The emulsion polymerization of styrene above the critical micelle concentration has been experimentally studied from a low final polymer content up to a high polymer content (similar to 50%). A maximum in the molecular weight (M) evolution has been observed in all cases. The presence or absence of such a maximum depends on the relative values of the rate of free-radical entry (rho) and the rate of chain transfer to the monomer (KtrCMp, where K-tr is the chain transfer to monomer rate coefficient and C-Mp is the monomer concentration in particles). If rho << KtrCMp, M is constant and equal to K-p/K-tr (where K-p is the propagation rate coefficient), except at very low particles sizes typical of the early stages of the reaction, in which the chain length is limited by the particle size. On the other hand, if rho >> KtrCMp, M is determined by both C-Mp and rho. It is proposed that rho is determined by the sum of the entry of the oligomeric radicals formed in the aqueous phase and those contained in particles that undergo limited coagulation. This coagulative entry can become very significant; therefore, reactor hydrodynamics can play a major role in the kinetic behavior observed. Disagreement between Clay and Gilbert's model and molecular weight distribution data can be ascribed, to a lesser or greater extent, to the degree of correctness of the quasi-steady state and instantaneous-termination approaches. (c) 2005 Wiley Periodicals, Inc.