Design of Vinylboron Monomers for the Comprehensive Structural Control of Poly(vinyl alcohol)s via Stereospecific Controlled Radical Polymerization and Subsequent Side-Chain Replacement

A series of vinylboron monomers was designed, wherein the boron atom is protected by N-alkyl-substituted anthranilamide moieties, in order to prepare poly(vinyl alcohol) (PVA) polymers via stereospecific RAFT polymerization and subsequent oxidation of the pendant boron groups with comprehensive control over primary structural factors, i.e., branching structure, molecular weight, tacticity, and block sequencing with other monomers. In contrast to vinylboron monomers that are protected by nonsubstituted anthranilamide moieties, those protected with N-alkyl-substituted anthranilamide moieties allowed suppressing backbiting reactions during the radical polymerization, thus enabling the generation of linear PVAs. In particular, when relatively bulky substituents such as isobutyl and neopentyl groups were installed on the anthranilamide protecting group, the polymerization proceeded in an isospecific fashion through helical conformation, yielding isotactic PVAs. Stereoblock PVAs and a vinyl alcohol-acrylamide block copolymer were also synthesized via RAFT polymerization, and the thermal properties specific to tacticity, e.g., dual melting behavior affected by the two segments, were observed.