Photodehydrochlorination of chlorinated poly vinyl chloride

Poly(vinyl chloride) can be chlorinated in the presence of UV radiation  

When chlorinated poly(vinyl chloride) (3.72) is exposed to UV radiation of wavelength 250 nm (from a mercury lamp), dehydrochlorination proceeds rapidly along the polymer chain leading to the formation of chlorinated polyene sequences (3.73) according to the following reaction [558, 563]  

The dehydrochlorination chain reaction is 10 times more eflBcient in chlorinated poly(vinyl chloride) than in non-chlorinated polymer, with a quantum yield of 0.12mol/photon in the absence of oxygen [564], There may be two possible explanations for this  

In the photolysis of chlorinated poly(vinyl chloride), the radical evolved by C—Cl photocleavage still possesses chlorine atoms in the P and P positions (—CHCl—CH—CHCl—) it will thus be able to initiate a second zip-dehydrochlorination, as well as the former initiated by the chlorine radical, with formation of twice the amount of HCl as in poly(vinyl chloride). This is illustrated by the overall reaction scheme shown in Fig. 3.42. 

In the photolysis of poly(vinyl chloride) the radical formed in the initiation step (—CHj—CH—CHj—) has no chlorine in the j8-position and is therefore more stable than the corresponding j8,j8 -chloro radical formed in chlorinated poly(vinyl chloride). Cage recombination is more likely to 

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