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

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.