Homoleptic phenoxy-imine pyridine zinc complexes: efficient catalysts for solvent free synthesis and chemical degradation of polyesters

The synthesis and structural characterization of six homoleptic zinc complexes supported by monoanionic phenoxy-imine pyridine ligands bearing different substituents on the phenoxy moiety are described. All complexes have been tested as catalysts for the ring-opening polymerization of l-lactide under mild or industrially relevant conditions using technical grade l-lactide and excess alcohol. Interestingly, the Zn(ii) catalysts were stable under severe conditions, showing activities comparable to those of industrial catalysts. The same complexes were also revealed to be active in the depolymerization by alcoholysis of polylactide and poly(ethylene terephthalate). For both processes, a structure-reactivity relationship has been found, depending on the substituents introduced on the ligand backbone. Tests for the catalyst recycling in solvent-free alcoholysis were performed, offering new opportunities for the implementation of a circular (bio)plastics economy through procedures of low environmental impact.