Chain transfer to monomer during the free-radical polymerization of vinyl chloride is shown to occur, in part, by a mechanism that begins with the abstraction of methylene hydrogen from the polymer by a propagating macroradicalThe resultant radical then donates a chlorine atom to the monomer to form an allylic structure that can contribute to the thermal instability of poly(vinyl chloride). Double-bond contents found by NMR spectroscopy and their correlation with mo-

During the preparation of poly(vinyl chloride) (PVC) under low monomer pressures, intramolecular hydrogen transfers (“backbites”) lead to the formation of small amounts of 1,3-bis(2-chloroethyl) branch (DEB) structures, as well as allylic chloride segments resulting from 1,6 shifts. The presence of these structural defects was established by the 13C NMR spectra (recorded at 125.77 MHz) of dechlorinated PVC specimens made by organotin hydride reduction. At 55−80 °C, the two backbites leading to DEB groups differ substantially in relative rate, in that the backbiting:addition rate ratio is larger for the second backbite by a factor of 15−16, irrespective of temperature. No convincing evidence was obtained for the presence of a chlorinated 2-ethyl-n-hexyl branch (EHB) segment resulting from double backbiting by an alternative route. Backbiting effects on dichlorobutyl branch concentrations and on chain transfer to monomer are discussed and quantified, and the synthesis of models for dechlorinated DEB and EHB structures is described.

Previously published concentrations (pbb's) of the 2,4-dichloro-n-butyl branches in poly-(vinyl chloride) (PVC) have been used to develop a simple theory that predicts pbb values as a function of monomer concentration and temperature during polymerization. The experimental values of pbb are shown to be consistent, quantitatively, with the well-known "back-biting" mechanism for dichlorobutyl branch formation. They also indicate that the activation energy for back-biting is greater than that for normal chain propagation by 4.4 kcal/mol. In monomer-swollen PVC at 45-80 °C, back-biting:propagation rate-constant ratios (ktJkp's) are invariant with monomer conversion up to total conversions of 83-91%, at least, and at 40 °C the value of kt/kp does not change significantly when the polymerization is carried out in 1,2-dichloroethane solution instead of in monomer-swollen polymer. These results are shown to provide strong evidence against diffusion control of the rate of chain propagation.