10.569 Synthesis of Polymers
Prof. Paula Hammond
Lecture 25: “Living” Cationic Polymerizations, Examples of Cationic
Polymerization, Isobutyl Rubber Synthesis, Polyvinyl Ethers
Cationic Polymerization
Some differences between cationic and anionic polymerization
• Rates are faster for cationic
(1 or more orders of magnitude faster than anionic or free radical)
C
•
is very reactive, difficult to control and stabilize
→ more transfer occurs
→ more side reactions
→ more difficult to form “living” systems
→ hard to make polymers with low PDI or block copolymers
• Living cationic only possible for a specific subset of monomers
• Most industrial cationic processes are not living
- recent developments are improving this
Kinetic Steps for Cationic Polymerization
Initiation: Use Acids
comes off
H A
• Protonic Acids (Bronsted): HA
strong, but without nucleophilic counterion
HClO4, CF3SO3H, H2SO4, CFCOOH
→ ClO4want to avoid
recombination
through counterion
+ H2C
CH
R
H3C
CH A
R
• Lewis Acids
Often as initiator/coordination complexes
helps stabilize counterions and prevent
recombination
BF3 + H2O ↔ [H+BF3-OH]
AlCl3 + RCl ↔ [R+AlCl4-]
SbF5 + HF ↔ [H+SbF6-]
Equilibrium between
anion-cation pair
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
Carbenium salts with aromatic stabilization
+
C Cl
SbCl5
C
SbCl6
Propagation
H
CH2
A
C
C
H
H2
C C
R
R
H
+
R
initiation species
H2C
CH2
CHR
vinyl monomer
A
Note: rearrangements can occur, especially if a more stable carbocattion can be
formed (e.g. tertiary carbocation)
(most common for 1-alkenes, α olefins)
e.g. 2 methyl butene
CH
H2C
CH
H3C
CH3
H
C C
H2
H3C C
H
CH3
secondary carbocation
C
H2
CH3
H2
C C
CH3
tertiary carbocation
This occurs via intramolecular hydride (H-) shifts
Usually slow: If Rp ≤ rearrangement rate, will get rearranged product
CH3
H2 H2
C C C
CH3
If Rp > rearrangement rate, will get random copolymer
H
CH3
H2 H2
C C C
H2
C
N
C
m
H3C CH3
CH3
As T↑, m↑ (less rearrangement)
Rate of rearrangement does not increase as fast as rate of propagation.
Hydride shift NOT common for conjugated monomers like: styrene, vinyl ethers and
isobutylene and other tertiary carbocations.
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 25
Page 2 of 7
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
Termination and Transfer (Several Possibilities)
A) Termination with counterion: kills propagating cation, kinetic chain (kt)
i) Combination
H
O
H
kt,comb
CH2
C O C CF3
CH2
+
CF3COO
C
R
R
ii) Anion Splitting
H
CH2
+
C
BF3OH
H
kt,s
CH2
C
OH +
BF3
R
R
B) Transfer or termination to impurity or solvent
O
To H2O, ROR, NR3,
C OR , etc.
Initiator
+
HMNM(IZ)
XA
ktr,s
HMNMA +
X(IZ)
Impurity
or Solvent
e.g.
more stable than
H
CH2
C
A
+ ROR
R
CH2
H
R
C
O
R
R
or
H
CH2
C
A
H
C
R
A
not as reactive,
acts as retardant
- will not propagate further
H
+ R-OH
R
CH2
C
R
OR +
H A
weak acid
will not initiate
All these processes
kill chain length.
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 25
Page 3 of 7
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
Transfer (Kinetic Chain Maintained)
A) Proton transfer to monomer
H
H
C
C
H
R
A
+
H2C
CH
H
H
C
C
R
H
+
H3C
C A
R
R
propagates
B) Hydride ion transfer from monomer
H
H
CH2
C
A +
H2C
R
C
CH2 CH2 +
R
R
propagates
C) Proton transfer to counterion
(“spontaneous termination”)
CH2
C
A
ktr,ci
k tr , M
kp
can be sizeable.
CH3
CH2 C
+
CH2
CH3
C A
R
In general, chain transfer to monomer is favorable so CM →
CH3
H2C
propagating
continues
H A
usually goes on
to initiate again
protic acid initiators
are possible
+Lewis acids are
less likely
Usually propagates
(does NOT kill chain)
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 25
Page 4 of 7
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
Kinetic Expressions
Inititation: Assume Lewis Acid Pair
I + ZY
1.
