Mechanism and Design Principles for Controlling Stereoselectivity in the Copolymerization of CO2/Cyclohexene Oxide by Indium(III) Phosphasalen Catalysts

Copolymerization of CO2 with cyclohexene oxide (CHO) creates a sustainable polymer that has been a target for homogeneous catalysis. In particular, indium(III) phosphasalen catalysts generate high proportions of carbonate linkages in isotactic product poly(cyclohexene carbonate) (iPCHC). We use theory here to characterize the initiation and propagation steps for these indium(III) catalysts, which involve mononuclear mechanisms for CO2 insertion and CHO ring opening that are distinct from copolymerization mechanisms previously reported for other metal- based catalysts. We find that phosphasalen ligand interactions with CHO and the carbonate-terminated growing chain lead to stereoselection for high levels of isotacticity and suggest further modifications to the ligand that might tune this.