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 epsilon-caprolactone were elucidated. The proton-exchange reactions of (C5H5)(3)Ln(THF) with the Salan ligand N,N'-(CH2Ph)(2)-N,N'-[CH2(2-OH-C6H2-Me-2-3,5)](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 Ln(2)O(2) 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 degrees 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 epsilon-caprolactone with the simultaneous addition of these two monomers, and the T-g values of the copolymers obtained increase linearly from -30.2 to +38.3 degrees 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.