DISPERSION POLYMERIZATION OF STYRENE IN POLAR-SOLVENTS .7. A SIMPLE MECHANISTIC MODEL TO PREDICT PARTICLE-SIZE

An integrated model for dispersion polymerization is developed to predict particle size from first principles. The key components of this model are a multibin kinetic model for unstabilized particle coalescence, the grafting mechanism of stabilization, and the radius of gyration of the grafted stabilizer chains. A critical point is defined where similar-sized particles stop coalescing with one another because the graft available equals the minimum graft required to stabilize the particles. Examples indicate that the particle count is determined within the first 1% of conversion, in agreement with experiment, and that the critical size particles are larger than 0.1 μm—significantly bigger than nuclei. In cases where full adsorption of graft is likely, absolute prediction of particle size is remarkably consistent with experiment. More generally, however, partial adsorption of graft is likely to affect the predicted scaling of diameter with stabilizer, initiator and monomer concentrations, and stabilizer molecular weight. Above the critical size, the size distribution narrows because the growing particles continue to capture dead polymer formed in solution. Observed loss of monodispersity by stabilization of secondary particles or coalescence of large particles is readily explained with this model. A critical factor influencing the final size distribution is the locus of polymerization and how it changes during the reaction. © 1990, American Chemical Society. All rights reserved.