Aluminoxane

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. 

Benvenuti M and Lenz RW. Polymerization and copolymerization of beta-batyrolactone and benzyl-beta-malolactonate by aluminoxane catalyst. J Polym Sci, Part A Polym Chem, 1991, 29, 793. 

In the view of molecular weight and molecular weight distribution we found that, the molecular weight of polymer obtained fix>m MMAO system p.ve the larger munber of molecular weight than the MAO system. This phenomenon was attributed to the present of TMA and TIBA in MAO and MMAO respectively. The molecular wei t distribution was independent with any almninoxane type. However, it essentially depended on typ i of cocatalyst (aluminoxane and borate)... 

The characteristics of C NMR spectra for all copolymers were similar. The triad distributions for all copolymo" from NMR monomer insertion are shown in Table 2. Based on the triad distribution of ethylene/l-hex aae copolymers in Table 2, we found that microstructurc of copolymer obtainrai fiom aluminoxane system was slightly different in monomer incorporation, but found significantly when borated system was applied. We suspected that this difference was arising fiom the diffaences in bimetallic complex active species between [aluminoxane] [catalyst] and [Borate] [catalyst] which had the electronic and gMmetric effects fiom the sterric effect of larger molecule of borate compare to the aluminoxane on the behaviors of comonomer insertion in our systems. 

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). 

Small amounts of water accelerate carboalumination of alkynes.228 This acceleration may be the result of formation of aluminoxanes. 

Allyl aryl ethers undergo accelerated Claisen and [1,3] rearrangements in the presence of a mixture of trialkylalanes and water or aluminoxanes. The addition of stoichiometric quantities of water accelerates both the trimethylaluminum-mediated aromatic Claisen reaction and the chiral zirconocene-catalyzed asymmetric carboalumination of terminal alkenes. These two reactions occur in tandem and, after oxidative quenching of the intermediate trialkylalane, result in the selective formation of two new C-C bonds and one C-0 bond (Eq. 12.70).153 Antibodies have also been developed to catalyze Claisen154 and oxy-Cope155 rearrangements. 

A remarkable feature of the methylalumination reaction is that the addition of water to the reaction mixture has an accelerating effect. An in situ generated aluminoxane species (similar to MAO) is most probably responsible for this effect. The methylalumination then proceeds at —23 °C without any loss of regioselectivity [63],... 

The zirconocene-catalyzed enantioselective methylalumination is accelerated by the addition of water to the reaction mixture. Styrene derivatives in particular, which are unreactive under the aforementioned conditions, readily undergo methylalumination at —5 °C. The reaction of water and AlMe3 most probably yields aluminoxanes. MAO was also shown to accelerate the reaction, although less markedly [78] (Scheme 8.39). 

A recent new discovery is the fact that the hydrolysis of branched /3-alkyl-substituted aluminoxanes are, in some cases, as effective as co-catalysts in olefin polymerization as MAO.63,64 For example, when combined with the the metallocenes, Cp 2ZrCl2, the hydrolysis products (Al/HzO = 2) of R3A1 (R = Bu and Oct) produced akylated ion pairs with high polymerization activities.65 The same combinations with Cp2ZrCl2 did not produce active catalysts, a result interpreted as due to the inhibition of /3-hydride elimination in the substituted metallocene derivatives. 

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. 

The synthesis of several other catalysts has been described in detail in the literature (25). An aluminoxane can be prepared by the reaction of Al2(S04)3 x 14 H2O and trimethylaluminum in toluene at 0°C (4). The alumoxane acts as an activating co-catalyst to form an alkylmetallocene cation. 

To the free valency of aluminum an organic group of halogen is attached. Alumoxanes are also addressed as aluminoxanes or alumina gels (27,28). 

W. Kaminsky, A. Bark, and M. Arndt, New polymers by homogeneous zirconocene/aluminoxane catalysts, Makromol. Chem., Macromol. Symp., 47 83-93,1991. 

When trialkylaluminums are treated with water under controlled conditions, linear or cyclic aluminoxanes (39, 40) are formed ... 

The bulky methylaluminoxane anion stabilizes the coordinatively unsaturated metal cation. Stabilization by noncoordinating anions such as carbosilane dendrimers is also viable.571 Aluminoxanes, however, are required to be used in large excess to be effective. Alternatively, the active catalyst can also be prepared by reacting a metal dialkyl with fluorinated boranes, borate salts or aluminate salts. 

The metallocene catalysts must be first activated by an aluminoxane co-catalyst, e.g., tetramethylaluminoxane (MAO) which is an oligomer, n being 10-15 (Fig. 9.5-2, right). Because a high excess of MAO is required for activation, a binary co-catalyst system was developed consisting of an organoborate and tri-isobutylaluminium (TiBA). Organoborate can be used in a stoichiometric ratio which reduces the costs and the residue of activator products in the polymer [5]. 

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