Radical formation during photodegradation

In general, antioxidants can either scavenge free radicals or prevent free radicals formation during photodegradation process. There are many types of antioxidants, such as hindered phenolic compounds, hindered amines, secondary aromatic amines, and phosphite compounds [30]. They can convert free radicals into stable compounds by chemical means, or prevent free radicals formation by converting their predecessors into stable compounds [31]. 

Tetrahydrofuran has been reported to exhibit an absorption maximum at 280 nm (52,56), but several workers have shown that this band is not produced by the purified solvent (30,41,57). Oxidation products from THF have been invoked in order to account for the appearance of the 280-nm band in PVC films that are solvent-cast from THF in air (57. 581. However, in some reported cases (56,59), this band was undoubtedly produced, at least in part, by a phenolic antioxidant (2.6-di-tert-butyl-p-cresol)(59) in the solvent. Since certain -alkylphenols have now been shown to be powerful photosensitizers for the dehydrochlorination of PVC (60), it is clear that antioxidant photosensitization might well have been responsible for some of the effects attributed previously (56) to THF alone. On the other hand, enhanced rates of photodegradation under air have also been observed for PVC films cast from purified THF (57), a result which has been ascribed to radical formation during the photooxidation of residual solvent (57,61). Rabek et al. (61) have shown that this photooxidation produces a-HOO-THF, a-HO-THF, and y-butyro-lactone, and they have found that the hydroperoxide product is an effective sensitizer for the photodehydrochlorination of PVC at X = 254 nm (61). 

The isopropylidene group cleaves to form biphenyl type photoproducts. This is because the chromophores, the substituents linked to the oxygen atom and the aromatic ring, may determine the extent of formation of allylic radical from the isopropylidene group, especially during photodegradation reactions. 

On the other hand, radicals are undoubtedly involved in the photodegradation of PVC under some experimental conditions. Recent ESR studies have provided evidence for the formation of alkyl and allyl-type radicals during the low-temperature UV irradiation of the polymer (43,72,87). Peroxy radicals were also observed when molecular oxygen was present (43,87). Other ESR work has shown convincingly that the radical -CHCI-CH2-CH-CH2-CHCI- results from the irradiation of PVC at liquid-nitrogen temperature (61,93) and is converted into a -CH2-CC1-CH2- radical at -110°C (93). 

Hydroperoxides represent, as it is generally accepted, ones of the most reactive species in the polymer photodegradation [41]. They can be induced during the processing of polymer, as well as during the subsequent exposure of polymers to light and heat in the presence of air. The present transitional metals accelerate the decomposition by radicalic process of the accumulation of hydroperoxides. In the case of polyolefin, the hydroperoxides photolysis leads to the formation of some carbonyl compounds and/or alcoxy radicals. 

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