Conjugated polyene sequences

Photolysis of PVC in the presence of oxygen causes oxidation of the polymer. However, under most (perhaps all) conditions, in both the presence and absence of oxygen, the photodegradation is complicated by scissions of carbon-chlorine bonds. Such scissions may lead to the formation of conjugated polyene sequences via sequential dehydrochlorination (Equation 1). The polyenes... 

The primary product which arises from the degradation of PVC, whether induced thermally, photochemically or by high energy radiation, is a distribution of conjugated polyene sequences of various lengths produced by a dehydrochlorination process which may be written -... 

The principal steps in the thermal degradation of VDC polymer are formation of a conjugated polyene sequence followed by carbonization. 

Rabek (I)). In the absence of air, photoyellowing of polystyrene has been attributed to the build up of conjugated polyene sequences and to the formation of species related to fulvene. Photooxidative yellowing has been ascribed to the formation of species related to acetophenone, benzalacetophenone, and 1,2-diones derived from the main chain of the polymer, as well as to dialdehydes or quinomethane structures resulting from photooxidation of the pendant phenyl moieties. 

The discoloration is due to a dehydrochlorination process restilting in the formation of long conjugated polyene sequences in the polymer chain [Eq. (7-10)]. Polyenes can give rise to photo-cross-linking reactions. 

Solvent cast, oriented PVC films exposed to monochromatic radiation also underwent dehydrochlorination when exposed to radiation in the wavelength range of X. > 340 nm. The optical density of polymer changes as conjugated polyene sequences are produced by photodehydrochlorination. Action spectrum of the change in optical density per incident photon showed [83] a logarithmic dependence on irradiation wavelength for X > 254 nm. 

A polymer peroxy (PO2 ) radical can react with one of the double bonds in the conjugated polyene sequence [1538] ... 

The chlorine radical reacts with poly(vinyl chloride). In the absence of oxygen the dechlorination reaction leads to the formation of conjugated polyene sequences [1121] ... 

Support for the conjugated alkene photosensitized degradation of poly(vinyl chloride) comes from the observation that photolysis of thermally degraded polymer (which contains conjugated polyene sequences) gives a substantially increased rate of dehydrochlorination [11]. 

Less than 0.0001% dehydrochlorination of PVC can be detected. The relationship between the conjugated polyene sequence length in) and the value [65] of Ava... 

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