Spring 2008
Chap 10. Non-Radical Addition Polymerization
Anionic Polymerization -the growing chain end bears a negative charge
The mechanism of anionic polymerization is a kind of repetitive conjugate addition reaction .
(the "Michael reaction" in organic chemistry)
Cationic Polymerization -the growing chain end bears a positive charge
The mechanism of cationic polymerization is a kind of repetitive alkylation reaction.
Hanyang Univ.
Spring 2008
Anionic Polymerization
General Scheme
Initiation:
B-Z + CH2=CHX
B-CH2-CH- Z+
X
Propagation:
M - Z+ + M
MM- Z+
Termination:
M- Z+ + HT
MH + ZT
Hanyang Univ.
Spring 2008
Anionic Polymerization
Styrene Polymerization
Initiation:
CH3CH2 CH
Li
+
CH2 CH
CH3CH2 CH CH2 CH Li
CH3
CH3
Propagation:
CH2 CH Li
+
CH2 CH
CH2 CH CH2 CH Li
Termination:
CH2 CH Li
+
H OH
CH2 CH2
+
Li OH
Hanyang Univ.
Spring 2008
Anionic Polymerization
Characteristics of an Ideal Anionic Polymerization
 Negative centers repel one another and thus termination by
recombination is not possible. An ideal polymerization is “living”,
which does not terminate until a terminator is added.
 Initiation is normally very fast relative to propagation and all chains
grow simultaneously. This leads to polymers with low polydispersity or
monodispersity.
Mw
1

1
 Theoretically:
Mn
xn
 The rate of polymerization for methacrylates and styrenes is high even at
-78 oC. This is partly for the high concentration of the anion centers.
 The degree of polymerization
K M 0
xn 
I 0
 K=1 or 2 depending on initiator used.
Hanyang Univ.
Spring 2008
Anionic Polymerization
Initiation by Electron Transfer
-.
+
THF
K+
K
-78 oC
-.
K+
.
CH2 CH K +
2
•
•
+
CH2 CH
K
+
.
+
CH2 CH K
+ CH CH2 CH2 CH K +
Polymerization mostly done in THF and not nonpolar solvents like
cyclohexane or benzene for the solubility the complex in THF.
The degree of polymerization is given by
2M 0
xn 
I 0
Hanyang Univ.
Spring 2008
Anionic Polymerization
Initiation by Nucleophilic Attack
CH3(CH2)2 CH2 Li +
+
CH2 CH THF
-78 oC
CH3
N Li
+
CH3(CH2)4 CH Li +
THF
CH2 C
CH3
N CH2 C
COOCH3
Li +
-78 oC
COOCH3
• Polymerization can be done in both polar and nonpolar solvents.
• The degree of polymerization is given by

M 0
xn 
I 0
Hanyang Univ.
Spring 2008
Anionic Polymerization
Initiation by Living Polymer
CH2 CH Li +
CH3
+
CH3
THF
CH2 C
COOCH3 -78
CH2 CH CH2 C Li +
oC
COOCH3
But not
CH3
CH2 C Li +
+
CH2 CH
COOCH3
Because the starting anion has to be a stronger Lewis base than the
resulting anion.
xn 
M 0
I 0
Hanyang Univ.
Spring 2008
Anionic Polymerization
Propagation
M Z
Covalent
Bond
M Z+
Contact Ion
Pair
M
-
Z
+
Solvent Separated
Ion Pair
M
-
+
Z
+
Free ions
Solvent polarity increases
kP increases
Polymer tacticity decreases
•
Kp can vary by orders of magnitude.
•
The polydispersity remains low because the rate of inter-conversion
between the different forms is much faster than the polymerization.
Hanyang Univ.
Spring 2008
Anionic Polymerization
Termination
By proton
M Z+
H+
MH + Z +
By CO2
M Z+
CO2
MCOO Z +
By using a limiting amount of 1,2-dibromoethane
2
M Z+
CH2Br CH2Br
M
M
K=2
By using a much excess of 1,2-dibromoethane
M Z+
CH2Br CH2Br
M
Br
Hanyang Univ.
Spring 2008
Anionic Polymerization
(1) proton donor
H2O or ethanol
Strong base is not enough for initiation.
H2O Ctr,s=10
H2O
low MW polymer
No living polymer
(2)
Strong base is not
enough for initiation.
H
CH2 C:-
+
C2H5OH
CH2
CH2
+
C2H5O-
Ctr,s=10-3 (small chain transfer constant)
EtOH
ethoxide
high MW product
no longer living.
Hanyang Univ.
Spring 2008
Anionic Polymerization
(3) Termination can occurred by hydride elimination without impurities.
a)
b) anionic species(active center) react with chain ends to form inactive
allylic anion.
. .CH2 CH
+
CH2 CH2
CH2 CH CH CH
-
+
..
CH2 CH CH CH
1,3 diphenylallyl anion is
very unreactive, highly
resonance stabilized
Hanyang Univ.
Spring 2008
Anionic Polymerization
Termination of polar monomer
In this case, although the initiator or active center attacks the monomer, that results the
non-polymerization.
CH3
CH3
CH2 C:- Li+
+
CH2 C
COOCH3
COOCH3
CH3 O CH3
CH2 C
C C CH2
+
+ Li CH3O
COOCH3
Hanyang Univ.
Spring 2008
Anionic Polymerization
Backbiting or intramolecular reaction
Cyclic trimer at the end of
chain
4) Hugginson-Wooding System
J.Chem. Soc. 1952
Polymerization of styrene conducted in liq. NH3 at bp -33C
(1) reaction rate ↑ as [I] and [M]2
I=K+NH2- rate ↑ as [NH2-] ↑ but as [K+] ↓
(2) MW

