Aluminium trialkyls

Initiation of polymerisation is said to be effected by zinc diethyl-water and aluminium trialkyl-water-acetyl acetone systems to give the structures indicated in Figure 19.12. 

Aluminium trialkyls and triaryls are highly reactive, colourless, volatile liquids or low-melting solids which ignite spontaneously in air and react violently with water they should therefore be handled circumspectly and with... 

W. R. Kroll, W. Larbig, u. 0. W. Steudel Metallorganische Verbin-dungen, XXXII. Zerfalls- und Austauschreaktionen der Aluminium-trialkyle. Liebigs Ann. Chem. 629, 53 (1960). 

Aluminium tri-n-alkyls are dimeric in solution, although - especially for higher alkyls - the fraction of monomer can become significant at higher temperatures [ 19] in the gas phase they are usually monomeric. Kinetic evidence indicates that olefin insertion involves a monomeric aluminium trialkyl this suggests a Cossee-type insertion mechanism. Kinetic data do not indicate the presence of an intermediate olefin 7c-complex [23]. However, if the olefin complexation energy at the 7c-complex stage is low, this would be expected. 

Scheme 3.11 Formation of calcium-carbon bonds by reaction of a calcium amide with an aluminium trialkyl (R— SiMe,) ... 

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. 

The polymerisation of olefins by catalysts of the Ziegler-Natta type represents a most important example of the insertion reaction. The Ziegler catalyst formed from TiCl4 and aluminium trialkyl. These component may react in a following way... 

Many soluble catalysts are known which will polymerize ethylene and butadiene. High activity soluble catalysts are employed commercially for diene polymerization but most soluble types are inefficient for olefin polymerization. A few are crystalline and of known structure such as blue (7r-C5H5)2TiCl. AlEtaCl [49] and red [(tt-CsHs )2TiAlEt2 ] 2 [50]. The complex (tt-CsHs )2TiCl2. AlEt2Cl polymerizes ethylene rapidly but decomposes quickly to the much less active blue trivalent titanium complex. Soluble catalysts are obtained from titanium alkoxides or acetyl acetonates with aluminium trialkyls and these polymerize ethylene and butadiene. Several active species have been identified, dependent on the temperature of formation and the Al/Ti ratio. Reduction to the trivalent state is slow and incomplete and maximum activity for ethylene polymerization occurs at about 25% reduction to Ti [51]. 

Active catalysts for butadiene polymerization are obtained from aluminium alkyl halides and soluble Co and Co salts and complexes. The structure of the organic grouping attached to the cobalt is not important, but compounds most widely employed are acetylacetonates and carboxylic acid salts such as the octoate and naphthenate. The activity of the catalyst and structure of the polymer are affected by the groupings in the complex. Catalysts from aluminium trialkyls and cobalt salts other than halides are relatively unstable and give syndiotactic 1,2-polybutadiene. If halogens are present, e.g., from CoClj or CoBrj,... 

Earlier we noted that R3B compounds are monomeric. In contrast, aluminium trialkyls form dimers. Although this resembles the behaviour of the halides discussed in Section... 

Bridge-Terminal Exchange of Aluminium Trialkyl Dimers, O. Yamamoto, K. Hayamizu, and M. Yanagisawa, J. Organometal. Chem., 73, 17 (1974). 

Butadiene. Os-mm p(dymerization of liquid butadiene monomer k possible in the presence of H-form alurruno-silicate obtained 1 treating a mixture of mordenite and clinoptiolite with acid, a trialkyi alununium compound and nickel acetyl acetone. 8—12 mesh natural zeolites are converted into add form by treatment with 1 N HCl for SO hours at 70 °C. A mixtute containing about 0.1 g of this activated zeolite, 0.1 ml aluminium trialkyl in toluerK and 2 mg nickel acetyl acetone, on being heated for 1 hour at 70 C and finally cooled at —78 C produces polybutadiene at room temperature (c -l,4,94.5 %, cts-1,2,0— %at fnms-1,4, S.6 %). 

The cleavage of (212 R = COMe or COPh) by lithium dialkylcuprates has been developed into a preparative route for the synthesis of y6-unsaturated ketones in yields of 60— 98 %. The cleavage of cyclopropyl ketones by lithium dialkylcuprates can also be effected by aluminium trialkyls, with nickel acetylacetonate as catalyst. ... 

Substantially trafi5-1,4-polyisoprenes have been obtained with several catalyst combinations of which an excellent example is that obtained from an aluminium trialkyl (e.g. A1(/-C4H9)3) with vanadium trichloride. The products have a trans- content in the range 99-100%. The addition of a titanium compound of the type Ti(OR)4 is reported to improve the efficiency and increase the polymerization rate. Such a material may be considered as a synthetic gutta percha. 

These salts could be regarded as containing complexes between EtsAl and ethyl anions, and they can be made from EtsAl and ethyl-sodium or -potassium. Aluminium trialkyls also form anionic complexes with a range of other anions, notably hydride and halide ions. The decrease in bond strength M—A with increasing atomic weight of A, discussed in Chapter 1 (p 4), operates here also in the sense that the reaction... 

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