We have studied the polymerization of water-soluble monomers by ultrasound. The monomers studied were acrylic acid and N-isopropyl acrylamide. Water solutions of these monomers were prepared at various concentrations, and irradiated by ultrasound of 1 MHz or 500 kHz. In the case of acrylic acid, poly(acrylic acid) was obtained at the lowest concentration of the monomer (5 vol%), while gellation was observed at the higher concentrations (10〜50 vol%). At still higher concentrations, no cavitation was observed, and no polymerization took place. In the case of 5 vol% solution, Mw increased up to the sonication time of 120 min, and then gradually decreased because polymer chains were cut by ultrasound. After 180 min of sonication, more than 40 % of the monomers were polymerized. In the case of N-isopropyl acrylamide, poly(N-isopropyl acrylamide) was obtained at all the concentrations studied (2.5, 5 and 10wt%). In the case of 2.5 wt% solution, Mw was larger than that of the higher concentration solutions, but started to decrease at very short sonication time (20 min or less). After the sonication of 180 min, all the solutions gave relatively similar Mw.
