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If, on the other hand, termination is a result of reactions of the growing chains with any of the
amino functions of the polymer and is a second-order reaction then:
P
n
þ
=
P
n
þ
ð
R
t
¼
d
½
d
t ¼ k
t
½
m
0
m
Þ
where m
0
is the original monomer concentration
5.13 Ring-Opening Polymerizations of Cyclic Sulfides
Ring-opening polymerizations of cyclic sulfides can be carried out by anionic, cationic, and coordi-
nated mechanisms [
187
-
189
]. These polymerizations are easier to carry out then those of the oxygen
analogs, because the sulfur-carbon bond is more polarizable. On the other hand, due to the larger size
of the sulfur atoms the rings are less strained than in the oxygen compounds. As a result, the sulfur
analog of tetrahydrofuran fails to polymerize. In cationic polymerizations, the propagating species are
sulfonium ions [
189
,
190
] and in anionic ones the sulfide anions. Goethals and Drigvers proposed the
following cationic mechanism for the polymerization of dimethyl thiethane [
189
]:
1. The initiation mechanismwith triethyl fluoroborate consists of alkylation of the monomer molecule
and formations of cyclic sulfonium ions. The reaction occurs instantaneously and quantitatively:
(C
2
H
5
)
3
O BF
4
C
2
H
5
+
S
S
BF
4
+
(C
2
H
5
)
2
O
2. The propagation reaction probably involves nucleophilic attacks at the
a
-carbon atom of the cyclic
sulfonium ions by the sulfur atoms from other monomer molecules:
S
S
BF
4
S
S
BF
4
The existence of the sulfonium ions among the propagating species was confirmed with NMR
studies [
191
].
3. Termination is presumed to occur through formations of unreactive sulfonium ions.
Two mechanisms of formation of sulfonium ions are possible: (1) by approaches to the catalyst's
electron accepting sites, (2) by abstraction of hydrides by methyl cations [
190
]:
S
S
CH
3
CH
4
+
R'
+
R'
S
S
+
S
R'
S
R'
