Complexed aluminium alkyls

The use of aluminium ethereates often gives polymers with better properties than are obtained when using non-complexed aluminium alkyls and it has been conjectured (Natta and Porri, 1969) that this might be due to the fact that the polymers are less cyclized than those obtained from the use of non-complexed alkyls. 

It is however to be expected that these two methods mi t give rise to different values for C. Complexed aluminium alkyl compounds (with at least one polymeric chain) should not be displaced from the active centres by CO. Such compo mds would be quenched in the usual manner by a tritiated alcohol yielding polymer chains containing tritium. This situation is easily understood if the polymerization sequence (c.f. Tait kinetic model) detailed in Figure 9 is studied. 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

A process related to ionic polymerisation where the catalyst system complexes of aluminium alkyls and titanium halides governs the way in which a monomer and a growing chain approach each other. 

Aluminium alkyls act as the electron acceptor and the electron donor is titanium halides and the combination, therefore, readily forms coordination complexes. 

Natta s bimetallic mechanism stipulates that when the catalyst and cocatalyst components are mixed, the chemisorption of the aluminium alkyl (electropositive in nature) occurs on the titanium chloride solid surface which results in the formation of an electron-deficient bridge complex of the structure shown... 

Gibson described the synthesis of four-coordinate cationic aluminium alkyls 1 which were reported to be well-defined aluminium polymerization catalysts [12]. However, the polydispersities of the products obtained were high (2.9-6.3), showing that there is not a single well-defined active species. The experiments were carried out in metal autoclaves, and Fe and Co complexes of pyridine-diimine ligands are extremely active in ethene polymerization [34], so a transition-metal impurity does not seem an unreasonable explanation. 

Of course, it is dangerous to exclude the possibility of aluminium polymerization on the basis of calculations. Reality is invariably more complicated than the simplified models put into computers. However, in view of the uncertainties surrounding existing systems, and the doubts thrown by calculations, any well-defined aluminium alkyl claimed to be active should at least be checked, as an isolated complex, for its propensity to olefin insertion vs. chain transfer, e.g,. using the Al-i-butyl/ethene experiment reported by Jordan [15], as explained above. 

With j -branched aluminium alkyls like Al(i-Bu)3, reduction is often the main reaction [52]. Ligand modification can be used to increase the amount of reduction still further, and also to control the diastereoselectivity. In this respect, the phenoxide-modified complex 7 appears to be particularly effective [53]. A recurring problem in diastereoselective reductions is that the product can epimerize through MPV reduction (see next section) of the starting material [54]. The kinetics of this complicated system have been analyzed in terms of the iso-inversion principle [55]. 

Lewis-acid complexes 6 are formed upon addition of the aluminium alkyl to oxovanadium(v) species and are mildly active catalysts for homogeneous ethylene polymerization (16). System 6 appeared to be a promising candidate for an... 

The detail of the structure of the polymerisation centre present in suppported Ziegler-Natta catalysts for a-olefin polymerisation has been the subject of much research effort (e.g./-/2) The catalyst consists of a solid catalyst MgC /TiC /electron donor and a co-catalyst, an aluminium alkyl complexed with an electron donor. Proposed mechanisms for the polymerisation involve a titanium species attached to magnesium chloride with the olefin coordinated to titanium. The detail of the site at which the titanium species is attached is an important area of study in understanding the mechanism of catalysis and several recent papers 10-12) have investigated the surface structure of magnesium chloride and the attachment of TiCl4, in particular the interaction of titanium species with the 100 and 110 planes of a and (3- magnesium chloride. 

Aluminium, thallium, indium and lithium form complexes with alkyl or aryl halides, which are decomposed by water, with the formation of hydrocarbons. 

Stable dihydride complexes of Pd and Pt have been isolated from the reaction of tricyclohexylphosphine with the metal acetylacetonates and aluminium alkyls.28 The pale yellow [PdH2(PCy3)2] has a trans structure from i.r. and n.m.r. studies, with v(Pd—H) occurring at 1740 cm-1. 

Wilke and his co-workers have shown that zera-valent complexes, especially of nickel, obtained by reduction with aluminium alkyls can be used in a wide variety of polymerisations such as trimerisation of butadiene to trans, tran, trans-cyclododecatriene. 

Both its solubility and any potential side reaction with reductant dictate the choice of precursor metal salt or complex. In many cases the reductant is an aluminium alkyl or electropositive metal (Goups 1,2, 12 or 13, possibly amalgamated with mercury), requiring the use of anhydrous metal salts or complexes. Carbonyls of higher nuclearity are... 

Aluminium alkyls will polymerize ethene to polyethene, albeit inefficiently. During studies of this process, Ziegler found that a nickel contaminant resulted in the exclusive formation of butene, i.e. dimerization (see above). This prompted a study of the reactions of various transition metal complexes in combination with aluminium alkyls. Recall (Figure 4.6) that aluminium alkyls readily transfer their o-organyl groups to the more electronegative transition metals. Amongst these, titanium... 

A similar catalytic process (Dimersol, Institut Frangais du Petrole), based on a nickel hydride formed in situ from a nickel complex and an aluminium alkyl, has been applied industrially to oligomerize ethylene, propylene, butenes or mixtures of the three. 

Titanium tetrachloride and aluminium triethyl form a hydrocarbon soluble complex at low temperatures which decomposes at —30°C to give the trichloride as a major product [32]. Complexes containing tetravalent titanium stabilized by adsorption on titanium trichloride apparently persist in catalysts prepared at Al/Ti ratios below 1.0 [33], but at higher ratios there are some Ti(II) sites present in the catalyst [34]. Analysis shows that at Al/Ti ratios above 1.0 the solid precipitate contains divalent titanium or even lower valency states of the metal [35]. Reduction of TiCl4 with AlEt2 Cl is less rapid and extensive than with AlEts and even at high Al/Ti ratios [36] reduction does not proceed much below the trivalent state. Aluminium alkyl dihalides are still less reactive and reduction to TiClj is slow and incomplete except at high Al/Ti ratios or elevated temperatures [37]. 

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,... 

In this stoichiometric reaction sequence, a Poisson distribution of a-oletin products is obtained. The main disadvantage of this process is the large amount of aluminium alkyls needed in an industrial plant. To overcome this drawback, improvements of the process were developed by several companies. Only the two most important examples, the Gulf process and the Ethyl process, will be described in more detail. Shell developed a different route based on a nickel complex catalyst. Though other processes based on different transition metal catalysts have been developed, only the three processes mentioned above became important [16]. 

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