A thermal responsive, water soluble polymer. The polymer comprises the co-polymerization product of: (a) 5˜95 wt % of N-isopropyl acrylamide (NIP); (b) 0.1˜80 wt % of 1-vinyl-2-pyrrolidinone (VPD); and optionally, (c) 0.1˜30 wt % of acrylic acid (AA). As the proportion of component (b) VPD increases, the Lower Critical Solution Temperature (LCST) and water solubility of the polymer increases. On the other hand, as the proportion of component (c) AA increases, the Lower Critical Solution Temperature (LCST) decreases and the COOH reactive groups increases, which impart high reactivity to the copolymer. By adjusting the proportion of the monomers, a broad range of LCST can be manipulated from about 20 to 80° C.