Rare earth metal ammonium tris(phenolate) catalysts for industrial poly(lactic acid) production

Abstract

Driven by the combined pressures of the climate crisis and increasing public and governmental consciousness of environmental contamination by petrochemical-derived plastic materials, demand for compostable, bio-based alternatives has increased dramatically in recent decades. Poly(lactic acid), prepared via the immortal catalytic ring-opening transesterification polymerisation of the cyclic monomer lactide, is the most widely commercialised of these alternative materials. The development of effective catalytic systems affording high rates of conversion, yet at loadings accommodating the stringency of food contact safety regulations, remains a high priority. Following our recent reports of both an industrially relevant zirconium-based system, and a related family of rare earth element-based catalysts able to dramatically out-perform the Zr species under mild conditions, this work examines the translation of the lanthanum- and yttrium-containing congeners to use under challenging, industrially relevant conditions (≤750 g-scale, 180 °C, solvent-free, commercial-grade lactide) via a simple benchmarking study. Kinetic data indicate that, in that setting, the La- and Y-based systems afford mutually similar rates when used at the same gravimetric metal loading. Polymer characterisation, including thermal stability studies, has revealed good catalytic control whilst affording a level of thermal stability of the final polymer sufficient for limited melt processing under commercially required temperatures without stabilisation to deactivate persistent catalyst residues. More sustained melt processing likely requires development of a suitable stabilisation protocol. While not out-performing the Zr-based system purely on kinetics, the catalysts assessed herein are credible candidates for application at-scale.