Radical Polymerization of Vinyl Boronate Involving Backbiting Chain Transfer and Postpolymerization Oxidation Affording Branched Poly(vinyl alcohol)s

We found that radical polymerization of vinyl boronic acid pinacol ester (VBpin) was accompanied by a backbiting chain transfer reaction to the polymer backbone, and thus, branched chains were generated via the propagation from the midchain radical. The subsequent oxidation for the boron pendant afforded a branched poly(vinyl alcohol) (PVA), which was certainly supported by NMR structural analyses. The synthesis of the branched PVA is inaccessible via radical polymerization of vinyl acetate (VAc) and subsequent saponification because the VAc polymerization favors the chain transfer reaction to pendant methyl groups over that to the backbone. Our effort was also directed to tuning the branch degree via copolymerization of VBpin with VAc for studies on effects of the branch degree on the physical properties, such as solubility in water and crystallization in the bulk state.