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.

Automated summary

This study demonstrates that trinuclear chromium complexes of salicylaldimine, in conjunction with a nucleophilic cocatalyst, are efficient catalysts for the copolymerization of epoxides and cyclic anhydrides, producing perfectly alternating polyesters. By investigating the distance between chromium species, it was found that trinuclear salphenCr(III)-complexes with distances of approximately 7.3 angstrom exhibit high catalytic activity even at low catalyst loadings. Additionally, in situ infrared spectroscopy studies suggest that the activation energy of the trinuclear Cr(III)-salphen catalyst is lower than that of the corresponding mononuclear catalyst due to an intramolecular synergistic effect among the metal atoms.