Investigating the Ring-Opening Polymerization Activity of Niobium and Tantalum Ethoxides Supported by Phenoxyimine Ligands.

Abstract

A variety of metal catalysts from around the periodic table have been studied for the ring-opening polymerization (ROP) of cyclic esters. Within this field, group V catalysts have been rarely explored. To better understand the effect the choice of metal and ligand has on ROP activity, a series of 10 niobium and tantalum alkoxide catalysts, supported by a range of phenoxyimine ligands, were synthesized. The electronics and steric bulk of the ligands were varied on the phenoxy group (tBu, Cl, and OMe) and the imine group (Ph; 2,6-diMePh; 2,6-diiPrPh; and 2,4,6-tritBuPh) to probe their effect on the catalyst structure and activity. Catalysts were characterized with 1D, 2D, and variable-temperature NMR techniques to determine their structure in solution. Single crystal X-ray diffraction studies were conducted to establish their solid-state structure. The 10 catalysts are pseudo-octahedral, and each shows ligand coordination through phenoxy-oxygen and imine-nitrogen (O,N). In the case of the o-vanillin ligand set, however, evidence was found for O,O-coordination of the ligand when the steric encumbrance of the imine-nitrogen was increased. Each catalyst was active for the ring-opening polymerization of both rac-lactide (LA) and ε-caprolactone (CL) in the absence of solvent at 140 °C. In the case of CL, the catalysts supported by chloro-containing ligands showed the most polymerization control based on final polymer molecular weight and dispersity. Ligand trends were less clear for the polymerization of LA, though in all cases the catalysts were more controlled than the parent homoleptic alkoxide [M(OEt)5; M = Nb or Ta]. The most promising catalyst in the family was tested for copolymerization activity of LA and CL in one pot. Copolymerization of the two monomers was successful and yielded random poly(caprolactone-co-lactide).