Versatile NHC-based zinc and magnesium complexes for the synthesis and chemical recycling of aliphatic polyesters and polycarbonates

Abstract

Backbone substituted NHC derivatives of zinc and magnesium, in the presence of alcohol initiators, have been shown to be effective catalysts for the ring-opening polymerization of different cyclic monomers, efficiently furnishing sustainable aliphatic polyesters and polycarbonates. Comparing the behaviour of complexes with the same ligands but different metals, the order of the activity was found to be dependent on the monomer, with magnesium complexes showing a higher activity in the ROP of ε-caprolactone (ε-CL), trimethylene carbonate (TMC) and 1-methyl-trimethylene carbonate (Me-TMC), while the zinc complexes polymerized L-lactide (L-LA) and 2,2-dimethyl-trimethylene carbonate (DTC) more efficiently than their magnesium counterparts. Also the symmetry of the backbone substituents on the NHC ligand influenced the activity in the ROP with the syn substituents inducing a higher activity for both metals and for all the monomers with respect to the ligand with the anti symmetry. Kinetic studies demonstrated the polymerization reactions of L-LA, ε-CL and TMC to proceed via a mechanism first order in monomer concentration, with the four zinc and magnesium complexes. The obtained polymers possessed controlled molecular masses and dispersities dependent on the metal (generally higher for polymers obtained by magnesium complexes). The results of MALDI-ToF and NMR analysis confirmed the controlled nature of the present catalytic systems, where side reactions, such as inter- and intramolecular transesterifications, were minimized during the polymerization. Zinc and magnesium complexes were also tested in preliminary alcoholysis experiments of PLLA samples, and found able to promote these reactions of polylactide upcycling.