[K+] and [NH2-]
(3) Polymer is formed without unsaturation.
Hanyang Univ.
Spring 2008
Anionic Polymerization
Initiation step
k
KNH2
K
NH2
Dissociation of initiator
[K  ][NH2  ]
k
[KNH2 ]
H

+
R i  k i [N H 2 ][M] 
CH
CH2
k iK[M][KNH2 ]

[K ]
k1
H2N
C:
CH2
 If [K+] , then Ri 

NH2
Hanyang Univ.
Spring 2008
Anionic Polymerization
Propagation
R p  k p [M  ][M]
Termination
Occurs by chain transfer
H
H2N
CH2 CH CH2 C:-
H
ktr,s
+ NH3
H2N
+ : NH2
CH2 CH CH2 C
H
n
n
Rtr=ktr,s[M-][NH3+]
Overall Rate using Steady state assumption. (RiRt).
Rp 
K  ki  k p [ M ]2 [ KNH 2 ]

ktr , s [ K ][ NH 3 ]
1
2
Ri  ki K [ M ][ KNH 2 ]
Rp 
1
2
2

ktr , s [ NH 3 ]
1
2
ki K k p [ M ] [ KNH 2 ]
ktr , s [ NH 3 ]
ki  k p [ M ]2 [ H 2 N  ]
1
2
If KCl is added 
Rp decreases
[K+]=[NH2-]
Hanyang Univ.
Spring 2008
Anionic Polymerization
In dehydrate state,
k p [M ]
[M ]
Xn 

ktr,s [ NH 3 ] Cs [ NH 3 ]
Chain transfer constant for solvent
Activation energy for Xn
E xn  E p  E tr  4kcal / mole
 temp
overall rate
 DP n
 Rate
E R  E i  E p  E tr  9kcal / mole
Hanyang Univ.
Spring 2008
Anionic Polymerization
In Flory
If there is no termination rxn, the narrow MW distribution can be obtained.
Mw
1
 1
Mn
Xn
 if X n  
Mw
1
Mn
5) Base Initiated Polymerization
- a strong nucleophile is required as the initiator
NO2 >

C
CH
O >
CH2
SO2 >
>>>
CO2

CN
>
SO
>
CH3
Hanyang Univ.
C2H5
Spring 2008
Anionic Polymerization
6) Practical Comments
purity import!
If we use metal as an initiator, the propagation rate is fast.
7) Propagation Kinetics
Comparing to the radical polymerization, the propagation doesn’t occur too fast
Rp  k p [ M  ][ M ]
For most of the living polymers
conc. of anion = conc. of initiator
[M:-] = [I]
[M] = is about 10-9 to 10-7 molar
[M:-] =
10-3 to 10-2 molar
kp for free radical case is 5103 l/molesec
Kp : depends on solvent and counter ion
Counter ion and active center can be separated by changing the solvent
 then reaction rate increases
Hanyang Univ.
Spring 2008
Anionic Polymerization
(1) Evaluation of Individual Propagation Rate Constants
R p  k p  [P  ][M]  k p  [P  (C  )][ M]
Propagation rate constant for free ion and ion pair.
[P-]: conc. of free ion
[P-(C+)]: conc. of ion pair

kp
app

k p [ P  ]  k  [ P  (C  )]

