Monoallyl compounds are not readily homopolymerized by a conventional free-radical mechanism. However, we successfully performed the radical polymerization of allylbigulanide hydrochloride in a concentrated acid solution (hydrochloric acid or phosphoric acid) in the presence of a radical initiator at 50 degreesC. The polymer product was precipitated from the reaction solution through the addition of an excess amount of acetone. The precipitated crude polymer [polyallylbiguanide (PAB)] was then purified by dialysis. PAB was confirmed by elemental analysis, infrared spectroscopy, and H-1 NMR. The molecular weight range of PAB was 10,340-113,200, and PAB exhibited a low polydispersity (weight-average molecular weight/number-average molecular weight = 1.04-1.68) by multi-angle laser light scattering. The polymerization of allylbiguanide was quite sensitive to the protonic concentration of the inorganic acid. (C) 2004 Wiley Periodicals, Inc.