Hydrogen‐bond‐assisted stereocontrol in the radical polymerization of N‐isopropylacrylamide with primary alkyl phosphate: The effect of the chain length of the straight ester group

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 0 °C, and TEP provided the highest syndiotacticity (racemo dyad = 65%) at −40 °C. On the other hand, lowering the temperature reversed the stereoselectivity of the propagation reaction so that isotactically rich poly(NIPAAm)s were obtained at −80 °C. 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 0 °C and a predominantly 1:2 complex at −80 °C 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.