Alternating Copolymers of Vinyl Alcohol and N -Alkylacrylamide: Synthesis and Physical Properties in Aqueous and Bulk Phases

In this work, we designed a divinyl monomer composed of phenyl acrylate and benzyl vinyl ether to synthesize N-alkylacrylamide–vinyl alcohol (VA) alternating copolymers. The polymerization under diluted condition and the intrinsic low reactivity ratio of vinyl ether for acrylate allowed selective cyclopolymerization. Moreover, the spacer design enabled an aminolysis reaction for the generation of N-alkylacrylamide units and deprotection of the benzyl ether bond for that of VA units. Systematic variation of the alkyl substituent of the amine used for aminolysis revealed an unusual thermal behavior of the resulting N-alkylacrylamide–VA alternating copolymers: the glass transition temperature increased with increasing alkyl chain length of the N-alkylacrylamide units. This trend is in sharp contrast to that typically observed for other vinyl polymers and alternating copolymers. Spectroscopic analysis suggested that this anomalous behavior originates from hydrogen-bonding interactions involving hydroxyl groups derived from the VA units. In aqueous solution, the copolymers also exhibited unusual thermoresponsive behavior: the thermoresponsive behavior and the cloud point significantly changed with pH, despite the absence of typical pH-responsive functional groups. This phenomenon is attributed to hydrogen-bonding interactions involving hydroxyl groups directly attached to the polymer backbone. These findings demonstrate that backbone-connected hydroxyl groups in alternating copolymers can participate in unique hydrogen-bonding networks, leading to unconventional thermal and solution properties.