This article concerns the modification of polyurethane using polyamide 6,6 prepolymer to improve the dyeability properties of the polyurethane copolymer with acid dye. First, the carboxyl-terminated polyamide 6,6 prepolymer was synthesized from adipic acid and 1,6-diaminohexane. The isocyanate-terminated polyurethane prepolymer was also synthesized from polytetramethylene glycol and 4,4'-diphenylmethane diisocyanate in N,N-dimethylformamide. The polyurethane prepolymer was then extended with a mixture of 1,4-butanediol and the polyamide 6,6 prepolymer (molar ratios of 1,4-butanediol to prepolymer being 100%, 75%, 50%, and 25%, respectively). Finally, the poly(urethane-amide) copolymers were dyed with acid dyes. the chemical, physical, and the dyeing properties of the poly(urethane-amide) coploymers are discussed. >From the experimental results, it is found that the inherent viscosity of poly (urethane-amide) coploymers is increased with the increasing amount of polyamide content. The structure is proven by infrared spectra, which exhibits the absorption peaks of urethane and amide groups as we expected. From the differential scanning calorimetry measurements, it is found that the poly(urethane-amide) coploymers have two-phase structures and good phase separation. There are four transition temperatures (T(g)s, T(g)h, T(m)s, and T(m)h), but only those copolymers in PTMG 2,000 series possess T(m)s. Moreover, the T(g)s is found to change with the length of soft segment, and the T(g)h is increased with the increasing amount of polyamide content. Also, the dyed copolymers exhibit higher T(g)h than those without dyeing of dye molecule, but the T(g)s is not obviously changed. For mechanical properties, it is indicated that both the modulus and the strength of the coploymers are higher than those of unmodified polyurethane, but they are lowered after being dyed with dye molecule due to further separation of intermolecular distance of the dyed polyurethanes. For dye uptake in dyeing properties, it is found to increase with increasing amount of polyamide content. For dye fastness, the dyed copolymers exhibit higher grade of water fastness than that of unmodified polyurethane. (C) 2003 Wiley Periodicals, Inc.