Alkyls in Polyethylene Catalyst Systems

Though aluminum and magnesium alkyls are most important, lithium, boron and zinc alkyls are also useful in niche polyethylene applications. While in-depth discussions of production, properties and applications of metal alkyls are outside the scope of this book, key industrial methods for production of 

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

As mentioned in section 4.2.2, Ziegler-Natta catalyst systems used in the polyethylene industry typically employ high ratios of A1 to transition metal in the polymerization reactor. Ratios of 30 are common. Hence, there is a large excess of aluminum alkyl to achieve the roles depicted in sections 4.2.2 and 4.2.3 and to scavenge poisons. 

Phillips supported chromium (II) catalyst, the most commonly used for high density polyethylene (HDPE) manufacture, possibly behaves in a similar manner, but the olefin insertion reaction is faster by several orders of magnitude. In the original Zeigler catalyst systems for HDPE, an aluminium alkyl is used to reductively alkylate the primary component, most frequently a titanium compound, to give the true catalytic species. 

As shown above, the first commercial Ziegler catalyst for the manufacture of polyethylene utilized triethyl aluminum (TEAL). However, early research involved extensive examination of various types of other metal alkyls that were used with titanium tetrachloride to promote ethylene polymerization. Some of these catalyst systems are summarized in Table 2.2. 

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