Zinc and magnesium catalysts for the synthesis for PLA and its degradation: Clues for catalyst design

Homoleptic and heteroleptic zinc and magnesium complexes bearing phenoxy-imine pyridine ligands were synthetized and characterized. Several structural parameters such as the structure of the ancillary ligand, the nature of the metal centre and the coordination geometry were investigated for identifying design elements purposeful to improve the catalytic activity in the ring opening polymerization (ROP) of lactide and in the reverse reaction of degradation of polylactide by alcoholysis. High efficiency and control degree were reached with zinc catalysts while magnesium ones showed lower activity and a weak control degree. NMR studies demonstrated that, for the homoleptic zinc complex, an activated-monomer mechanism was active. Differently, a coordination/insertion mechanism was observed for the related homoleptic zinc complex. Significantly, the complexes that showed the best activity in the ROP of lactide revealed to be the most efficient also in the degradation reaction.