Molecular design of single site catalyst precursors for the ring-opening polymerization of cyclic ethers and esters. 2. Can ring-opening polymerization of propylene oxide occur by a cis-migratory mechanism?

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 biphenoxi de 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, h ave been isolated and characterized. These dimers are broken up by donor li gands, and the molecular structure of ethylidenebis(4,6-di-tert-butylphenox ide 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 insolubl e white precipitate. In the donor solvent THF monomeric species are formed, and (O similar to similar toO)AlX(THF) has been crystallographically chara cterized, 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 t o similar toO)AlEt(THF), 5, in the presence of THF by displacement of one e thyl and one ethoxide ligand. Compounds 1, 2, 3, 4, 5, 6, [(O similar to si milar 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, polymeriza tion catalyst precursors. The polymers have been examined by MS techniques and NMR spectroscopy, and these results are compared with polypropylene oxi de, 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. Polymerizatio n of S-PO and 50:50 mixtures of S-PO and rac-PO reveals that the stereoirre gular 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 earli er 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 t o 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 c ompared to coordinate catalysis polymerizations of PO employing the Union C arbide calcium amide-alkoxide system and (porphyrin)AlCl and lead us to pre dict that a cis-migratory ringopening polymerization process is not likely to be developed for polymerization of PO.