AbstractThe chain transfer constants of normal and tertiary mercaptans with polystyrene radicals at 50°C. are, respectively, 19 and 4.5, with polybutadiene radicals, respectively, 19 and 3.7. The values are somewhat larger at 5°C., corresponding to an activation energy for transfer about 1 kcal./mole smaller than for propagation. At equilibrium 0.58 g. ofn‐octyl, 0.62g. oftert‐octyl, 0.25 g. ofn‐tetradecyl, and 0.26 g. oftert‐hexadecyl mercaptan can be dissolved per gram of polystyrene in emulsion, but the rate of attainment of equilibrium is extremely slow with the higher molecular mercaptans. The small apparent transfer constants of the higher mercaptans in emulsion polymerization are due to the slow rate of transport to the polymer particles. This transport is greatly accelerated by the addition of alcohol, which makes the distribution toward the water layer more favorable. In general the solubility in water of the mercaptan (or modifier in general) is the most important factor, the diffusion coefficient being of only secondary significance in a comparison of compounds of approximately the same molecul