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

Abstract

ε-Caprolactone (ε-CL) adducts of cationic, amine tris(phenolate)-supported niobium(V) and tantalum(V) ethoxides initiate the ring-opening polymerization of lactones. The Ta(V) species prepared and applied catalytically herein exhibits higher activity in the ring-opening polymerization (ROP) of εcaprolactone than the previously reported, isostructural Nb(V) complex, contradicting literature comparisons of Nb(V)-and Ta(V)-based protocols. Both systems also initiate the ROP of δvalerolactone and rac-β-butyrolactone, kinetic studies confirming retention of higher activity by the Ta congener. Polymerizations of rac-β-butyrolactone and δ-valerolactone were previously unrealized under Group V-or Ta-mediated conditions, respectively, although the former has afforded only low molecular weight, cyclic poly-3hydroxybutyrate. Cationic ethoxo−Nb(V) and −Ta(V) δ-valerolactone adducts are also reported, demonstrating the facility of δvalerolactone as a ligand and the generality of the synthetic method. Both δ-valerolactone-bearing complexes initiate the ROP of εcaprolactone, δ-valerolactone, and rac-β-butyrolactone. Accordingly, we have elucidated trends in reactivity and investigated the initiation mechanism for such systems, the insertion event being predicated upon intramolecular nucleophilic attack on the coordinated lactone by the adjacent alkoxide moiety. This mechanism enables quantitative, stoichiometric installation of a single monomer residue distinct from the bulk of the polymer chain, and permits modification of polymer properties via both manipulation of the molecular architecture and tuning of the polymerization kinetics, and thus dispersity, through hitherto inaccessible independent control of the initiation event.

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

“…As expected these values are significantly longer than the cationic tris(phenolate)-supported niobium and tantalum catalysts (1.833(2) and 1.835(4), respectively) reported by McCormick and Davidson, due to the difference in available positive charge at the metal centers. 45,46…”

“…By contrast heavy group V metals, niobium and tantalum, have rarely been studied. 39–46 Kim and coworkers attempted the first ROP of l - and rac -LA with tantalum amine tris(phenolate) alkoxide complexes under neat conditions. However, these complexes were inactive, potentially due to the lack of sufficient positive charge on the metal center.…”

“…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.…”

“…Compared to other metals around the periodic table, little is known about niobium and tantalum ROP catalysts. For instance, only the work of McCormick and Davidson thus far demonstrates the intentional synthesis of single-initiator alkoxides 45,46 and, the differences between niobium and tantalum catalysts are still an open question. Indeed, the limited comparative studies available in the literature appear to depend on the choice of monomer.…”

“…Chakraborty and coworkers reported that niobium catalysts are more active for the ROP of l - and rac -LA, 44 while McCormick and Davidson reporting the opposite for CL. 46 However, the basis for these differences, or if they are consistent between systems and conditions, remain unknown. Exploring these systems is critical to determining if catalyst synthesis can be used to improve their ROP activity and control above that of more earth-abundant elements, such as neighboring titanium and zirconium.…”