AN EXPERIMENTAL INVESTIGATION ON THE EVOLUTION OF THE MOLECULAR-WEIGHT DISTRIBUTION IN STYRENE EMULSION POLYMERIZATION

Styrene ab initio emulsion polymerizations were conducted at 70 degree s C in an automated reaction calorimeter. Two polymerizations were per formed, one above and the other below the critical micelle concentrati on (CMC) of the surfactant, thus ensuring differing polymerization kin etics between the two: the system below the CMC gave large particles t hat were expected to follow pseudobulk kinetics, while that above the CMC gave small particles that were expected to follow zero-one kinetic s. The evolutions of the molecular weight distributions (MWDs) were ch aracterized by removing samples periodically during the course of the reactions and analyzing with gel permeation chromatography. Interpreta tion of the data used average molecular weights, the GPC MWDs, and the number MWDs, as functions of conversion. It was found that all of the number MWDs (plotted as In (number of polymer chains) vs. molecular w eight of polymer chains) were concave-up at low molecular weights and become nearly linear at molecular weights (greater than or equal to 3- 4 x 10(6)); this linearity is expected from theory. The slope of the h igh molecular weight region was consistent with theory for the dominan t mode for chain stoppage: termination and transfer for the pseudobulk system and (predominantly) chain transfer to monomer for the zero-one system. The most likely explanation for the concavity of the number M WDs is a heterogeneity of radicals: some surface anchored with sulfate end groups and others (with hydrogen end groups arising from transfer to monomer and/or reentry) being more mobile. Thus, two types of term ination are proposed: slow reaction-diffusion for the less mobile surf ace anchored chains, and rapid short-long (center of mass) termination for the more mobile hydrogen-terminated chains. (C) 1997 John Wiley & Sons, Inc.