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.

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