Changing up BN-Polystyrene: Effect of Substitution Pattern on the Free-Radical Polymerization and Polymer Properties

Azaborinine-substituted polymers are poised to dramatically expand the diversity and functionality of polystyrenes via BN for CC substitution, but this new class of materials remains underexplored. Although formally isosteric to the respective all-carbon polystyrene derivatives, polarization due to the difference in electronegativity between B and N can have a distinct effect on polymerizability and the resulting polymer properties. Discussed herein is the successful preparation of a series of new isomeric azaborinine-substituted polymers via standard free-radical polymerization. We investigate the effects of the substitution pattern of vinylated B-mesityl azaborinines, that is, the position of the vinyl group relative to the BN moiety, on the polymerization reactivity and physical properties of the respective polymers. Copolymerization experiments with the parent styrene, as well as meta- and para-mesitylstyrene as direct all-carbon analogues of the azaborinine monomers, reveal that depending on the substitution pattern the more polarized azaborinines are incorporated to a lesser (B-Mes in para-position) or larger extent (B-Mes in meta-position) than the all-carbon analogues. Computational studies offer insights into the subtle electronic effects that result in this differential reactivity. We also compare the polymer properties to those of the all-carbon polystyrene analogues.