Mathematical model for molecular weight of continuous emulsion polymer

Abstract

A non-steady state model to predict the average molecular weight is developed for the emulsion polymerization of vinyl acetate from a continuous stirred tank reactor which shows oscillatory behavior in conversion. particle size, average molecular weight and polydispersity index. The kinetics of vinyl acetate polymerization is complex because of chain transfer to polymer. Chain transfer to monomer and terminal double bond reaction. The major mechanisms included in the proposed model are (i) Particles are generated by micellar and homogeneous nucleation. (ii) The desorption and readsorption of monomeric radicals from and into polymer particle. (iii) Gel effect at high conversion. (iv) Particles shrinkage during the monomer starved period. The differences between terminal and non-terminal double bond species and the effects of branch point are taken into consideration. Weight average molecular weight is correctly predicted by the proposed model but number average molecular weight is overestimated the experimental data due to complex behavior of emulsifier. Transfer to polymer and terminal double bond reactions are the major reactions that control the polydispersity index. The effect of emulsifier concentration. Initiator concentration and mean residence time over polymer properties are determined. The model equations are developed via balance equations over all species in the system and solved numerically by fourth order Runge-Kutta method.