Metal catalysts for closing the cycle of PLA lifetime

Polymers have become ubiquitous materials in our daily lives on account of their low-cost production and versatile properties. However, with the surge in demand, some critical issues relating to their use are emerging. Plastics are largely derived from petrochemical feedstocks and many of the materials that we employ are limited to a single use, despite having an extremely long life. As a result, plastic waste has been accumulating in the environment for years.[1] In response to this issue, scientific research is working towards new biodegradable polymers produced from renewable resources. The best candidate is polylactide (PLA) which, thanks to its good mechanical properties, can be used in various fields. Currently, the recycling of PLA is not actually planned, and its waste management strategy is still aligned with a linear economic model. A desirable end-of-life fate for PLA is the chemical recycling through alcoholysis where the product, alkyl lactate (A-La), can be used as a green solvent or converted into lactide, thus accomplishing a circular economy process.[2] My work has been focused on the identification of the best design of the catalytic species in terms of the structure of the auxiliary ligand, nature of the metal center and coordination geometry to promote the synthesis and chemical degradation of PLA. Indeed, new zinc and magnesium complexes supported by the pyridylphenoxyimine-based ligand have been synthesized.[3] In the lactide polymerization, high activity and efficient control were observed for Zn-based complexes even under industrially relevant conditions (technical grade monomer, solvent-free conditions, and high temperature). The Zn heteroleptic complexes showed the highest activities, resulting among the most efficient catalysts reported in the literature and comparable to the industrial tin catalyst. The same complexes were successfully employed for the degradation of post-consumer PLA. Different mechanisms have been highlighted depending on the conditions adopted (in solution or solvent-free conditions).[4]