Ring opening polymerization of lactide promoted by Zinc and Magnesium complexes with a N-heterocyclic carbene-phenoxy-imine hybrid non-innocent ligand

Abstract

Zinc and magnesium complexes of N-heterocyclic carbene (NHC)-phenoxy-imine ligands derived from chiral (S,S)-trans-1,2-diaminocyclohexane have been synthetized. Either monomeric or dimeric species have been obtained depending on the reaction conditions. All complexes were tested as catalysts for the ring opening polymerization (ROP) of lactide showing moderate activity and control degree. MALDI-ToF-MS spectra of PLA highlighted that magnesium complex promoted the formation of cyclic PLA, whereas zinc analogous and its monomeric species gave linear PLA, suggesting that different polymerization mechanisms occur depending on the metal center. Moreover, according to MALDI-ToF-MS spectra, in the absence of additional initiators, polymers arising from zinc catalysts would bear the NHC-phenoxy-imine hybrid ligand as terminal unit. Computational studies were performed in order to shed light on catalyst behaviors and on the mechanisms involved in the ROP of lactide. Free energy profiles of the initiation step suggest that microstructural differences lie in the ligand role. Indeed, in the absence of additional initiators, the ligand participates to the initiation involving its NHC moiety when wrapped around magnesium and its phenoxy-imine moiety when bound to zinc.