Binuclear chromium–salan complex catalyzed alternating copolymerization of epoxides and cyclic anhydrides

Mono- and bi-nuclear chromium(III)–salan complexes are efficient catalysts for the alternating copolymerization of terminal epoxides [such as epichlorohydrin (ClPO) and glycidyl phenyl ether (GO)] and cyclic anhydrides [e.g. maleic anhydride (MA) and succinic anhydride (SA)] to afford the corresponding copolymers with >99% ester content. The binuclear complex c bearing a binaphthol linker showed significantly higher activity than the mononuclear chromium–salan complexes a and b. For example, the catalytic activities (based on chromium concentration) of complex c for MA/ClPO and MA/GO copolymerizations are 4.1 and 7.1 times that of complex a, respectively. An intramolecular bimetallic synergistic effect, in which the alternating chain-growth and dissociation of propagating copolymer species take turns at the two metal ions of the binuclear catalyst c, was suggested to make a contribution to the enhanced catalytic activity. Importantly, when using the binuclear complex c as a catalyst for MA/(S)-GO copolymerization, a highly regioregular ring-opening step was observed with a concomitant >99% retention of configuration at the methine carbon center of epoxide incorporated into the polyester.