Trinuclear salphen-chromium(III)chloride complexes as catalysts for the alternating copolymerization of epoxides and cyclic anhydrides

Although the alternating copolymerization of epoxides and cyclic anhydrides is a promising route to aliphatic polyesters, improved catalysts are required to realize commercialization of this process. Herein, trinuclear chromium complexes of salicylaldimine, in conjunction with a nucleophilic cocatalyst, are demonstrated as excellent catalysts for epoxide/cyclic anhydride copolymerization, selectively affording perfectly alternating polyesters. The effect of the distance between the chromium species is investigated by varying the bridging skeleton in a series of trinuclear salphen-Cr(III)Cl complexes for obtaining different Cr-Cr distances. Trinuclear salphenCr(III)-complexes with Cr-Cr distances of approximately 7.3 angstrom are found to be efficient copolymerization catalysts, even at high temperatures and extremely low catalyst loadings. In particular, a high activity of 10,620 h(-1) is obtained for the copolymerization of cyclohexene oxide (CHO) and phthalic anhydride (PA) under a low catalyst loading (<0.01 mol%) at 100 degrees C. In situ infrared spectroscopy studies suggest that the activation energy of the trinuclear Cr(III)-salphen catalyst for CHO/PA copolymerization is 15 kJ mol(-1) lower than that of the corresponding mononuclear catalyst owing to an intramolecular synergistic effect among the metal atoms.