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

Abstract

Abstract Synthetic routes to a series of new (salen)CoX (salen = N , N ′‐bis(salicylidene)‐1,2‐diaminoalkane; X = Br or pentafluorobenzoate (OBzF 5 )) species are described. Several of these complexes are active for the copolymerization of propylene oxide (PO) and CO 2 , 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 )CoOBzF 5 (salen‐ 1 = N , N ′‐bis(3,5‐ di‐ tert ‐butylsalicylidene)‐1,2‐diaminocyclohexane) catalysts with [Ph 4 P]Cl or [PPN]Y ([PPN] = bis(triphenylphosphine)iminium; Y = Cl or OBzF 5 ) cocatalysts exhibited turnover frequencies up to 720 h − 1 for rac ‐PO/CO 2 copolymerization, yielding PPC with greater than 90% head‐to‐tail connectivity. Additionally, the ( R , R )‐(salen‐ 1 )CoOBzF 5 /[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