1
IONIC POLYMERIZATION
I. Basics
A. Ionic polymerization is a type of chain polymerization. (Polymer chain
grows at one end rather than two ends.)
B. Cationic polymerization
1. Lewis acid induces formation of carbocation that propagates when
reacting with olefin.
2. Vinyl monomer must be substituted to stabilize carbocation.
H2C
C
+
CH 3
CH 3
CH 3
CH 3
CH 3
F3B
H2
C
F3B
BF3
H2
C
C
CH 3
H2
C
CH 3
+
C
H2C
C
CH 3
CH 3
C
CH 3
C. Anionic polymerization
1. Strong base (amide ion) or alkyl lithium induces carbanion formation.
2. Electron withdrawing groups must stabilized carbanion.
H2C
CH
H2C
C
H2C
CH
NH 2+
H3C
H
C
H2
C
CH
D. Ionic polymerizations can also known as living polymerizations.
1. Counterion “caps” chain. (gegenion)
2. Adding more monomer causes chains to grow.
2
2. Kinetics of Cationic Polymerizations
A. Initiation
1.
First-order in monomer and initiator.
I + M  M+ i  ki  I M
2.
Initiators
a. Lewis acids: BF3, AlCl3, AlBr3, w/H2O
b.
Triphenylmethylhalides
C
+
C
Cl
c. Tropylium halides
+
Cl
B. Propagation
1.
First-order in monomer and carbocation.
M + Mn+  Mn+1+
2.
Stabilization of carbocation
a. Hyperconjugation
H3C
H3C
C
H3C
 p  k p  M   M  
CH 2
>
C
H
CH 2
>
H2C
CH2
Cl
Cl
3
b.
HC
Resonance stabilization in styrenes
CH 3
HC
HC
CH 2
HC
CH 2
CH 2
OCH3 > CH3 > H > Cl for para substitution on styrene.
c. Stabilization with oxonium ion
H
O
+
H2C
d.
CH 2
H
O
O
H2C
CH 2
H3C
H2
C
O
CH
CH 2
Solvent stabilization
iii. High dielectric constant solvents help keep carbocation and
gegenion separate
i. Often monomer is its own solvent.
ii. CH2Cl2, CH3Cl, C6H5NO2
iii. High dielectric constant solvents help keep carbocation and
gegenion separate.
iv. Aprotic polar solvents are best.
C. Chain Transfer
1. Undesirable; charge transfer from chain to monomer prematurely ends
propagation.
M + Mn+  M+ + Mn
 Tr  k Tr  M   M  
2. Proton traps reduce chain transfer, increase molecular weight, decreases
polydispersity.
Example: 2,5-di(t-butyl)pyridine
N
D. Termination
1. First-order in carbocation.
Mn+ + G-  Mn
 t  k t  M  
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E. Degree of Polymerization
1. Assume that i   t .
k i  I M   k t  M     M   
k i  I M 
kt
2. Assume that the rate of polymerization is the rate of propagation.
 p  k p  M   M   k p  M 

k i  I M 
kt
k p k i  I M 
2

kt
3. Without chain transfer, DP is ratio of propagation rate to termination rate.
p
k p  M   M  
kp
M
kt
k t  M 
4. If chain transfer is dominant, DP is ratio of propagation to chain transfer
rate. Note DP becomes independent of monomer concentration.
DP 
t
DP 

p
 tr



k p  M   M  
k tr  M   M 


kp
k tr
5. If the initiation rate is much greater than the propagation rate, i >> p,
 M   M0 ek It
 M 0   M 
DP 
 I0
i
5
3. Kinetics of Anionic Polymerizations
A. Initiation
1. Rate law is same as cationic polymerization, first-order in monomer and
initiator.
2. Initiators
a. Bases: OH-, NH2-, H2O
b.
Alkyllithium compounds: t-butyl lithium
c. Alkali metals
Na + M  Na+ + M-
B. Propagation
1. Rate law same as cationic polymerization, first-order in monomer and
carbanion.
2. Stabilization of carbanion
a. Electron-withdrawing groups, R = CN, CO2R, C6H5
H3C
CH
R
b.
Solvating power of solvent
glyme (ethylene glycol dimethyl ether) > tetrahydrofuran > benzene > cyclohexane
C. Chain Transfer
1. Carbanion can induce proton transfer from oligomer and as well as
monomer.
 Tr  k Tr  M   M  
2. Chain transfer reduces molecular weight; therefore it is generally
undesirable.
3. Chain transfer is slowed at low temperature.
D. Termination
1. Often has no specific termination step, polymer is living polymer.
2. Chain transfer is slowed at low temperature.
3. Termination is proton transfer from solvent e.g., liquid ammonia.
 t  k t  NH3   M  
6
E. Degree of Polymerization
1. Assume that i   t
k i  NH 2   M   k t  NH 3   M     M   
k i  NH 2   M 
k t  NH3 
2. DP is ratio of propagation rate to termination rate.
 p  k p  M M   k p  M 

DP 
p
t

k i  NH 2   M
k t  NH3 
k p  M   M  
k t  M   NH3 


kp
k p k i  NH 2   M 
2

M
k t  NH3 
k t  NH3 