sr
L 1.0
R
fl
1r
";,~~~IVf
1
Cs+'
I
I'
'V
I
I
4
\
I
I
I
I
"
I
I
-"
2
3
I
I
I
I
I
'
ij
B-H
Figure 4-10: HNMR of Cobalt Dicarbollide, Cesium Salt, in D-Acetone
110
-
I
1
ppm
M
~0
"C
*
1
Cs+ l
1
- - IJ- - - - -I - lll- -
80
i
I
60
I
I I
j I
ii-
40
I
I-
i i
20
I
i' j i
0
Figure 4-11: '3CNMR of Cobalt Dicarbollide, Cesium Salt, in D-Acetone
111
1
i
iT
ppm
-1
CS+1
Cs,
Y
h
4
la
0
00;
(4
tr -1 I'
JO
5
_ __ _ _ _ _ _ ___ ___
I I
I
l
0
I
rI
-5
_ ___ _ _ _ _ __
II
I I
I
-10O
_ ___ ___ _ ___ __ _ __
l'1I ,I
II
-20
Figure 4-12: 'llBNMR of Cobalt Dicarbollide, Cesium Salt, in D-Acetone
112
ppm
-1
-1
Br
Li+
THF
B
Br
Figure 4-13: Synthesis of Bromobutyl Cobalt Dicarbollide
4.3
Poly-Nor-11-CDC: Sandwiching, Attachment,
Polymerization
This approach involves attaching a cobalt dicarbollide funtionality to a norbornenebased monomer and then polymerizing. It has similar advantages to the previous
approach, in that a soluble polymer product can be produced. Additionally, a broader
range of monomer structure is possible, since the chemically harsh sandwiching step
occurs before the norbornene group is attached. A disadvantage of this route is that
cobalt dicarbollide is more difficult to attach to a norbornene-based monomer than
carborane is, so the monomer synthesis yields are much lower.
Similar to carborane, cobalt dicarbollide can be deprotonated to undergo nucleophilic substitution [90, 14]. Unfortunately, deprotonated cobalt dicarbollide preferentially reacts with the olefin bond of norbornene-based monomers. Thus, it is not
possible to use routes analogous to those described in Chapter 2 to produce a cobalt
dicarbollide functionalized norbornene monomer.
We have taken the approach of first functionalizing cobalt dicarbollide with an
alkyl bromide (see Figure 4-13), and then reacting that to a norbornene alcohol.
Although norbornene methanol is a primary alcohol, the steric influence of the norbornene structure hinders the substitution reaction and leads to the formation of an
elimination side product (see Figure 4-14). Since we were unable to separate the
113
/ OH
+
NaH
THF, 500C
NaH
THF, 0OC
50%
Figure 4-14: Mixed Elimination and Substitution Products Resulting from the Reaction of Norbornene Methanol and Bromobutyl Cobalt Dicarbollide
114
~n0 0
OH
OH
Figure 4-15: Synthesis of an Ester Linked Norbornene Alcohol
cobalt dicarbollide functionalized norbornene product from the alkene cobalt dicarbollide side product, this mixture of synthetic products was used as described in the
section below and a different norbornene alcohol was investigated.
The commercially available norbornene acid can be converted to an acid chloride and then reacted with a diol to produce an ester linked norbornene alcohol (see
Figure 4-15). Since the alcohol functionality in this monomer is distant from the norbornene moity, the alcohol should be react as a typical primary alcohol. The reaction
of alkyl bromide cobalt dicarbollide with this alcohol produces a cobalt dicarbollide
functionalized norbornene with a long spacer chain (see Figure 4-16). However, this
reaction proceeds with very low yield and only minute amounts of the functionalized
norbornene were recovered. The Grubbs catalyst seems to react with the norbornene
olefins of this monomer based on HNMR (Figure 4-30), but the resulting amount of
material was not enough to fully characterize. This synthetic route is impractical
unless the product yield can be improved.
4.4
Poly-CDC-siloxane: Polymerization, Sandwiching, Attachment
This is the approach used by the Los Alamos group, in which cobalt dicarbollide is
grafted onto a polymer. Their products were limited by the selection of commercially available resins which can react with deprotonated cobalt dicarbollide. In our
research, we investigated a variation of this approach which extends the pool of commercially available polymers which can be functionalized. Instead of attaching cobalt
115
O
o//\/OH~
+
-1
NaH
THF
-I
Figure 4-16: Synthesis of Nor-11-CDC
116
dicarbollide directly onto a polymer, we attached a cobalt dicarbollide derivative.
First, cobalt dicarbollide was attached to a spacer chain with a reactive end. Then,
the reactive end of the cobalt dicarbollide derivative was grafted onto a polymer.
Although we were able to use hydrosiloxanes, an alternative variety of commercially
available polymer, the resulting product proved to be unsatisfactorily liquid.
We had not planned to try this route, but some materials serendipitously synthesized during investigations of the previous approach (see Figure 4-14) were appropriate for grafting onto silane polymers. Additionally, there was the possiblilty the the
alkene componene might preferentially react, leaving purified cobalt dicarbollide functionalized norbornene monomer. We found that both components of the previously
synthesized mixture of terminal olefins and norbornene olefins react with hydrosilanes
in the presence of hexachloroplatinic acid. Unfortunately, the norbornenes reacted
preferentially. The resulting polymer was a liquid. This route could have produced a
solid product if a higher molecular weight hydrosilane polymer had been used.
4.5
Poly-Nor-Si-Anion: Polymerizing the Carborane Anion Monomer
The first batch of carborane anion monomer was less pure than the second batch.
Attempts to polymerize the product from the first batch with a Schrock molybdenum catalyst in a THF/toluene mixture were unsuccessful. This monomer also did
not react with the Grubbs ruthenium catalyst in methanol. However, the Grubbs
catalyst did react with this monomer in THF, as shown by the disappearance of the
strained olefin bonds of the monomer and the appearance of the olefin bonds typical of norbornene-based polymers. GPC analysis using THF eluent and polystyragel
columns did not show any polymeric peaks. Viscometry measurements suggest an
oligomeric product. Additionally, GPC analysis using THF with 50 mM LiBr as eluent suggests that the product was a mixture of oligorneric and low molecular weight
material. It is not clear if some of the low molecular weight signal from the GPC
117
traces were artifacts of interactions between the polymer and the column packing
materials.
Reaction of the second batch of purer carborane anion monomer with the Grubbs
ruthenium initiator in THF produced polymer with a cleaner HNMR spectra than the
first batch. The polymer spectra showed the expected broad peaks, the correct ratio of
olefin hydrogens to methyl silyl hydrogens, and the characteristic broad, overlapping
carborane boron hydrogen peaks. (see Figure 4-17). This material did not have a
high enough solubility in THF with 50 mM LiBr to obtain GPC data. The polymer
is somewhat soluble in THF and acetone, but very soluble in methanol.
4.6
Experimental Details
Reagent grade sodium hydride and potassium hydroxide were purchased commercially
and used without further purification. Reagent grade THF and absolute ethanol were
used as purchased. Cobalt dicarbollide was purchased as the cesium salt from Boron
Biologicals and also synthesized according to the literature procedure (ref).
4.6.1 Degrading o-Carborane
Carborane must be degraded for to its open-faced form in order to make cobalt
dicarbollide compounds. The degradation of o-carborane into the C2 B9H-2 anion was
carried out following the literature procedure [47] on a scale of 2 to 10 g o-carborane
per batch. In a typical reaction, 10.0 g o-carborane was added to a solution of 7.78
g KOH (2 equivalents) in 120 ml ethanol. The solution was refiuxed overnight. An
additional 40 ml ethanol was added, and CO 2 from dry ice was bubbled thought the
solution for 7 hours. The white precipiate was filtered off. The filtrate was dried and
then dissolved in 70 ml water. The product was precipitated with a solution of 8.6
g trimethyl ammonium chloride in 50 ml water. The solid product was filtered off
and recrystallized from 450 ml boiling water, with a 94% yield. The NMR spectra
for degrade o-carborane, trimethylammonium salt, are showi in Figures 4-18, 4-19,
4-20.
118
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Figure 4-17: HNMR of Poly-Si-Anion in D-Methanol
119
2
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34 %3i
t
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2
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2
B-H
Figure 4-18: HNMR of Degraded o-Carborane, Trimethylammonium Salt, in DAcetone
120
1
am
I
et
m'
CIV
2
+1 2
NH
12
H
*
-1
C
2
80
60
40
20
0
Ppm
Figure 4-19: 13CNMR of Degraded o-Carborane, Trimethylammonium Salt, in DAcetone
121
Co
0
NH
I
K)
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01
-1
H
U
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5
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peak assignments tentative
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Figure 4-20: llBNMR of Degraded o-Carborane, Trimethylammonium Salt, in DAcetone
122
4.6.2
Poly-Nor-C-Carb-Co
Nor-C-carb was polymerized to approximately 100 repeat units with the Grubbs
catalyst.
A solution of 0.6 g KOH (2.2 equivalents)
in 2 ml ethanol (7.2 equivalents)
was added to a solution of 1.2 g polymer in 40 ml THF and stirred overnight. The
solution developed an orange/yellow color. The solution was dried to a brownish,
brittle film and then crushed into small pieces and washed with 2 times 100 ml water.
