Copolymerization of cyclohexene oxide and carbon dioxide using (salen)Co(iii) complexes: synthesis and characterization of syndiotactic poly(cyclohexene carbonate)

Abstract

Synthetic routes to a series of new (salen)CoX (salen = N,N′-bis(salicylidene)-1,2-diaminoalkane; X = halide or carboxylate) species are described and the X-ray crystal structures of two (salen-1)CoX (salen-1 = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = Cl, I) complexes are presented. (R,R)-(salen-1)CoX (X = Cl, Br, I, OAc, pentafluorobenzoate (OBzF₅)) catalysts are active for the copolymerization of cyclohexene oxide (CHO) and CO₂, yielding syndiotactic poly(cyclohexene carbonate) (PCHC), a previously unreported PCHC microstructure. Variation of the salen ligand and reaction conditions, as well as the inclusion of [PPN]Cl ([PPN]Cl = bis(triphenylphosphine)iminium chloride) cocatalysts, has dramatic effects on the polymerization rate and the resultant PCHC tacticity. Catalysts rac-(salen-6)CoX (salen-6 = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminopropane; X = Br, OBzF₅) have high activities for CHO/CO₂ copolymerization, yielding syndiotactic PCHCs with up to 81% r-centered tetrads. Using Bernoullian statistical methods, PCHC tetrad and triad sequences were assigned in the ¹³C NMR spectra of these polymers in the carbonyl and methylene regions, respectively.