Rates of termination in polymerizations initiated by benzoyl peroxide and by ammonium persulfate

Polymerizations of styrene have been carried out in solution using as initiators benzoyl peroxide or ammonium persulfate. It has been shown that the ammonium persulfate initiated polymerizations proceed at a faster rate and produce polymers of higher molecular weight than those initiated by benzoyl peroxide, although all the conditions for polymerization remained identical in both cases. This behavior is explained in terms of a reduced rate of termination. It is suggested tentatively that the reduction in the rate of termination may be due to the mutual electrostatic repulsion of the chains caused by the presence of electrically charged end groups arising from initiation by the sulfate ion radical. Calculations are presented showing the essential conditions that have to be fulfilled to make the explanation tenable.