Polyene sequences in degraded

Figure 2. Frequency distributions of polyene sequences in PVC and degraded copolymers...

Scheme 25 Diels-Alder crosslinking of polyene sequences in PVC degradation...

Braun [22] showed from ozonolysis that for fractions of bulk PVC the number of internal double bonds and the rate of thermal degradation, although dependent on each other, were independent of the molecular weight. This clearly demonstrated the role of internal unsaturation on the stability of the polymer. After careful chlorination of the double bonds, an increase in thermal stability was observed and the number of double bonds as determined by oxidation with potassium permanganate were reduced. It was also shown that one polyene sequence was formed from each isolated double bond. 

These remarks represent only the barest outline of at least two aspects of PVC degradation which have been the focus of attention for several years and remain incompletely understood namely the mechanism involved and the related problem of the involvement of HC1. Several excellent reviews give more comprehensive summaries of the earlier work (10, 11, 12). More recent work has made it clear that under appropriate conditions the presence of HC1 can affect the initiation, propagation and termination steps as well as influencing the distribution of polyene sequence lengths. In addition it can undergo photochemical addition reactions with the polyenes, i.e. the reverse of the dehydrochlorination process, as well as forming colored polyene/HCl complexes. These various possibilities will be considered in turn. 

These results emphasise the important role played by HC1 not only as a catalyst for the dehydrochlorination process but in influencing the distribution of polyene sequences which result from the primary part of the degradation process and the photochemical cross-linking reactions of the polyenylic cations. 

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

In degrading pure PVC, relatively short polyene sequences containing up to about 10-12 conjugated double bonds are formed primarily. The share of longer polyene sequences is small, the upper limit being about 25-30 conjugated double bonds. 

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