Poly(N-isopropylacrylamide) (pNIPAAm), a well-studied, biologically inert polymer that undergoes a sharp aqueous thermal transition at 32 degrees C, has been a subject of widespread interest for possible biological applications. A major hindrance to its successful application is due to the difficulty of maintaining a sharp transition when the polymer is modified for a physiological transition temperature, especially in isotonic solutions. Current copolymer blends raise the transition temperature but also make the transition significantly broader. We have combined the use of reversible addition-fragmentation chain transfer (RAFT) polymerization with tacticity control to synthesize well-defined pNIPAAm that demonstrates sharp transitions under physiological conditions. By selecting a RAFT agent with appropriate end groups, controlling molecular weight, and increasing the racemo diad content, we were able to increase the thermal transition temperature of pure pNIPAAm to a sharp transition at 37.6 degrees C under isotonic conditions. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012