Alumoxane formation

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... 

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. 

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... 

BR with narrow MMDs (Mw/Mn > 3.5) and a low solution viscosity can also be obtained by the use of a multi-component catalyst system which comprises the following six components (1) Nd-salt, (2) additive for the improvement of Nd-solubility, (3) aluminum-based halide donor, (4) alumoxane, (5) aluminum (hydrido) alkyl, and (6) diene. The solubility of the Nd-salt is improved by acetylacetone, tetrahydrofuran, pyridine, N,N-dimethylformamide, thiophene, diphenylether, triethylamine, organo-phosphoric compounds and mono- or bivalent alcohols (component 2). The catalyst components are prereacted for at least 30 seconds at 20 - 80 °C. Catalyst aging is preferably performed in the presence of a small amount of diene [397,398 ]. As the additives employed for the increase of the solubility of Nd salts exhibit electron-donating properties it can be equally well speculated that poisoning of selective catalyst sites favors the formation of polymers with a low PDI. 

The synthesis of the first dinuclear alumoxane (4) containing a terminal hydroxide was accomplished (26) soon after the discovery of aluminum dihydroxide 3, just by changing the reaction stoichiometry (Scheme 3). It is assumed that excess LAII2 generates LAl(OH)2 and LAI (0H)NH2. Condensation of the two leads to the elimination of ammonia and the formation of the alumoxane dihydroxide [LAl(0H)]2([r-0). 

The alumoxane cocatalysts have at least two functions alkylation of the metallocene component, which takes place within seconds even at -60 °C (eq. (2)) and formation of the active species by abstraction of Me" (eq. (3)). The resulting active species is discussed as being a 14e (= 14 valence electron) cationic alkylmetallocenium ion formed by dissociation of the metallocene alumoxane complex [26,27]. The [alumoxane-Me]" anion is regarded as weakly or non-co-ordinating. Nearly every zirconocene atom is active, forming a single-site catalyst [28, 29]. 

On the other hand, the well-known 5-methylheptatrienyl-butadiene-cobalt(I) complex [Co( /, 7 -CH3C7Hio)( 7 -C4H6)] has been proved recently to be a very highly active catalyst for the formation of syndiotactic 1,2-polybutadiene [51]. The activity increases strongly with the acceptor properties of the solvent in the order heptane < toluene < dichloromethane < carbon disulfide, and can be extremely enhanced by the addition of alumoxane. 

R. L. Callender and A. R. Barron, Formation and evaluation of highly uniform aluminate interfece coatings for sapphire fiber reinforced ceramic matrix composites (FRCMCs) using carboxy late-alumoxane nanoparticles, J. Mater. Sci., 36 4977-A987 (2001). 

Erker, G Fritze, C. Selectivity control by temperature variation during formation of isotactic vs. syndiotactic polypropylene on a titanocene/alumoxane catalyst. Angew. Chem., Int. Ed. Engl. 1992, 31, 199-202. 

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