From the reactions between 2,2'-ethylidenebis(4,6-di-tert-butylphe and 2,2'-methylidenebis(4-dimethyl-6-di-tert-butylphenol) and Et2AlCl the biphenoxide complexes [(O similar to similar to CHMe similar to similar toO)AlCl]2, 1, and [(O similar to similar to CH(2)similar to similar toO)AlCl](2), 2, have been isolated and characterized. These dimers are broken up by donor ligands, and the molecular structure of ethylidenebis(4,6-di-tert-butylphenoxide 3, has been structurally characterized. Racemic 5,5'-6,6'-tetramethyl-3,3'-di-tert-butyl-1,1'-biphen-2,2'-diol and Et2AlCl react in hexane to give [(O similar to similar toO)AlCl](2), compound 6, as a hydrocarbon insoluble white precipitate. In the donor solvent THF monomeric species are formed, and (O similar to similar toO)AlX(THF) has been crystallographically characterized, X = 20% Cl and 80% Et occupancy. Refluxing in THF favors X = Cl, compound 4. The reaction of Et2Al(OEt) with the biphenol gives (O similar to similar toO)AlEt(THF), 5, in the presence of THF by displacement of one ethyl and one ethoxide ligand. Compounds 1, 2, 3, 4, 5, 6, [(O similar to similar to CHMe similar to similar toO)Al(O'Pr-d ((OPr)-Pr-l-d(7))](2), and [Cp2Zr(OEt)(OEt2)(+) [HB(C6F5)(3)](-) act as propylene oxide, PO, polymerization catalyst precursors. The polymers have been examined by MS techniques and NMR spectroscopy, and these results are compared with polypropylene oxide, PPO, formed by base catalysis and by porphyrin- and salen-AlCl catalyst precursors. The new Al compounds and the cationic zirconium alkoxide give close to 50:50 HH to TT junctions with end groups C-CI, OH, and =CH2 being identified by MS and NMR. Polymerizations employing [(O similar to similar to CHMe similar to similar toO)Al((OPr)-Pr-l-d(7))](2) give HO-(PO)n-(OPr)-Pr-l-d(7) oligomers, in addition to vinyl-terminated species. Polymerization of S-PO and 50:50 mixtures of S-PO and rac-PO reveals that the stereoirregular polymer is formed by a stereoselective ring-opening step. An analysis of the HH and TT junctions at the triad level is made, extending the earlier assignments of Tonelli and Schilling. This analysis leads us to suggest that polymerization occurs by a cationic coordinate mechanism wherein ring opening occurs by backside attack on an activated PO molecule which leads to inversion at the methine carbon. The rac-biphenoxide-Al complexes show a preference for ii and i linkages in (HT)(HT)(HT) units. These results are compared to coordinate catalysis polymerizations of PO employing the Union Carbide calcium amide-alkoxide system and (porphyrin)AlCl and lead us to predict that a cis-migratory ringopening polymerization process is not likely to be developed for polymerization of PO.