Exit in the emulsion polymerization of vinyl acetate

The following model is proposed to explain the independence of polymerization rate on monomer concentration which is often observed in the emulsion polymerization of vinyl acetate: that radical loss is by transfer to a monomeric species which is very slow to propagate and whose radical activity is lost by exit and termination, either in the aqueous phase or when it enters a particle containing a growing radical. Since the transfer step is rate-determining, the rate of this process is proportional to monomer concentration, which cancels the dependence on monomer concentration in the overall polymerization rate expression; this model also predicts that the radical loss rate coefficient should be either k(tr)C(p) or 2k(tr)C(p), depending on which is the actual fate of the desorbed radical (where k(tr) is the rate coefficient for transfer to monomer and C-p is the concentration of monomer in the particles). Experimental radical loss rates, determined directly by gamma-radiolysis relaxation studies, are quantitatively in accord with the hypothesis.