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 ( 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).

Coordination of ε-Caprolactone to a Cationic Niobium(V) Alkoxide Complex: Fundamental Insight into Ring-Opening Polymerization via Coordination–Insertion

“…consideration in developing any process for which they might be employed. 63−66 In addition to a recent report from Plaman and Durr regarding the catalytic application of phenoxyimino Nb and Ta ethoxides, 56 and Al-Khafaji and co-worker's work encompassing the use of calixarene-supported NbCl 5 as a ROP initiator, 57 reports from the groups of Chakraborty and Redshaw concerning, respectively, the use of Nb and Ta imino phenoxides and tetraphenolate-supported Nb and Ta species, 51,55 and a poorly controlled trihydridoniobocene system reported by Otero and co-workers, 54 remain the only examples of these metals being successfully applied to the polymerization of ε-caprolactone. Several vanadium-catalyzed protocols have also been reported for the ROP of cyclic esters, including for εcaprolactone.…”

“…Complexes of the heavier group five metals, niobium and tantalum, have received little attention as initiators for the ROP of cyclic esters, with only a small handful of reports in the literature. ,– Indeed, the authors are aware of no established industrial catalytic processes mediated by Nb-based species, although this metal does have various specialist metallurgical uses, , as well as having more recently garnered interest in relation to potential energy material applications. – Importantly, ethical sourcing of Nb and Ta is a key consideration in developing any process for which they might be employed. – …”

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

“…However, these complexes were inactive, potentially due to the lack of sufficient positive charge on the metal center. 39 Since this initial report, several groups have successfully polymerized both CL and LA using a range of different catalyst platforms – including neutral metallocene 43 and phenoxyimine 40,42 supported systems, as well as zwitterionic 44 and cationic 45,46 systems.…”

Unexpected Periodicity in Cationic Group 5 Initiators for the Ring-Opening Polymerization of Lactones

“…37−42 There is only one example of the niobium-or tantalum-catalyzed ROP of δvalerolactone, δ-VL, that concerning an unusual trihydroniobocene system reported by Otero and co-workers. 41 Moreover, the authors are aware of only five reports describing application of structurally analogous Nb and Ta complexes to the polymerization of cyclic esters [37][38][39][40]43 and from only three of those can any comparison of relative activity be drawn. 37−39 Pampaloni and co-workers prepared Nb and Ta tetrakis-(ethoxide) complexes supported by an α-amino acid-derived (Lphenylalanine) ancillary ligand and applied them to the solventfree ROP of L-LA and rac-LA.…”

“…Examples in the literature of protocols for the ring-opening polymerization of cyclic esters catalyzed by complexes of the Group 5 metals are scarce, and to our knowledge, these elements have never been applied to the ROP of β-BL. − There is only one example of the niobium- or tantalum-catalyzed ROP of δ-valerolactone, δ-VL, that concerning an unusual trihydroniobocene system reported by Otero and co-workers . Moreover, the authors are aware of only five reports describing application of structurally analogous Nb and Ta complexes to the polymerization of cyclic esters − , and from only three of those can any comparison of relative activity be drawn. − …”