Trialkylaluminium catalysts

A catalyst of aluminium and other metal alkyls in combination with metallic halides, e.g., the catalyst obtained by the interaction of a trialkylaluminium with titanium tetrachloride. Such catalysts, discovered by Karl Ziegler, are used in making stereospecific polymers. 

Sen reported that (C6F5)2AlR (2) (generated in situ) is an ethene polymerization catalyst (precursor) [13]. Moreover, the system also catalyzes copolymerization of ethene and propene. This latter fact, in particular, is remarkable, since a j5-branched alkyl (formed after propene insertion) should undergo very easy jS-elimination and hydrogen transfer to ethene, as discussed above. Thus, for this system to work the intrinsic chain transfer barriers of (QF5)2AlR should be much higher than those of trialkylaluminium. 

Exceptionally bulky methylaluminium bis(4-bromo-2,6-di-rerf-butyl)phenoxide is a useful catalyst in producing high stereoselectivity in the cycloaddition of trialkylsilylketene with aldehydes to give cis-3,4-disubstituted-2-oxetanones (95T4011). The complex formed from a trialkylaluminium and the chiral bissulfonamide (31)... 

Many of the catalysts obtained from such precursors and trialkylaluminium as the activator exhibit high activity in the polymerisation of ethylene, but are not, however, useful for the polymerisation of propylene and higher a-olefins. 

A new class of super highly active catalysts based on an MgCl2 support have been developed since the late 1970s. These catalysts are characterised by the appearance of surface-active species of practically one type. Their productivities are up to 2000 kg of polypropylene (with an isotactic index of 95-98 %) per gram Ti/h. In addition, lower Al/Ti ratios, i.e. ratios of the trialkylaluminium activator to titanium species, are required, and the usual reciprocal correlation between polypropylene isotactic index and yield is virtually absent [28,38],... 

It may be interesting that even a homogeneous zirconocene catalyst can be activated by common trialkylaluminium when trimethylsilanol (Mc2SiOH) is used as a modifier [210]. 

The first report on the coordination polymerisation of epoxide, leading to a stereoregular (isotactic) polymer, concerned the polymerisation of propylene oxide in the presence of a ferric chloride-propylene oxide catalyst the respective patent appeared in 1955 [13]. In this catalyst, which is referred to as the Pruitt Baggett adduct of the general formula Cl(C3H60)vFe(Cl)(0C3H6),CI, two substituents of the alcoholate type formed by the addition of propylene oxide to Fe Cl bonds and one chlorine atom at the iron atom are present [14]. A few years later, various types of catalyst effective for stereoselective polymerisation of propylene oxide were found and developed aluminium isopropoxide-zinc chloride [15], dialkylzinc-water [16], dialkylzinc alcohol [16], trialkylalumi-nium water [17] and trialkylaluminium-water acetylacetone [18] and trialkyla-luminium lanthanide triacetylacetonate H20 [19]. Other important catalysts for the stereoselective polymerisation of propylene oxide, such as bimetallic /1-oxoalkoxides of the [(R0)2A10]2Zn type, were obtained by condensation of zinc acetate with aluminium isopropoxide in a 1 2 molar ratio of reactants [20-22]. 

A wide range of acyclic trisubstituted enones readily undergo ECA with both commercially available trialkylaluminium reagents and the in situ-generated aryl (dialkyl)aluminium reagents. Very low catalyst loadings are sufficient (0.5-3.0 mol %) and products are formed in good yields (33-95%) and exceptional enantio-selectivities (80 to 99%) (Scheme 28) [65]. 

For both mechanisms of the coordination polymerization the polymer chain grows away from the surface of the catalyst as a result of attaching the next, pre-coordinated monomer molecules. The alkyl group, coming from trialkylaluminium or another metaloorganic cocatalyst becomes to be the final group of the polymer chain. 

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