Alternating copolymerization of propylene oxide and carbon dioxide with highly efficient and selective (salen)Co(III) catalysts: Effect of ligand and cocatalyst variation

Synthetic routes to a series of new (salen)CoX (salen = N,N′‐bis(salicylidene)‐1,2‐diaminoalkane; X = Br or pentafluorobenzoate (OBzF5)) species are described. Several of these complexes are active for the copolymerization of propylene oxide (PO) and CO2, yielding regioregular poly(propylene carbonate) (PPC) without the generation of propylene carbonate byproduct. Variation of the salen ligand, as well as the inclusion of organic‐based ionic or Lewis basic cocatalysts, has dramatic effects on the resultant (salen) CoX catalytic activity. Highly active (R,R)‐(salen‐1)CoOBzF5 (salen‐1 = N,N′‐bis(3,5‐ di‐tert‐butylsalicylidene)‐1,2‐diaminocyclohexane) catalysts with [Ph4P]Cl or [PPN]Y ([PPN] = bis(triphenylphosphine)iminium; Y = Cl or OBzF5) cocatalysts exhibited turnover frequencies up to 720 h−1 for rac‐PO/CO2 copolymerization, yielding PPC with greater than 90% head‐to‐tail connectivity. Additionally, the (R,R)‐(salen‐1)CoOBzF5/[PPN]Cl catalyst system demonstrated a krel of 9.7 for the enchainment of (S)‐ over (R)‐PO when the copolymerization was carried out at low temperatures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5182–5191, 2006 Salen‐based (salen = N,N′‐bis(salicylidene)‐1,2‐diaminoalkane) cobalt catalysts for the alternating copolymerization of propylene oxide and CO2 provide a viable route to regioregular poly(propylene carbonate) without formation of propylene carbonate byproduct. The structural properties of the salen ligand in these catalysts and the inclusion organic‐based ionic and Lewis‐basic cocatalysts dictate the copolymerization activity and regioselectivity. Once optimized, catalyst turnover frequencies up to 720 h−1 for propylene oxide/CO2 copolymerization were observed for the generation of regioregular poly(propylene carbonate).