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.

Zirconium complexes of phenylene-bridged {ONSO} ligands: coordination chemistry and stereoselective polymerization of rac-lactide.

Sequential tetradentate dianionic thio-imine diphenolate ligands featuring an ortho-phenylene core and their zirconium complexes are described for the first time. Ligands that include different combinations of bulky-alkyl groups and halo groups on the two phenol arms were prepared by a substitution/condensation reaction sequence. An unexpected fac-fac wrapping mode was found in the solid state for the ligands in the octahedral [{ONSO}Zr(O(t)Bu)2] complexes. The complexes were all fluxional, and the barrier for enantiomer interconversion was found to depend on the phenolate substituents. The complexes were found to catalyze the polymerization of rac-lactide to poly(lactic acid) in solution with polymer tacticities varying from heterotactic to atactic which showed correlation to the nature of phenolate substituents but not to the degree of complex fluxionality.