Monte Carlo simulation of tri-functional branching and tetra-functional crosslinking in emulsion polymerization of butadiene

A direct Monte Carlo method is used to simulate the effect of tri-functional long chain branching and tetra-functional crosslinking on molecular weight distribution in emulsion polymerization of butadiene. Butadiene polymerization, due to high extent of reaction with internal or pendant double bonds of polymer chains, can be used as a model to study the effect of tetra-functional crosslinking on polymer microstructure. In this simulation, elementary reactions included propagation, chain transfer to monomer, termination by disproportionation, transfer to C-H bond (BN3) and reaction with internal or pendant C=C bond (CL4) of growing and dead polymer chains. The initial polymerization volume of the simulation was 10(5) nm3. The ratio of monomer to initiator concentration and initiator to polymer particles were 500 and 2.5, respectively, and the number of simulated polymer particles were 400. For simulated conversions in the range of 20-75% a bimodal molecular weight distribution was observed. The maximum of the second peak of the bimodal distribution moved to higher molecular weights as the conversion was increased. As the conversion was increased from 20 to 75%, the increase in the number average molecular weight of the polymer was linear but a slight increase in the slope of the weight average molecular weight was observed. More importantly, as the conversion was increased, a relatively sharp change in the slope of the weight fraction of the second peak of the molecular weight distribution curve was observed at approximately 20% conversion. According to the results, in polymerization systems with high extent of tetra-functional crosslinking, the development of the molecular weight distribution in emulsion polymerization is different from bulk systems. (C) 2001 Published by Elsevier Science Ltd.