A novel thermo-responsive smart copolymer that can selectively respond to specific ions, poly[ (N-isopropylacrylamide) -co-(benzo-15 -crown-5 -acrylamide)], has been synthesized and characterized. The copolymer exhibits a negative shift of the lower critical solution temperature (LCST) for phase transition that is specifically responsive to certain alkali metal ions. The order of significance of the LCST shift that is induced by ions is K+ > Cs+ > Na+ > Li+. The greater the number of crown ether units in the copolymer, or the larger the ion concentration, the higher the sensitivity and selectivity of the copolymer for cation recognition. Because of its novel ion-responsive characteristics, the proposed smart copolymer is a promising new candidate material for sensors, actuators, switches, and so on.