Chemistry Reference
In-Depth Information
Photo chlorination can achieve 90% conversion [
156
,
157
]. Typical commercially chlorinated poly
(vinyl chloride), however, has a chlorine content of 66-67%. The material contains methylene groups
and is in effect a copolymer of vinyl chloride and 1,2-dichloroethylene.
Chlorination raises the softening temperature of poly(vinyl chloride). The products exhibit poorer
heat stability than the parent material and are higher in melt viscosity. High-temperature chlorinations
in chlorinated solvents at 100
C result in extensive substitutions of the methylenic hydrogens as well.
The reactions, however, are accompanied by extensive chain scissions. The products are soluble in
solvents like acetone and methylene chloride and have low softening points, low impact strength, and
poor color stability.
Chlorination of poly(vinylidene chloride)
can be carried out with sulfuryl chloride using azobisiso-
butyronitrile [
155
]. The reaction appears to proceed in three steps:
1. Formation of SO
2
Cl
free radicals by abstraction.
2. Formation of free radicals on the polymeric backbones:
Cl
Cl
Cl
Cl
SO
2
Cl
+
Cl
Cl
Cl
Cl
3. Transfer reactions of the active sites:
Cl
Cl
Cl
Cl
+
SO
2
Cl
2
+
SO
2
Cl
Cl
Cl
Cl
Cl
Cl
Chlorination of poly(vinyl fluoride)
yields a product with 40-50% chlorine content [
158
].
Fluorination
of poly(vinylidene fluoride) was reported [
160
]. When mixtures of fluorine and
nitrogen gases are used, the reactions are limited by the amount of diffusion of fluorine into the
polymer network. X-ray photoelectron spectroscopy shows presence of -CF
2
-, -CHF-, and -CH
2
-
groups in the product [
161
].
Many attempts at other modifications of poly(vinyl chloride) were reported in the literature. Often
the reactions are based on expectations that the polymers will react like typical alkyl halides.
Unfortunately, in place of nucleophilic substitutions, the polymers often undergo rapid and sequential
eliminations of HCl along the chains. Nevertheless, many substitution reactions are still possible and
can be successfully carried out. One example is a replacement of 43% of the chlorine atoms with
azide groups [
162
]:
Cl
Cl
Cl
N
3
NaN
3
The reaction can be carried out at 60
C for 10 h and the polymer does not cross-link. It proceeds
faster in DMF than in other solvents. Once substituted, the polymer becomes photosensitive and
