A Binary Silicon‐Centered Organoboron Catalyst with Superior Performance to That of Its Bifunctional Analogue

This work reported that a silicon‐centered alkyl borane/ammonium salt binary (two‐component) catalyst exhibits much higher activity than its bifunctional analogue (one‐component) for the ring‐opening polymerization of propylene oxide, showing 7.3 times the activity of its bifunctional analogue at a low catalyst loading of 0.01 mol %, and even 15.3 times the activity at an extremely low loading of 0.002 mol %. By using 19F NMR spectroscopy, control experiments, and theoretical calculation we discovered that the central silicon atom displays appropriate electron density and a larger intramolecular cavity, which is useful to co‐activate the monomer and to deliver propagating chains, thus leading to a better intramolecular synergic effect than its bifunctional analogue. A unique two‐pathway initiation mode was proposed to explain the unusual high activity of the binary catalytic system. This study breaks the traditional impression of the binary Lewis acid/nucleophilic catalyst with poor activity because of the increase in entropy. In general, two‐component catalysts containing Lewis pairs are inferior to one‐component ones due to the lack of intramolecular synergy. A silicon‐centered bimolecular catalyst has been developed that exhibits a rare higher activity than its unimolecular analogue, even at low loadings. A unique two‐path initiation mode was proposed to explain its exceptional catalytic performance in the ring‐opening polymerization of propylene oxide.