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

Graphical Abstract

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).

Abstract

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 k_rel 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