DISPERSION POLYMERIZATION OF STYRENE IN POLAR-SOLVENTS .1. GRAFTING MECHANISM OF STABILIZATION BY HYDROXYPROPYL CELLULOSE

The true steric stabilizer in hydroxypropyl cellulose (HPC)-stabilized dispersion polymerization of styrene in polar solvents is grafted HPC-polystyrene, formed in situ, which ends up on the particle surface. The pyrene-labeled HPC experienced a molecular weight increase similar in magnitude to the molecular weight of polystyrene formed by dispersion polymerization. The pyrene-containing polymer also had solubility properties consistent with graft. Kinetic analysis for grafting via chain transfer in solution polymerization predicts a grafting probability of about 1 in 200 polystyrene chains, so the particles must be large (on the order of 0.1 μm) before being sterically stabilized. Precipitation experiments were designed to separate nucleation and stabilization from concurrent polymerization. Precipitation of dispersion-polymerized particles from dioxane with methanol produced new particles with the same morphology as the original particles. The size of these precipitated particles was inversely proportional to the addition rate of nonsolvent to the 0.4 power, but the size distributions were very wide. In contrast, precipitation of commercially available polystyrene from dioxane in the presence of HPC did not produce stable particles. These experiments also suggest that nuclei coalesce extensively until they are sterically stabilized by graft. © 1990.