MOLECULAR-WEIGHT DISTRIBUTIONS IN THE THERMALLY INITIATED EMULSION POLYMERIZATION OF STYRENE

In both thermal emulsion polymerization of styrene in the temperature range 60-80-degrees-C, and peroxodisulfate-initiated polymerization at 60-degrees-C, weight-average to number-average molecular-weight ratios (MBAR(w)/MBAR(n)) approach 1,5 when potassium octadecanoate is used as emulsifier. A low activation energy for thermal initiation (almost-equal-to 66,0 kJ/mol) was deduced which may indicate a catalytic effect of the emulsifier during the thermal initiation process. Participation of the emulsifier is probably attributed to a transfer of one of two monomer radicals, produced thermally, to the emulsifier, with subsequent desorption to the aqueous phase, leaving one radical in the polymerization locus.