New insight into the formation of structural defects in poly(vinyl chloride)

The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both H-1 and C-13 NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects

New Insight into the Formation of Structural Defects in Poly(Vinyl Chloride)

The monomer conversion dependence of the formation of the various types of defect structures in radical suspension polymerization of vinyl chloride was examined via both 1H and 13C NMR spectrometry. The rate coefficients for model propagation and intra- and intermolecular hydrogen abstraction reactions were obtained via high-level ab initio molecular orbital calculations. An enormous increase in the formation of both branched and internal unsaturated structures was observed at conversions above 85%, and this is mirrored by a sudden decrease in stability of the resulting PVC polymer. Above this threshold-conversion, the monomer is depleted from the polymer-rich phase, and the propagation rate is thus substantially reduced, thereby allowing the chain-transfer processes to compete more effectively. In contrast to the other defects, the chloroallylic end groups were found to decrease at high conversions. On the basis of the theoretical and experimental data obtained in this study, this decrease was attributed to copolymerization and abstraction reactions that are expected to be favored at high monomer conversions. Finally, a surprising increase in the concentration of the methyl branches was reported. Although a definitive explanation for this behavior is yet to be obtained, the involvement of transfer reactions of an intra- or intermolecular nature seems likely, and (in the latter case) these could lead to the presence of tertiary chlorine in these defects.

Experimental and Theoretical Evaluation of the Reactions Leading to Formation of Internal Double Bonds in Suspension PVC

The number of internal double bonds in poly(vinyl chloride) (PVC) samples was studied as a function of molecular weight at various monomer conversions. These defect structures were found to exhibit end-group-like characteristics: their concentration per chain was largely constant as a function of molecular weight. This tendency was independent of the degree of conversion. An intramolecular mechanism for formation of unsaturated structures and their location between carbons 5-6 were confirmed via 13C NMR studies. High-level ab initio calculations showed that a 1-6 hydrogen transfer reaction was the most likely origin for these structures, though a second mechanism involving backbiting of the 1-2 Cl shifted head-to-head radical followed by β-chlorine elimination and then transfer to monomer could also contribute at lower conversions. From the experimental analysis and theoretical calculations, it emerged that this backbiting reaction is stereoselective, with the isotactic conformation appearing to be more resistant. However, from the ab initio calculations and earlier results of other research groups it also seems likely that hydrogen abstraction from chloroallylic end groups and further propagation of such radical is a concurrent route to internal double bonds. The evidence collected in this paper point to hydrogen abstraction reactions, especially backbiting and abstraction from chloroallylic end groups, as reactions for which inhibition should have a beneficial effect on the thermal stability of PVC.