The aluminum chloride complexes [(O similar to similar toO)AlCl](2), I, [(O similar to CHMe similar toO)AlCl](2), II, and TPPAlCl, III, where O similar to similar toO = 5,5',6,6'-tetramethyl-3,3'-di-tert-butyl-1,1'-biphenoxide, O similar to CHMe similar toO = 2,2'-ethylidenebis-4,6-di-tert-butylphenoxide, and TPP = tetraphenyl porphyrin, are shown to be inactive in initiating the ring-opening polymerization, ROP, of lactides, LA (L and rac). Upon addition of propylene oxide, PO, they lead to block oligomers/polymers of the form (PPO) (PLA),. The Union Carbide catalyst system known as calcium amide-alkoxide, [Ca(NH2)((OPr)-Pr-i)], is active in ROP of lactides at room temperature as well as PO. Block copolymers of PO and LA can be formed by the initial ROP of PO followed by the addition of LA but not from reactions involving the reverse order of the addition of PO/LA. The alkoxide aluminum compounds [(O similar to CHMe similar toO)Al(mu -(OPr)-Pr-i)](2), IV, and TPPAlOMe, V, are active in PO polymerization at room temperature but will effect lactide ROP only upon heating to 80 degreesC. The stereoselectivity exhibited in ROP of PO by III and V and the Union Carbide catalyst system is not observed for ROP of rac-LA. These results are discussed in terms of the intimate mechanism of ring opening of PO and lactides, where it is proposed that coordinate catalysts differ in their operation. Single-site catalysis is favored for lactides but coordinate base catalysis for ROP of PO requires the cooperation of two metal sites. Copolymers of PO and LA are thus prepared as block copolymers by consecutive reactions. The copolymers have been characterized by H-1, C-13 NMR spectroscopy, by mass spectrometry (GPC/ESI/MS and LD/FTMS), and by gel permeation chromatography.