Zinc N-heterocyclic carbene complexes and their polymerization of d,l-lactide

A series of zinc complexes of monodentate N-heterocyclic carbenes (NHCs) and a new sterically bulky bidentate pyridyl-NHC ligand have been synthesized and characterized by spectroscopic and X-ray crystallographic methods. Dinuclear alkoxide complexes of monodentate NHC complexes with 2,4,6-trimethylphenyl substituents appear to form monomeric species in solution and show good control and activity for lactide polymerization, including mild stereoelectivity as indicated by formation of heterotactic-enriched polylactide in d,l-lactide polymerizations. Kinetics studies revealed an overall second order rate law, first order in [LA] and [catalyst]. Efforts to obtain Zn–alkoxide complexes of a more sterically hindered NHC with 2,6-diisopropylphenyl groups were unsuccessful due to Zn–NHC bond scission. Ligand dissociation was also observed in attempts to prepare Zn–alkoxide complexes of the bidentate pyridyl-NHC system, despite its chelating nature.