Chemistry Reference
In-Depth Information
The complexes can initiate the polymerization of conjugated monomers by a
very rapid electron transfer process:
H
CH
2
HC
C
CH
2
HC
CH
2
-
-
(11-11)
+
+
+
Na
+
Na
The styryl radical anion may dimerize to a dianion that is capable of growing
at both ends:
H
C
-
H
C
H
C
+
+
+
2
CH
2
Na
Na
-
CH
2
CH
2
-
Na
(11-12)
This initiation process is similar to alkali metal initiation in this respect. The
monomer in these systems often has a lower electron affinity than the polycyclic
hydrocarbon, but dimerization of the monomeric radical anion (reaction 11-12)
drives the equilibrium of reaction (11-11) to the right.
11.2.3
Anionic Propagation
Anionic polymerizations are generally much faster than free-radical reactions
although the
k
p
values are of the same order of magnitude for addition reactions
of radicals and solvated anionic ion pairs (free macroanions react much faster).
The concentration of radicals in free-radical polymerizations is usually about
10
2
9
10
2
7
M
while that of propagating ion pairs is 10
2
3
10
2
2
M
. As a result,
anionic polymerizations are 10
4
10
7
times as fast as free-radical reactions at the
same temperature.
11.2.3.1
Heterogeneous Initiation/Homogeneous Propagation
In polymerizations by alkali metals or insoluble organometallics, the initiation
step occurs at a phase interface while subsequent propagation reactions may occur
in a homogeneous medium. The overall kinetics of such reactions are often very
complex and specific to the particular systems. Useful generalizations are more
likely to be provided at the present time by systems in which the initiation and
propagation processes are both homogeneous. Such polymerizations are discussed
next.
11.2.3.2
Homogeneous Initiation and Propagation
Propagation reactions can be studied intensively in homogeneous systems because
conditions can be arranged so that initiation is very fast and there are virtually no
termination or chain transfer reactions.
