Alkylating Agent for Creation of Active Centers

In this case, the aluminum alkyl is functioning as a cocatalyst, sometimes also called an activator. Titanium alkyls, believed to be active centers for polymerization, are created through transfer of an alkyl from aluminum to titanium, known as alkylation. Molar ratios of cocatalyst to transition metal (Al/Ti) are typically 30 for commercial polyethylene processes using Ziegler-Natta catalysts (lower ratios are used for polypropylene). The vast majority of aluminum alkyls sold into the polyethylene industry today is for use as cocatalysts. With TEAL, the most widely used cocatalyst, alkylation proceeds as in eq 4.8  

The titanium alkyl active center may be associated with (or stabilized by) an aluminum alkyl (see discussion on p. 41-42 and eq. 3.6). 

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