Substitution Reactions of Halogen-Bearing Polymers

Procedures for commercial chlorinations of poly(vinyl chloride) 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]. 

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 aU the CH2 groups have reacted. After that, in the second stage the original -CHCl groups are attacked. Some unreacted -CH2- groups, however, remain [155]. 

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 scissimis. The products are soluble in solvents like acetone and methylene chloride and have low softening points, low impact strength, and poor color stability. 

Photochlorination can achieve 90% conversion. Typical commercially chlorinated poly(vinyl 

CHAPTER 8 chloride), however, has a chlorine content of 66-67%. The material contains methylene groups and 

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