Aluminum Alkyls in Ziegler-Natta Catalysts

Aluminum alkyls have been produced commercially since 1959 using technology originally licensed by Karl Ziegler. Aluminum alkyls are typically pyrophoric and explosively reactive with water (3, 9, 10). Considering such hazards, it is remarkable that thousands of tons of aluminum alkyls are produced each year and have been supplied for decades to the polyolefins industry worldwide with relatively few safety incidents. (However, see section 4.7.) 

Adding equations 4.1 and 4.2 gives the overall reaction for the direct process shown in eq 4.3  

However, the reaction shown in eq 4.3 does not take place in the absence of pre-formed triethylaluminum. 

Triethylaluminum has also been produced industrially by the so-called exchange process illustrated in eq 4.4 with triisobutylaluminum and ethylene  

Both the direct and exchange processes may be run continuously. However, economics favor the direct process and the direct product also has fewer contaminants. The exchange process is no longer used for triethylaluminum. 

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Since the alkyl ligand is incorporated in the final polymer produced, it seemed feasible that the polymerization system outlined above could be adapted to effect an anion to Ziegler-Natta transformation process in order to prepare novel block copolymers. Thus lithium living polymers could be used in lieu of lithium alkyls—such combinations, used to examine the effect of alkyl size on catalyst solubility, had already shown catalytic activity—and polymer-aluminum derivatives could be prepared to replace the small molecule aluminum alkyls in Ziegler-Natta catalyst combinations. The latter derivatives could be synthesized by successive alkylation of aluminum halides with living anionic polymers. ... 

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