Methylaluminoxane metallocene activation

Metallocenes such as Cp2TiCl2 and Cp2ZrCl2 alone are capable of polymerising styrene to an atactic polymer (involving a free radical propagation mechanism) [97]. The same metallocenes activated with methylaluminoxane form active catalysts for the polymerisation of styrene their productivity and syn-diospecificity, however, are not very high. In contrast, when activated with aluminium alkyls, these metallocenes do not afford catalysts that might be active in the polymerisation of styrene [98,99]. 

Metallocene catalysis is an alternative to the traditional Ziegler-Natta vanadium-based catalysis for commercial polyolefin production, e.g. the use of metallocene-catalyzed ethylene alpha-olefin copolymers as viscosity index modifiers for lubricating oil compositions [23]. The catalyst is an activated metallocene transition metal, usually Ti, Zr or Hf, attached to one or two cyclopentadienyl rings and typically activated by methylaluminoxane. Metallocene catalysis achieves more stereo-regularity and also enables incorporation of higher alpha-olefins and/or other monomers into the polymer backbone. In addition, the low catalyst concentration does not require a cleanup step to remove ash. 

In order to enhance the activity of coordination catalysts we typically add a cocatalyst. The cocatalyst works synergistically with the catalyst to allow us to tailor the tacticity and molecular weight of the product while also enhancing the rate of the reaction. An example of a commercially used cocatalyst is methylaluminoxane used in conjunction with metallocene catalysts. 

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]. 

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... 

Titanocene and zirconocene dichlorides (Cp2MtCl2 with Mt = Ti, Zr) were the first metallocenes studied [Breslow and Newburg, 1957 Natta et al., 1957a], The metallocene initiators, like the traditional Ziegler-Natta initiators, require activation by a Lewis acid coinitiator, sometimes called an activator. AIRCI2 and A1R3 were used initially, but the result was initiator systems with low activity for ethylene polymerization and no activity in a-olefin polymerization. The use of methylaluminoxane (MAO), [A1(CH3)0] , resulted in greatly improved activity for ethylene polymerization [Sinn and Kaminsky, 1980], The properties of MAO are discussed in Sec. 8-5g. MAO has two functions alkylation of a transition metal-chloride bond followed by abstraction of the second chloride to yield a metallocenium... 

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 ... 

Metallocene Catalysts. Higher a-olefins can be polymerized with catalyst systems containing metallocene complexes. The first catalysts of this type (Kaminsky catalysts) include metallocene complexes of zirconium such as biscyclopentadienylzirconium dichloride, activated by methylaluminoxane. These catalysts polymerize a-olefins with the formation of amorphous atactic polymers. Polymers with high molecular weights are produced at decreased temperatures and have rubber-like properties. 

The breakthrough in metallocene catalyst development occurred in the early 1980s when a metallocene catalyst, instead of an aluminium alkyl, was combined with methylaluminoxane (MAO) [8, 9, 10]. This catalyst system boosted the activity of metallocene-based catalyst and produced uniform polyethene with the narrow molar mass distribution typical for single-site catalysts. Efforts to polymerise propene failed, however the product was found to be fully atactic, indicating complete lack of stereospecificity of the catalyst [10]. 

In connection with the above characteristic features of metallocene methy-laluminoxane catalysts, it must be emphasised that alternative, potentially cheaper alkylaluminoxanes, such as ethyl or z-butyl derivatives, which are more soluble in aliphatic hydrocarbons than methylaluminoxane, or other alkylaluminium compounds used as activators for metallocene procatalysts, show inferior activity. 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

Extensive efforts have also been made to develop olefin polymerisation catalysts based on metallocenes with only one ligand of the cyclopentadienyl type. Ethylene-,dimethylsilylene- or tetramethyldisilylene-bridged mono(l-tetra -methylcyclopentadienyl), mono(l-indenyl) or mono(9-fluorenyl)-amidotita-nium complexes, such as dimethylsilylene(l-tetramethylcyclopentadienyl)(t-butyl)amidotitanium dichloride [Me2Si(Me4Cp)N(/-Bu)TiCl2] (Figure 3.10), have recently attracted both industrial and scientific interest as precursors for methylaluminoxane-activated catalysts, which polymerise ethylene and copolymerise ethylene with 1-butene, 1-hexene and 1-octene [30,105,148-152]. 

Most of the supported metallocene catalysts reported so far were devised to immobilise the metallocene on the surface of inorganic carriers, utilising the ionic interaction between the Cl ligands of the metallocene and the surface active site [schemes (19) to (21)]. Similarly, in the methylaluminoxane-pretreated catalyst, the metallocene is immobilised by an analogous ionic interaction [scheme (22)]. Therefore, it is obvious that catalyst precursors formed according to schemes (19) to (22) can be easily activated with common aluminium trialkyls. 

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 model would explain the inability of metallocene-alkylaluminium halide systems to promote the polymerisation of propylene and higher a-olefins [94] it is obvious that there is insufficient capability of the more weakly coordinating a-olefins to form reactive, olefin-separated ion pairs by displacement of an aluminate anion from the metal centre. At any rate, the limitation of homogeneous catalysts to the polymerisation of only ethylene was a crucial obstacle to progress in this field for many years. This impediment was fortunately overcome, however, by a series of serendipitous observations [90-95, 100,101,103] that led, around the 1980s, to the discovery by Kaminsky, Sinn et al. [90, 91,94,95,100,101] that metallocenes are activated for catalysing the polymerisation of propylene and other a-olefins (without a, a-disubstituted olefins) by methylaluminoxane [30],... 

One should assume here, in accordance with general opinion, that the productive reaction complex in methylaluminoxane-activated group 4 metallocene catalysts is the [Cp 2Mt(R).olefin]+[Alx(R) i. 1OxX2] complex, which is generated by displacement of an [A1x(R)a. 1OxX2] ion from its [Cp 2Mt(R)]+ counterion by the coordinating olefin molecule [30]. 

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]. 

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