Two novel magnesium aryloxides have been prepared and their catalytic activities toward ring-opening polymerization (ROP) of epsilon-caprolactone and L-lactide have been investigated. The reaction of 2,2'-(2-methoxybenzylidene)-bis(4,6-di(1-methyl-1-phenylethyl)phenol) (MEMPEP-H-2) (1) and 2,2'-methylene-bis(4,6-di(1-methyl-1-phenylethyl)phenol) (MMPEP-H-2) with "Bu2Mg yield dimeric magnesium complexes [Mg(mu-MEMPEP)(THF)](2) (2) and [Mg(mu-MMPEP)(THF)](2) (3), respectively. Catalytic studies of complexes 2 and 3 illustrate that both 2 and 3 are good catalysts in epsilon-caprolactone and L-lactide polymerization. Theoretical study of the ROP mechanism of epsilon-caprolactone catalyzed by 2 demonstrates that the initiator, benzyl alcohol, is activated by the formation of a hydrogen bond with the phenoxy oxygen of MEMPEP2- ligand. (c) 2005 Elsevier Ltd. All rights reserved.