Cationic Polymerization
Concepts
General Mechanism
Initiation
E+
In
E
R
E+ +
R
Propagation
E
E
R
+
R
R
E
R
R
n
R
R
Cationic Polymerization
Concepts
General Mechanism
Termination
Nu
E
R
n
R
Nu-
R
R
H
E
R
E
n
R
-H+
R
β-elim
E
R
n
R
R
n
R
R
Cationic Polymerization
Chemistry
Initiators
1. Proton Acids
e.g.
CF3SO3H
H2SO4
H3PO4
(want a counterion that’s not nucleophilic)
R
R
H O
O S O
OH
H
R
-
O
O S O
OH
NOT
H
O
O S O
OH
1
Cationic Polymerization
Chemistry
Initiators
2. Lewis Acids
e.g.
AlCl3
BF3
SbCl5
TiCl4
ZnCl2
RAlCl2
With Lewis acid initiators, you most often need a co-initiator
e.g. a “protogen”
BF3
+
H+[BF3OH]-
H2 O
or a “cationogen”
(CH3)3C+
AlCl3 + (CH3)3CCl
+
(AlCl4)-
Cationic Polymerization
Chemistry
Initiators
(BF4-)
S
3. Photoinitiators
(activated by UV)
+
3
(PF6-)
I+
*
(BF4-)
hν
S
+
(BF4-)
S
+
+
Y-H
any solvent
(e.g. S-H)
1
(BF4-)
N2 + (BF4-)
S
H
+
+Y
-
2
S
H+ = initiator
(BF4-)
Chemistry
Cationic Polymerization
Initiators
3. Iodine
H
H
H
R
H
C I
R
I2
I2
I
H
H
H
I
R
H
C I I I
R
H
C I I I
R
R
H H H
2
C C C I
R
R
H H H
2
C C C I I I
R
R
2
Cationic Polymerization
Kinetics: Initiation
1. Single Cation Initiatior
ki
Y+Z- + M
YM+ Z-
+ -
Ri = ki [M] [Y Z ]
2. Cation Initiator Complex
I + ZY =
Y+ (IZ)-
Ri = ki [M] [complex]
BF3 + H2O → H+BF3OHK = [H+BF3OH-]/[BF3] [H2O]
therefore: complex = [H+BF3OH-] = K [BF3] [H2O]
Ri = ki [M] {K [BF3] [H2O]}
Cationic Polymerization
Propagation
Y
R1
H2
C C + (IZ)
R1
cation and counterion
R2
R2
R1
insertion between
+
YMn+(IZ)- + M
kp
Y
R2
R1
H2
C C + (IZ)
R2
YMnM+(IZ)-
Rp = kp [M] [M+]
Cationic Polymerization
Propagation
Regiochemistry
Markowninkov addition – form the most stable carbocation
E+
E+
H
H
+
R
R
e- donating R groups stabilize a cation thus increasing reactivity,
and affect regiochemistry by directing the cation α to the donating group:
R = alkyl
aryl
halide
OR
3
Cationic Polymerization
Propagation
Factors affecting rate:
1. R groups: rate of propagation (and rate of initiation) fastest
with monomer that yields most stable carbocation
OMe
Me
H
Cl
H
Note: most stable carbocation reacts fastest
2. Counterion: larger and less tightly bound counterions give larger kp
ClO4-
I3-
TiClOH-
Cationic Polymerization
Examples
Solvent
ε
catalyst
kp
(dm3 mol-1sec-1)
CH2Cl2
9.72
HClO4
17.0
CH2Cl2
9.72
TiCl4/H2O
6.0
CH2Cl2
9.72
I2
0.003
Cationic Polymerization
Propagation
3. Solvent Effects
Species
B- A+
B A
covalent
intimate
ion pair
-
B
A
solvent separated
ion pair
+
B-
A
+
free ions
4
Cationic Polymerization
Propagation
Solvent
ε
catalyst
kp
(dm3 mol-1sec-1)
CCl4
2.3
HClO4
.0012
40/60 : CCl4/CH2Cl2
5.16
HClO4
0.44
40/60 : CCl4/CH2Cl2
7.0
HClO4
3.20
CH2Cl2
9.72
HClO4
17.0
CH2Cl2
9.72
TiCl4/H2O
6.0
CH2Cl2
9.72
I2
0.003
4. Temperature: Ea only 2-3 kcal/mole; that’s low therefore reaction is fast
and side reactions have higher Ea → use low temps for polym
Cationic Polymerization
Propagation
Rearrangements: aka Isomerization Polymerization
Not so bad: high DP when a regular pattern can be formed via simple
rearrangement routes involving stable cations:
E
+
E+
-H+
H
H
both cations can propagate,
the lower the temp,
the more tertiary formed
Cationic Polymerization
Propagation
Rearrangements: aka Isomerization Polymerization
Not so good: low DP when rearrangements are complex:
-
H shift
H- shift
CH3- shift
H- shift
5
Cationic Polymerization
Termination
Recombination of Ion Pairs – can control with solvent polarity
B- A+
-
B
A
B-
A
+
B A
+
kt
YMn+(IZ)-
YMn-(IZ)
Rt = kt [M+]
Cationic Polymerization
Chain Transfer
