Ziegler–Natta catalyst

A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:

Heterogeneous supported catalysts based on titanium compounds are used in polymerization reactions in combination with cocatalysts, organoaluminum compounds such as triethylaluminium, Al(C2H5)3. This class of catalyst dominates the industry. Homogeneous catalysts usually based on complexes of the group 4 metals titanium, zirconium or hafnium. They are usually used in combination with a different organoaluminum cocatalyst, methylaluminoxane (or methylalumoxane, MAO). These catalysts traditionally contain metallocenes but also feature multidentate oxygen- and nitrogen-based ligands. Ziegler–Natta catalysts are used to polymerize terminal alkenes (ethylene and alkenes with the vinyl double bond):

n CH2=CHR → −[CH2−CHR]n−;

History The 1963 Nobel Prize in Chemistry was awarded to German Karl Ziegler, for his discovery of first titanium-based catalysts, and Italian Giulio Natta, for using them to prepare stereoregular polymers from propylene. Ziegler–Natta catalysts have been used in the commercial manufacture of various polyolefins since 1956. As of 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these and related (especially Phillips) catalysts worldwide exceeds 100 million tonnes. Together, these polymers represent the largest-volume commodity plastics as well as the largest-volume commodity chemicals in the world. In the early 1950s workers at Phillips Petroleum discovered that chromium catalysts are highly effective for the low-temperature polymerization of ethylene, which launched major industrial technologies culminating in the Phillips catalyst. A few years later, Ziegler discovered that a combination of titanium tetrachloride (TiCl4) and diethylaluminium chloride (Al(C2H5)2Cl) gave comparable activities for the production of polyethylene. Natta used crystalline α-TiCl3 in combination with Al(C2H5)3 to produce first isotactic polypropylene. Usually Ziegler catalysts refer to titanium-based systems for conversions of ethylene and Ziegler–Natta catalysts refer to systems for conversions of propylene. Also, in the 1960s, BASF developed a gas-phase, mechanically-stirred polymerization process for making polypropylene. In that process, the particle bed in the reactor was either not fluidized or not fully fluidized. In 1968, the first gas-phase fluidized-bed polymerization process, the Unipol process, was commercialized by Union Carbide to produce polyethylene. In the mid-1980s, the Unipol process was further extended to produce polypropylene. In the 1970s, magnesium chloride (MgCl2) was discovered to greatly enhance the activity of the titanium-based catalysts. These catalysts were so active that the removal of unwanted amorphous polymer and residual titanium from the product (so-called deashing) was no longer necessary, enabling the commercialization of linear low-density polyethylene (LLDPE) resins and allowed the development of fully amorphous copolymers. The fluidized-bed process remains one of the two most widely used processes for producing polypropylene.

Stereochemistry of poly-1-alkenes Natta first used polymerization catalysts based on titanium chlorides to polymerize propylene and other 1-alkenes. He discovered that these polymers are crystalline materials and ascribed their crystallinity to a special feature of the polymer structure called stereoregularity.

The concept of stereoregularity in polymer chains is illustrated in the picture on the left with polypropylene. Stereoregular poly(1-alkene) can be isotactic or syndiotactic depending on the relative orientation of the alkyl groups in polymer chains consisting of units −[CH2−CHR]−, like the CH3 groups in the figure. In the isotactic polymers, all stereogenic centers CHR share the same configuration. The stereogenic centers in syndiotactic polymers alternate their relative configuration. A polymer that lacks any regular arrangement in the position of its alkyl substituents (R) is called atactic. Both isotactic and syndiotactic polypropylene are crystalline, whereas atactic polypropylene, which can also be prepared with special Ziegler–Natta catalysts, is amorphous. Ultimately, the stereoregularity of the polymer is determined by the catalyst used to prepare it. Most polymers produced with a Ziegler-Natta catalyst are enantioselective site-controlled, meaning the stereochemistry of each added monomer is dependent primarily on the stereochemistry of the catalyst (barring stereo errors) rather than the stereochemistry of the previous monomer. Thus, the chirality of the catalyst greatly influences tacticity. For example, achiral bis(2-phenylindenyl)zirconium dichloride produces isotactic polymers whereas its chiral variant produces syndiotactic polymers. The attachment of bulky ligands to the metal center results in steric hindrance, designed to restrict the orientation from which the incoming alkene can attack the metal center. Unique behaviors of the catalyst can also strongly impact tacticity. For example, the open site of VCl4/Al(C2H5)2Cl alternates between the axial and equatorial site with every monomer addition. As a result, each successive propylene monomer will insert into the vanadium-carbon bond from opposite sides. The alternating active site geometry thus produces a polymer chain with alternating enantiomeric configurations, producing syndiotactic polypropylene. See #Mechanism of Ziegler–Natta polymerization for the detailed mechanism.

Heterogeneous catalysts The first and dominant class of titanium-based catalysts (and some vanadium-based catalysts) for alkene polymerization can be roughly subdivided into two subclasses, both heterogeneous catalysts:

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