Autoacceleration

Abstract: The rate of bulk polymerization of methyl methacrylate and styrene was determined directly, continuously and over the whole range of conversion with a differential scanning calorimeter (DSC) operated isothermally. At the later stages of the accelerated polymerization of methyl methacrylate, a previously unknown inflection or peak in the rate of polymerization was observed. The variation of the rate after the onset of the gel effect, including this peculiar inflection, was interpreted on the basis of the diffusion behavior of monomer molecules and polymeric radicals in the polymer–monomer system, their diffusion rates being predicted from the free volume theory. The final conversion at which no further polymerization proceeds was determined for both monomers. It was affirmed quantitatively that the final conversion has a close relation with the transition of the polymer–monomer system from a viscous liquid to a glassy state.

Abstract: Reversible addition-fragmentation chain transfer (RAFT) polymerization has been shown to be a facile means of synthesizing comb, star, and graft polymers of styrene. The precursors required for these reactions were synthesized readily from RAFT-prepared poly(vinylbenzyl chloride) and poly(styrene-co-vinylbenzyl chloride), which gave intrinsically well-defined star and comb precursors. Substitution of the chlorine atom in the vinylbenzyl chloride moiety with a dithiobenzoate group proceeded readily, with a minor detriment to the molecular weight distribution. The kinetics of the reaction were consistent with a living polymerization mechanism, except that for highly crowded systems, there were deviations from linearity early in the reaction due to steric hindrance and late in the reaction due to chain entanglement and autoacceleration. A crosslinked polymer-supported RAFT agent was also prepared, and this was used in the preparation of graft polymers with pendant polystyrene chains. (C) 2002 Wiley Periodicals, Inc.

Abstract: I. INTRODUCTION Emulsion polymerization often has significant advantages over homogeneous bulk or solution polymerization. Aside from the obvious physical advantages when a polymer latex is the final product as in paints or other coatings, there are distinct processing advantages. For example, low viscosity and good temperature control can often be achieved in an emulsion polymerization when this is impossible in bulk. In addition, high molecular weight polymer can usually be produced at much higher reaction rates in an emulsion polymerization reactor than in a bulk polymerization. For these and other reasons, emulsion polymerization has become a multibillion dollar/year processing operation in terms of sales in the United States alone.