Synthesis and Characterization of Dinuclear Salan Rare-Earth Metal Complexes and Their Application in the Homo- and Copolymerization of Cyclic Esters.

Abstract

Four rare-earth-metal aryloxo complexes stabilized by a tetradentate Salan ligand were prepared, and their catalytic properties for the (co)polymerization of lactides and ε-caprolactone were elucidated. The proton-exchange reactions of (C5H5)3Ln(THF) with the Salan ligand N, N'-(CH2Ph)2- N, N'-[CH2(2-OH-C6H2-Me2-3,5)]2 (LH2) in a 1:1 molar ratio, and subsequently with 1 equiv of p-methylphenol, gave the rare-earth-metal aryloxides [LLn(OC6H4-4-CH3)(THF) n]2 [ n = 0 and Ln = Y (1), Sm (2), and Nd (3); n = 1 and Ln = La (4)] in good isolated yields. These complexes were fully characterized by elemental analysis, IR, and NMR spectroscopy (for complexes 1 and 4). Solid-state structures of complexes 1-4 were confirmed by single-crystal X-ray diffraction analysis. Complexes 1-4 have dinuclear solid-state structures, with a Ln2O2 core bridging the Salan ligands. The coordination geometry around each of the metals is a slightly distorted octahedron in complexes 1-3, whereas it is a capped trigonal prism in complex 4. It was found that complexes 1-4 can initiate efficiently the homopolymerization of l-lactide (l-LA) and rac-lactide ( rac-LA) at 30 °C in tetrahydrofuran. The increasing activity of these complexes is in agreement with increasing ionic radii. A kinetic study revealed that seven-coordinated lanthanum complex 4 is more active for rac-LA polymerization compared with l-LA. A further study revealed that complex 4 was also an efficient initiator for the random copolymerization of l-LA and ε-caprolactone with the simultaneous addition of these two monomers, and the Tg values of the copolymers obtained increase linearly from -30.2 to +38.3 °C with an increase of the percentage of LA units. A mechanism study revealed that transesterification plays a crucial role in the formation of a random copolymer.