Controlled Ring-Opening Polymerization of β‑Butyrolactone Via Bifunctional Organoboron Catalysts

Ring-opening polymerization affords an efficient strategy to convert β-butyrolactone into poly­(β-hydroxybutyrate) (PHB) which is a promising and widely used biodegradable polyester. Although numerous metallic catalysts have been well established, the toxic residual metal contaminants of them impede the application of PHBs especially in the biomedical and microelectronic field. Herein, we explore a series of bifunctional organoboron catalysts with varying frameworks and nucleophiles for preparing PHBs. The modular design of these well-defined catalysts permits synthetically facile steric and electronic interference to systematically optimize catalytic activity. The optimized catalyst achieved ideal reactivity with an excellent polymer selectivity (>99%) in a controlled manner. It was further confirmed that the BBL ring is opened at the alkyl–oxygen bond with the concomitance of the inversion of chiral centers, resulting in polyesters terminated by carboxylate. Additionally, side reactions were comprehensively investigated and a mechanism was proposed.