A series of monomeric amide or aryloxide complexes of the form LCaX, where L = CH[CMeNC6H3-2,6-Pr-2(i)]2, CH[CMeNC6H4-2-OMe](2), a bulky tris-pyrazolyl borate, Tp(iPr) or T p(tBu) or 9-BBN-pz(2) and X = N(SiMe3)(2) or OC6H3-2,6-Pr-2(i), has been prepared and characterized and investigated in the ring-opening polymerizations of lactide. The compounds (Tp(tBu))CaX in THF are shown to be highly active and stereoselective. The propylene oxide complex (Tp(tBu)) Ca(OC6H3-2,6-Pr-2(i)).(PO) has been isolated and structurally characterized (single-crystal X-ray) and shown to be inert to the polymerization of PO.