Robust, Thermally Stable and Impurity-Tolerant Aluminum-Based Catalyst System for Polylactide Production under...

Ring-opening polymerization of l-lactide (l-LA) is a powerful approach that leverages a renewable monomer to generate high-molecular-weight polylactide (PLA). It is essential to replace the industrially used cytotoxic tin-(II) bis-(2-ethylhexanoate) [Sn-(Oct)₂] with low/nontoxic alternatives. However, most of benign catalysts addressed solution polymerizations that are typically performed at 25-100 °C. Robust, thermally stable and protonic agent-tolerant catalysts for industrial PLA production are still lacking. Herein, we designed and synthesized tetracoordinate aluminum methyl complexes bearing (amidoalkyl)-pyridine-phenolate (AmPyPh) pincers for industrial l-LA polymerizations. The catalytic activity could be modulated by the electronic and steric effects of the substituents in different positions. The structure-catalytic performance relationships were further investigated with assistance of density functional theory calculations. These (AmPyPh)-AlMe could effectively promote l-LA polymerization under industrial-relevant conditions (150-180 °C, in melt and bulk). These novel Al complexes could suppress the epimerization side reaction under harsh conditions and afforded semicrystalline PLLA. The molecular weight could be modulated by the monomer/catalyst feed ratio and the loading dosage of chain transfer agent. (AmPyPh)-AlMe showed comparable catalytic activity to Sn-(Oct)₂ in the presence of 20 equiv of benzyl alcohol but exhibited a much higher degree of control over polymerization and afforded PLLA with narrower distribution. Technical grade l-LA without further purification could also be polymerized rapidly thanks to the good tolerance to protonic agents at high temperature.