Methylaluminoxane MAO

the most widely used Lewis acid coinitiator (activator) (Eq. 8-52), is obtained by a controlled hydrolysis of trimethylaluminum (TMA). In spite of considerable research, the detailed structure of MAO remains unclear [Chen and Marks, 2000 Kissin and Brando-lini, 2003 Pedeutour et al., 2001 Wang et al., 2001 Ystenes et al., 2000]. MAO is probably a mixture of linear, cyclic, and three-dimensional structures containing the repeat unit XXXVII with n = 5-20. 

The exact composition in terms of the relative amounts of linear, cyclic, and three-dimensional structures and molecular weight probably varies with the detailed method of preparation. Most workers favor a three-dimensional spherical cagelike structure as the structure responsible for MAO s coinitiator property. However, this may be an oversimplification, and more than one structure may be responsible for the observed activation of metallocenes by MAO. After activation of a metallocene initiator, MAO forms the basis of the counterion, (ClMAO) or (CH3MAO). MAO normally contains TMA in two forms free TMA and 

MAO is needed in large excess relative to the metallocene initiator, usually IO2 IO4 1, to achieve high activities and stable kinetic profiles. MAO is usually added first in a polymerization system, and a portion may actually serve the function of destroying deleterious impurities prior to the addition of the metallocene initiator. Otherwise, the impurities would destroy the metallocene if the metallocene were added first. 

The structure of MAO is poorly defined and varies with preparation conditions, but it performs well in activating the metallocene initiator. The use of MAO is complicated by its lack of long-term storage stability. It is usually supplied by manufacturers as a cloudy solution of MAO in toluene MAO has very low solubility in aliphatic solvents. Precipitation is often observed on long standing, especially if the container is frequently opened and exposed to moisture and oxygen. This precipitation, if not too extensive, may not affect the utility of the MAO as a coinitiator. A modified MAO, known as MMAO, offers some improvement in storage stability and improved solubility in aliphatics. MMAO is prepared by controlled hydrolysis of a mixture of trimethylaluminum and triisobutylaluminum. 

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Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

In a typical polymerization, the immobilized precatalyst, toluene, and methylaluminoxane (MAO) at a ratio of 800 1 Al Ti were added to the reactor in a drybox. The mixture was allowed to stir for 20 minutes to allow for sufficient activation of the catalyst. The reactor was then removed from the glovebox, placed in a 25 C water bath, then ethylene at 60 psi was introduced. The polymerization was allowed to continue for 10 minutes, then terminated by adding acidic ethanol. The precipitated polymers were washed with ethanol, then dried at 70 C. 

Thus, in the presence of methylaluminoxane (MAO) at 23°C, (C5H5B-N(/-Pr)2)2 ZrCl2 polymerizes ethylene with an activity of 105 kg ofpolyethylene/(h [Zr] mol), similar to that observed with well-studied Cp2ZrCl2 as the catalyst. It is believed that MAO is functioning in its usual role in these Ziegler-Natta polymerizations (methylation of Zr and abstraction of methyl to form a highly reactive Zr cation).41... 

Figure 29 Structures of typical metallocenes and proposed structure of methylaluminoxane (MAO).

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]. 

Manufacturing systems, molecular, 17 58 MAO/metallocene ratio, 16 94. See also Methylaluminoxane (MAO)... 

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

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... 

These catalysts were used in combination with methylaluminoxane (MAO) for ethylene-norbornene co-polymerization and compared with isopropylidene[9-fluorenylcyclopentadienyl]zirconium dichloride catalyst activity under identical conditions. 

Substituents have a similar strong effect on the selectivity of dimerization induced by organiactinades (Th, U).533-535 In this case head-to-head or head-to-tail dimers may be formed. Methylaluminoxane (MAO), in turn, induces the selective formation of head-to-tail dimers in very high yields (>97%).536... 

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 a typical ethylene copolymerization condition, the comonomer (i.e. p-methylstyrene) was mixed with solvent (toluene or hexane) and methylaluminoxane (MAO) (30 wt% in toluene) needed in a Parr 450 ml stainless autoclave equipped with a mechanical stirrer. The sealed reactor was then saturated with 45 psi ethylene gas at 30 or 53 °C before... 

Somewhat greater success has been reported in respect of homo-oligo-merization of olefins, though once again reports are few, and focused predominantly on nickel. The complexes TpxNiCl (Tpx = TpMs 13, TpMs 14, TpMs 15 Ms = mesityl, Tpw = HB(pzN4s)2(pz5Ms), TpMs = HB(pzMs) (pz5 ) were screened in the presence of methylaluminoxane (MAO) cocatalyst at 0 °C under 30 bar ethylene. Under these conditions, both 13 and 14 showed appreciable activity, with very high selectivity for 1-butene (95-96%) within the C4 fraction, equivalent to 81% selectivity overall.13 In contrast, 15 was found to be completely inactive under these conditions. 

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]. 

Arndt, M. and Kaminsky, W., Microstructure of Poly(cycloolefin)s Produced by Metallocene/Methylaluminoxane (mao) Catalysts . Macromol. Symp., 97, 225-246 (1995). 

By changing the NHC ligands to NHCs possessing a hemilabile pyridine linkage, Jin and coworkers were able to use Ni(II) - NHC complexes as catalysts for the polymerization of norbornene and ethylene in the presence of methylaluminoxane (MAO) as a cocatalyst [47]. The Ni complexes were prepared via Scheme 7. Although the free carbenes of 16 could not be generated successfully, the desired Ni compounds (17) could be prepared via the... 

Because of their acceptable thermal stability ansa-metallocenes are suitable catalysts for high pressure polymerization at temperatures fairly above 100°C. In this investigation a modified silyl-bridged bis(tetrahydroindenyl)zirconocene in toluene as the solvent was used. The cocatalyst was methylaluminoxane (MAO) which was available in a 10 wt% solution also in toluene. The polymerization experiments were performed in a continuously operated laboratory unit equipped with a stirred autoclave (Figure 1). It is described in detail in [6],... 

Using separate prepared methylaluminoxane (MAO) as cocatalyst together with metallocenes for olefin polymerization Kaminsky, Sinn... 

An enormous increase (factor up to 1 million) in activity was found in 1975 at the University of Hamburg when water was added in a ratio of A1(CH3)3 H20 = 1 2 and, in 1977, using the isolated reaction product of methylaluminoxane (MAO) together with titanocenes and zirconocenes (Cp2Ti(CH3)2, Cp2Zr(CH3)2, Cp2ZrCl2) as catalysts for ethene polymerization [26,27]. In these combinations, metallocenes become more active than commercially used Ziegler catalysts. 

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],... 

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