Two RAFT agents, suitable for inducing living radical polymerization in water, have been synthesized. Both RAFT agents were shown to be effective over the temperature range 25-70 degrees C. One RAFT agent was functionalized. with a pyridyl disulfide group. RAFT efficacy was demonstrated for the polymerizations of N-isopropyl acrylamide (NIPAAM) and poly(ethylene oxide)-acrylate (PEG-A) in both water and acetonitrile. The kinetic data indicates that the pyridyl disulfide functionality is largely benign in free radical polymerizations, remaining intact for subsequent reaction with thiol groups. This result was confirmed by studying conventional radical polymerizations in the presence of hydroxyethyl pyridyl disulfide. The utility of the pyridyl disulfide functionality at the terminus of the polymers was demonstrated by synthesizing polymer-BSA conjugates. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7207-7224, 2008