Bifunctional yttrium(III) and titanium(IV) NHC catalysts for lactide polymerisation.

Lewis acidic yttrium(III) and titanium(IV) derivatives of anionic, metal-tethered carbenes apparently act as bifunctional catalysts for the polymerisation of D,L-lactide, using a combination of Lewis acid and base functionalities to initiate ring opening of the cyclic monomer; the alcohol- and amino-functionalised carbenes from which they derive provide models for the first insertion step, and also display metal-free polymerisation catalysis to generate polylactic acid.