Page 194 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Ionic Chain-Reaction and Complex Coordination Polymerization 157

−
where C = :NH
2
CN CN CN CN
| | | |
−
H N−CH −CH: + H C CH H N−(−CH −CH−) −CH CH : − (5.32)
n
2
2
2
2
2
2
2
Carboanion Acrylonitrile Macrocarbanion
−
R = k [M][M ] (5.33)
p
p
with termination occurring through solvent transfer gives:
CN CN CN CN
| | | | (5.34)
H N–(–CH –CH–) –CH –CH : + NH H N–(–CH –CH–) –CH –CH + :NH –
–
n
2 2 2 2 3 2 2 n 2 2 2
−
R = k [NH ][M ] (5.35)
tr
tr
3
As in the case with cationic polymerizations, the number of growing chains is constant so that a

steady state exists such as the R = R . This is useful because it is difficult to determine the concen-
i
tr
−
tration of [M ] so that it can be eliminated as follows:
−
k [C][M] = k [NH ][M ] (5.36)
i
3
tr
k [C][M]
−
i
and [M ] = ----------------- (5.37)
3
k [NH ]
tr
Substitution into Equation 5.33 gives
k [M] [C][M] k [M] [C]
k
2
– p i
R = [M][M ] = =
k
p p k [NH ] [NH ]
tr 3 3 (5.38)
DP can be described as follows:
–
R k [M][M ] k [M]
DP = p = p = (5.39)
–
R k [NH ][M ] [NH ]
tr tr 3 3
Using the same approach we did with the cationic polymerization, we have for the rate of
propagation
E (overall) = E + E − E (5.40)
tr
p
i
and for the dependence of chain length
E (overall) = E − E (5.41)
p
tr
Thus, the rate of propagation and the molecular weight are both inversely related to the concen-
tration of ammonia. The activation energy for chain transfer is larger than the activation energy
for propagation. The overall activation energy is about 160 kJ/mol. The reaction rate increases and
molecular weight decreases as the temperature is increased as shown in Figure 4.4. The reaction rate

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