N-Isopropylacrylamide (NIPA) homopolymers having carboxylic acid end-groups were synthesized by using mercaptoacetic acid as chain transfer agent. Polymerization was achieved in ethanol using 2,2'-azoisobutyronitrile as initiator. Viscosity-average molecular weights of these homopolymers were in the range of 13 x 10(3)-40 x 10(3). Change in the initiator concentration did not cause any significant change in the molecular weight, while decrease in the concentration of the chain transfer agent resulted polymers with higher molecular weights. The carboxylic acid-ended poly(NIPA) was then copolymerized with polyethyleneimine (PEI, MW 2 000) using a water soluble carbodiimide. Viscosity-average molecular weights of these copolymers were in the range of 23 x 10(3)-57 x 10(3). With respect to carboxyl-ended poly-(NIPA), the block copolymers exhibited a pH-dependent temperature sensitivity and higher LCST values in acidic pH.