The reactions of SO2 with various oxiranes, propylene
sulfide, and oxetane have been studied.
It was found that SO2 activates these monomers toward
ring opening polymerization. The spontaneous
reactions lead to the formation of cationic active centers, on which
the heterocyclic monomer homopropagation and various cyclization processes mainly proceed. Anionic
alternating copolymerization of SO2
with ethylene oxide or propylene oxide was observed after the
spontaneous termination of cationic active
species. Zwitterions with active anionic sites are formed in the
presence of pyridine. They can initiate
the alternating copolymerization of SO2 and oxiranes.
The relative reactivity of the oxiranes in the
addition
to anionic species increases in the following order: epichlorohydrin
< glycidyl acetate < propylene oxide
< butyl glycidyl ether < ethylene oxide < 1,2-cyclohexene oxide.
Styrene oxide, oxetane, and propylene
sulfide undergo spontaneous reactions also in the presence of pyridine.
In systems with the former two
monomers the alternating copolymerization proceeds simultaneously with
the processes initiated by
cationic centers; propylene sulfide, however, does not form copolymers
with SO2. Monomers of low ring
strain like THF or trioxane do not undergo ring opening in the presence
of SO2 and pyridine.
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