Thermosensitive and water-soluble copolymers were prepared through the copolymerization of acryloyloxypropyl phosphinic acid (APPA) and N-isopropyl acrylamide (NIPAAm). The thermosensitivity of the copolymers and copolymer/metal complexes was studied. The APPA-NIPAAm copolymers with less than 11 mol % APPA moiety had a lower critical solution temperature (LCST) of approximately 45 degreesC, but the APPA-NIPAAm copolymers with greater than 21 mol % APPA moiety had no LCST from 25 to 55 degreesC. The APPA-NIPAAm copolymers had a higher adsorption capacity for Sm3+, Nd3+, and La3+ than for Cu2+, Ni2+ and Co2+. The APPA-NIPAAm (10:90) copolymer/metal (Sm3+, Nd3+, or La3+) complexes became water-insoluble above 45 degreesC at pH 6-7, but the APPA-NIPAAm (10:90) copolymer/metal (Cu2+, Ni2+, or Co2+) complexes were water-soluble from 25 to 55 degreesC at pH 6-7. The temperature at which both the APPA-NIPAAm copolymers and the copolymer/metal complexes became water-insoluble increased as the pH values of the solutions increased. The APPA-NIPAAm copolymers were able to separate metal ions from their mixed solutions when the temperature of the solutions was changed; this was followed by centrifugation of the copolymer/metal complexes after the copolymers were added to the metal solutions. (C) 2004 Wiley Periodicals, Inc.