Chemistry Reference
In-Depth Information
or polyethylene is a commercial process. The reaction resembles chlorination in
the step of hydrogen abstraction by chlorine radicals. It is catalyzed by pyridine [
148
-
150
] and can be
pictured as follows [
147
]:
Chlorosulfonation
SO
2
Cl
+
HCl
+
SO
2
SO
2
Cl
Cl
2
+
Cl
Cl
+
Cl
Cl
2
+
The amount of SO
2
vs. chlorine in the reaction mixture affects the resultant ratios of chlorosul-
fonation vs. chlorination of the polymer. These ratios and the amounts of conversion vary with the
temperature [
149
].
Commercially produced chlorosulfonated polyethylene contains approximately 26-29% chlorine
and 1.2-1.7% sulfur [
151
]. The material is an elastomer that remains flexible below
50
C. It is
commonly cross-linked (vulcanized) through the sulfonyl chloride groups. Heating it, either as a solid
or in solution, to 150
C results in loss of SO
2
and HCl. If the material is heated for 2 h at 175
C, all
SO
2
Cl groups are removed. In gamma radiation-induced chlorosulfonations and sulfoxidations of
polyethylene powders [
153
] at room temperature, the ratios of SO
2
Cl groups to Cl groups decrease
with increases in radiation.
Polypropylene can be chlorosulfonated to the extent of containing 6% chlorine and 1.4% sulfur
without embrittlement. The reaction can be done in CCl
4
at 55
C. There is apparently less degrada-
tion than in a direct chlorination reaction [
148
,
152
].
9.4.2 Substitution Reactions of Halogen-Bearing Polymers
Procedures for commercial
vary. Low temperature chlorinations
are done on aqueous dispersions of the polymers that are reacted with chlorine gas in the presence of
swelling agents, like chloroform. These are light catalyzed reactions, usually carried out at about
50
C. They result in substitutions of methylene hydrogens [
158
,
159
].
chlorinations of poly(vinyl chloride)
Cl
Cl
hν
HCl
+
Cl
2
Cl
Some breakdown of the polymers accompanies the reactions in suspension [
154
]. Irregularity in
the structures and the release of HCl significantly contribute to this. It appears that the degradations
are initiated by HCl that is released as a result of the chlorination. In the process, double bonds form.
Immediately upon their formation, they become saturated with chlorines.
A study was conducted on solution chlorination of poly(vinyl chloride) in the presence of free
radicals [
155
] generated from azobisisobutyronitrile. The reaction appears to proceed in two stages.
The first one takes place until 60% of all the CH
2
groups have reacted. After that, in the second stage
the original -CHCl groups are attacked. Some unreacted -CH
2
- groups, however, remain [
155
].
