Polyene sequence

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. 

Head-to-head units formed in a molecule have not only been considered as initiation sites for the dehydrochlorination but also as termination points for the growing polyene sequences [19,66,68]. Head-to-head units can either be formed through termination by combination [Eq. (19)] or by head-to-head addition during propagation [Eq. (20)]. 

In the stabilization of PVC, the principal mode of action of the various stabilizer systems has been explained in terms of the Frye and Horst mechanism, i.e., substitution of labile chlorines by more stable groups. Evidence for other actions, such as HCl neutralization, addition to polyene sequences, and bimetallic complex formation have also been given. Despite the wide acceptance of the Frye and Horst mechanism, researchers have frequently contended that this could not be the dominant mechanism in the stabilization of PVC. 

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 -... 

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. 

Absorptions in the region 270-415 nm would be expected to contain contributions frcm polyene sequences of the general structure (-CH=CH-)-> -, where n = 3-15 since the distribution... 

When a particular range of polyene sequences in PVC is selected by irradiating with a narrow band of frequencies, it is not surprising that the maximum amount of photobleaching is observed at a wavelength which is close to that of the irradiating light. 

Effect of HC1 on Polyene Sequence Lengths Produced by Thermal Degradation... 

In order to investigate how HC1 affects the polyene sequences we (38) have investigated the effect of trifluoroacetic acid (TFA) on the polyenes introduced chemically into PVC using the method of Shindo (59). TFA was chosen since its concentration in solution can be controlled and set at higher levels than is possible with HC1 thus allowing the possibility of producing high concentration of intermediates which can be detected. 

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. 

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