Polyesters derived from the ring-opening polymerization (ROP) of lactides formed from inexpensive renewable resources constitute one important class of biodegradable and biocompatible polymers. Herein, the preparation and characterization of a series of closely related magnesium and zinc compounds that are active for ROP of lactides are reported using three β-diiminate ligands L1, L2 and L3 where L1 = CH(CMeNC6H3-2,6-iPr2)2, L2 = CH(CMeNC6H4-2-tBu)2 and L3 = CH(CMeNC6H4-2-OMe)2. For the L1 ligand, L1Mg(NiPr2)•(THF), L1Zn(NiPr2), L1Mg(OtBu)•(THF), L1Zn(OtBu) and L1Zn(OSiPh3)•(THF) have been synthesized. All compounds initiate and sustain ROP of lactides. For a related series L1MX(THF)n, where n = 0 or 1, the reactivity follows the order M = Mg > Zn and X = OtBu > NiPr2 > N(SiMe3)2 > OSiPh3. Heterotactic polylactide (PLA) is produced from ROP of rac-lactide by the zinc catalysts in CH2Cl2. The magnesium catalysts produce atactic and heterotactic PLA in CH2Cl2 and THF, respectively. The resting states for Zn and Mg are proposed to be L1Zn(η2-OCHMeC(O)OP) and L1Mg(μ-OP)2MgL1. For the L2 ligand, L2MgNiPr2•(THF) and L2ZnNiPr2 are prepared. In solution, L2ZnNiPr2 exists as a mixture of syn- and anti-rotamers while L2MgNiPr2•(THF) exists only as a syn-rotamer. In the zinc compound, the syn-conformer is shown to be more reactive than the anti–conformer and is responsible for the polymerization. Heterotactic PLA is obtained in CH2Cl2 for the zinc compound and in THF for the magnesium compound. For the L3 ligand, L3MgN(SiMe3)2, [L3MgOtBu]2, L3ZnN(SiMe3)2, L3ZnOiPr and L3CaN(SiMe3)2•(THF) are prepared. The OMe groups of ligand L3 show little affinity toward zinc and in the ROP of rac-lactide give PLA with only moderately enhanced heterotactic tetrads. The OMe groups are shown to bind to Mg reversibly and display significant enhancement of heterotactic PLA. Herein, the first single-site calcium complexes of L1 and tris(pyrazolyl/indazolyl)borate ligands are reported and shown to initiate and sustain ROP of lactide: L1CaNR12•(THF), TpR2CaNR12•(THF)n, TpMenCaNR12, (9-BBN)Bp(+)-CamCaNR12•2THF where TpR2 = [η3-HB(3-R2pz)3], R1 = SiMe3, R2 = tBu oriPr, n = 0 or 1, Men = menthone-derived indazole and Cam = camphor-derived indazole. All calcium complexes polymerize rac-lactide in THF giving mostly atactic PLA except for TptBuCaNR12 where heterotactic PLA is produced. TptBuCaNR12 reacts with 2,6-diisopropylphenol giving TptBuCa(O-2,6-iPr2C6 H3). The order of heterotactic selectivity is THF > PO ≈ CHO > CH2Cl2. The single-site calcium complexes are not active for ROP of epoxides and the compound TptBuCa(O-2,6-iPr2C6H3 )•(PO) is reported and structurally characterized by a single crystal X-ray diffraction study.