4.6 Article

Enolic Schiff base aluminum complexes and their catalytic stereoselective polymerization of racemic lactide

Journal

CHEMISTRY-A EUROPEAN JOURNAL
Volume 14, Issue 10, Pages 3126-3136

Publisher

WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.200701473

Keywords

aluminum; lactide; ring-opening polymerization; Schiff bases; stereoselectivity

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A series of enolic Schiff base aluminum(III) complexes LAIR (where L = NNOO-tetradentate enolic Schiff base ligand) containing ligands that differ in their steric and electronic properties were synthesized. Their single crystals showed that these complexes are five -coordinated around the aluminum center. Their coordination geometries are between square pyramidal and trigonal bipyramidal. Their catalytic properties in the solution polymerization of racemic lactide (rac-LA) were examined. The modifications in the auxiliary ligand exhibited a dramatic influence on the catalytic performance. Lengthening the backbone from C-2 alkylene to C-3 alkylene resulted in remarkable enhancement of both the stereoselectivity and the polymerization rate because of the increasing flexibility of the diimine backbone. Electron-withdrawing substituents in the diketone also highly improved the activity and the stereoselectivity. Among these complexes, 4b had the highest activity and the stereoselectivity owing to the C-3 alkylene backbone and the two gem-methyl groups on the middle carbon atom. The value of the polymerization rate constant (k(p)) catalyzed by 4b in 70 degrees C was 1.90 L mol(-1) min(-1), the activation energy of the polymerization (35.4 kJ mol(-1)) was calculated according to the Arrhenius equation. Other factors that influenced the polymerization, such as the polymerization time, the temperature, and the monomer concentration, are also discussed in detail.

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