The water filtrate was yellow. The solids were then washed with 2 times 20 ml diethyl
ether. The ether remained colorless and the solids swelled in the ether. The polymer
solids were cast into a film and dried. (See Figures 4-21, 4-22, 4-23 for NMR spectra
of the degraded polymer.)
The recovered polymer product (1.227 g) was dissolved in 50 ml THF and added
to a stirred solution of 0.265 g NaH (2.5 equivalents) in 50 ml THF. A solution of 0.850
g trimethylammonium salt of C2 B 9 H1 2 (1 equivalent) in 10 ml THF was added to the
stirred solution. This was an error, as at least 3 equivalents of NaH were required
to do all three things: deprotonate the C2 B9 H- groups on the polymer, react with
the trimethylammonium on the free C2B 9 H12,and deprotonate the free C2 B 9H2. As
the trimethylammonium is probably most reactive, and the free C2 B9 H-2 next most
reactive, this error means that half of the functional groups on the polymer may not
have been deprotonated.
The solution was refluxed for 3 hours, cooled, and added to 1.427 g CoC12 (2.5
equivalents) in 90 ml THF and stirred under nitrogen. The solution was refiuxed for 2
hr and spontaneously separated into two phases. The clear blue top phase and cloudy
blue bottom phase were separated. The 50 ml of bottom phase was poured into 800
ml water and precipitated as a greenish solid. This solid swelled in THF and DMSO,
but was insoluble in toluene, methylene chloride, ether, acetone, methanol, and dilute
aqueous HC1. The solids were swelled in 25 ml THF for 3 days and reprecipitated in
350 ml methanol. The swelling and reprecipitation was repeated.
The top phase was concentrated from 150 ml to 50 ml and then poured into 300
ml water. Only a small amount of orange/black tacky solid precipitated. This was
123
E
0.
CL
01
'4
Cu
L
a)
r
I,.
1r
U)
CM
t,
CV
Figure 4-21: HNMR of Degraded Poly-Nor-C-Carb in D-THF
124
'
0
Q
to
0
Or
r
,I
Figure 4-22: 13CNMR of Degraded Poly-Nor-C-Carb in D-THF
125
0
a
a.
V '-
?
I
=0
l
C)
c0
0
OO
r
FU
Poly-Nor-C-Carb in D-THF
Figure 4-23: "lBNMR of Degraded
126
Table 4.1: Elemental Composition of Poly-Nor-C-Carb-Co
Element
wt% Found
wt % Pendant
wt% X-Linked
wt% Poly-Nor-C-Carb
carbon
36
32
hydrogen
boron
cobalt
7
19
17
7
43
13
43
8
35
11
48
9
43
0
Na+K
chloride
nitrogen
total
1
2
0
82
5
0
0
100
5
0
0
102
0
0
0
100
dissolved in 10 ml THF and reprecipiated in 200 ml methanol. Then, the solids were
dissolved in 5 ml THF and reprecipited
in 150 ml methanol.
Since cobalt dicarbollide is soluble in methanol, the repeated precipitations should
remove any cobalt dicarbollide side product.
We have found that polynorbornene
mixed with cobalt dicarbollide in THF is adequately cleaned by 1 to 2 precipitations
in methanol.
Elemental Analysis
Elemental analysis by Galbraith Laboratories confirmed that some kind of organometallic complex was formed in this reaction. However, the low amount of sodium or potassium is puzzling, as is the low amount of boron found. The elemental mass percents
do not add up to 100, reflecting some kind of underreporting which may be caused by
the unusual chemical and thermal stability of carborane compounds [133]. Table 4.1
shows the element content of the product compared to the content of the poly-C-carb
starting material and the theoretical fully pendant and fully cross-linked models.
Spectroscopy
The insoluble powder was mixed with KBr powder and compacted into a pellet under
pressure. IR spectra of cobalt dicarbollide and polynorbornene on KBR pellets were
compared (see Figure 4-24). The characteristic difference between the IR spectra
127
of cobalt dicarbollide and carborane is that cobalt dicarbollide shows a broad peak
around 3400 cm~ l . This peak is often associated with organometallic conmpounds.
Unfortunately, this peak is attributed to the presence of adsorbed water. Thus, the
IR results cannot confirm the presence of pendant cobalt dicarbollide functionalities.
Suggested Modifications
I suggest that certain modifications be made to this synthesis if it is repeated.
1. The poly-nor-C-carb should be dissolved in a THF/ethanol
little THF as possible. Upon standing, THF/cthanol/KOH
mixture with as
solutions develop a yellow
color, presumably due to some reaction between the chemicals. However, solutions
with more THF develop this color faster than solutions with less THF. The polymer
should be dissolved in a minimal amount of pure THF, and then ethanol added. If
the polymer begins to precipitate, more THF can be added.
2. If the ratio of free carborane to polymer bound carborane is kept at 1:1, at
least 3 and preferably 4 equivalents of NaH should be used instead of 2.5.
3. It may be desirable to increase the ratio of free carborane to polymer bound
carborane to reduce polymer crosslinking. A 2:1 or 3:1 ratio could be used. The
amount of NaH should be increased proportionally, since the free carborane requires
NaH for both trimethylammonium removal and carborane deprotonation.
4.6.3
Poly-Nor-C-CDC
Degradation of Nor-C-Carb
This reaction was carried out at scales of 1 to 3 g monomer. In a typical reaction,
1 g of nor-C-carb was added to a solution of 0.5 g KOH (2.2 equivalents)
in 8 ml
ethanol. The solution was refluxed 5 hours and then CO2 from dry ice was bubbled
through the solution for 2 hours. The solid precipitates were filtered off, and the
filtrate dried. The filtrate was dissolved in 4 ml water and the product precipitated
with 0.52 g trimethyl ammonium chloride (1.4 equivalents) in 2 ml water. The solid
128
3600
3400
3200
3000
280U
bW
Wavenumbers (cm-l)
Figure 4-24: IR Spectra of Polynorborne, Poly-Nor-C-CDC-Co, and Cobalt Dicarbollide
129
product was filtered off and recrystallized by evaporation of either water/methanol
or water/ethanol with yields of 70-75%. The product is soluble in methanol, ethanol,
acetone, THF.
Sandwiching
The sandwiching reaction between the monomer and free carborane was carried out
at scales between 0.2 and 2.0 g of the trimethylammonium norbornene carborane
salt. The larger scale is easier to work with. For that batch, 2.0 g of the trimethylammonium salt of the degraded nor-C-carb was dissolved in 23 ml of anhydrous THF
and 0.384 g NaH (2.4 equivalents) was added. At the same time, 2.3048 g of the
trimethylammonium salt of degraded carborane (1.78 equivalents) was dissolved in
31 ml anhydrous THF and 0.686 g NaH (2.4 TMA-carborane equivalents) was added.
Both solutions evolved gas and became slightly yellow. The solutions were refluxed
under nitrogen for 3 hours, then cooled for 2 hours.
An slurry of 2.0 g CoC12 (2.3 equivalents) in 50 ml anhydrous
THF was stirred.
A
0.75 ml aliquot of the CoC12 slurry was mixed with a 0.5 ml aliquot of the degraded
nor-C-carb solution. This mixture became rose colored, and then black within ten
minutes. The rest of the degraded nor-C-carb and carborane solutions were mixed
together and added to the CoC12 slurry. This mixture became orange and then black
immediately. Both mixtures were stirred overnight.
The aliquot mixture was dried and dissolved in 1 ml water. It was then filtered,
and the solids rinsed with 1 ml water. The aqueous filtrate was precipiated with a
few drops of aqueous cesium chloride. A pinkish red solid product was filtered off.
Nitrogen was bubbled through the larger mixture for 30 minutess, and thef' it
was dried to a tar. The material was dissolved in 100 ml water and filtered. The
solids were rinsed 3 times with 15 ml water. The filtrate was precipiated with aqueous
cesium chloride. The product first separated from solution as an oily tar, but solidified
upon standing in the water solution over two days . The solid was broken up into
a powder and filtered off. The solid was partially dissolved in 10 ml chloroform and
filtered. The remaining solids were stirred and rinsed 3 times in 10 ml chloroform.
130
The chloroform filtrates were combined and dried to a reddish, glassy solid containing
both the nor-C-CDC product and some cobalt dicarbollide. The chloroform insoluble
solids were orange. Analysis of the insoluble fraction in a smaller batch reaction
identified it as cobalt dicarbollide.
The bis(methylnorbornene) cobalt dicarbollide is soluble in acetone, THF, methanol,
and methylene chloride.
The nor-C-CDC monomer is soluble in acetone, THF,
methanol, toluene, methylene chloride, and chloroform but insoluble in hexane.
Polymerization
Based on HNMR peak integration, the content of the nor-C-CDC reaction after purification was about 80% nor-C-CDC with the balance being cobalt dicarbollide. We
assumed an 80 mol% nor-C-CDC content for polymerization calculations (see Figure 4-9).
We considered two polymerization systems: the Grubbs catalyst in THF and the
Schrock catalyst in toluene. Both solutions contained 0.1 g of the nor-C-CDC mixture
in 2 ml solvent, and catalyst solution was added for a target polymerization length
of 50 repeat units. This translated to 0.285 ml of a 32 mg in 3 ml Grubbs solution
and 0.312 ml of a 13 mg in 2 ml Schrock solution.
