Alumoxane

The metallocene catalyst is used m combination with a promoter usually methyl alumoxane (MAO)... 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

The detailed mechanism of the catalysis is not known, but it is believed that the Lewis acid character of the zirconium is critical.223 The reaction is further accelerated by inclusion of partially hydrolyzed trialkylaluminum reagents known as alumoxanes.224... 

Sinn H, Kaminsky W, Hoker H (eds) (1995) Alumoxanes, macromolecular symposia 97. Huthig Wepf, Heidelberg... 

Poly (acetyl acetonate alumoxane) results from they hydrolysis of dialkoxyaluminium acetyl acetonate ... 

Barron, A. R. Alkyl Alumoxanes Synthesis, Structure and Reactivity. In Metallocene-based Polyolefins-, Scheirs, J., Kaminsky, W., Eds. Wiley Chichester, 2000 Vol. 1, pp 33-67. 

The controlled hydrolysis of aluminum alkyl and aryl compounds yields oligomeric species of the formula (RA10) . They are called alumoxanes. Alkyl substituted alu-... 

The first step in the hydrolysis reaction is the formation of the water adduct [Eq. (23)] [69], which subsequently eliminates alkane to yield the hydroxide R2A10H. At elevated temperatures, further alkane elimination is observed with the formation of alkyl-alumoxane [67, 68]. Structurally characterized alumoxanes are listed in... 

The most frequent structures of organo-alumoxanes are hexameric cage structures (41-45) [67] (Figure 3.5-11) (Table 3.5-3). The Al O core can be described as a drum-like hexagonal prism with alternating A1 and O atoms. The AI3O3 hexago-... 

Ewen was the first to report the synthesis of stereoregular propene polymers with soluble Group 4 metal complexes and alumoxane as the co-catalyst [13], He found that Cp2TiPh2 with alumoxane and propene gives isotactic polypropene. This catalyst does not contain an asymmetric site that would be able to control the stereoregularity. A stereo-block-polymer is obtained, see Figure 10.6. Formation of this sequence of regular blocks is taken as a proof for the chain-end control mechanism. 

The interaction may not be quite as strong as in the case of 2,1 insertion discussed above, but there will always be a tendency of the growing chain to arrive at an isotactic stereochemistry when 1,2 insertion occurs. One example of chain-end control leading to isotactic polymer was reported by Ewen [13] using Cp2TiPh2/alumoxane as the catalyst. The stereoregularity increased with lower temperatures at -45 °C the isotactic index as measured on pentads amounted to 52 %. The polymer contains stereoblocks of isotactic polymer. At 25 °C the polymerisation gives almost random 1,2 insertion and an atactic polymer is formed. 

Above we mentioned the results reported by Ewen [13] who found that Cp2TiPh2/alumoxane gives a polypropene with isotactic stereoblocks. Naturally, this achiral catalyst can only give chain-end control as it lacks the necessary chiral centre for site control. In the 13C NMR the stereoblocks can be clearly observed as they lead to the typical 1 1 ratio of mmmr and mmrm absorptions in addition to the main peak of mmmm pentads. These are two simple examples showing how the analysis of the 13C NMR spectra can be used for the determination of the most likely mechanism of control of the stereochemistry. Obviously, further details can be obtained from the statistical analysis of the spectra and very neat examples are known [18],... 

As already pointed out, recent advances in the synthesis and structural characterisation of discrete molecular alumoxanes have lately been the subject of review [8]. While developments in the crystallisation and structural... 

An unusual and complicated array of heterocycles results from the inclusion of hydroxide moieties into a similar ferf-butylaluminium oxide system to [(f-Bu)A10]8 [50]. In this context, the product which results from the hydrolysis of [(f-Bu)Al(/T3-0)]6 [24] - [(f-Bu)Al]6(/T3-0)4(/r3-0H)4 - is best viewed as comprising an octahedron of aluminium centres each face of which is /X3-capped by either an oxide or a hydroxide group (Fig. 11) [63]. This species was the first to exhibit penta-coordinate Al-centres in an alumoxane context. Moreover, the polyhedral architecture incorporated an interstitial void which, it was suggested, might facilitate the formation of inclusion complexes. The predilection for (AlO) (n = 2) metallocycles does not hold for the tetracyclic array of n = 3 rings displayed by the mixed oxide-hydroxide... 

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