We have used an azobenzene derivative substituted with a 2-chloropropionyl group as an initiator for the atom transfer radical polymerization of N-isopropylacrylamide. The resulting polymers, which possessed azobenzene moieties at one of their termini, had molecular weights of 1.7-12.0 kDa. Aqueous solutions of these polymers displayed low critical solution temperatures that alternated reversibly in response to E-to-Z photoisomerization cycles of the terminal azobenzene moieties. The difference between the transition temperatures after exposure to UV and visible irradiation increased linearly up to a value of over 10 degrees C upon decreasing the molecular weight of the polymer.