Magnesium (Mg) and zinc (Zn) complexes incorporating tridentate anilido-aldimine ligand, (E)-2, 6-diisopropyl-N-( 2-((2-(piperidin-1-yl)ethylimino)methyl)phenyl)aniline (AA(Pip)-H, 1), were synthesized and structurally characterized. The reaction of AA(Pip)-H (1) with (MgBu2)-Bu-n or ZnEt2 in equivalent proportions afforded the monomeric complex [(AA(Pip))(MgBu)-Bu-n] (2) or [(AA(Pip))ZnEt] (3), respectively. The coordination modes of these complexes differ in the solid state: Mg complex 2 shows a four-coordinated and distorted tetrahedral geometry, whereas Zn complex 3 adopts a trigonal planar geometry with a three-coordinated Zn center. Complexes 2 and 3 are efficient catalysts for the ring-opening polymerization of beta-butyrolactone (beta-BL) in the presence of 9-anthracenemethanol (9-AnOH). The polymerization of beta-BL with the Zn catalyst system is demonstrated in a living fashion with a narrow polydispersity index, PDI = 1.01-1.10. The number-averaged molecular weight (M-n) of the produced poly(3-hydroxybutyrate) (PHB) is quite close to the expected M-n over diverse molar ratios of monomer to 9-AnOH. A greater ratio of monomer to alcohol catalyzed by Zn complex 3 served to form PHB with a large molecular weight (M-n > 60000). An effective method to prepare PHB-beta-PCL and PEG-b-PHB by the ring-opening copolymerization of b-BL catalyzed by zinc complex 3 is reported. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5339-5347, 2010