After a few minutes, the toluene solution had visible orange precipitate.
solutions were stirred overnight.
precipitated in 20 ml hexane.
Both
The toluene was decanted from the solids, and
The THF solution was also precipitated in 20 ml
hexane. All solids were dried under vacuum at 60 C overnight.
HNMR showed
that the product of the Grubbs catalyst reaction did not contain monomer olefin
hydrogen peaks, but instead had typical polymer olefin hydrogen peaks (see Figure 425). Similarly, the small amount of solid which had precipitated from the Schrock
polymerization reaction was identified as polymer. However, the rest of the Schrock
reaction mixture was remained nor-C-CDC monomer.
131
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IN I
a
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Figure 4-25: HNMR of Poly-Nor-C-CDC in D-Acetone
132
4.6.4
Poly-Nor-11-CDC
Norbornene butanol ester
Norbornene acid (Frinton Laboratories) and thionyl chloride (Fluka) were purchased
and used without further purification. To convert the acid into an acid chloride, 5.0
g of the norbornene acid was dropped into 4.7 g neat thionyl chloride (1.1 equivalents) and stirred. After 1 hour, the solution was slightly yellow. After 3 hours, the
solution was darker yellow. The mixture was put under vacuum overnight to remove
unreacted thionyl chloride. The remaining liquid was dropped into a solution of 32.6
g butanediol (10 equivalents) and stirred for 3 hours over an ice bath. Upon addition
of 3.66 g triethyl amine (1 equivalent) dripped into the solution, a white precipitate
was formed and white gas was evolved. The solution was stirred overnight under
nitrogen. The white solids were filtered off and the yellow filtrate evaporated to an
oil. The yellow oil was dissolved in 100 ml chloroform and shaken with 3 times 100
ml water. The chloroform phase was evaporated to about 6 ml of dark yellow oil.
The product was purified using column chromotography with silica gel as adsorbent
and either a 1:2 mixture of diethyl ether/petroleum
acetate/petroleum
ether or a 1:2 mixture of ethyl
ether as eleuent. The eleuent mixture can be gradually increased
in the more polar solvent after the first two components are eleuted in order to reduce tailing. The product fraction is the last fraction eleuted, after any unreacted
butanediol or norbornene acid. Use of ethyl acetate instead of diethyl ether reduces
column cracking.
NMR of the clear oil product shows peaks for hydrogens of carbons adjacent to
oxygen in approximately the correct ratio with the olefin hydrogen peaks (see Figure 426). Aliphatic hydrogen peaks corresponding to the middle carbon hydrogens from
the spacer chain are also visible. Although this is not definative proof of structure,
it is adequate comfirmation for our purposes, since this type of acid chloride/alcohol
reaction is very common and generally proceeds as expected.
133
E
E
I
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L68 O
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/
l
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6E09'0
OOO'[
I-
0
1-
0)
0O
-
(- ·
!",
I
ooou
i
i
wOa
leJ6alui
Figure 4-26: HNMR of Norbornene Butanol Ester in D-Acetone
134
bromobutylcobalt dicarbollide
A dibromoalkane can be reacted in large excess with deprotonated cobalt dicarbollide to make a a bromoalkylcobalt dicarbollide. We chose dibromobutane, available
from Aldrich and used either after distilling from CaH or without further purification.
There was no noticible difference in yield depending on purification of the
dibromobutane.
Between 1 and 1.28 7uivalents of n-butyl lithium solution were added to a solution
containing 1.0 g of the cesium salt of cobalt dicarbollide in 20 ml anhydrous THF. The
butyl lithium solution used was originally 10 M in hexanes but was later diluted by the
addition of 5 ml THF. There was no noticible difference in yield between the batches
to which different amounts of butyl lithium had been added. Upon deprotonation,
the cobalt dicarbollide solution changes from orange to dark purple/black. The black
solution was then added to 4.75 g dibromobutane (10 equivalents) in 20 ml THF.
The solution became orange again and was stirred for 3 hours. After the unreacted
NaH was filtered off, the solution was dried to a thick tar. The orange tar was rinsed
with 3 times 30 ml petroleum ether to remove unreacted dibromobutane and then
dissolved in water and precipiated with aqueous cesium chloride.
HNMR shows that approximately 55 mnol%of the cobalt dicarbollide reacted with
the dibromobutane while the rest was unreacted (Figure 4-27). If the solid mixture
is rinsed with chloroform, bromobutylcobalt dicarbollide is dissolved along with some
cobalt dicarbollide. The chloroform solubles are approximately 80 mol% bromobutyl
cobalt dicarbollide. HNMR of the product mixture shows the three non-equivalent
carborane hydrogens from the bromobutyl cobalt dicarbollide in the correct ratio with
the hydrogens on the bromide substituted carbon. Additionally, the other aliphatic
hydrogen peaks can be seen superimposed on the broad boron hydrogen peaks. Unreacted cobalt dicarbollide can be identified by the single carborane carbon hydrogen
peak.
135
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C
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ppm
1--,2
1.2.3
II
7
4-6
4-6
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Figure 4-27: HNMR of Bromobutyl Cobalt Dicarbollide, Cesium Salt, in D-Acetone
136
Nor-11-CDC
Butanol norbornene ester can be reacted with the bromobutyl cobalt dicarbollide to
form a cobalt dicarbollide funtionalized norbornene monomer with 11 atoms between
the functional group and the norbornene structure. It was found that sodium hydride
undergoes reaction with bromobutyl cobalt dicarbollide. Thus, the preferred reagent
addition order is to mix the norbornene ester with sodium hydride before adding the
bromobutyl cobalt dicarbollide. This reaction was done on a small scale, using 0.1 g
of bromobutylcobalt dicarbollide. Since this starting material was never completely
separated from byproduct cobalt dicarbollide, the 0.1 g refers to the calculated amount
of bromobutyl cobalt dicarbollide reagent present in a larger amount of mixed product.
For example, if the 55 mol% bromobutylcobalt dicarbollide mixture is used, a total
of 1.8 g material must be added in order to achieve a calculated 0.1 g of reagent.
A ratio of 2:1 norbornene ester to bromobutyl cobalt dicarbollide was used. Accordingly, 0.071 g of norbornene butanol ester was dissolved in 3 ml anhydrous THF
and treated with 8 mg sodium hydride (1 ester equivalent) under reflux for 2 hours.
The bromobutyl cobalt dicarbollide mixture was dissolved in 4 ml anhydrous THF,
added to the norbornene ester solution, and stirred overnight.
The solution was
dried to a solid, dissolved in 20 ml water, and then precipitated with aqueous cesium
chloride.
The product was purified by column chromotography using silica gel adsorbent
and a mixture of 3:1 ethyl acetate/hexane eleuant. The product eleutes as the second
component. The first component is clear, but the product fraction is reddish. The
third fraction is orange. Generally, we have found that cobalt dicarbollide functionalized compounds are difficult to purify with column chromatography because the
influence of the anionic group tends to dominate, causing different cobalt dicarbollide
containing compounds to eleute together. Also, these compounds tend to tail extensively and require very polar eleuents. However, in this case, the product could be
purified and was recovered in about 20% yield.
HNMR of the product shows the expected olefin hydrogen peaks and three nonequiv-
137
alent cobalt dicarbollide carbon hydrogen peaks, although the area of the cobalt dicarbollide carbon hydrogen peaks is lower than expected (see Figure 4-28) The product
is soluble in acetone and THF. In contrast to the norbornene methanol based reaction
(described in Section 4.6.5), there was no sign of elimination side product.
Polymerization
Upon treatment with the Schrock fluorinated catalyst, HNMR of the product still
showed olefin hydrogen peaks but all of the cobalt dicarbollide carbon hydrogen peaks
were gone (see Figure 4-29). The exact reaction is unknown, but it seems evident
that the Schrock fluorinated catalyst is inappropriate for polymerizing this monomer.
The polymerization attempt was carried out in 1 ml THF, using 27 mg nor-11-CDC
mixture and 0.021 ml of a 55 mg in 2 ml THF solution of the Schrock fluorinated
molybdenum catalyst (MW 765.5). This would nominally product 50 repeat units.
It is unknown by what mechanism a catalytic amount of the molybdenum compound
could cleave or destroy all the cobalt dicarbollide groups.
Upon treatment with the Grubbs catalyst, HNMR shows that the product spectrum's olefin hydrogen peaks are much reduced in comparison to the pendant cobalt
dicarbollide carbon hydrogen peaks (see Figure 4-30). This suggests that the Grubbs
catalyst polymerized this monomer. The polymerization attempt was done in 0.3 ml
THF, using about 5 mg nor-11-CDC mixture and 0.007 ml of a solution of 33 mg
catalyst in 4 ml THF. This would nominally produce 100 repeat units.
Reaction of olefin bonds and deprotonated cobalt dicarbollide
Reaction between deprotonated cobalt dicarbollide and carbon-carbon double bonds
was first observed during attempts to synthesize allyl cobalt dicarbollide by reaction
with allyl bromide. HNMR analysis of the product showed no remaining unsaturated
hydrogen peaks.
Since the olefin bonds of norbornene are part of a strained cyclic structure, they
sometimes demonstrate different reactivity than terminal olefins. Thus, it was not certain that deprotonated cobalt dicarbollide would react with norbornene olefin bonds.
