AbstractThe mechanisms of mass transfer between miniemulsion droplets and polymer particles in the miniemulsion copolymerization of styrene‐methyl methacrylate were studied. A mathematical model was developed for the process. This model includes the mass transfer of highly water‐insoluble compounds from miniemulsion droplets to polymer particles by both molecular diffusion and collision between droplets and particles. According to previous results, the concentration of the monomers in the different phases were assumed to be under thermodynamic equilibrium. Both homogeneous nucleation and heterogeneous nucleation were accounted for. The mathematical model also includes the calculation of the average number of radicals per particle when a mixture of oil‐soluble initiator and water‐soluble initiator was used. This model was used to analyze a previously reported experimental result of seeded copolymerizations carried out in the presence of miniemulsion droplets containing a water‐insoluble inhibitor at different stirring speeds. The simulation results indicated that at 30 rpm, about 66% of the mass transfer of the highly water‐insoluble inhibitor occurred by diffusion and the rest by collision. On the other hand, at 120 rpm only about 20% of the inhibitor's mass transfer was by molecular diffusion, whereas 80% occurred b