Group 4 Metal Complexes of Phenylene–Salalen Ligands in rac‐Lactide Polymerization Giving High Molecular Weight Stereoblock Poly(lactic acid)

Phenylene–salalens—sequential tetradentate dianionic {ONNO}‐type ligands that include an ortho‐phenylene group bridging between an imine and amine internal donors bound to phenol arms with a broad variety of substitution patterns are described. Zirconium and hafnium complexes of the type [{ONNO}M(O‐tBu)2] were formed as single diastereomers, which, according to crystallographic structures, featured the fac(around the amine)‐mer(around the imine) wrapping mode. The reactivity and stereoselectivity in rac‐lactide (rac‐LA) polymerizations were found to depend on the substitution pattern: complexes featuring small groups on the imine‐side phenol and bulky groups on the amine‐side phenol exhibited the favorable combination of high activity and high isoselectivity (Pm≤0.91). Isotactic stereoblock copolymers of high molecular weights were prepared. The polymers crystallized in the stereocomplex phase according to thermal (differential scanning calorimetry) and crystallographic analysis. Around the block: Structural variations of phenylene–salalen ligands lead to Zr and Hf complexes that are found to be active and isoselective catalysts for polymerization of rac‐lactide, yielding stereoblock poly(lactic acids) (PLAs) of high molecular weights, which crystallize in a stereocomplex phase.