Catalytic activity of a series of Zn(II) phenoxides for the copolymerization of epoxides and carbon dioxide

A series of zinc phenoxides of the general formula (2,6-R2C6H3O)(2)Zn(base) (2) [R = Ph, Bu-t, Pr-i, base = Et2O, THF, or propylene carbonate] and (2,4 ,6-Me3C6H2O)(2)Zn(pyridine)(2) have been synthesized and characterized in t he solid state by X-ray crystallography. All complexes crystallized as four -coordinate monomers with highly distorted tetrahedral geometry about the z inc center. The angles between the two sterically encumbering phenoxide lig ands were found to be significantly more obtuse than the corresponding angl es between the two smaller neutral base ligands, having average values of 1 40 degrees and 95 degrees, respectively. In a noninteracting solvent such a s benzene or methylene chloride at ambient temperature, the ancillary base ligands are extensively dissociated from the zinc center, with the degree o f dissociation being dependent on the base as well as the substituents on t he phenolate ligands. That is, stronger ligand binding was found in zinc ce nters containing electron-donating tert-butyl substituents as opposed to el ectron-withdrawing phenyl substituents. In all instances, the order bf liga nd binding was pyridine > THF > epoxides. These bis(phenoxide) derivatives of zinc were shown to be very effective catalysts for the copolymerization of cyclohexene oxide and CO2 in the absence of strongly coordinating solven ts, to afford high-molecular-weight polycarbonate (M-w ranging from 45 x 10 (3) to 173 x 10(3) Da) with low levels of polyether linkages. However, unde r similar conditions, these zinc complexes only coupled propylene oxide and CO2 to produce cyclic propylene carbonate. Nevertheless, these bis (phenox ide) derivatives of zinc were competent at terpolymerization of cyclohexene oxide/propylene oxide/CO2 with little cyclic propylene carbonate formation at low propylene oxide loadings. While CO2 showed no reactivity with the s terically encumbered zinc bis(phenoxides), e.g., (2,6-di-tert-butylphenoxid e)(2)Zn(pyridine)(2), it rapidly inserted into one of the Zn-O bonds of the less crowded (2,4,6-trimethylphenoxide)(2)Zn(pyridine)(2) to provide the c orresponding aryl carbonate zinc derivative. At the same time, both sterica lly hindered and sterically nonhindered phenoxide derivatives of zinc serve d to ring-open epoxide, i.e., were effective catalysts for the homopolymeri zation of epoxide to polyethers. The relevance of these reactivity patterns to the initiation step of the copolymerization process involving these mon omeric zinc complexes is discussed.