Olefin Polymerizations Catalyzed by Late
Transition Metal Complexes
Maurice Brookhart
University of North Carolina
Polyolefins
Polyethylene,
CH2
Polypropylene, CH3CH
CH
CH2
CH2
CH2
Polystyrene,
n
n
n
6 x 1010 lbs/yr
3 x 1010 lbs/yr
1 x 1010
lbs/yr
Total : 100 billions / year
16lbs / person on Earth / year !
• Inexpensive monomers
• Little waste in production
• Attractive physical properties, long term stabilities
Polymer Microstructure — Key to Properties
Polypropylene
H3C
H3 C
H
isotactic
H3C
H
H3C
H
H3C
H
H
Tm = 160°C
Stereoregular
Tm = 165°C
syndiotactic
Completely amorphous
atactic
Polyethylene
High Density PE (HDPE) Tm= 136°C
R
R
Linear Low Density PE (LLDPE)
Tm = 115~130°C
Low Density PE (LDPE) Tm= 105~115°C
Polyolefins Primarily Produced via
Metal-Catalyzed Processes
Catalyst Structures Control:
— polymer microstructures
— polymer molecular weights, molecular weight distributions
— comonomer incorporation
Early Metal Catalysts (Ti, Zr, Cr)
isotactic polypropylene
syndiotactic polypropylene
atactic polypropylene
Late Metal Catalysts (Pd, Ni, Co)
O
O
O
alternating CO /
O
copolymer
high % crystallinity
Ph Ph Ph
syndiotactic polystyrene
Ph Ph Ph
Tm ~ 250oC
General Mechanism for Polymer Formation
Initiation
LnM
H
LnM
H
migratory
LnM
insertion
Chain Growth (RP)
LnM
migratory
LnM
LnM
insertion
etc
Chain transfer (RCT)
LnM
LnM
LnM
P
H H
-H
elim.
P
+
LnM
LnM
P
H
H
new chain starts
etc.
LnM
RP >> RCT => High Polymer
RP ~ RCT => Short Chains (oligomers)
RP < RCT => Only C4
Olefin Polymerizations Using Late Metal
Catalysts (Ni, Pd)
Why Late Metals ?
1. Potentially different enchainment mechanisms =>
new microstructures
2. Less oxophilic — functional group compatible
G
G
G
But…
1. Normally lower insertion barriers
2. Chain transfer competitive with propagation =>
dimers, short chain oligomers
α–Diimine Based Catalysts
R
R
R'
A
R = H, Me, acenaphthyl
R'
N
M
R'
R' = -iPr, -Me, aryl, halogen
N
H3C
R'
solv.
CF3
A =
B
4
M = Ni, Pd
CF3
■ High molecular weight polymers with unique microstructures from:
● ethylene
● α – olefins
● cyclopentene
● trans-1,2-disubstituted olefins
■ Copolymers of ethylene with certain polar vinyl monomers
Catalysts Modeled on α–Diimine Systems
R Ph
N
R
Ar
R
N
N
N
P
M
X
R
Ar
M
X
R
R
MMAO
Daugulis, Brookhart
M = Fe, Co
Bennett
Small, Brookhart
Gibson
R
R
N
N
M
R
G
R
M = Ni, Pd
R
R
R
Ni
N N
N N
N
O
R
Grubbs
Johnson (DuPont)
R
R
M
R
M = Ni, Pd
N
O
Killian (Eastman)
Ni
R
Hicks, Jenkins, Brookhart
Polyethylene
n
Early Metal Catalysts
Ti (IV), Zr (IV), Cr/SiO2
linear PE
semicrystalline
Tm ~ 136 °C
High Density PE, HDPE
1-10% incorporation, LLDPE
R
R
R
Tm = ~ 115 - 130 oC
+
Mn > 105
R
amorphous PE
R
N
N
Pd
R
hyperbranched,
R
Solv
R'
30 °C
~500 TO/hr
~100 branches / 1000 C's
5
Mn 10 - 10
R
Tm: 25° - 135° C
R
N
N
Ni
R
Br
R
Br
/ Et2AlCl
6
30 °C
~1-3 x 106 TO/hr
5 - 80 branches / 1000 C's
increasing [C2H4] decreases branching
increasing T increases branching
Poly (α–Olefins)
Early Metal Catalysts
R
1,2-insertion
R
Ar
R
N
R
R
R
R
+
"chain-straightened" (1, 3 enchainment)
N Ar
M
chain-straightened, primarily C1, C4 branches
(1,6 enchainment)
1,2–Disubstituted Olefins
cis-1,3enchainment
R
Ar
R
N
+ A-
N Ar
M
X
Y
chain
straightening
1,3 insertion
Mechanistic Studies
Generation of Cationic Alkyl Complexes
R
R
N X
M
N X
R
R
N
CH3
M
N
CH3
2 R'MgX
N
M = Pd
stable at 25 °C
CH3
M
N
CH3
R' = -CH3
-CH2CH3
-CH2CH2CH3
-CH2CH(CH3)2
H(OEt2)2+ BAr'4Et20
N
M = Ni
stable only below
ca. -20 °C
CH3
M
N
OEt2
+
BAr'4-
1H, 13C
NMR Studies – Pd(II)
+
Ar
Me
N
Pd
N
OEt2
Ar
+
Ar
C2H4 (excess)
N
CD2Cl2
-80 °C
N
Me
Pd
Ar
-30 °C
k1
+
Ar
poly
N
Pd
N
Ar
catalyst
resting state
+
Ar
N
kp, -30 °C
Pd
N
Ar
Insertion Kinetics – Ni(II)
+
Ar
Me
N
Ni
N
1. 20 eq C2H4
-130 °C
2. -110 °C
Ni
Ar
+
Ar
Pr
N
Ni
Ar
+
Ar
-70 °C
N
ksub. insert.
