Ultrasonically Initiated Polymerization

Ultrasound consists of high-frequency sound waves and is widely used in medical purposes and chemical reactions, especially polymerization. The emulsion polymerization of some monomers can occur without the conventional free radical initiators under ultrasonic irradiation. In this paper recent experimental results are reviewed in three sections: (1) nature and source of the free radical for the initiation process; (2) effects of different types of cavitation; and (3) dependence of the polymerization rate and the polymer molecular weight on acoustic intensity, inert gas flow rate, type of surfactant and its concentration, and initial monomer concentration. Since the polymerization of monomer and degradation of polymer occur simultaneously, the ultrasonically initiated emulsion polymerization is complicated. The experimental results show that ionic surfactants play a very important role in obtaining a high polymer yield, and high conversions can be reached in a short time with usage of a high purge rate of N2. An increase in ultrasound intensity leads to an increase in polymerization rate in the range of cavitation threshold and cavitation peak values. Lower monomer concentration favors enhancement of polymerization rate.