Chemistry Reference
In-Depth Information
Poly(alkylene sulfide)s are exceptionally oil-resistant elastomers. They also exhibit good resis-
tance to solvents and to weathering. On the other hand, these elastomers lack the strength of synthetic
rubbers and possess an unpleasant odor.
Aliphatic polysulfides also form from reactions of di thiols with alkyl dihalides. High molecular
weight polymers, however, are hard to form [
109
]:
+
n
n
Br
R'
Br
NaS
R
SNa
S
R
S
R'
n
+
2N NaBr
Similar polymers form through addition of di thiols to diolefins [
109
]. The reaction can take place
either by a free-radical mechanism or by an ionic one. A free-radical mechanism requires presence of
peroxides to achieve counter Markownikoff additions across the double bonds:
n
HS
+
n
peroxide
SH
S
S
6
6
n
It is a step-growth polymerization process. The additions to the double bonds involve a series of
hydrogen exchange reactions [
108
]:
R
2
+
R
2
I
2
HS
SH
+
HS
S
2HI
+
R
HS
S
R
2
S
SH
2
R
HS
SH
R
+
R
S
SH
2
etc.
HS
S
When additions take place by ionic mechanisms, they are catalyzed by either acids or bases. These
additions follow the Markownikoff rule:
R
+
n
n
HS
SH
R
S
S
n
7.10 Polyurethanes
The polyurethanes [
126
,
132
] are sometimes also called “isocyanate polymers.” They are
characterized by the urethane linkage. Other functional groups, however, may be present in the
polymer as well. The urethane linkages can be produced in polymers by several different routes.
Among these, the most common ones are through reactions of the isocyanate groups with compounds
