Rational Optimization of Bifunctional Organoboron Catalysts for Versatile Polyethers via Ring-Opening Polymerization of Epoxides

Quaternary ammonium and phosphonium borane bifunctional
catalysts
have shown high catalytic performance in ring-opening polymerization
(ROP) of epoxides to produce polyether. Herein, we systematically
investigate a series of well-defined organoboron catalysts by varying
the electronic and steric properties of the Lewis acidic boron (B)
centers, manipulating the steric hindrance on the ammonium cation
(N+), adjusting the distance between B and N+, and regulating the nucleic B number of the catalysts. The investigation
on the dinuclear catalysts indicated that the reactivity of a given
catalyst could be speculated by its B–N–B angle and
the B···B distance. We found that the increase of Lewis
acidity and the number of B centers of the organoboron catalysts are
useful for a high catalytic activity for ROP of epoxides. The Lewis
acidity of the B centers was determined using the acceptor numbers,
showing an order of borinane (23.4) > BBN (21.7) > BCy2 (18.8) > Bpin (15.5). Moreover, we demonstrated the production
of
various telechelic polyols in the presence of different chain transfer
agents using the organoboron catalysts. The produced telechelic samples
have a well-defined terminal functionality with controllable molecular
weight. Lastly, these organoboron catalysts were utilized to produce
block copolymers, allyl-terminated macromonomers, and random copolymers.