Chemistry Reference
In-Depth Information
There are indications of a “living” chain-growth mechanism in boron trifluoride diethyl etherate
initiated polymerizations of propylene sulfide [
192
] at conversions of 5-20%. In these early stages of
polymerization the molecular weight corresponds to that calculated for typical “living” polymers.
This is believed to take place through formations of stable sulfonium ions:
R
monomer
O
BF
3
+
S
S
BF
3
S
R
S
BF
3
Higher conversions in thiirane polymerizations, however, proceed with chain scission transfer
mechanism under the influence of BF
3
(C
2
H
5
)
2
O[
192
]. This is indicated by a change in the molecular
weight distribution, a bimodal character. When the reaction is complete there is a marked decrease in
the average molecular weight of the polymer. When thietane polymerizes with triethyl-oxonium
tetrafluoroborate initiation in methylene chloride, the reaction terminates after only limited conversion
[
193
]. This results from reactions between the reactive chain ends (cyclic sulfonium salts) and the
sulfur atoms on the polymer backbone. In propylene sulfide polymerization, however, terminations are
mainly due to formations of 12-membered ring sulfonium salts from intramolecular reactions [
193
].
When the polymerizations of cyclic sulfides are carried out with anionic initiators, many side
reactions can occur. On the other hand, common anionic initiators, like KOH yield optically active
polymers from optically active propylene sulfide [
194
]. An example of a side reaction is formation
[
192
] of propylene and sodium sulfide in sodiumnaphthalene initiated polymerizations. Such reactions
are very rapid even at
78
C. A similar reaction was shown to take place with ethyllithium [
195
]:
S
S
+
+
C
2
H
5
Li
C
2
H
5
Li
Other side reactions that occur in butyllithium-initiated polymerizations are cleavages of the
polysulfides [
192
]:
S
C
4
H
9
+
+
Li
S
S
LiC
4
H
9
+
+
C
4
H
9
Li
S
S
High molecular weight polymers can be prepared from ethylene sulfide with a diethylzinc-water
catalyst [
196
]. The polymers form in two steps. Initially insoluble crystalline polymers form at room
temperature with a high catalyst to monomer ratio. These product polymers, that contain all of the
catalyst act as seeds for further polymerizations. Though the final polymers are insoluble, the
molecular weights are estimated to be high. At a conversion of 20% the molecular weights are believed
to be about 900,000 [
196
]. When diethylzinc is prereacted with optically active alcohols, optically
active poly(propylene sulfide)s form [
197
-
199
]. Cadmium salts are also very effective catalysts for
polymerization of thiiranes. The polymers of substituted thiiranes have high stereoregularity.
