Group Metal Alkyls

Finally, chain polymerisation can occur via coordination, as is the case for polymerisation involving Ziegler-Natta catalysts. These catalysts are complexes formed between main-group metal alkyls and transition metal salts. Typical components are shown in Table 2.1. 

Main group metal alkyls Transition metal salts... 

Main-group metal alkyls and aryls are valuable tools in preparative organic and inorganic chemistry, and many industrial applications are known for these reactive substances. In addition, they have attracted considerable attention from a more fundamental and theoretical point of view, because the high Lewis acidity at the metal center is the source of singular structures and bonding (1-3). 

Both homogeneous and heterogeneons catalysts are effective for this reaction. Homogeneons catalysts are nsnally composed of a transition metal compound combined with a main group metal alkyl cocatalyst, or they consist solely of a well-defined transition metal carbene complex. The most common transition metals nsed in these catalysts are Mo, W, and Re, although other metals from groups 4-9 have also been used. Literally thousands of different... 

The ease of reversal of alkene insertion is evident from the numerous syntheses of transition metal-hydride complexes using main group metal alkyls as the source of hydride. The hydride in the products of such reactions usually arises from -hydride abstraction or elimination from intermediate unstable transition metal alkyls. This idea is reinforced by the greater effectiveness of secondary alkyls such as isopropyl or cyclohexyl compounds. However, it has been shown that in at least one case the hydride results from hydrolysis of a Pt-Mg bond, not from the alkyl formed from reaction of a Pt-Cl bond with a Grignard reagent. Several of the reactions listed in Table 1 are spontaneously reversible. Reactions where -hydride elimination has been used in the synthesis of hydrides are listed in Table... 

Since the preparation of RBeOR from the reaction of BeR2 with R OH by Coates et al. (6), it was shown that reactions of metal alkoxides with organo-lithium or other main group metal alkyls, such as Grignard reagents or aluminium alkyls, can yield (4a) either substitution (Eq. 59) or addition products (Eqs. 60 and 61) the latter often results in formation of heterometallic alkoxides, for example,... 

Table 2.1 Components of typical Ziegler-Natta catalysts Main group metal alkyls Transition metal salts...

In contrast to the stable 6-Group metal alkyls, with their eompara-tiveljf strong metal-carbon bonds the transition metals do jiot form stable alkyl coinpounds Jaffe and have dis-... 

Bimolecular chain (polymeryl group) transfer to a Group 11-13 (main-group) metal alkyl component according to Eq. (3.4). 

Figure 3.9 Activation of a Group 4 metallocene precatalyst with a main-group metal-alkyl cocatalyst.

Ziegler Catalysts. For his work in the discovery of a new class of highly active catalysts for polymerization of ethylene, propylene, and dienes, Karl Ziegler shared the 1963 Nobel Prize in Chemistry with Guilio Natta whose contributions were predominantly related to polypropylene. Today, these catalysts together with the Phillips catalyst are responsible for the majority of the world s polyethylene production. Loosely defined, Ziegler catalysts are polyethylene catalysts derived from transition-metal halides and main group metal alkyls (46,50-53). In modem... 

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