Bimodal molecular mass distribution in surfactant-free emulsion polymerization as a consequence of coagulative nucleation

It is demonstrated that the often observed broadness of the molecular weight distribution obtained from latex particles synthesized by means of surfactant-free emulsion polymerization results from the multistage kinetics. The initial stage of the polymerization, by which the primary particles are formed, is of the 01-kind which means that it can be assumed that the particles are so small that at any moment of time there is no more than one radical chain per particle. After the aggregation of primary particles into secondary particles, the so called coagulative nucleation step, the polymerization kinetics in the subsequently coalesced secondary particles is of the pseudo-bulk kind which means that there are so many radicals in the particles that the polymerization process proceeds as in bulk. The important consequence is that the molecular weight of the 01-process is about one order of magnitude larger than that of the pseudo-bulk process. Hence, each of these polymerization modes and the aggregation stage in-between leave their traces in the molecular weight distribution as is shown by experiments aimed at prolonging the initial formation kinetics. Bimodal molecular weight distributions are found both for ionic and non-ionic initiators.