Chemistry Reference
In-Depth Information
80
C almost quantitative conversions to high molecular weight polymers
take place [
190
]. Similar conditions yield polystyrene [
191
]. 4-vinyl pyridine polymerizes in liquid
ammonia in the same manner [
191
]. Initiation results from reduction of the monomer by direct electron
transfer at the cathode to form a red-orange vinyl pyridyl radical-anion:
By cooling the solutions to
CH
2
CH
+
e
N
N
Another example is polymerization of isoprene in an electric field in tetrahydrofuran solution.
Here too, a “living” polymer forms [
192
].
4.4.2 Propagation in Anionic Chain-Growth Polymerization
The propagation reaction consists of successive additions of monomer molecules to the active centers
of the growing chains:
X
X
X
M
+
M
X
No matter what the mechanism of initiation is, the propagation reaction takes place strictly
between the monomer and the growing polymeric chain with or without a counterion.
When the reaction occurs in non-polar solvents the propagation step is not hampered as much by a
tendency of ion pairs to cluster into aggregates, as is encountered in initiation. For instance, in
butyllithium initiated polymerizations of styrene in benzene, the propagation step is much faster than
the initiation [
193
,
194
]. This is probably due to an absence of aggregates. Some association between
the growing polymeric chains, however, does occur [
193
]. It may be shown as follows:
R
2
Li
Li
R
Li
R
These association equilibriums, however, are mobile in character [
193
]. The driving force in the
propagation reaction is similar to that in the initiation. In non-polar solvents the reaction with the
incoming monomers are similar to those in the initiation step. The monomers coordinate with the cations
at the end of the chains first. This is followed by intramolecular rearrangements that lead to regenerations
of new metal carbon linkages:
H
H
C
C
H
+
Li
Li
Li