138
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Figure 4-28: HNMR of Nor-11-CDC in D-Acetone
139
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800
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Figure 4-29: HNMR of Nor-11-CDC after Treatment with a Schrock Catalyst in
D-Acetone
140
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CY
V-
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wad
01.
D-Acetone
Figure 4-30: HNMR of Poly-Nor-11-CDC in
141
Following a reaction scheme analogous to that of nor-6-carb, deprotonated cobalt
dicarbollide was reacted with methylnorbornene bromobutyl ether.
The product was separated into fractions using column chromotography with a
1:3 mixture of acetone and ethyl acetate. None of the five orange colored fractions
collected showed the typical norbornene olefin hydrogen peaks in their HNMR spectra.
Cobalt dicarbollide itself showed no reactivity toward methylnorbornene bromobutyl
ether, but the deprotonated cobalt dicarbollide species did. The chemical structure
of the reaction product or product mixture was undetermined.
Reaction Description
A solution of 1.85 g cobalt dicarbollide (cesium salt) in 45
ml THF was treated with 0.386 ml of 10 M BuLi in 5 ml THF. The orange solution
became black. After stirring 10 minutes, an equimolar amount of methylnorbornene
bromobutyl ether (1.0 g) was added. The solution became orange immediately, and
was stirred for 45 minutes. The solvent was then evaporated, and the residue extracted with 100 ml ether and 150 ml water. The ether phase dried to a tacky orange
solid. This solid was dissolved in a heated solution of 100 ml of 1M NaCL and 50 ml
acetone, and aqueous CsCl was added. As the solution cooled, a dark orange liquid
separated out. This orange product mixture was dissolved in 5 ml methylene chloride, applied to a silica gel column, and rinsed with methylene chloride. Rinsing with
methylene chloride did not remove any of the orange product, but did eleute a small
amount of clear oil. The orange product was eleuted with methanol and then dried.
The product was then separated into 5 fractions using column chromotography with
a 1:3 mixture of acetone and ethyl acetate. These fractions did not represent distinct
spots on TLC, but an arbitrary division of the components of one long smear into
fractions of equal volume. The crude product showed one long smear on TLC; each
fraction showed a shorter subsection of the long smear.
142
4.6.5
Poly-CDC-Siloxane
Mixed substitution and elimination product
This reaction was carried out at two temperatures, 0 °C and 50 °C, to determine
if the reaction temperature affect the ratio of substitution to elimination product.
Often, low temperatures favor formation of substitution products. In our case, the
product mixtures of the two reactions were found to have the same composition.
The 0 °C reaction was carrried out as follows. The 50 °C reaction was similar,
except for the temperature of reaction. Norbornene methanol (0.3 g) was dissolved
in 5 ml anhydrous THF and stirred with 0.06 g sodium hydride (1 equivalent) for 30
minutes at 65 °C. Slight orange discoloration was observed. The pot was cooled to 0
°C and a solution of 0.68 g of a mixture consisting of approximately 80% bromobutylcobalt dicarbollide (cesium salt) mixed with 20% cobalt dicarbollide (cesium salt) in
2 ml THF was added. This represents approximately 2 equivalents of the norbornene
methanol per bromo reagent. The pot was stirred for 30 minutes at 0 °C.
The solution was poured into a mixture of 30 ml water and 30 ml diethyl ether.
Aqueous cesium chloride was added to the water phase. The ether phase was dried
to 5 ml and fractionated with column chromotography.
Silica gel was used as an
adsorbant. Diethyl ether was run through the column to eleute unreacted norbornene
methanol, then acetonitrile was used to eleute the orange product.
HNMR of the product mixture showed norbornene olefin hydrogen peaks as well
as the unsaturated hydrogen peaks of the elimination product (see Figure 4-31).
Separate cobalt dicarbollide carbon hydrogen peaks were also visible for both products. Based on the integration areas of the norbornene vs alkene peaks, the 0 °C
reaction was found to be 53% elimination product and the 50 °C reaction was 55%
elimination product, a negligible difference.
Thin layer chromatography using silica gel plates and various solvent eleuents
was unable to separate the two products.
It is likely that there is a method to
separate these products, but we were not able to find one. Since alkenes poison
the polymerization catalysts ued for norbornene monomers [111, 41], this product
143
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E
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s
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a
a
.
E
Figure 4-31: HNMR of Elimination (Butene Cobalt Dicarbollide) and Substitution
(Methoxynorborne Cobalt Dicarbollide) Products in D-Acetone
144
mixture is not useful for ROMP.
Alternative methods of reaction, perhaps using a two phase system with phase
transfer catalyst, might produce less elimination product. We did not investigate any
alternative methods.
hydrosiloxane coupling
Unsaturated hydrocarbons are known to react with hydrosilane groups, sometimes
with the addition of a catalyst such as hexachloroplatinic acid. Thus, both components of the reaction product mixture described above could be grafted onto a
polymer with hydrosilane groups. We investigated this grafting reaction to see if it
would either produce a useful cobalt dicarbollide functionalize material or serve as
a way to purify the substitution product by removing the elimination product.The
norbornene product was found to react more readily than the alkene product, thus
reaction with hydrosilane polymer cannot be used to purify the norbornene monomer.
The hydrosilane polymer used was purchased from Geleste, and is a siloxane
copolymer of molecular weight 1900-2000, consisting of 25-30% methylhydrosilane
and the balance dimethylsilane.
(See Figure 4-32 for the HNMR spectum.)
The
mixed elimination and substitution product (0.48 g, estimated 39 mol% elimination
and 36 mol% substitution with the balance cobalt dicarbollide) was dissolved in 20
ml anhydrous toluene and 0.13 g of the polymer added. This amount of polymer,
assuming 25% hydrosilane, is 1.45 equivalents on a substitution product basis. A
drop of 0.12 M hexachloroplatinic acid in THF was added, and the solution stirred
at 65 °C for 3 days.
HNMR shows that the product no longer contains hydrosilane groups or norbornene olefin groups (Figure 4-33). Methyl silyl hydrogen peaks, alkene hydrogen
peaks, and cobalt dicarbollide carbon hydrogen peaks are still visible. The silixane copolymer remained liquid after the norbornene cobalt dicarbollide monomer
was grafted onto it, and we were unable to chromatographically separate the grafted
polymer from the remaining elimination product. Probably this difficulty would be
avoided if a higher molecular weight, solid polymer starting material were used, but
145
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0
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m
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4
IM ..... ISIIMN0.U..Izrum
i6
Figure 4-32: HNMR of Polydimethylsilane/Polymethylhydrosilane
-
146
o
<D
i
ppm
in D-Chloroform
D
c
none was available.
The product mixture from the hydrosiloxane grafting reaction was mixed with 10
ml THF and 0.09 g more hydrosiloxane polymer. One drop of the 0.12 M chloroplatinic acid in THF was added, and the solution was heated to 65 C overnight. HNMR
showed disappearance of the alkene hydrogen peaks as well as the hydrosilane hydrogen peaks (see Figure 4-34) The hydrosilation reaction easily attaches unsaturated
groups to the functional polymer, but because the polymer has low molecular weight,
it remains liquid after grafting. This product might be useful as a polyelectrolyte,
but we did not investigate possible applications.
4.6.6
Poly-Nor-Si-Anion
Two batches of this monomer were synthesized. The first batch had some impurity
containing silylmethyl groups. Possibly, excess butyl lithium had reacted with the
dimethylchlorosilylnorbornene. The second batch was more pure, as shown by melting
point measurements and HNMR results.
Schrock Catalyst
The Schrock catalyst did not polymerize this monomer. A solution of 0.113 g of the
cesium salt of the carborane anion monomer (batch 1) in 5 ml anhydrous THF was
dried overnight stirring over 4 Amolecular sieves. The solution was filtered and 1
ml THF added. To the monomer, 0.3 ml of a 13 mg/2ml in toluene solution of the
Schrock molybdenum catalyst was added, for a target length of 75 repeat units. The
solution was stirred overnight and terminated with an excess of bezaldahyde. HNMR
showed the product to be identical to the monomer.
Grubbs Catalyst, Methanol
The norbornene anion monomer is soluble in polar solvents such as methanol, acetone,
and THF. Since methanol was the superior solvent for the monomer, we wanted to
try using it as the polymerization solvent.
147
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Figure 4-33: HNMR of Mixed Elimination/Substitution
Polyhydrosilane in D-Acetone
148
Products Reacted with a
In
-*V
ru
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(I
-
v
,a
a
Figure 4-34: HNMR of First Hydrosilation Reaction Products Reacted with More
Polyhydrosilane in D-Acetone
149
Mixtures of methanol and toluene (ratios from 1:4 to 4:1) were previously tested as
polymerization solvents for a norbornene ether using the Grubbs ruthenium catalyst.
Our results were consistent with the literature, confirming that protic solvents such
as methanol can be used for polymerization. However, an attempt to use methanol
as the polymerization solvent for this monomer was unsuccessful.
A solution of 0.2 g of the cesium salt of the carborane anion monomer (batch 1) in
5.0 ml of methanol was treated to remove dissolved oxygen. The solution was sealed
under a rubber septum and nitrogen was introduced into the solution through a needle
while another needle vented from the headspace. A toluene solution (0.483 ml) of the
Grubbs ruthenium catalysts solution (32 mg/2ml) was injected for a target length
of 50 repeat units. During the first 10 minutes, nitrogen degassing was continued.
