Sterically stabilized emulsion polymerization of styrene

Abstract

Batch emulsion polymerizations of styrene initiated by ammonium peroxodisulate (APS) and 2,2′-azobisizobutyronitrile (AIBN) at a broad monomer concentration range were investigated. The role of the thick emulsifier layer formed by a nonionic emulsifier was investigated in the sterically stabilized emulsion polymerization of styrene. The polymerization rate vs. conversion dependence was characterized by two or three nonstationary rate intervals. The rate of polymerization increases with the monomer concentration and the increase is much more pronounced at lower monomer concentrations. The deviation from the micellar model was ascribed to the high oil-solubility of non-ionic emulsifier and the chain transfer and desorption events. The values of desorption rate constant k′des for the sterically stabilized emulsion polymerization of styrene are slightly than those for the electrostatically stabilized emulsion polymerization of styrene. The number of polymer particles is nearly independent of the monomer concentration at ca. 20-30% conversion. The dependence of the final polymer particle number vs. the monomer concentration is described by a curve with a maximum. The primary radical termination decreases the rate of polymerization, the number of polymer particles and the final conversion at the high concentration of both APS and AIBN. In the former APS system the less hydrophobic oligomeric radicals with depressed entry rate into particles appear. In the latter AIBN system the growing radicals are deactivated by primary radicals derived from AIBN. The similar polymerization behavior for both initiators results from the dominant chain transfer and radical exit events. The molecular weight of polystyrene is inversely proportional to the initiator concentration.