Cobalt Catalysts for the Alternating Copolymerization of Propylene Oxide and Carbon Dioxide:  Combining High Activity and Selectivity

Synthetic pathways to (salcy)CoX (salcy = N,N ‘-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = halide or carboxylate) complexes are described. Complexes (R,R)-(salcy)CoCl, (R,R)-(salcy)CoBr, (R,R)-(salcy)CoOAc, and (R,R)-(salcy)CoOBzF₅ (OBzF₅ = pentafluorobenzoate) are highly active catalysts for the living, alternating copolymerization of propylene oxide (PO) and CO₂, yielding poly(propylene carbonate) (PPC) with no detectable byproducts. The PPC generated using these catalyst systems is highly regioregular and has up to 99% carbonate linkages with a narrow molecular weight distribution (MWD). Inclusion of the cocatalysts [PPN]Cl or [PPN][OBzF₅] ([PPN] = bis(triphenylphosphine)iminium) with complex (R,R)-(salcy)CoCl, (R,R)-(salcy)CoBr, or (R,R)-(salcy)CoOBzF₅ results in remarkable activity enhancement of the copolymerization as well as improved stereoselectivity and regioselectivity with maximized reactivity at low CO₂ pressures. In the case of [PPN]Cl with (R,R)-(salcy)CoOBzF₅, an unprecedented catalytic activity of 620 turnovers per hour is achieved for the copolymerization of rac-PO and CO₂, yielding iso-enriched PPC with 94% head-to-tail connectivity. The stereochemistry of the monomer and catalyst used in the copolymerization has dramatic effects on catalytic activity and the PPC microstructure. Using catalyst (R,R)-(salcy)CoBr with (S)-PO/CO₂ generates highly regioregular PPC, whereas using (R)-PO/CO₂ with the same catalyst gives an almost completely regiorandom copolymer. The rac-PO/CO₂ copolymerization with catalyst rac-(salcy)CoBr yields syndio-enriched PPC, an unreported PPC microstructure. In addition, (R,R)-(salcy)CoOBzF₅/[PPN]Cl copolymerizes (S)-PO and CO₂ with a turnover frequency of 1100 h^-1, an activity surpassing that observed in any previously reported system.