An Alkenyl Boronate As a Monomer for Radical Polymerizations: Boron As a Guide for Chain Growth and As a Replaceable Side Chain for Post-Polymerization Transformation

The ability of isopropenyl boronate pinacol ester to serve as a monomer in radical polymerizations was established and exploited for the synthesis of polymers that are difficult to access using other polymerization techniques. Although the monomer exhibits an α-methyl-substituted unconjugated structure, which is usually unfavorable for radical propagation, both free and controlled radical polymerizations smoothly afford the corresponding polymers. A density-functional-theory-based investigation revealed that the boron atom moderately stabilizes the radical species, which leads to the suppression of the degradative chain transfer to the α-methyl groups, and thus guides the reaction towards the radical polymerization. The boronyl pendants, which are directly attached to the polymer backbone, can be replaced with -OH or -NH to yield poly(α-methyl vinyl amine) or poly(α-methyl vinyl alcohol), which has been inaccessible by conventional synthetic methods.