Reducing Agent for the Transition Metal

This function can be effectively illustrated with a catalyst synthesis used in an early commercial polypropylene process, now obsolete. The catalyst system employed ethylaluminum sesquichloride (EASC) for prereduction of TiCl in hexane (eq 4.5). EASC reduces the oxidation state of titanium and TiClj precipitates as the P (brown) form. Reduction is believed to proceed through an unstable alkylated TP species (eq 4.5) which decomposes to TP (eq 4.6). Lower oxidation states (Ti+ ) may also be formed. These reactions are exothermic and very fast. 

By-product ethylaluminum dichloride (EADC) is soluble in hexane, but is a poor cocatalyst. EADC must be removed (or converted to a more effective cocatalyst) before introduction of monomer. Eor example, ethylaluminum dichloride can be easily converted (as in eq 4.7) to diethylaluminum chloride by redistribution reaction with triethylaluminum (see reference 3 and literature cited therein for discussions of aluminum alkyl redistribution reactions). 

Aluminum alkyls are still used industrially for prereduction of transition metal compounds. However, far more is used in the role of cocatalyst, described in the next section. 

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Metal Alkyls in Polyethylene Catalyst Systems 49 4.2.1 Reducing Agent for the Transition Metal... 

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