[M ]
at Eq. P  (C  )
K
 Rp  k p
app
[ M  ][ M ]
P  C 
1 [ P  (C  )]

K [ P  ][C  ]
if more ions have been added,
[ P  ]  [C  ]



[ P ]  ( K [ P (C )])
1
2
Hanyang Univ.
Spring 2008
Anionic Polymerization
* How to measure kp, kp, K ?
C0
C

[M ]
log
slope  k papp.
t
k app

p

p
slope  ( k  k ) K
1
2
intercept  k p
1
[M  ] 2
Hanyang Univ.
Spring 2008
Anionic Polymerization
A salt that must be soluble in THF with common ion to gegen ion is added to reaction mixture.

[P ] 
K[M  ]

[C ]
The salt was added at high conc.
Conc. of the added salt is [CZ]
[C+][CZ]
K[ M  ]
 [P ] 
[ CZ ]

[P

(C

)]  [M
Hence
k papp  k p 
 originally
k papp

the conc. of living and the conc. of free ion

K[M  ]
]
[ CZ ]
(k p  k p )K
[ CZ ]
k p [P  ]  k  [P  ( C  )]

[M ]
k papp
slope  (k p  k p )K
int  k p
[CZ ]
Then able to get kp-, kp, K from the two graphs.
Hanyang Univ.
Spring 2008
Anionic Polymerization
Effect of gegen ion on Anionic Polymerization of Styrene
THF
Dioxane
kp
K107
kp-
kp
Li+
160
2.2
6.5104
0.94
Na+
80
1.5
3.4
K+
60~80
0.8
19.8
Rb+
50~80
0.1
21.5
CS+
22
0.02
24.5
- Why kp- is the same value?
; kp- is much more larger than kp
Thus we can say that reactivity of free ion is much greater than that of ion pairs.
- In the case of dioxane?
;In dioxane which is tend not to be solvated, it has reverse tendency comparing to the case of THF.
Solvation is not important in dioxane.
Cs is too high and there is no difference.
Hanyang Univ.
Spring 2008
Anionic Polymerization
Li+ genenion in aromatic hydrocarbon
R p  k p [M : Li ][M]
Look at difference.
R i  k i [RLi][M]
Unassociated species
Let’s say we are using the BuLi initiator.
solvation as well as  is important!
Although, the 1,2 diethoxyethane reduce the , kp varies 1~1000 fold because of highly
solvating ether .
Reactivity of free ion < Reacitivity of ion pair
In aromatic hydrocarbon, unassociated species dominate rate.
Depends on the unassociated species in very low conc.
Covalent character
Hanyang Univ.
Spring 2008
Anionic Polymerization
Evidence — the viscosity measurement before and after term, we find that living
polymer is associated after termination, viscosity drops.
[RLi] 
[( C 4H9Li) 6
1
]6
1
]6
Ri  [

1
6
K1

[M : Li ] 
1
K 22


[(M : Li ) 2
1
]2
1
order in the R 
2
1
order in initiation rate
6
Because initiators and ion pairs are reduced,
Polymerization reaction in Aliphatic HC is lower than inaromatic HC.
Hanyang Univ.
Spring 2008
Anionic Polymerization
Lenz P.437 Table 13-9
Effect of solvent and gengenion on Copolymerization of Styrene and isoprene at 25C
% Styrene in copolymer
Solvent
Na+ counter ion
Li+ counter ion
Nonsolvent
66
15
Benzene
66
15
Triethyl ether
77
59
Ethyl ether
75
68
THF(highly
saturating solvent)
80
80
Generally sodium is
more ionic than
lithium
Hanyang Univ.
Spring 2008
Cationic Polymerization
The growing chain bears a positive charge.
The active sites are either carbenium ions or oxonium ions.
Electron donating groups are needed as the R groups
because these can stabilize the propagating species by resonance.
Ex)
Hanyang Univ.
Spring 2008
Cationic Initiators
Proton acids with unreactive counterions
Lewis acid + other reactive compound:
* To use Lewis acid effectively as initiators, use the co-initiator.
.F.
F : .B.
F
+
C2H5Cl
C2H5 + [BF3Cl]
cationogen
Hanyang Univ.
Spring 2008
Cationic Polymerization
Typical Initiator Systems
Co-initiator
Initiator
SnCl4
H2O
AlCl3
HCl
H2SO4
H2SO4
Order of reactivity
AlCl3 > AlRCl2 > AlR2Cl >AlR3 
HCl > CH3COOH > C6H5NO2 >
Ex)
BF3
+
H2O
More acidic initiators are the most
effective in initiating polymerization
OH
ke
> H2O >> CH3OH > CH3COCH3
+
BF3OH H
C
C
+
BF3OH H
+
 
