Hydrogen-bond-assisted syndiotactic-specific radical polymerizations of N-alkylacrylamides: The effect of the N-substituents on the stereospecif¡ cities and unusual large hysteresis in the phase-transition behavior of aqueous solution of syndiotactic poly(N-n-propylacrylamide)

The radical polymerizations of N-alkylacrylamides, such as N-methyl-(NMAAm), Ar-rc-propyl-(NNPAAm), N-benzyl-(NBnAAm), and N-(1-phenylethyl)acrylamides (NPhEAAm), at low temperatures were investigated in the absence or presence of hexamethylphosphoramide (HMPA) and 3-methyl-3-pentanol (3Me3PenOH), which induced the syndiotactic specificities in the radical polymerization of AT-isopropylacrylamide (NIPAAm). In the absence of the syndiotactic-specificity inducers, the syndiotacticities of the obtained polymers gradually increased as the bulkiness of the iV-substituents increased. Both HMPA and 3Me3PenOH induced the syndiotactic specificities in the NNPAAm polymerizations as well as in the NIPAAm polymerizations. The addition of 3Me3PenOH into the polymerizations of NMAAm significantly induced the syndiotactic specificities, whereas the tacticities of the obtained polymers were hardly affected by adding HMPA. In the polymerizations of bulkier monomers, such as NBnAAm and NPhEAAm, HMPA worked as the syndiotactic specificity inducer at higher temperatures, whereas 3Me3PenOH hardly influenced the stereospecificity, regardless of the temperatures. The phase-transition behaviors of the aqueous solutions of poly(NNPAAm)s were also investigated. It appeared that the poly (NNPAAm) with racemo dyad content of 70% exhibited unusual large hysteresis between the heating and cooling processes.