Chemistry Reference
In-Depth Information
Cis
and
trans
crotonamides can also polymerize by hydrogen transfer polymerization. Sodium
t
-butoxide in pyridine yields identical polymers from both isomers [
217
]. Also, hydrogen transfer
polymerization of acrylamide with optically active, basic catalysts yields optically active polymers
[
218
]. The reactions can be carried out in toluene, using optically active alcoholates of amyl alcohol. The
initiating ability of themetal ions is in the following order, Na
Al [
218
]. Optically
active polymethacrylamide forms with optically active barium and calcium alcoholates, but not with the
other cations [
218
]. In this reaction, however, the asymmetric synthesis takes place through an intermo-
lecular hydrogen transfer rather than through an intramolecular hydrogen migration [
218
]:
>
Ba
>
Ca
>
Mg
>
O
O
NH
2
HNH
Me
OR
Me
OR
H
2
N
O
O
H
2
N
O
+
N
Me
OR
H
2
N
O
H
4.4.3 Termination in Anionic Polymerization
Termination reactions in anionic polymerization, particularly with non-polar monomers and in non-
polar solvent are not common. If carbanion quenching impurities are absent, many polymerization
reactions may not terminate after a complete disappearance of the monomer. Styryl anion, (one of the
most stable ones) for instance, can persist for a long time, such as weeks, after the monomer is
consumed. Addition of more monomer results in a continuation of the reaction and a further increase
in the molecular weight. The anionic “
living
” polymers retain their activities considerably longer
periods of time than do the cationic “living” ones [
180
].
The termination steps in anionic polymerizations can result from deliberate introductions of
carbanion quenchers, such as water or acids, or from impurities. Terminations, however, can take
place in some instances through chain transferring a proton from another molecule like a solvent or a
monomer or even from a molecule of another polymer. In some solvents, like liquid ammonia,
transfer to solvent is extensive, as in styrene polymerization by amide ions [
219
].
In addition, in some polymerizations termination might occur from the following reactions:
1. Elimination of a hydride-ion to form an unsaturated end.
2. Isomerization to an inactive anion.
3. Some irreversible reaction of the active center with a molecule of a monomer or a solvent.
It was observed, for instance that hydrogen transfer from monomer to the growing chain can be a
way of termination in polymerizations of polar monomers, like acrylonitrile [
220
]:
N
N
N
Me
N
Me
+
+