C C
H3C
C
C
isobutylene
H3C C C C
+
C BF3O H
+
C B F3OH
+
C


C C
kp
C
Hanyang Univ.
Spring 2008
Cationic Polymerization
Termination
C C
C B F3OH
C
C
C
H
+
HB-F3OH
C
Problem : temination reactions occur randomly.
Kinetics


R i  k i M  H B F3 O H   k i K e M H 2 OBF3 


R p  k p M 
 BF3OH
Rt  kt 
R i  R t S S

 BF3OH 
 BF3OH  k i Ke MH2OBF3 
kiKe
MH 2OBF3 
kt
[ * ] can control rxn
Rp 
k pkiKe
kt
H2OBF3 M 2
*
*
Hanyang Univ.
Spring 2008
Cationic Polymerization
 Xn 
Rp
R t  R tr
15
C C
  k tr 
17
C
C
16
M
33
C 6 B F3OH
3
Xn 
kt 
M
C
7
C
1

kp
8
k tr
C
C
22
C
21
27
C
+
30
C C 32B F3OH
C
34
k p M
k t  k tr M
kt  0
ktr  0
X n  const. ,
X n  M 
Hanyang Univ.
Spring 2008
Chain Transfer Reactions
-Cationic vinyl polymerization is plagued by numerous side reactions,
which lead to chain transfer mostly.
Ex)
• Difficult to achieve high MW
(*initiator can give rise to many separate
chains because of chain transfer)
• These side reactions can be minimized
But ! not eliminated by running the reaction
at low temperature
Hanyang Univ.
Spring 2008
Cationic Polymerization
1) Ring opening polymerization
(1) Mechanism
O R CH2
+
O
CH2
R
carbon type polymzn.
+
. . CH2
.O.
R
+
ORCH2ORCH2 O
CH2
R
Example of ROR
: cyclic amides, sulfides, acetals, esters, lactam, alkanes, …
(2) Polymerizability
- unstable ring or the ring which cannot be cyclized easily are very reactive
* 3,4 and 7-11 membered ring is the most reactive ring
5,6 membered rings are stable and polymerize slowly,
but, it still possible to be polymerized.
**3-membered ring is the most easiest to be polymerized
Hanyang Univ.
Spring 2008
Cationic Polymerization
(3) polymerization of THF(Polytetrahydrofuran)
PF4 + (PF6) 2 PF5
PF4 + (PF6) -
+
+
PF4 O
O
PF6
-
gegenion
if H2O exist in the co-catalyst, the polymerization rate increases.
If the living polymerization is possible to occur, thus the termination or transfer also could be occurred.
O (CH2)4
+
O
A-
(CH2)4
+O
(CH2)4
+ (CH2)4
O
A (CH2)4
O(CH2)4O(CH2)4
O
O(CH2)4O(CH2)4
O(CH2)4
+
+
O- (CH2)4
A
Hanyang Univ.
Spring 2008
Cationic Polymerization
(4)Kinetics
Initiation
+
I
initiator
+
ZY
coinitiator
Y (IZ)
-
+
M
K
+
Y (IZ)
ki
-
+
YM (IZ)
Ri  k i [Y  ( IZ )  ][ M ]  Kk i [ I ][ ZY ][ M ]
ex) styrene, stannic-chloride-H2O System [SnCl4OH-]H+
Propagation – can has a low activation energy and can be polymerized rapidly
Mn
+
O
+ O
Mn O
(CH2)4O

or
Simple propagation reaction
H
+
CH2 C [SnCl4OH]
-
+
R
strong initiator
H2C CH
R
CH2CHCH2
R
R p  k p [M  ][ M ]
The total rate of polymerization may actually increases by decreasing the temperature,
which means that the termination has a high activation energy.
Hanyang Univ.