Chain Transfer to Monomer: most frequently encountered
A. β-Proton transfer
Y
H
R1
C
C + (IZ)
H
R1
-
R1
ktr, M
+
H
C
Y
R2
R
2
R2
R1
C
R1
H
(IZ)-
+
R2
R2
B. Hydride Transfer
H
Y
R1
H2
C C + (IZ)
Y
R2
R
+
R1
ktr, M
+
2
-
HMnM (IZ) + M
R2
ktr, M
(IZ)-
R1
H2
C CH
+
R2
R2
+
Mn+1 + HM (IZ)
-
Rtr, M = ktr, M [M] [M+]
Chain Transfer
Cationic Polymerization
Chain Transfer to Polymer: EAS or hydride transfer
1. Electrophilic Aromatic Substitution: “backbiting”
(IZ)-
(IZ)-
H
2. Hydride Transfer
(IZ)
H
-
+
-
(IZ)
H
+
6
Cationic Polymerization
Chain Transfer
Spontaneous: aka: Chain Transfer to Counterion; this is actually termination
Y
H
R1
C
C + (IZ)
R1
-
R
H
Y
C
2
H
C
R
+
H+(IZ)-
A
+
+ X (IZ)
OH
+
+ H (IZ)
2
Cationic Polymerization
Chain Transfer
Transfer Agents, impurities, Solvent (S) (e.g. H2O)
Y
Y
H
R1
C
C + (IZ)
H
R2
H
R1
C
C + (IZ)
H
R2
-
-
+ + X A
Y
+
+ H OH
Y
ktr, S
HMnM+(IZ)+ + XA
Kinetics
1
H
R
C
C
H
R2
H
R
C
C
H
R2
1
HMnMA + X+(IZ)-
Cationic Polymerization
Overall (first considering unimolecular termination)
Ri = Kkt [I] [ZY] [M]
Rp = kp [YM+(IZ)-] [M]
Rt = kt [YM+(IZ)-]
Apply steady state: Ri = Rt
[YM+(IZ)-] =
K ki [ZY] [M]
kt
DP
DP =
Rp
Rt
=
kp[M]
kt
Note: Increase in I,
no change to DP
7
Cationic Polymerization
Kinetics
Overall with Chain Transfer
Rts = kts [YM+(IZ)-]
Rp = ktr, M [YM+(IZ)-] [M]
Rtr, S = ktr, S [YM+(IZ)-] [S]
DP
DP =
Rp
Rt + Rts + Rtr, m + Rtr, S
kp [M]
=
kt + kts + ktr, m[M] + ktr, S [S]
Cationic Polymerization
Energetics
Composite activation energy for rate of polymerization:
ER = Ei + Ep - Et
And for DP (= kinetic chain length)
EDP = Ep - Et
Often:
Ep << Ei , Et or Etr
Therefore,
ER = negative
When this is the case, there is an
increase in rate with a decrease in temp.
Because, R = Ae-Ea/RT becomes R = AeEa/RT
Cationic Polymerization
Living Polymerization
Polymerizations in which propagating centers do not undergo termination
or transfer; i.e. propagating center has low reactivity such that termination and
transfer reactions are effectively suppressed.
1. If you add more monomer, polymer keeps growing
2. Propagating center should persist throughout polymerization.
For Cationic, need to balance the stability of the carbocation with
counterion, solvent polarity, and temperature.
H H H
2
C C C I
R
R
H
C I
R
R
optimum conditions = HI, toluene, -40◦C
For
N
8
Cationic Polymerization
Telechelic Polymers via Cationic Polymerization
Telechelic Polymers: have relatively low mass (M n ≤ 20,000) with
functional end groups. These can be used for
further reaction to form block or network polymers.
How do you make these? With cationic polymerization, a good technique is
by the “Inifer” method. Inifer is short for
initiation-chain transfer. BCl + RCl
R+BCl43
R+BCl4-
+
R
H2
C
CH3
CH3
BCl4RCl
new polymer
R+BCl4-
+
R
H2
C
H2
C
CH2
R
CH3
Cl
telechelic
CH3
initiator
R
CH3
H2
C
CH3
OH
CH3
Cationic Polymerization
Types of Polymers Produced via Cationic Polymerization
Polymer
Application
Polyisobutylene
Polybutenes
Adhesives, Sealants, Insulating oils,
Lubricating oil, Moisture barriers
Isobutylene-Isoprene Copolymer
(Butyl Rubber)
Inner tubes, Engine mounts, Chemical
tank liners, Hoses, Gaskets
Isobutylene-Cyclopentadiene
Copolymer
Ozone resistant rubber
Polyterpene Resins
(Natural Product Based)
Inks, Varnishes, Paints, Adhesives
Coumarone-Indene Resins
(Coal Based)
Flooring, Coatings, Adhesives
Poly(vinyl ethers)
Polymer modifiers, Tackifiers,
Adhesives
Cationic Polymerization
Examples
9
Cationic Polymerization
Examples
10