The influence of n-dodecyl mercaptan (n-DDM) on the molecular weights distribution of the styrene/methyl methacrylate/butyl acrylate (St/MMA/BuA) terpolymer was studied. First, a kinetics emulsion terpolymerization model based on Nomura's model for copolymerization was developed. The predicted data by this model was compared with those obtained for batch, semicontinuous polymerization, as well as seeded experiments for the St/MMA/BuA system. A composition near to the unitary azeotrope of methyl methacrylate (MMA) was calculated and used to estimate the M-w of a terpolymer uniform in composition synthesized by a seeded process. It was established that a higher proportion of BuA makes the model of weight fraction of polymer W-p in the particle more suitable for simulating the gel effect on the propagation rate coefficient k(p). Likewise, this model of gel effect upon kp is significant for both the semicontinuous processes with lower monomer addition rate and the seeded process. (C) 2015 Elsevier Ltd. All rights reserved.