2. Y(IZ)
Y(IZ)
+ M
ki
⎡Y + ( IZ )− ⎤
⎦
⇒ K = ⎣
ZY
⎡⎣I ⎤⎡
⎤
⎦⎣ ⎦
YM(IZ)
often rate limiting
If step 2 is rate determining, then
−
Ri = ki ⎡Y + ( IZ ) ⎤ ⎡⎣M ⎤⎦
⎣
⎦
ZY
= ki K ⎣⎡I ⎦⎣
⎤⎡ ⎦⎣
⎤⎡M ⎦⎤
*Ri could be determined based on step 1. Then the expressions would be different.
Propagation
kp
YMj(IZ) + M
YMj+1(IZ)
Assumption: chain length has
little effect on reactivity
some number
of monomer
−
Rp = kp ⎡YM +j ( IZ ) ⎤ ⎣⎡M ⎦⎤ = kp ⎣⎡M + ⎦⎤ ⎣⎡M ⎤⎦
⎣
⎦
Let [M+] = total concentration of
all-size propagating carbocations
(ion pairs + free ions)
Termination
Must determine primary means of termination (solvent, impurities, counterion
combinations, or all?)
Example case: termination by counterion combination
kt,comb
−
YMIZ ⇒
YM(IZ)
Rt = kt ,comb ⎡YM + ( IZ ) ⎤ = kt ,comb ⎡⎣M + ⎤⎦
⎣
⎦
If we assume steady state [M+]
Rt = kt ⎡⎣ M + ⎤⎦ = ki K ⎣⎡M ⎤⎡
⎤⎡ZY ⎦⎤ = Ri
⎦⎣I ⎦⎣
Steady state assumption is that Rt = Ri
ki K ⎡⎣M ⎤⎡
Ri
⎦⎣I ⎤⎡
⎦⎣ZY ⎤⎦
+
⎣⎡M ⎦⎤ = k =
kt
t
Going back to Rp
Rp =
Ri k p ⎣⎡M ⎦⎤
kt
R
with ⎡⎣ M + ⎤⎦ = i
kt
=
Kki k
2
p
⎡⎣I ⎤⎡
⎦⎣ZY ⎤⎡
⎦⎣M ⎤⎦
kt
second order in [M]
First order in Ri
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 25
Page 5 of 7
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
(unlike free radical)
PN : No transfer (to monomer, solvent, counterion)
PN =
Rp
Rt
=
kp ⎣⎡M ⎦⎤
kt
PN : If transfer occurs
Rtr,M: to monomer → create new propagating chain
→ create new cationic species
Rtr,S: to solvent
Rtr,Ci: to counterion → recreate initiation
PN =
PN =
1
PN
=
Rp
Rt + Rtr ,Ci + Rtr ,M + Rtr ,S
Rtr ,Ci = ktr ,Ci ⎡⎣M + ⎤⎦
with Rtr ,M = ktr ,M ⎡⎣M + ⎤⎦ ⎣⎡M ⎦⎤
Rtr ,S = ktr ,S ⎣⎡M + ⎦⎤ ⎣⎡S ⎦⎤
k p ⎡⎣M ⎤⎦
kt + ktr ,Ci + ktr ,M ⎡⎣M ⎤⎦ + ktr ,S ⎡⎣S ⎤⎦
k
⎡S ⎤
kt
+ tr ,Ci + CM + CS ⎣ ⎦
k p ⎣⎡M ⎦⎤ k p ⎣⎡M ⎤⎦
⎣⎡M ⎤⎦
ktr ,M
ktr ,S
kp
kp
Suppose transfer to solvent or impurity does not result in further propagation.
e.g. NR3 ⇒
HMNM(IZ)
HMNM
+ NR3
This must be included in
steady state [M+] expression
Kk i k p [I ][ZY ][M ]2
Rp =
k t + k tr ,S [S ]
R
(IZ)
N
R
R
Is stable
No further propagation
term from transfer like
*does not effect PN expression (do not include in PN calc)
*Note: all of the above assumes the 2nd initiation step is rate determining.
Validity of Steady State Assumption
Not really valid
- rxn rates very rapid (seconds – minutes)
- often Ri > Rt
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 25
Page 6 of 7
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.
- [M+] slowly increases with time
- [M+] reaches maximum late in polymerization
then decreases with further conversion
Application of equations is merely an approximation of what really happens.
10.569, Synthesis of Polymers, Fall 2006
Prof. Paula Hammond
Lecture 25
Page 7 of 7
Citation: Professor Paula Hammond, 10.569 Synthesis of Polymers Fall 2006 materials, MIT
OpenCourseWare (http://ocw.mit.edu/index.html), Massachusetts Institute of Technology, Date.