At the end of 10 minutes, the solution was green. After approximately 6 hours, the
solutions was yellow. After stirring overnight, the solution was dried. HNMR showed
the reaction product to be identical to the monomer.
Grubbs Catalyst, THF
The unreacted monomer from the methanol polymerization attempt was added to 5
ml of THF. Then 0.688 ml of a 32mg/3ml ruthenium catalyst solution was added
for a target length of 50 repeat units. This calculation assumed that 0.19 g of material was left from the 0.2 g originally used for the methanol reaction above. Within
minutes, the solution darkened. The reaction was stirred overnight and terminated
with an excess of ethyl vinyl ether. HNMR confirmed that the olefin bonds of the
norbornene moity had reacted, but impurities remained, as evidenced by the unexplained methylsilyl hydrogen peaks. Since this batch of monomer was not very pure
to begin with, and since this particular sample of the monomer had been recycled
from a previous polymerization attempt, the polymerization experiment was repeated
using monomer of higher purity.
0.05 g of the monomer from the 2nd batch was dissolved in 1 ml THF and 0.439
ml of a 33mg/3ml ruthenium catalyst solution was added for a target length of 20
repeat units. The solution turned brown and became slightly cloudy. The reaction
150
Table 4.2: Viscometry Results
Solution
THF
monomer, THF
Flow Time, seconds
67
71
product, THF
73
methanol
91
monomer, methanol
94
product, methanol
97
was stirred overnight and spontaneously separated into two phases. The top phase
was clear and light brown. A small amount of a second clear, blue/grean phase was
on the bottom of the vial. The solution was shaken up and dropped into 10 ml
hexane. A sticky tar precipitated and the hexane was decanted.
HNMR shows a
clean polymer spectrum with the characteristic polynorbornene olefin bond peaks in
the correct ratio with the broadened methylsilyl hydrogen peaks.
Viscometry
Solutions of the carborane anion monomer and the product of the first polymerization
attempt of this monomer using the Grubbs catalyst in THF were analyzed by capillary
viscometry. Each solution was 10 mg/i ml; THF and methanol were used as solvents.
Measurements were taken in a room temperature water bath. These measurements
were not fitted to viscocity equations, instead the monomer and potential polymer
product flow times were simply compared. Results showed little increase in viscocity
for the solution containing reaction product (see Table 4.2). Flow times were very
reproducible in multiple measurements.
151
Chapter 5
Cobalt Dicarbollide and Carborane
Anion Polymers: Potential
Applications
The cobalt dicarbollide and carborane anion groups have similar properties. As discussed in Chapter 1, both molecules are voluminous anions with good thermal and
chemical stability. Due to these shared characteristics, polymers functionalized with
either cobalt dicarbollide or the carborane anion may have applications for selectively
binding cesium. Additionally, such polymers may be interesting polyelectrolytes for
use in lithium batteries.
Chapter 4 discussed the synthesis of novel cobalt dicarbollide and carborane anion
functionalized polymers.
In this chapter, we discuss preliminary testing of these
materials for application as solid ion-exchangers and polyelectrolytes.
5.1
Cesium Ion-Exchange
Several novel polymers were examined for suitability as solid-phase cesium separation
materials.
In waste treatment applications, these polymers might be formed into
filters or small column packing beads. For our preliminary tests, the soluble materials
were simply cast into films. The cross-linked powder that resulted from the attach152
polymerize-sandwich scheme (poly-nor-C-carb-Co) was sealed into a "teabag" made
from a non-woven, porous polypropylene material chosen for its inertness to acid and
metal ions.
5.1.1
Cobalt Dicarbollide and Carborane Anion Polymers for
Cesium Binding
Poly-nor-C-CDC and poly-nor-Si-anion homopolymers were considered as potential
cesium ion-exchange materials. Because these materials are soluble in polar solvents
such as methanol, there was concern that they would also be partially soluble in
water. Some hydrophilicity is desirable in ion-exchange materials, to ensure contact
between the polymer functional groups and aqueous ions. However, material loss due
to solubility in water would negate one of the advantages of using a solid extractant
instead of a second solvent phase extractant.
One way to control polymer solubility characteristics is copolymerization. Both
nor-C-CDC and nor-Si-anion were copolymerized with norbornene to produce less hydrophilic materials. Additionally, a sample of the poly-nor-C-CDC/poly-norbornene
copolymer was cross-linked for further reduction in solubility.
Copolymerizing Nor-Si-Anion
The nor-Si-anion monomer was copolymerized with norbornene to form a polynorbornene(90)/poly-nor-Si-carb(30) polymer. This copolymer does not phase separate from
THF during polymerization, as the homopolymer does. The copolymer is also less
readily soluble in methanol than the homopolymer and somewhat less brittle. HNMR
confirms that approximately 25% of the copolymer is poly-nor-Si-carb from comparison of the olefin hydrogen and methylsilyl hydrogen peaks (see Figure 5-1).
Copolymerizing Nor-C-CDC
Copolymerization of the cobalt dicarbollide monomer with norbornene was investigated. The resulting copolymers were also brittle, orange solids but are less brittle
153
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Figure 5-1: HNMR of Polynorbornene(90)/Poly-Nor-Si-Anion(30)
in D-THF
154
Block Copolymer
a
than the homopolymer material. A 100:35 copolymer was soluble in toluene but only
slightly soluble in acetone while a 100:45 copolymer was fully soluble in acetone and
partially soluble in toluene. This demonstrates that copolymerization can be used to
tailor solubility.
GPC in THF with 50mM LiBr showed a coplymer peak with narrow weight distribution. A broad, lower molecular weight peak also appeared. HNMR suggests that
somewhat less than the expected amount of nor-C-CDC was incorporated into the
material (15% instead of 26% for the nominally 35:100 copolymer). (See Figure 5-2
for the spectrum. Figure 5-3 shows the HNMR spectrum of polynorbornene for comparison.) Since the nor-C-CDC monomer had never been completely separated from
cobalt dicarbollide coproduct and the ratio of the two products was only estimated
with HNMR, it is possible that the actual amount of nor-C-CDC added to the polymerization was less than calculated, and that cobalt dicarbollide was responsible for
the large low molecular weight peak.
Crosslinking the Copolymer
The copolymer was found to undergo cross-linking
to an insoluble resin when held at 65 C for four hours, probably due to reaction
of the unsaturated bonds in the polynorbornene blocks. Since polynorbornene has
a low Tg, it will heat crosslink at fairly low temperatures.
Temperature triggered
cross-linking might be useful, as the material could be processed into the desired
shape while soluble and then easily crosslinked.
Copolymer Morphology
Polynorbornene(100)-polynor-C-CDC(35) block copoly-
mers were examined by TEM for microphase separation. Both a thin film cast from
toluene and microtomed samples from a thick film cast from THF showed no microphase separation.
After annealing at 45-75 C for three hours, the thin film
sample still showed no microphase separation.
This indicates that the cobalt di-
carbollide functional group is not polar enough to significantly alter the polymer's
Flory-Huggins interaction parameter (X). This is somewhat surprising, since the
presence of the cobalt dicarbollide group does significantly impact polymer solubility.
155
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Figure 5-2: HNMR of Polynorbornene/Poly-Nor-C-CDC
Toluene
156
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Block Copolymer in D-
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Figure 5-3: HNMR of Polynorbornene in D-Ibluene
157
a
Ca
However, there have been other examples of copolymers with an organometallic block
that does not phase separate from an organic block [115].
The carborane anion copolymer was not examined for microphase separation. It
is expected that this block copolymer would be similar to the cobalt dicarbollide
functionalized material.
It has been reported that aqueous ions are able to diffuse through microphase
separated films containing one hydrophobic and one hydrophilic phase. Phase separation may result in better ion diffusion through the material since the hydrophilic
phase can form channels or pathways. However, our copolymer materials probably
have a high enough average hydrophilicity that such channels are unnecessary for the
diffusion of aqueous ions.
A Potential Crosslinking Agent for Poly-Nor-C-CDC
When cobalt chloride is combined with a solution of degraded nor-C-carb monomer,
an organometallic sandwich compound with norbornene groups attached to both
carborane fragments is formed. This difunctional monomer is a potential crosslinking
agent because both norbornene groups may be polymerized. Such crosslinking agents
would be useful for producing insoluble resin particles suitable for use in ion-exchange
columns. Because this monomer also contains the cobalt dicarbollide group, adding
such a crosslinking agent would not dilute the binding site density or decrease the
material's binding capacity.
Other studies have shown that microstructure can be controlled by the order of
polymerization.
We tested both simultaneous polymerization of crosslinking agent
and nor-C-CDC monomer as well as the "linear arm first" method which has been
found to produce microporous particles [12, 125].
These preliminary experiments showed that the bisnorbornene monomer does
not have good crosslinking activity. Polymerization of a 1:2 mixture of crosslinking monomer and nor-C-CDC results in soluble polymers with remaining unreacted
norbornene olefin bonds. HNMR suggest that only one norbornene group from each
difunctional monomer reacts. This is true when both monomers are mixed together
158
initially and when the monofunctional monomer is polymerized first. If crosslinking is
desired, either a different difunctional monomer should be used in combination with
nor-C-CDC or the heat triggered method could be used with the a polynorbornene
copolymer.
