| From the reactions between 2,2'-ethylidenebis(4,6-di- tert-butylphenol) and 2,2'-methylidenebis(4-di-methyl-6-di- tert-butylphenol) and Et2AlCl the biphenoxide complexes [(O∼∼CHMe∼∼O)AlCl]2, 1, and [(O∼∼CH 2∼∼O)AlCl]2, 2, have been characterized. These dimers are broken up by donor ligands and in THF give [(O∼∼CHMe∼∼O)AlCl(THF)], 3. Racemic 5,5'-6,6'-tetra-methyl-3,3 '-di-tert-butyl-1,1'-biphen-2,2 '-diol and Et2AlCl react in hexane to give [(O∼∼O)AlCl] 2, 6. In THF, monomeric species such as (O∼∼O)AlX(THF) (X = Et, 5, Cl, 4) are formed. Compounds 1, 2, 3, 4, 5, 6 act as propylene oxide, PO, polymerization catalyst precursors. The polymers have been examined by MS techniques and NMR spectroscopy. The new Al compounds give close to 50:50 HH to TT junctions. Polymerization of S-PO and 50:50 mixtures of S-PO and rac -PO reveal that the stereoirregular polymer is formed by a stereoselective ring-opening step. An analysis of the HH and TT junctions at the triad level suggests that polymerization occurs by a cationic coordinate mechanism by back-side attack on an activated PO molecule, leading to inversion at the methine carbon.; The aluminum chloride complexes mentioned above are shown to be inactive in initiating the ring-opening polymerization, ROP, of lactides, LA. Upon addition of PO, they lead to block oligomers/polymers of the form (PPO) n(PLA)m. The copolymers have been characterized by 1H, 13C NMR spectroscopy, by mass spectrometry (GPC/ESI/MS and LD/FTMS) and gel permeation chromatography. The Union Carbide catalyst system, [Ca(NH2)(OiPr)], is active in ROP of lactides at room temperature as well as PO. Compounds [(O∼CHMe∼O)Al(mu-O iPr)]2 and TPPAlOMe (TPP = tetraphenylporphyrin) are active in PO polymerization at room temperature but will effect lactide ROP only upon heating to 80°C. The stereoselectivity exhibited in ROP of PO by the porphyrin and the Union Carbide catalyst systems 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. (Abstract shortened by UMI.)... |
