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.