Poly-Nor-C-Carb-Co
The cross-linked powder was judged to be unsuitable for use in its synthesized form
and could not be heat-processed into other forms. After 0.1 g of the powder was sealed
into a bag and soaked in a 1 wt% nitric acid solution for 1 day, the solution became
noticibly orange and foamed when shaken. The solution color change and foaminess
indicates that the material partially dissolved in the dilute acid. Additionally, significant amounts of the fine powder escaped from the bag. Because this material was
such a fine powder, it was difficult to separate from solution. This material was not
investigated further.
5.1.2
Solubility in Nitric Acid
Strong nitric acid treatment is commonly used to remove cesium ions from cobalt
dicarbollide so that the binding agent can be reused. Thus, it is desirable for a
cobalt dicarbollide functionalized polymer to be stable in acid, except for undergoing
cesium-hydrogen ion exchange.
Approximately 30 mg each of poly-nor-Si-anion, polynorbornene(90)/poly-norSi-anion(30), poly-nor-C-CDC, polynorbornene(100)/poly-nor-C-CDC(45),
cross-linked polynorbornene(100)/poly-nor-C-CDC(45)
and heat
were tested. These materials
were soaked in 10 ml of 40 wt% (7.9 M) nitric acid to test for solubility and reactivity.
After 4 days, the films were rinsed, dried, and weighed to determine mass loss. All
of these polymers were synthesized in the cesium salt form, and it was expected that
this strong acid treatment would convert them into their acid form for future metal
ion binding tests. Therefore, some weight loss due to cesium-hydrogen counterion
exchange was expected. The acid solutions were sampled using ICP-MS to measure
159
Table 5.1: Weight Loss After 40 wt% Nitric Acid Treatment
Material
Original
Weight, mg
Post-Acid
wt%
% Dissolved
poly-nor-anion(20)
poly-nor-anion(30)/poly-norbornene(90)
42.5
29.6
52.5
78.4
36
1
poly-nor-CDC(50)
31.0
61.9
33
poly-nor-CDC(45) /poly-norbornene( 100)
poly-nor-CDC(45)/poly-norbornene(100)
29.3
28.3
64.5
71.4
31
23
heat cross-linked
the amount of cesium in solution.
It was anticipated that the acid form of the polymers would be more hydrophilic
than the cesium form. While hydrophilicity is desirable because it facilitates ionexchange, extreme hydrophicity could result in solubility and loss of the polymer.
Copoiymerization with norbornene and heat cross-linking were tested as ways to
reduce solubility. Table 5.1 lists the materials's original weight and percent remaining
after soaking in acid. Additionally, the percent of polymer dissolved is listed. The
amount of polymer dissolved was calculated based on the material weight change, the
amount of cesium detected, and the assumption that any cesium bound to dissolved
polymer would also be detected (see Section 5.1.3 for calculation details).
These results show that both homopolymers have significant solubility in strong
acid. For poly-nor-Si-anion materials, copolymerization with norbornene essentially
eliminates solubility. For poly-nor-CDC, copolymerization did not significantly lower
solubility. However, cross-linking in addition to copolymerization did reduce material
solubility.
These experiments do not indicate whether the measured solubility is due extraction of low molecular weight fractions in the material or to polymer dissolving. For
the cross-linked material, it is likely that some soluble fraction of the material was extracted and that subsequent acid treatment would not result in further material loss.
Experiments with sequential acid treatments would be needed to check for subsequent
material loss.
160
Material Color Change
After the acid treatments, all the materials evidenced some color change. The three
cobalt dicarbollide functionalized materials darkened somewhat from their original
orange color. The carborane anion materials were originally gray. Afterwards, the
homopolymer material was orange and the copolymer light orange. The cause of
these color changes was not determined.
Solution Color Change
The acid solutions also evidenced subtle color changes. The solutions in contact with
the carborane anion materials were very slightly yellow. The solution in contact
with the cobalt dicarbollide functionalized homopolymer was slightly pink, while the
solutions in contact with the cobalt dicarbollide copolymers were slightly yellow. All
the solutions had some small tendency to foam. Although bubbles formed when the
solutions were shaken, they quickly subsided. This effect was not quantified, but the
solution of the poly-nor-anion/poly-norbornene
copolymer seemed to be less foamy
than the others. These changes were probably the result of partial dissolution of the
polymers in the acid.
5.1.3
Cesium Stripping
The acid solutions were analyzed for cesium content to measure the effectiveness of
nitric acid stripping for regenerating these ion-exchange materials.
test of 2.7 mg of the poly-nor-CDC(35)/poly-norbornene(100)
A preliminary
material in 40 wt%
nitric acid indicated that approximately 80% of the cesium would be stripped from
the polymer. This calculation was made on the basis of the HNMR estimate of the
cobalt dicarbollide content (15 mol%), which was lower than the nominal content .
Table 5.2 shows the percent of cesium stripped from each material by the acid
treatment. The percentage of cesium stripped was calculated based on the amount of
cesium calculated to be originally present in the materials and the measured concentration in the acid solutions. In the case of the copolymers, the calculated amount
161
Table 5.2: Cesium Removal by Strong Nitric Acid
Material
poly-nor-anion(20)
poly-nor-anion(30)/poly-norbornene(90)
poly-nor-CDC(50)
poly-nor-CDC(45)/poly-norbornene(100)
poly-nor-CDC(45)/poly-norbornene(100)
heat cross-linked
Percentage of Cesium Stripped
60
110
35
36
43
of cesium originally present was based on the nominal copolymer rotios of functional
to non-functional monomer (45:100 for the cobalt dicarbollide copolymer and 30:90
for the carborane anion copolymer. The original cesium content is probably accurate
for the homopolymers and the carborane anion copolymer, but is probably overestimated for the cobalt dicarbollide copolymer (since HNMR analysis of the 35:100
batch indicated a lower percent of functional groups than expected). Thus, the actual percentage stripped from the cobalt dicarbollide copolymers may be higher than
the table shows.
These results show that cesium can be stripped from the carborane anion materials
more easily than from the cobalt dicarbollide materials. Additionally, the decrease in
hydrophilicity due to copolymerization does not adversely affect stripping. Actually,
copolymerization increases stripping efficiency in the carborane anion material. Heatinduced crosslinking also increases stripping. The anomalous result for the carborane
anion copolymer (over 100% cesium stripped) was probably caused by experimental
error during material weighing or solution sampling.
It is surprising that stripping efficiency should be improved by copolymerization or
cross-linking, since it might be expected that measures undertaken to reduce solubility
might also reduce the access of aqeous cations to the polymer functional groups.
Calculation
Assuming that all cesium associated with dissolved polymer is detected by ICPMS, and that the polymer dissolves uniformly (i.e the copolymer composition of the
162
remaining solid is the same as that of the dissolved material), we have the following
mass balance equations
Md = Afoc* (1- Pd) * Ps + Moc * Pd
Mr = Mo * (1 - Pd) * ((MWcs* (1 - Ps) + MWh * (Ps))/MWcs)
where Md = amount cesium detected in solution, Moc = amount of cesium in the
original material, Pd = fraction of material dissolved, Ps = fraction of cesium stripped
from the undissolved material, Mr = remaining material mass, Mo = original material
mass, MWcs = molecular weight of original material per cesium, MWh = molecular
weight of theoretical material per hydrogen if all cesium were exchanged for hydrogen.
We can solve for Pd and Ps. Md, Mo, and Mr are measured experimentally.
Moc is calculated from Mo and MWcs. MWcs and MWh are calculated from the
molecular structure of the materials. The largest potential inaccuracy is that since
the exact composition of the cobalt dicarbollide containing copolymers is unknown,
MWcs and MWh must be based on the nominal composition. Mo and Mr are subject
to experimental uncertainty stemming from the milligram quantities being weighed.
Mr is subject to error mainly due to inaccuracies in sample preparation for ICP-MS
analysis. Since the actual acid solutions are too concentrated for the instrument used,
samples were diluted by 4x105 times for analysis and some volumetric measurement
uncertainty was introduced. The error in Mr could be reduced by making up multiple
dilutions from each sample.
5.1.4
Cesium and Sodium Binding
After the materials were partially converted into the hydrogen form, they were exposed to solutions containing cesium and sodium ions. The solutions were 5.57x10-3 M
in both ions, and a sufficient volume of solution was used so that each sample of material was exposed to an amount of each cation equal to the total number of binding
sites present. The solution was approximately 0.4 wt% nitric acid, which is about
163
Table 5.3: Cesium and Sodium Binding by Carborane Anion and Cobalt Dicarbollide
Functionalized Polymers
Material
Theoretical
Kd Cs Kd Na
Capacity, meq/g ml/g
ml/g
3.4
209
<42
1.7
124
<17
poly-nor-CDC(50)
2.3
183
<27
poly-nor-CDC(45)/poly-norbornene(100)
1.6
77
<19
poly-nor-CDC(45)/poly-norbornene(100)
heat cross-linked
1.6
71
<17
poly-nor-anion(20)
poly-nor-anion(30)/poly-norbornene
(90)
0.06M.
