Highly Stable Heterometallic Catalysts for Polyester Depolymerization and Polymerization at High Temperatures

Chemical depolymerization to monomer is a valuable tool in the quest to prevent the accumulation of single-use plastic, especially poly(ethylene terephthalate), PET, in the environment. However, the protic nucleophiles and high temperatures usually required in this process make the development of rationally designed, stable catalysts very challenging. Herein, we report a series of heterobimetallic catalysts, combining Zn(II) and Mg(II) with K(I) and Na(I) to form "ate" complexes that are stable in air and at high temperatures, which can be applied in the depolymerization of PET and poly(lactide), PLA. The catalysts present outstanding stability and remain active after 4 successive PET additions over several days on the bench. Reaction studies reveal the influence of the metal coordinated in the inner pocket of the ligand upon the activity of the metal coordinated in the outer pocket, with different heterobimetallic catalysts being found to be optimal for the depolymerization of PET and PLA respectively. Their potential for application in industrially relevant bulk polymerization of rac-lactide is also demonstrated and provides key insights for rational heterometallic catalyst design in the (de)polymerization of PET and PLA.