Aluminoxane, methyl

The first production of syndiotactic polystyrene has been credited to research workers at Idemitsu Kosan in 1985 who used cyc/opentadienyl titanium compounds with methyl aluminoxane as catalyst. 

The corresponding iron-catalyzed oligomerization of ethylene was developed by Gibson and coworkers [125]. A combination of an iron precatalyst with MAO (methyl aluminoxane) yields a catalyst that affords ethylene oligomers (>99% linear ot-olefin mixtures). The activity of ketimine iron complexes (R = Me) is higher than that of the aldimine analogs (R = H) and also the a-value of the oligomer is better (Scheme 41). 

The silica-supported chromate can be activated directly to a very efficient ethylene polymerization catalyst by ethylene itself or by reduction under CO, to yield active Cr(ll) bisiloxy species, ](=SiO)2Cr] [8]. While the silsesquioxane Cr derivative on its own does not lead to an active polymerization catalyst under ethylene (albeit only low ethylene pressure were tested), the silsesquioxane chromate ester can yield an active polymerization catalyst by addition of methyl-aluminoxane as co-catalyst. Comparison between the two catalytic systems is therefore possible but suffers from the lack of molecular definition of the active homogeneous species obtained after activation with the alkylating agent (Scheme 14.11). 

As previously mentioned, the catalyst precursors in aprotic solvents generally contain a Pd-alkyl moiety for immediate insertion of CO without the need of activators. In some cases, bis-halide Pd" precursors have been employed in conjunction with activators such as methyl aluminoxane (MAO), which is able to replace one halide ligand with a methyl group and to create a free coordination site at the metal [17]. 

It is now well recognised that the active species is a cationic complex, or more precisely a solvent-separated or tight ion pair, the structure of which depends on the mode of catalyst activation. Early spectroscopic and synthetic studies on metallocene dimethyl precursors helped to outline the principal reaction pathways, these have been reviewed [16, 21, 23]. Some of this chemistry is briefly summarised here since it presents the background for the understanding of later studies on methyl-aluminoxane (MAO) systems. 

Kaminsky W, Kulper K, Brintzinger HH, FRWP Wild, Polymerization of Propene and Butene with a Chiral Zirconocene and Methyl Aluminoxane as Cocatalyst, Angew Chem Int Ed Engl, 24,507 (1985)... 

Two approaches have been followed to generate a supported methyl-aluminoxane 1) the reaction of a carrier containing hydroxyl groups (starch,... 

Miyanaga [2] prepared moderate molecular weight polyglycidol ethers, (II), by polymerizing the corresponding glycidol ether with samarium triisopropoxide, samarium tris(tetramethyl heptanedionate), and yttrium tris(tetramethyl hep-tanedionate) with methyl aluminoxane. 

Yields are often low in lab preps (usually <60%). The product is called methyl-aluminoxane (MAO) or, less commonly, polymethylaluminoxane (PMAO). MAO is an ill-defined, complex composition, virtually insoluble in aliphatic hydrocarbons. MAO is typically supplied as a toluene solution containing 13% Al, which corresponds to 28% concentration of MAO. 

Rioka, M. Tsutsui, T. Ueda, T. Rashiwa, N. Stereospecific Polymerization of a-Olefin with an Ethylene Bis(l-indenyl)hafnium Dichloride and Methyl-aluminoxane Catalyst System. In Catalytic Olefin Polymerization, Studies in Surface Science and Catalysis-, Reii, T., Soga, R., Eds. Elsevier New York, 1990, p 483. 

Cyclic olefin copolymer (COC, poly(norbomene-co-ethylene)) catalyzed by ansa-ethylenebis-(indenyl)zirconium dichloride (Et[Ind](2)ZrCl2)/methyl-aluminoxane (MAO) gives rise to phenomenal increase of Tg upon annealing. Unaveraged solid-state NMR resonance of the CH carbons in the norbor-nene (53 and 49 ppm) evidenced the different conformation conversions. ... 

Chiral zirconocene complexes have also been studied as catalysts for the hydrogenation of nonfunctionalized olefins115. Using homogeneous Ziegler Natta-type catalyst systems derived from [ethylenebis(4,5.6,7-tetrahydro-l-indenyl)]zirconium complexes and methyl aluminoxane [A1(CH3)0] , 2-phenyl-l-butene was hydrogenated in 36% optical yield (20 bar H2, benzene, 25 °C). Under the same conditions, the reaction of styrene with D2 gave optically active 1,2-dideuteroethylbenzene with 65% ee. 

Sinn, H., Bliemeister, J., Clausnitzer, D., Tikwe, L., and Winter, H., Some New Results on Methyl Aluminoxane , in Transition Metals and Organometallics as Catalysts for Olefin Polymerization (W. Kaminsky and H. Sinn, eds.). Springer-Verlag, Berlin, 1988. 

Copper ethylene chemistry is well developed, but reports of polymerizations based upon copper are rare. The amidinate ligand, A/,A/ -ditrimethylsilyl-benzamidinato, has been reported to support copper-catalyzed ethylene polymerizations. It is prepared from hexamethyldisilazane, benzonitrile, and tri-methylsilyl chloride. The resulting copper chloride complex, 2, when activated with methyl-aluminoxane, produced polyethylene with Mv = 820 000 and = 138 In another patent. 

The most recent catalysts for syndiotactic polypropylene are complexes, like i-propyl(cyclopen-tadienyl-l-fluorenyl)hafiiium dichloride with methyl aluminoxane. Another, similar catalyst is /-propyl(/7 -cyclopentadienyl- / -fluorenyl)zirconium dichloride with methyl aluminoxane. These catalysts yield polymers that are high in syndiotactic material (the zirconium-based compound yields 86% of racemic pentads)Commercial production of syndiotactic polypropylene is in the planning stage. What catalytic system will be used, however, is not known at this time. Some of the properties of the two isomers, isotatic and syndiotactic polypropylenes, are compared in Table 5.5. 

Asymmetric synthesis cyclopolymerization of 1,5-pentadiene (261) was performed with an optically active metallocene catalyst. The polymer (262) obtained by (S)-ethylenebis(tet-rahydroindenyl)zirconium (S)-binaphtholate ([a] 435+1848°) in the presence of methyl aluminoxane (MAO) showed molecular rotation [(p] 4os-49.3°, and NMR analysis showed that the polymer had -68% tram structure. An optically active copolymer consisting of cyclic 262 units and linear units formed by 1,2-insertion shows LC phases. ... 

Baugh et al. [22] synthesized and characterized a series of nickel(II) and iron(II) complexes of the general formula [LMX2] containing bidentate (for M = Ni) and tridentate (for M = Fe) heterocycle-imine ligands. Activation of these pre-catalysts with methyl aluminoxane yields active catalyst systems for the oligomerization/polymerization of ethylene. Compared to a-diimine nickel and bis (immo)pyridine iron catalysts, both metal systems provide only half of the steric protection and consequently the catalytic activities are significantly lower. 

Hanaoka, Oda, and coworkers report [23] that single-site polymerization catalysts are of considerable interest industrially today, because they afford highly controllable polymerization performances based on precise design of catalyst architecmre and their industrial applications. Among them, they point to constrained geometry catalyst and phenoxy-induced complex, they call phenics-Ti, that are used together with methyl aluminoxane... 

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