CHAIN TRANSFER BEHAVIOR OF FRACTIONATED COMMERCIAL MERCAPTANS IN EMULSION POLYMERIZATION OF STYRENE

The chain transfer behavior of fractionated commercial tertiary mercap tans was investigated in batch and semicontinuous emulsion polymerizat ion of styrene over the entire monomer conversion range. Four mercapta ns were obtained by fractionation, which contained 9, 11, 12, and 13 c arbons, respectively. The effect of the mercaptans was evaluated in te rms of the consumption rates of the monomer and the chain transfer age nts, the number average degree of polymerization, DP(n), and the polyd ispersity index, I, of the polymer. The batch experiments showed that the chain transfer efficiency decreases with increasing carbon number, which is due primarily to a lower diffusion rate of the chain transfe r agent to the reaction sites (growing latex particles) through the aq ueous phase. The partitioning ratio of the chain transfer agents betwe en the aqueous phase and the monomer droplets also contributes, to a l esser extent, to the efficiency. The number average degree of polymeri zation and the polydispersity index are primarily controlled by the ra tio of the mercaptan consumption rate over that of the monomer. In ord er to obtain a polymer with a constant DP(n) and a narrow I, this rati o should be as close to unity as possible, as is the case for C-11. Ot herwise, too high a ratio causes a severe increase in DP(n) at the end of polymerization, and too low a ratio leads in the opposite directio n. The semicontinuous experiments confirmed the batch results. (C) 199 4 John Wiley & Sons, Inc.