Early Commercial Ziegler-Natta Catalysts

Titanium tetrachloride was the logical choice as the raw material for early Ziegler-Natta catalysts. TiCl is a clear, colorless, hygroscopic liquid that fumes upon exposure to ambient air. TiCl (aka tickle 4 ) was (and still is) manufactured in enormous volumes as a precursor to titanium dioxide used as a pigment for paint. Consequently, TiCl was readily available and relatively inexpensive. Also, TiCl had been shown by Ziegler and coworkers to produce some of the most active polyethylene catalysts. Though sometimes called a catalyst, it is more accurate to call TiCl a precatalyst, since it must be reduced and combined with a cocatalyst to become active. 

Early (1960-1965) commercial Ziegler-Natta catalysts were produced by reduction of TiCl using metallic aluminum, hydrogen or an aluminum alkyl. Simplified overall equations are listed below  

TiCl + /3 A1 TiCIj /3 AICI3 [also written (TiCl3)3 AlCl3] 

The predominant product in each case was titanium trichloride (aka tickle 3 ), an active catalyst for olefin polymerization. The preferred cocatalyst was diethyl-aluminum chloride (DEAC). TiCl from eq 3.1 contains co-crystallized aluminum trichloride. TiCl from eq 3.3 may contain small amounts of complexed aluminum alkyl. Products from eq 3.1 and 3.2 were supplied commercially by companies such as Stauffer Chemical and Dart (both now defunct). Catalyst from eq 3.3 was manufactured on site by polyolefin producers, usually in an inert hydrocarbon such as hexane. 

TiClj is a highly colored solid that exists in several crystalline forms, designated as alpha (a), beta (p), gamma (y) and delta (S). The a, y and S forms have layered crystal structures and are violet. The P form has a linear structure and is brown. 

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Currently, most of the commercial iPP is made using fourth-generation Ziegler-Natta catalysts with different internal and external donors that increase the productivity to 25 kg polypropylene/g catalyst and isotacticity from 95% to 99%. Shell Oil Co. and Montedison started commercializing these catalytic systems in the early 1980s. Shell reported the use of benzoic acid as internal and external donors, and Montedison utilized alkyl phthalates and silyl ethers as internal and external donors, respectively [3]. 

Ziegler-Natta polymerization is used extensively for the polymerization of simple olefins (e.g. ethene, propene and 1-butene) and is the focus of much academic attention, as even small improvements to a commercial process operated on this scale can be important. Ziegler-Natta catalyst systems, which in general are early transition metal compounds used in conjunction vyith alkylaluminum compounds, lend themselves to study in the chloroaluminate(iii) ionic liquids, especially those with an acidic composition. 

Alkene polymerization is one of the most important catalytic reactions in commercial use. The Ziegler-Natta catalysts, for which Ziegler and Natta won the Nobel Prize in 1963, account for some 15 million tonnes of polyethylene and polypropylene annually. These catalysts are rather similar to the metathesis catalysts in that mixtures of alkylaluminum reagents and high-valent early metal complexes are used. The best known is TiCl3/Et2AlCl, which is active at 25°C and 1 atm this contrasts with the severe conditions required for thermal polymerization (200 C, 1000 atm). Not only are the conditions milder, but the product shows much less branching than in the... 

Natta and Ziegler found early on that vanadium halides could be treated with aluminum alkyls to form catalysts competent for alkene polymerizations. A variety of simple precursors in various oxidation states, for example, VCI3, VCI4, and VOCI3. can be treated with aluminum alkyl halides to produce active catalysts. Unlike the titanium and chromium systems, the vanadium catalysts can be single sited with narrow MWDs and more importantly narrow composition distributions. Also unlike the other systems, vanadium incorporates a-olefms at rates somewhat slower but comparable to ethylene. For this reason, vanadium catalysts are used commercially to make EPMs and EPDMs that can have at least 50 mol.% ethylene with the balance being predominantly other olefins and much smaller amounts of diolefins (Figure 9). 

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