N
R
Ni
Ar
-80 °C
k1st insertion
N
Ar
N
Me
N
CDCl2F
OEt2
+
Ar
Activation Barriers to Insertion (ethylene)
G (1st insertion)
G (subseq. insertions)
+
Me
N
Ni
13.6 kcal/mol (-81 oC)
14.0 kcal/mol (-72 oC)
18.4 kcal/mol (-20 oC)
18.6 kcal/mol (-20 oC)
N
+
N
Me
Pd
N
G‡ (Pd-Ni) ca. 5 kcal/mol
Mechanistic Model
insertion
insertion
R'
N
N
R
R
M
M
N
N
M
N
methyl branch
resting state
N
ethyl branch
turnoverlimiting
R
N
R
N
M
N
R
M
N
H
N
R'
N
M
N
M
N
"chain running"
Blocking of Axial Coordination Sites
Chain Transfer Mechanisms
(1) Associative Displacement (retarded by blocking axial postions)
N
N
N
M
N
+
R
M
H
H
R
(2) Chain Transfer to Monomer (suggested by Ziegler calculations)
H3C
N
N
Ni
Ni
H3C
N
H3C
N
N
H
Ni
H3C
N
Mechanistic Model
insertion
insertion
R'
N
N
R
R
M
M
N
N
M
N
methyl branch
resting state
N
ethyl branch
turnoverlimiting
R
N
R
N
M
N
R
M
N
H
N
R'
N
M
N
M
N
"chain running"
Formation of Agostic Ethyl Complex
BAr'4
N
Pd
N
H(OiPr2)2BAr'4
N
CDCl2F, -80 oC
N
CH3CH3
1
H 2.2 ppm
13
C  38.5 ppm
Hc
Hc
C
C
Pd
 -8.9 ppm
t, 2JHH = 16 Hz
1
JCH = 67 Hz
1
JCH = 153 Hz
Hb
Hb
Ha
1
H 1.4 ppm
13
1
C  19.3 ppm
JCH = 155 Hz
Pd
H
(-130 oC)
Dynamics of Agostic Ethyl Complex
Hc H c
N
*
Pd
Hb
Hb
Ha
N
*
N
Hb Hb
N
Pd
Pd
N
*
Ha
Hc
Hc
Ha
N
k = 1450 s-1, -108 oC
G‡ = 7.1 kcal/mol
H  H
N
H
H
Ni
N
H H
H
N
N
Ni
Ni
N
H
H
H
k = 170 s-1, 16 °C
G‡ = 14.0 kcal/mol
N
H
Cationic Metal Alkyl Intermediates –
Ethylene Trapping Experiments
N
Pd
H(OEt2)2+ BAr'4-80 °C
N
N
N
Pd
Pd
H
N
1
N
-80 °C
-65 °C
20
N
Pd
Pd
N
N
1
H
N
20
-25 °C
insertion
(several 100 1,2 shifts prior to insertion)
(via reversible
loss of C2H4)
Cationic Metal Alkyl Intermediates –
Ethylene Trapping Experiments
N
Ni
N
-80 °C
X
N
Ni
N
-80 °C
N
N
Ni
N
Ni
N
etc.
etc.
no equilibration
prior to insertion
Mechanistic Model
insertion
insertion
R'
N
N
R
R
M
M
N
N
M
N
methyl branch
resting state
N
ethyl branch
turnoverlimiting
R
N
R
N
M
N
R
M
H
N
H
N
R'
N
M
N
M
H
N
H
"chain running"
Commercial Copolymers of Ethylene and Polar
Vinyl Monomers
● Radical Initiation
● High temperatures, very high ethylene pressure
CO2Me
OAc
CO2Bu
CN
CO2H
Si(OMe)3
CO2H
Examination of Pd and Ni Diimine Catalysts for
Copolymerizations of Ethylene and:
OR
1.
