Handbook of Solvents - George Wypych - ChemTech - Ventech!

786 Michelle L. Coote and Thomas P. Davis
wider solid angle. 80 The heavier mass of the complex would also serve to increase the
pre-exponential factor.
In addition to the four terminal model reactions, four complex addition reactions and
an equilibrium constant are required to describe the system.
k ij
�
RMi ⋅+ Mj ⎯⎯→RMiMj ⋅ where:, i j = 1or 2
k iij
RMi ⋅+ MiMj ⎯⎯→RMiMiMj ⋅ where:, i j = 1or 2and
i ≠ j
k iji
RMi ⋅+ MiMj ⎯⎯→RMiMjMi ⋅ where:, i j = 1or 2and
i ≠ j
� � K
M1 + M2 ←⎯→M1M2 The composition and propagation rate can be expressed in terms of the following parameters.
31
where:
F
F
1
2
f
=
f
kp
=
�
1
�
2
( AB) rf + ( AC) f
( ) + ( )
� �
2 1 1 1 1 2 2
� �
1 2 2 2 2 1 1
AB r f A C f
( 2
2
1) 1( 2)
+ ( 1
2
2) 2( 2)
+ ( + )
� � ( Arf 2 1 1 / k11)
+ ( Ar 1 2f2 / k22)
� � � �
1 2 2 1 1 2
A B r f AB r f AC A C f f
A 1 =1+r 1s 1cQf 1 � and A2 =1+r 2s 2cQf 2 �
B 1 =1+s 1c(1+r 1c -1 )Qf 2 � and B2 =1+s 2c(1+r 2c -1 )Qf 1 �
C 1 =1+r 1s 1c(1+r 1c -1 )Qf 1 � and C2 =1+r 2s 2c(1+r 2c -1 )Qf 2 �
2Qf i � = {[Q(fj -f i) +1] 2 +4Qf i} 1/2 - [Q(f i -f j) +1] and Q = k[M]
f i is feed composition of Mi
f i � = [Mi � ]/[M]
r i =k ii/k ij; r ic =k iij/k iji; s ic =k iij/k ii where: i,j =1or2andi≠j
The applicability of the MCP model to strongly alternating copolymerization has been
a long standing point of contention. In essence, there are two opposing accounts of the
strongly alternating behavior observed in copolymers of electron-donor-acceptor (EDA)
monomer pairs. In the first account, this behavior has been attributed to the fact that the
transition state is stabilized in cross-propagation reaction and destabilized in the
homopropagation. Deviations from the terminal model are caused merely by penultimate
unit effects. In the second account -the MCP model- the strongly alternating behavior is a
result of propagation of a 1:1 comonomer complex which, as seen above, also leads to deviations
from the terminal model. An intermediate mechanism, which will be discussed
shortly, is the MCD model in which the complex dissociates during the propagation step.
The main approach to discriminating between these models has been to compare their ability
to describe the copolymerization data of various explicit systems, and to study the effect
of added solvents on their behavior. Unfortunately, both approaches have led to inconclusive
results.