Molecular weight distribution (soluble and insoluble fraction) in emulsion polymerization of acrylate monomers by Monte Carlo simulations

A Monte Carlo simulation model for the semibatch emulsion polymerization of acrylate monomers was developed. The model accounts for the complex kinetics of acrylate monomers (presence of chain-end and midchain radicals), the compartmentalization of emulsion polymerization systems and the development of the entire molecular weight distribution (MWD) as well as the branching density. It was found that the MWD produced in the semibatch process was bimodal (a sharp and extremely high Mw peak and a broad and lower Mw mode), the midchain radicals were predominant during the polymerization, and most of the branches were short and produced by the backbiting mechanism. Interestingly, the bimodality of the MWD could be avoided under certain experimental conditions (those who led to a decrease of particles with more than two radicals), but this did not prevent the formation of polymer chains of molecular weights above 10(7) g/mol (which are typically insoluble). The model also shows the importance of the presence of midchain radicals in the microstructure of the polymer formed. Thus, the fraction of termination by combination and disproportionation of the midchain radicals had a significant influence on the location of the high molecular weight peak. The predictions of the Monte Carlo model were in good agreement with experimental data and a previously developed deterministic mean-field theory model for the seeded semibatch emulsion polymerization of n-butyl acrylate.