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

5 years ago

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

Israel Goldberg, Moshe Kol, Tomer Rosen, Vincenzo Venditto, Ayellet Stopper

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.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701007