A novel tridentate anilido-aldimine ligand, [o-C6H4(NHAr)-HC= NCH2CH2NMe2] (Ar = 2,6-(Pr2C6H3)-Pr-i, L-H, 1), has been prepared by the condensation of N, N-dimethylethylenediamine with one molar equivalent of 2-fluoro-benzaldehyde in hexane, followed by the addition of the lithium salt of diisopropylaniline in THE Magnesium (Mg) and zinc (Zn) complexes supported by the tridentate anilido-aldimine ligand have been synthesized and structurally characterized. Reaction of L-H (1) with an equivalent amount of (MgBu2)-Bu-n or ZnEt2 produces the monomeric complex [(LMgBu)-Bu-n] (2) or [LZnEt] (3), respectively. Experimental results show that complexes 2 and 3 are efficient catalysts for ring-opening polymerization of epsilon-caprolactone (CL) and L-lactide (LA) in the presence of benzyl alcohol and catalyze the polymerization of epsilon-CL and L-LA in a controlled fashion yielding polymers with a narrow polydispersity index. In both polymerizations, the activity of Mg complex 2 is higher than that of Zn complex 3, which is probably due to the higher Lewis acidity and better oxophilic nature of Mg2+ metal. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47:4927-4936, 2009