Poly(N-isopropylacrylamide) is a well-known thermosensitive polymer exhibiting a phase transition at 32 C. Here, ultrasonic polymerization of the monomer, N-isopropylacrylamide, in an aqueous solution is reported as a novel reaction. Ultrasound irradiation of an N-isopropylacrylamide aqueous solution for approximately 30 min produced the polymer. The yield and molecular weight (M w ) of the polymer were measured for various temperatures and monomer concentrations. At 15 C (below the critical temperature), the reaction yield did not increase after 1 h. At 33 C (above the critical temperature), however, polymerization continued even 6 h after sonication, producing twice the yield obtained at 15 C. The M w of the polymer obtained at 33 C was 5 Â 10 4 g/mol, approximately five times that at 15 C. #