O
O
2.
O
R
OR
3.
Si
OR OR
Problems Connected with
G
Copolymerization
1. Monomer Binding through the Functional Group
R
L
M
R
L
+
M
G
L
L
G
2. β-Elimination of G
R
L
M
L
R
L
L
M
M
G
L
R
G
L
G
3. Weak Competitive Binding of
R
L
G
M
M
+
L
R
L
+
L
G
G
K >> 1
4. Strong Chelate Formation Following Insertion
R
L
insertion
M
L
G
isomerization
L
L
M
L
M
L
G
K << 1
G
5. High Barrier to Insertion of Open Chelate
G
L
M
G
R
insertion
M
slow
R
G1‡
R
G2‡
L
L
L
M
L
L
R
insertion
fast
L
M
L
‡
G1‡ > G2
Examples: G = -CN ; -Br, -Cl
CH3
N
CN
CH3
N
Pd
Pd
N
N
OEt2
NC
Ittel, Johnson, Brookhart, Chem. Rev. 2000
CH3
N
X = Br, Cl
Pd
N
X
Sen, et. al. 2002
Jordan, et. al. 2003
N
Pd
N
N
elim
Pd
X
N
N
X
X
Pd
N
N
Pd
X
N
2+
Ethylene / Acrylate Copolymerization - Pd
CH3
N
Pd
N
NCCH3
CO2CH3
CH2Cl2, T = 35 oC
P(C2H4) = 2 atm
MA = 25 vol%
CO2CH3
TOF = ca. 100 TO/h (slow!)
Branched Copolymer
6 mol% MA incorporation
103 branches/1000 C
Methyl Acrylate Insertion
O
OCH3
N
Pd
N
CH3
-80 oC
2,1-insertion
O
N
-60 oC
Pd
OCH3
N
OCH3
O
N
Pd
N
O
N
Pd
N
-30 oC
OCH3
Mechanism of Copolymerization
O
OCH3
N
OCH3
N
O
+C2H4
N
-C2H4
N
Pd
Pd
N
2,1-ins.
G = 16 kcal/mol
‡
P
rearrangement
Pd
N
K ~ 0.02 M-1
P
resting state
O
P
OCH3
25 °C
insertion
‡
G ~ 18 kcal/mol
N
Pd
chain growth
N
O
chain running
P
OCH3
Examination of Pd and Ni Diimine Catalysts for
Copolymerizations of Ethylene and:
OR
1.
O
O
2.
O
R
OR
3.
Si
OR OR
Ethylene / Alkoxy Vinyl Silane Copolymers
Versipol Group - DuPont
R'
random copolymer
R'
Si(OR)xR'y
R''
N
N
M
R' R L R'
/
linear to highly branched
R''
25 - 120 °C
up to 32 mol%
comonomer incorporation
600 psi C2H4, 60 °C
N
N
Ni
Me3Si
SiMe3
5 vol%
Si(OEt)3
toluene
5 eq. B(C6F5)3
5 eq. LiB(C6F5)4
PE copolymer
0.42 mol% silane
10 Me branches / 1000 C
Tm = 121 °C
Mn = 25.5 K, Mw/Mn = 2.9
110 kg PE / gm Ni
Vinyl Alkoxy Silane Insertion Chemistry -
N
N
Si(OEt)(Me)2
OEt2
CD2Cl2
-60 °C
Ni
Me
no 2-alkene
complexes
observed
N
N
N
Ni
Si
Si(OEt)(Me)2
N
Ni
O
O Et
Et
15%
2,1 insertion
Si
85%
1,2 insertion
Evidence for Reversible C2H4 Coordination
N
N
N
Ni
O Et
Si
N
Ni
+ C2H4
CDCl2F
EtO
Si
Keq ca. 0.035 M-1, -120 °C
 4.68
 4.59
 3.96
 3.73
Advantages of Vinyl Alkoxy Silane
Comonomers
1. Insertion barriers of vinyl alkoxy silanes into Pd-R and
Ni-R bonds are similar to ethylene insertion barriers.
2. Chelates resulting from vinyl alkoxy silane insertions
are readily opened with ethylene.
3. Open chelates readily insert ethylene.
4. Relative binding affinities favor ethylene, but not to a
prohibitive extent.