Polylactide (PLA) is an attractive polymeric material due to its origin from annually renewable resources and its biodegradability. The ring-opening polymerization (ROP) of lactide initiated by Lewis acidic and oxophilic metal-based catalysts constitutes the method of choice to access PLA in a controlled and stereoselective manner. The design and synthesis of ligand-supported metal complexes to act as effective ROP initiators of lactide monomers have been the subject of numerous investigations over the past decades. In view of their oxophilic nature, well-defined group 4 metal complexes supported by polydentate supporting ligands have appeared as active initiators for lactide ROP. This perspective summarizes various classes of structurally well-defined group 4 metal initiators developed for lactide ROP. It also provides observed trends regarding their catalytic performance. Whenever appropriate and possible, catalyst structure–ROP performance (i.e. activity, control and stereoselectivity) relationships are rationalized.

Magnesium bis(hydridotriphenylborate), a solvent-separated ion pair [Mg(thf)₆][HBPh₃]₂, is a versatile catalyst for hydroboration of polar unsaturated substrates including CO₂.