Highly Selective and Reactive (salan)CrCl Catalyst for the Copolymerization and Block Copolymerization of Epoxides with Carbon Dioxide

Abstract

Chromium(III) derivatives of the tetradentate phenoxyamine ligand H2salan, where salan = the N,N′-dimethylated bis(aminophenoxide) ligand or the saturated version of the corresponding salen ligand, in the presence of 1 equiv of [PPN]N3 (PPN=Ph3P═N+═PPh3) are shown to effectively catalyze the copolymerization of cyclohexene oxide and carbon dioxide. X-ray crystallographic analysis reveals the structure of the complex to be different from that of its salen analogue, with an all cis arrangement of the nitrogen and oxygen atoms. Although these catalysts are selective for copolymerizing propylene oxide and CO2 at ambient temperature with a high degree of regioselectivity, the copolymerization of cyclohexene oxide and CO2 requires higher temperatures (e.g., 60 °C). Nevertheless, the random polymerization of cyclohexene oxide and propylene oxide with CO2 at ambient temperature provides a terpolymer with a nearly statistical distribution of monomer units. In addition, these catalysts have been shown to be efficient at producing diblock copolymers of poly(propylene carbonate) and poly(cyclohexylene carbonate) as well as triblock copolymers of poly(propylene carbonate)/poly(cyclohexylene carbonate)/poly(vinylcyclohexylene carbonate).