Simple and Efficient Zinc Catalysts for Synthesis and Chemical Degradation of Polyesters

A series of zinc complexes supported by tridentate imino-phenolate ligands with different substituents on the phenolate ring and bearing a pyridyl ring as an additional coordinative fragment were synthesized with the aim of elucidating structure-to-activity relationships in the ring-opening polymerization (ROP) of cyclic esters. For all complexes, a dynamic behavior associated with the conformational flexibility of the metallacycles involving the pyridinic nitrogen was observed. In the presence of 2-propanol, all zinc complexes promoted, with high efficiency and control, the ROP of rac-lactide (rac-LA), epsilon-caprolactone (epsilon-CL), and pentadecalactone (PDL). A significant effect of the substituents was observed for which electron-donating groups increased the polymerization rate, while sterically encumbered groups had detrimental effects. All complexes were highly efficient catalysts in the alcoholysis of aliphatic polyesters such as polylactide, polycaprolactone, and polypentadecalactone at room temperature without the use of solvents and in the glycolysis of commercial polyethylene terephthalate waste samples offering new contributions for a circular economy of plastics.