Usually the binding of a dissolved ion by a solid material is described by the
distribution coefficient Kd, defined as the amount of bound ion per weight solid
divided by the amount of dissolved ion per volume solution. Kd is often quoted in
units of ml/g. The distribution coefficient is useful in comparisons between materials
of unknown composition. The theoretical capacity of an ion-exchange material is often
described in meq/g, the millimoles of binding site per gram material. Table 5.3 lists
the theoretical capacities of the materials tested and their distribution coefficients for
sodium and cesium as measured in 0.06M nitric acid.
The amount of sodium uptake by these materials was too small to be detected
with our techniques.
Thus, the distribution coefficients for sodium could not be
calculated. Instead, we list the minimum detectable Kd, assuming that our technique
could detect a 5% decrease in solution sodium content. The actual Kd for sodium
is less than this miniumum detectable level. Different experimental conditions would
be needed to measure the sodium binding constants so that the cation selectivity of
these materials could be quantified. However, our results show that these materials
are all selective for cesium over sodium.
Much research has been done recently on resorcinol-formaldehyde or phenolic
resins, as well as composite materials consisting of inorganic cesium binding materials in a polymer matrix
[9, 89, 23, 114, 99, 113, 76, 124]. However, these resins
are designed for alkaline wastes and are only effective at high pH. Often, cation bind164
ing decreases with increasing acid concentration. Kd for cesium is strongly affected
by pH and sodium concentration. This makes it somewhat difficult to compare our
results with most literature values. For organic resins in alkaline media, distribution
coefficients ranging from 10 to 5x10 4 have been measured
[9, 89, 114, 113, 763. The
cesium distribution coefficients for our materials are comparable to some resins in
acid and better than others [133].
Cesium capacity for our materials is comparable to some organic resins [9] but
lower than others [23, 114]. However, our resins are able to bind cesium at pH of 1-2,
while phenol or resorcinol resins are only useful at pH greater than 10.
Functional Group Efficiency
In order to compare the cesium binding efficiency of the functional groups in each of
our materials, Table 5.4 lists the actual capacity, ratio of Kd to theoretical capacity,
ratio of Kd to actual capacity, and ratio of cesium stripped to cesium bound. The
actual capacity for each material was calculated with the equation
* (1 - Ps) + MlWh* (Ps))
Ps/(MAWcs
where Ps = fraction of cesium stripped from the undissolved material, MWcs =
molecular weight of original material per cesium, and MWh = molecular weight of
theoretical material per hydrogen if all cesium were exchanged for hydrogen. Kd/Ct
(Ct = theoretical capacity) gives an indication of how well each material binds cesium
given theoretical binding site density, while Kd/Ca (Ca = actual capacity) gives
a measurement of how well each material binds cesium given the actual available
binding site density. The ratio of cesium stripped by nitric acid to cesium bound
by the material indicates what percentage of available (hydrogen form) binding sites
actually exchanged hydrogen for cesium under the experimental conditions.
From a comparison of Kd/Ct, the nor-Si-anion copolymer and the cobalt dicarbollide homopolymer materials show the best performance. However, considering Kd/Ca
or amount bound/amount stripped, we can conclude that the cobalt dicarbollide func165
Table 5.4: Cesium Binding Efficiency of Carborane Anion and Cobalt Dicarbollide
Functional Groups in Polymers
Material
poly-nor-anion(20)
poly-nor-anion(30)/
Actual Capacity
Kd Cs/Ct
Kd Cs/Ca
bound/
meq/g
ml/meq
ml/meq
stripped
1.7
2.0
62
72
121
64
0.53
0.26
0.7
0.5
79
49
270
156
1.11
0.72
0.6
46
119
0.54
poly-norbornene(90)
poly-nor-CDC(50)
poly-nor-CDC(45)/
poly-norborenen(100)
poly-nor-CDC(45)/
poly-norbornene(100)
heat cross-linked
tional group is better at binding cesium than the carborane anion group. Furthermore,
each functional group is most effective in the homopolymer, with copolymerization
significantly lowering the likelyhood of an available functional group binding to dissolved cesium. Being in a cross-linked material also lowers the binding effectiveness
of the cobalt dicarbollide functional group.
5.1.5
Conclusions
We have found that both carborane anion functionalize polymers and cobalt dicarbollide functionalized polymers remove cesium ions from acidic solution. Copolymerization and crosslinking can be used to control undesirable polymer solubility in strong
nitric acid, but there is a trade-off in binding efficiency. Strong nitric acid can be
used to strip cesium off the polymers. These materials are all selective for cesium
over sodium.
Our polymers were all functionalized with metal-binding groups attached near the
polymer backbone. It would be interesting to see whether the use of a spacer chain
improves cesium binding efficiency and decouples binding efficiency from material
solubility.
166
5.2
Solid Electrolytes
Solid-state batteries need a solid electrolyte separating anode and cathode. Polymeric
materials have a number of advantages for use as solid electrolytes. Unlike inorganic
crystals or glasses, polymers can maintain good physical contact with electrodes under the stresses caused by battery operation. They also have much lower minimum
use temperatures than molten salt electrolytes. Although polymer electrolytes usually have significantly lower ionic conductivity than these other materials, they can
be formed into thin films so that larger area compensates for lower conductivity.
Polymer electrolytes have been studied particularly for rechargable lithium battery
applications
[31].
Many polymer electrolytes contain a polymer doped with a lithium salt. Poly(ethylene
oxide) polymers (or non-crystalline variations on PEO) are commonly used because
the ether groups are able to coordinate lithium ions strongly enough for good dispersion yet weakly enough to allow mobility. The lithium salt used must be able to
dissociate in the polymer to allow movement of the cation and anion [32]
Other polymer electrolytes are actually polyelectrolytes, with immobile charged
groups attached to the polymer structure and mobile counterions. This single ion conductivity has the advantage that salt layers cannot build up at the electrolyte/electrode
interfaces. Such layers can interfere with conductivity [32]. Since most polyelectrolytes are brittle polymers, they often need to be either plasticized or incorporated
into a copolymer [31]. In addition to use in lithium batteries, polyelectrolytes may
have applications
in fuel cells or thin film electrochromatic
devices [33].
We have synthesized polyelectrolytes functionalized with carborane anions or
cobalt dicarbollide. Since cobalt dicarbollide and the carborane anion are unusually diffuse anions, they easily dissociate from their counterions. The combination of
an polymer-bound anion with a dissociated cation makes the the lithium salts of these
polymers good candidates for use in lithium batteries. These brittle polymers could
be plasticized by blending with PEO or other rubbery, ionically conductive polymers.
Alternatively, our polymers could be modified to contain PEO-like blocks. There
167
are a number of norbornene-based, ether functionalize monomers (describe in the
appendix) which could be copolymerized with our anionic monomers, Alternatively,
our polymers might be used as the basis for an interesting comb copolymer with PEO
side-chains.
5.2.1
Ionic Conductivity with the Small Molecule Lithium
Salts
Before testing our polyelectrolyte materials, we considered both the carborane anion
and cobalt dicarbollide lithium salts for use as electrolytes in a poly(oligo(oxyethylene)
methacrylate) (POEM) matrix. These experiments were conducted in cooperation
with Philip Soo in Professor Anne Mayes's group.
Typically, polymer electrolyte
candidate for use in a lithium battery would have a salt such as LiCF3 SO3 mixed
into a PEO-type polymer matrix. LiCF3 SO3 dissociates well in PEO, but is known
to undergo decomposition reactions with the lithium anode. Carborane salts may be
more stable.
The carborane anion was obtained as the trimethylammonium salt, while cobalt
dicarbollide was synthesized as the cesium salt. We converted these compounds into
their lithium forms before testing so that our results could be compared to other
lithium conductivity data. Lithium is a cation of interest for battery applications,
because lithium batteries have a much higher theoretical energy density than nickelcadmium or lead-acid batteries [33].
The lithium salt of the carborane anion was found to be very hygroscopic. Both
salts were dried under vacuum at elevated temperatures before testing. The cobalt
dicarbollide salt was tested at a lithium to ethylene oxide ratio of 1:20. The carborane
anion salt was tested at ratios of 1:10, 1:21, and 1:30. Figure 5-4 shows the plots of
conductivity versus temperature for POEM doped with these salts.
These results indicate that the lithium salts of cobalt dicarbollide and the carborane anion work as dopants for ionically conductive polymers. Measured onductivity
for these samples was somewhat lower than for samples using LiCF 3 SO3 [128], but
168
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Cobalt Dicarbollide and the Carborane Anion in POEM
169
may have been adversely affected by water contamination (in the case of the carborane anion). We anticipate that a polymer functionalized with either moity would
also work as a lithium dopant in a POEM matrix.
5.2.2
A Polyelectrolyte Lithium Salt
The carborane anion polymer was selected for testing as a polyelectrolyte lithium
salt. Since cobalt dicarbollide is known to react with alkali metals, it would not be
appropriate for use with a lithium anode. Poly-nor-Si-anion is glassy, so it would not
be a good ion conductor on its own but might make a suitable dopant for a PEO-type
polymer. We were unable to test blended samples, as a solution of the lithium salt of
poly-nor-Si-anion and POEM in methanol demonstrated gross phase separation upon
drying.
