Poly , photodehydrochlorination

Poly(vinyl halides) - Photodehydrochlorination occurs in PVC with a foam backing as determined by Raman spectroscopy. For PVC stabilised with Zn/Ca stearates phooxidation indicates that dehydrochlorination dominates over oxidation at the inner layers beyond 100 microns " . The effect of polyene formation also appears to be a function of light intensity as well as oxygen diffusion rate. 

In poly(vinyl chloride) containing polyene structures photodehydrochlorination is initiated by intramolecular energy transfer from polyene excited singlet states to the allylic C—Cl bond (reaction 3.326) and/or eventually to the allylic C—H bond (reaction 3.327). Triplet states are less likely to be involved because of a very efficient intersystem crosslinking in polyenes [564]. 

Oxygen in general accelerates dehydrochlorination of poly(vinyl chloride), especially in thermal degradation [551, 552, 890]. Activation energies of photodehydrochlorination in air and nitrogen are a(air) = 19.6 kJ mol and (nitrogen) = 32.1 kJ mol respectively. 

Polyene structures formed during the photodehydrochlorination of poly(vinyl chloride) samples are bleached in the dark as well under UV/VIS irradiation. Bleaching in the dark is a much slower process that in the presence of light. Oxygen plays an important role in bleaching processes in the dark and during light irradiation [771, 1674, 1791, 1881, 1882]. 

Fig. 3.43. Absorption spectra of photodehydrochlorinated chlorinated poly(vinyl chloride) (C-PVC) and after UV irradiation (15 min) and after laser irradiation (488 nm) in air for 0.1 s [554]. (Reproduced with permission from [554] published by John Wiley Sons, 1971.)...

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