Effects of ionic strength on the emulsifier-free emulsion polymerization of styrene in water

The influence of ionic strength on the emulsifier-free emulsion polymerization of styrene in water has been studied using potassium sulfate as electrolyte. By increasing ionic strength from 0.01 to 0.61mol/L the EFEP process is depressed in that the rate of polymerization and the final achievable conversion are decreased, the polymerization-active period is shortened. Particle size distribution becomes more polydisperse, large particles can be produced at intermediate ionic strength, and the formation of low M.W. polystyrene molecules is favored while the large ones depressed. No latex is obtained at extremely high ionic strength. It is proposed that higher ionic strength requires higher surface potential which necessates larger particle size and causes enhanced coagulation, which is the major particle growth mechanism. The already reduced entry of ionic radicals into particles is completely inhibited once the larger particles reach the final size, causing sudden termination of the process.