DICHLOROBUTYL BRANCH FORMATION AND THE QUESTION OF DIFFUSION-CONTROLLED PROPAGATION IN THE POLYMERIZATION OF VINYL-CHLORIDE

Previously published concentrations ((rho)over bar>(BB)'s) of the 2,4- dichloro-n-butyl branches in poly-(vinyl chloride) (PVC) have been use d to develop a simple theory that predicts <(rho)over bar>(BB) values as a function of monomer concentration and temperature during polymeri zation. The experimental values of <(rho)over bar>(BB) are shown to be consistent, quantitatively, with the well-known ''back-biting'' mecha nism for dichlorobutyl branch formation. They also indicate that the a ctivation 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 inva riant with monomer conversion up to total conversions of 83-91%, at le ast, and at 40 degrees C the value of k(b)/k(p) does not change signif icantly when the polymerization is carried out in 1,2-dichloroethane s olution instead of in monomer-swollen polymer. These results are shown to provide strong evidence against diffusion control of the rate of c hain propagation.