Synthesis and catalytic activity of single-site group V alkoxide complexes for the ring-opening polymerization of ε-caprolactone.

The synthesis, characterization, and ring-opening polymerization (ROP) activity of a family of niobium and tantalum alkoxide catalysts was studied. The final catalysts are made in a two-step synthesis, first by reacting the desired homoleptic metal ethoxide with a phenolketoimine ligand to form a series of synthetic intermediates, followed by reaction with catechol to produce a catalytic platform with a single ethoxide initiator. By using two separate ligands, the electronic properties of the catalyst can be tuned, and the molecular weight of the polymer can be increased. It was found that synthetic intermediates adopted a mer geometry both in solution and in the solid state. This mer geometry was retained for the final catechol derivatives, however in one case, where catechol was substituted for 3-methoxycatechol, the molecule adopted a highly distorted fac geometry. Catalytic ROP activity of the synthetic intermediates and final catechol derivatives with ε-caprolactone was studied through a kinetic analysis. In all seven cases studied the reactions proceeded through the expected coordination-insertion mechanism, following pseudo first-order kinetics and increasing in Mn linearly vs. conversion. The single-initiator catechol derivatives increased the Mn by three times compared to that of the three-initiator synthetic intermediates with little decrease in the overall reaction rate. Both the nature of the ligand and metal were found to impact the rate of reaction in these systems. By switching from an electron donating ligand to an electron withdrawing ligand, the rate was found to nearly double. Tantalum species were faster than their niobium counterparts by ∼3 times in the synthetic intermediates and ∼1.5 times in the catechol derivatives. This observed periodicity supports recent literature findings in this area.

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

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 ( t Bu, Cl, and OMe) and the imine group (Ph; 2,6-diMePh; 2,6-di i PrPh; and 2,4,6-tri t BuPh) 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).