The radical polymerizations of N-isopropylacrylamide (NIPAAm) were carried out in toluene at low temperatures in the presence of phosphoric acid esters such as trimethyl phosphate, triethyl phosphate (TEP), tri-n-propyl phosphate, and tri-n-butyl phosphate (TBP). Syndiotactically rich poly(NIPAAm)s were obtained from -60 to 0degreesC, and TEP provided the highest syndiotacticity (racemo dyad = 65%) at -40degreesC. On the other hand, lowering the temperature reversed the stereoselectivity of the propagation reaction so that isotactically rich poly(NIPAAm)s were obtained at -80degreesC. In particular, TBP exhibited the most isotactic specificity (meso dyad = 57%). Job's plots for NIPAAm-TBP mixtures revealed that NIPAAm and TBP formed a 1:1 complex at 0degreesC and a predominantly 1:2 complex at -80degreesC through a hydrogen-bonding interaction. Therefore, the stereospecificity of NIPAAm polymerization should depend on the stoichiometry of the hydrogen-bond-assisted complex. Thus, the mechanism for this polymerization system was discussed. (C) 2004 Wiley Periodicals, Inc.