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?

Abstract

From the reactions between 2,2′-ethylidenebis(4,6-di-tert-butylphenol) and 2,2′-methylidenebis(4-dimethyl-6-di-tert-butylphenol) and Et 2AlCl the biphenoxide complexes [(O∼∼CHMe∼∼O)AlCl]2, 1, and [(O∼∼CH2∼∼O)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)AlCl(THF), 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∼∼O)AlCl]2, compound 6, as a hydrocarbon insoluble white precipitate. In the donor solvent THF monomeric species are formed, and (O∼∼O)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∼∼O)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∼∼CHMe∼∼O)Al-(O i Pr-d7)]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-Cl, OH, and )CH 2 being identified by MS and NMR. Polymerizations employing [(O∼∼CHMe∼∼O)Al(O i Pr-d7)]2 give HO-(PO)n-O i Pr-d7 oligomers, in addition to vinylterminated 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. † Dedicated to Dr. Walter Reichle, Corporate Research Fellow, Union Carbide, on the occasion of his retirement. * Corresponding author. E-mail Chisholm@chemistry. ohio-state.edu.

Use of Metal Catalysts Bearing Schiff Base Macrocycles for the Ring Opening Polymerization (ROP) of Cyclic Esters

“…The Al-Al distance in 3 may permit one metal centre to be used as a Lewis acid whilst the other can attack the carbonyl group of the monomer, akin to the situation postulated for the ROP of propylene oxide [21,22].…”

Alkylmono(cyclopentadienyl)titanium Complexes Containing the 2,2′‐Methylenebis(6‐tert‐butyl‐4‐methylphenoxido) Ligand – Studies on the Nature of the Catalytic Species Present in α‐Olefin Polymerisation Processes

“…The thf adduct monomer species [AlCl(mbmp)(thf)] obtained by reaction of mbmpH 2 with AlClEt 2 in thf has been described previously, although the dimeric species was not characterised. [41] Complexes 16 and 17 are insoluble in aliphatic hydrocarbons (hexane, pentane), scarcely soluble in aromatic solvents and highly water-sensitive. They can be stored for months under an inert gas without decomposition.…”

Microstructure Analysis of a CO2 Copolymer from Styrene Oxide at the Diad Level

“…[2] Therefore, the assignment of the stereo-and/or regiochemistry information of a polymer is one of the most-important tasks in the field of stereospecific polymerization catalysis. [3,4] In the alternating copolymerization of CO 2 with terminal epoxides, [5] there are also considerations of regiochemistry of the epoxide ring-opening and stereochemistry of the carbonate unit sequence in the resulting polymer chain. [6] In 2004, Chisholm and co-workers synthesized a series of oligoether carbonates, R(PO) n OCO 2 (PO) n R (R= Me, Et, or H; PO = propylene oxide ring-opened unit; and n = 1, 2, 3, 4, 10), as potential models for the microstructural assignment of poly(propylene carbonate) chains by NMR spectroscopy (Scheme 1).…”