INFLUENCE OF STRUCTURE ON THE PHOTO-DEGRADATION OF PVC .4. A CONCLUSIVE APPROACH TO THE MECHANISM OF PHOTO-OXIDATION AND PHOTO-DEHYDROCHLORINATION

The photo-oxidation and the photo-dehydrochlorination of a series of PVC samples having different isotactic contents have been studied in the presence of oxygen. The build-up of carbonyl groups was found to decrease as the isotactic content was increased. Conversely, the dehydrochlorination quantum yield appeared to increase linearly as the isotactic content increased. It is proposed that the former result is due to a less favoured hydrogen abstraction reaction at the isotactic triads, resulting from a higher shielding of the hydrogen atoms at CHCl groups. The latter result is associated with both the occurrence of some chlorine atoms of low dissociation energy located at a definite type of isotactic triad and the favoured hydrogen abstraction reaction at the CH2 group in syndio and heterotactic triads. The conclusions are discussed on the basis of the local conformations of the various triads. In particular, the role of the isotactic GTTG- triad conformation in the mechanisms above is emphasized. © 1989.