Previously published concentrations ((rho)over bar>(BB)’s) of the 2,4-dichloro-n-butyl branches in poly-(vinyl chloride) (PVC) have been used to develop a simple theory that predicts <(rho)over bar>(BB) values as a function of monomer concentration and temperature during polymerization. The experimental values of <(rho)over bar>(BB) 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 degrees C, back-biting:propagation rate-constant ratios (k(b)/k(p)’s) are invariant with monomer conversion up to total conversions of 83-91%, at least, and at 40 degrees C the value of k(b)/k(p) 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.