Methylaluminoxane active species

The key to highly active metallocene catalysts is the use of cocatalysts. In an activation step, the cocatalyst creates out of the metallocene a polymerization-active species. At first, methylaluminoxane (MAO) was usually used to activate metallocenes. Nowadays an alternative activation via borane and borate is becoming more and more important [20, 24, 25]. 

Occasional regioerrors appear significantly to inhibit the polymerisation of a-olefins by methylaluminoxane-activated metallocene catalysts [114, 138, 253— 261], In order to reduce the number of secondary Zr-CH(R)-CH2 species, and therefore to accelerate the polymerisation, advantage has been taken of the chain transfer reaction with hydrogen ... 

This mechanism, although understandable in a conceptual sense, is not fully understood in a mechanistic sense. For instance, the exact nature of active species and the role of the activator and/or counterion is a subject of debate this concerns methylaluminoxane-activated group 4 metallocene systems in particular. Methylaluminoxane may act to generate the active species and remove impurities from the polymerisation system as well as playing a more fundamental role such as assisting in the insertion of each monomer unit or reactivating dormant sites [358]. 

The base-free dimethyl Sc complex 121 was a highly active catalyst precursor for ethylene polymerization under B(C6F5)3, trityl borate, or methylaluminoxane (MAO)-type activation. The catalytic activity of 121 was similar to those observed of Group 4 metallocene complexes [81]. Generally, cationic scandium complexes are believed to be the active species. Activation of the catalyst was studied by reacting 120 and 121 with various equivalences of B(C6F5)3. The monomeric bulky rBu-substituted dimethyl complex 121 reacted with 1 equiv of B(C6F5)3... 

Zirconocene derivatives are used to catalyse a range of organic hydrogenation and C—C bond-forming reactions. In the presence of methylaluminoxane, chiral complex A catalyses asymmetric hydrogenations (see Section 26.4), with the active species being a cationic zirconium hydrido complex. 

Finally, the methylaluminoxane (MAO) [Al(Me)0] can be involved in several ways in the generation of a cationic zirconocene species. It can serve as a methylation agent (conversion of Zr-Cl to Zr-Me), and as a Lewis acid in abstracting methyl groups. In the generation of polymerization catalysts from Cp2ZrCl2 and MAO, the active species has the supposed composition [Cp2ZrMe]+ [Me-MAO]". ... 

The active species for the PBI complexes is not as well characterized as in the nickel and palladium systems. It is assumed to be a cationic alkyl complex formed by reaction of the dihalo precatalyst with a cocatalyst such as methylaluminoxane (MAO). The resulting active species polymerizes ethylene at unusually high rates to form linear high-density polyethylene. Even at ethylene pressures as low as 1 atm, the polymerization is extremely exothermic and the crystalline polymer product rapidly precipitates from solution. Computational chemistry is proving to be of utility in understanding the mechanistic aspects of this chemistry. - Lower barriers to insertion, relative to the nickel a-diimine complexes, support the higher activity. 

Syndiotactic polystyrene has also been described [110 113], A mixture of methylaluminoxane (MAO) and cyclopentadienyltitanium(III)chloride was used as catalyst, whereby the active species was postulated to be a cationic complex [CpTi(III) (Polymer)Sty]" [114], The stereocontrol in this catalyst is induced by the phenylgroups of the growing polymer chain and not by the symmetry of the catalyst as in most type of coordination catalysts. 

Most of the spectroscopic investigations discussed above were carried out on well-defined metallocene systems, either isolated species or those generated from a well-defined metallocene alkyl precursor activated with one equivalent of a borane or borate activator. Most practical polymerisation catalysts, on the other hand, include a scavenger, usually an aluminum alkyl, and may contain ill-defined activators such as methylaluminoxane (MAO), usually at high MAO/Zr ratios. Such systems are less amenable to quantitative studies nevertheless, the identifications of species such as those depicted in Schemes 8.5-8.8 has enabled similar compounds to be identified in more complex mixtures. An idea of the possible mode of action... 

Metallocene complexes require activation to be transformed into active catalysts. This is done by organoaluminoxanes, usually by methylaluminoxane (MAO), which provide maximum activity.570 During activation first the metal is methylated followed by a carbanion abstraction to form a metallocene monomethyl cation with a free coordination site (65), which is the actual active catalytic species ... 

In the case of chlorine-free catalysts such as Mg(OH)2/Ti(OBu)4—[A1 (Me)0]x and Si02/Ti(0Bu)4—[Al(Me)0]x, heterogeneous species are assumed to promote the syndiospecific polymerisation of styrene [67,68]. In a polymerisation system with the latter catalyst, best results were achieved when treating the carrier with [AI(Mc)0]x prior to supporting Ti(OBu)4 (no further activation with methylaluminoxane was needed). The polymerisation rate reaches a maximum at an Al/Ti molar ratio of 20 this is much lower than the value of the Al/Ti molar ratio required to reach the maximum polymerisation rate in the respective homogeneous system, i.e. the system without a carrier [54]. 

A key to the high polymerization activity of metallocenes are the cocatalysts. Methylaluminoxane (MAO) is mostly used and is synthesized by controlled hydrolysis of trimethyl aluminium [30]. Other bulky anionic complexes which show a weak coordination, such as borates, play an increasing role too. One function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds even at — 60 °C [31]. Excess MAO leads to the dialkylated species as NMR measurements show. In order for the active site of form, it is atleast necessary that one alkyl group is bonded to the metallocene [32],... 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

As previously mentioned, one of the primary motivations for the development of site-isolated aminosilicas is to construct a better molecular-level understanding of immobilized catalysts through the use of a more uniformly reactive surface. Within the area of a-olefin polymerizations, another parameter that negatively affects the ability to study well-defined surfaces is the use of methylaluminoxane (MAO) as a catalyst activator. The exact structure of the MAO species has been postulated to exist in a number of different forms, which makes it difficult to elucidate the exact nature of its interactions with the surface [21]. To address this issue, a well-defined sulfonic acid organic/inorganic hybrid material was developed to serve as both a support and a catalyst activator for homogeneous a-olefin polymerization catalysts [22]. 

The most common activator is known as MAO (methylaluminoxane). MAO is a complex mixture of chemical species, but has the rough C A1 0 stoichiometry of 1 1 1. MAO is prepared from the careful reaction of trimethylaluminum with water. As described earlier, the Rappe group has developed a MAO model based on structural studies and analogy to AIR(NR ) clusters. The model consists of a (AIMeO)9 cluster that has abstracted a Me and has had the resulting two-coordinate oxygen coordinated by the Lewis acidic AlMes. In the following discussion, we refer to this counteranion model as MA09. 

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