5.2.3
Potential Pathway to an Unusual Comb Copolymer
A variety of norbornene-based ether functionalized monomer are available for copolymerization with nor-Si-anion or nor-C-CDC to produce a polyelectrolyte with ether
groups for lithium ion conductivity. This approach is similar to that of Douglass,
combining the lithium ion conductivity of PEO, the low charge density of a carboranebased anion, and the synthetic flexibility of ROMP [21]. Douglass et al. encountered
difficulties with monomer purification.
Additionally, they used the B12 H1 1SMeR-
anion, which may have its negative charge localized at the S atom instead of being distributed over the boron cage [106]. Although it is not yet clear whether this
approach will work, it seems promising.
We propose a route to an unusual comb copolymer containing PEO blocks for
lithium ion transport and the lithium salt of the carborane anion. This copolymer
would be similar to a poly-nor-C-carb/PEO
comb copolymer (see Figure 5-5) except
it would also contain nor-Si-Anion units in the backbone. Such a copolymer could
be synthesized by a combination of ROMP and anionic polymerization.
First, a
carborane functionalized monomer could be deprotonated at the carborane carbon
170
7
0
-
Figure 5-5: Proposed Synthesis of Poly-Nor-C-Carb/PEO
Comb Copolymer
and used to oligomerize a short chain of ethylene oxide units. Then, this monomer
could be
opolymerized with nor-Si-anion. Alternatively, a poly-nor-Si-anion/poly-
nor-Si-carb copolymer could be synthesized, and then the carborane carbons could be
deprotonated with butyl lithium and ethylene oxide chains grown from the carbanion
sites. Either block or random copolymers could be made.
These schemes have the advantage that the PEO side chain length could be sys-
tematically varied to achieve the optimal ether/lithium salt ratio for conductivity.
Probably the PEO segments would be prevented from crystallizing, as they would be
relatively short and interspersed with the other components. (Crystallinity is undesirable, as it raises the minimum use temperature.) These polyelectrolytes would be
single ion conductions, since the anionic groups are attached to the polymer backbone.
We conducted a preliminary investigation into this synthetic route. Because ethylene oxide is a gas, it is more difficult to handle than a liquid like propylene oxide.
Thus, we substituted propylene oxide in these experiments. The reaction of propylene
oxide with carborane carbanion has been reported before.
171
Our results showed that nor-Si-carb can be deprotonated at the carborane carbon with butyl lithium. This carbanion can react to add propylene oxide. Similarly,
the pendant carboranes of poly-nor-Si-carb can be deprotonated with butyl lithium
to create carbanions which add propylene exide. Apparently, attachment to a norbornene framework or polymer backbone does not prevent carborane carbanions from
reacting with propylene oxide. However, HNMR spectra of these products suggest
that only one propylene oxide attached at each carbanion (see Figures 5-6 and 5-7).
We hypothesize that water or other impurities in the propylene oxide prevented
anionic polymerization. If these experiments are repeated, the propylene oxide starting material should be more rigorously purified.
5.3 Experimental Details
5.3.1
Copolymerization
Polynorbornene( 100) /Poly-Nor-C-CDC (45)
A solution of 0.031 g norbornene in 1 ml anhydrous THF was polymerized to a target
length of 100 repeat units with 0.271 ml of 30 mg/3 ml Grubbs catalyst.
After 1
hr, 0.1 g of nor-C-CDC product (approximately 80 mol% nor-C-CDC and 20 mol%
cobalt dicarbollide) was added for a target length of 45 repeat units. After stirring
overnight, the polymer solution was precipitated in 20 ml hexane. The hexane was
decanted off and the solids dried under vacuum. Approximately 64% of the mass was
recovered.
Polynorbornene(90)/Poly-Nor-Si-Anion(30)
A solution of 0.033 g norbornen in 1 ml anhydrous THF was polymerized to a target
length of 90 repeat units with 0.322 ml of a 30mg/3ml Grubbs solution. After 4 hours,
0.05 g nor-Si-anion (cesium salt) was added for a target length of 30 repeat units.
After stirring overnight, the solution was noticibly viscous. The polymer was diluted
with 1 ml THF and precipitated in 20 ml hexane. The fluffy solid was recovered by
172
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173
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174
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gravity filtration.
5.3.2
Testing a Potential Crosslinking Agent
Bis(nor-C)-CDC was considered as a potential crosslinking agent for poly-nor-C-CDC.
In one experiment, 0.05 g of the nor-C-CDC product was mixed with 0.0266 g bis(norC)-CDC in 1 ml anhydrous THF. 0.146 ml of 30 mg/3 ml Grubbs catalyst solutions
was added. In another experiment, 0.05 g of the nor-C-CDC product in 0.5 ml THF
was first polymerized with 0.146 ml of 30 mg/3 ml Grubbs solution and 0.0266 g
bis(nor-C)-CDC added after 3.5 hours. In each case, there was an approximately 1:2
ratio between bis(nor-C)-CDC and nor-C-CDC and a 1:50 ratio between catalyst and
nor-C-CDC. Each polymerization solution was stirred overnight and then precipitated
in 17 ml hexane. The solids were let settle, and the hexane decanted. Figures 5-8
and 5-9 show the HNMR spectra for these reactions.
5.3.3
Sodium and Cesium Analysis
Cesium analysis was done by Barry Grant from the Department of Earth, Atmospheric, and Planetary Science. A quadrapole ICP-MS with microconcentric nebulizer, the VG Elemental Fison PQ2 Plus, was used. Samples were diluted 1/4x10 5
because the ICP-MS was set up to do ppb analysis. A 1% nitric acid solution was
used as the the sample matrix. Although MilliQ water was used for these experiments, some cesium was detected in the blank 1% nitric acid in water sample. Thus,
sample concentrations were calculated by correcting for the blank cesium content and
then calibrating with a 1.02 ppb cesium standard. The standard was run every 2 or
3 samples so that machine sensitivity drift could be measured and corrected for.
Sodium analysis was done with an ICP-AES. This machine could not be used for
cesium analysis because the cesium emission is very weak. Samples were diluted 1/2
and concentrations were calculated based on standard solutions.
175
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in D-Acetone
176
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Later in D-Acetone
177
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5.3.4
Converting to Lithium Salts
Cobalt Dicarbollide
A solution of 0.1 g trimethylammonium chloride was dissolved in 1 ml methanol and
added to a solution of 0.238 g cobalt dicarbollide (cesium salt) in 1 ml methanol.
Some solids began to precipitate. Water was added until the solution was colorless,
with orange precipitates. The solids were filtered off and rinsed with 10 ml water.
The cobalt dicarbollide (trimethylammonium salt) was dissolved in 6 ml anhydrous THF and treated with a solution of 0.051 ml 10M butyl lithium in 1 ml THF.
The butyl lithium solution was dripped into the stirred cobalt dicarbollide solution
until a persistent color change was observed. With each drop, the solution became
black and then orange again. The black color signals deprotonation of cobalt dicarbollide. Butyl lithium is more reactive towards the trimethylammonium cation than
cobalt dicarbollide, so all the trimethylammonium reacts first, leaving the cobalt
dicarbollide in its lithium salt form.
After 1 day, the solution was dried and dissolved in 2 ml water. The product was
extracted with 3x3 ml diethyl ether. The ether phases were collected and dried under
vacuum.
Carborane Anion
The carborane anion was purchase in its trimethylammonium salt form. A solution of
0.2 g carborane anion (trimethylammonium salt) in 5 ml anhydrous THF was treated
with 0.128 ml 10M BuLi in 1 ml THF (1.3 equivalents). The solution started to show
some color change to yellow at the end of the addition.
The lithium salt is insoluble in ether, so it could not be purified by water/ether
extraction.
Insetad, the solution was dried and redissolved in acetone. Insolubles
were filtered off, and the filtrate dried under vacuum.
178
5.3.5
Reaction of Propylene Oxide with Norbornene Carborane Carbanions
Reaction with Poly-Nor-Si-Carb
A solution of 0.1 g of poly-nor-Si-carb(100) in 5 ml anhydrous THF was deprotonated
with a solution of 0.034 ml 10M butyl lithium in 2 ml THF. Upon addition of the
BuLi, the polymer precipitated in large pieces from solution.
It is likely that the
poly-carbanion is insoluble in THF. To the stirred solution, 0.98 g of propylene oxide
in 3 ml THF were added. After 90 minutes, the polymer had redissolved but the
solution was cloudy. A solution of 0.34 ml 1N HCl in 3 ml THF was added to the
polymer solution. The polymer solution cloudiness mostly disappeared. The solution
was dried to about 2 ml and precipitated in 100 ml methanol. Polymer precipitated
as a rubbery tar. This material was dried overnight in a vacuum oven.
Reaction with Nor-Si-Carb Monomer
Propylene oxide was stirred with NaH and filtered. 0.1 g nor-Si-carb was dissolved
in 5 ml anhydrous THF and deprotonated with a solution of 0.034 ml 10M BuLi in
2 ml THF. After 15 min, 1.0 g of the treated
propylene oxide was added.
After 30
minutes, the solution was yellow. After stirring overnight, a solution of 0.34 ml 1 N
HC1 in 2 ml THF was added to the monomer solution. Upon addition of the acid,
the monomer solution became cloudy and soem white precipitate was formed. The
solution was dried in a vacuum oven overnight, -hen dissolved in acetone and filtered.
The filtrate contained the product.
179
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191
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