Hydrogen-bond-assisted stereocontrol in the radical polymerization of N-isopropylacrylamide with secondary alkyl phosphate: The effect of the bulkiness of the ester group

摘要

The radical polymerization of N-isopropylacrylamide (NIPAAm) in toluene at low temperatures was investigated in the presence of triisopropyl phosphate (TiPP). The addition of TiPP induced a syndiotactic specificity that was enhanced by the polymerization temperature being lowered, whereas atactic polymers were obtained in the absence of TiPP, regardless of the temperature. Syndiotactic-rich poly (NIPAAm) with a racemo dyad content of 65% was obtained at -60 degrees C with a fourfold amount of TiPP, but almost atactic poly(NIPAAm)s were obtained by the temperature being lowered to -80 degrees C. This result contrasted with the result in the presence of primary alkyl phosphates, such as tri-n-propyl phosphate: the stereospecificity varied from syndiotactic to isotactic as the polymerization temperature was lowered. NMR analysis at -80 degrees C revealed that TiPP predominantly formed a 1:1 complex with NIPAAm, although primary alkyl phosphates preferentially formed a 1:2 complex with NIPAAm. Thus, it was concluded that a slight increase in the bulkiness of the added phosphates influenced the stoichiometry of the NIPAAm-phosphate complex at lower temperatures, and consequently a drastic change in the effect on the stereospecificity of NIPAAm polymerization was observed. (c) 2005 Wiley Periodicals, Inc.