A Ziegler–Natta catalyst is a catalyst used in the synthesis of polymers of 1-alkenes (α-olefins). Three types of Ziegler–Natta catalysts are currently employed:
Ziegler–Natta catalysts are used to polymerize terminal 1-alkenes (ethylene and alkenes with the vinyl double bond):
n CH2=CHR → −[CH2−CHR]n−
German Karl Ziegler, for his discovery of first titanium-based catalysts, and Italian Giulio Natta, for using them to prepare stereoregular polymers, were awarded the Nobel Prize in Chemistry in 1963. Ziegler–Natta catalysts have been used in the commercial manufacture of various polymeric materials since 1956. In 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these catalysts worldwide exceeds 100 million metric tons. Together, these polymers represent the largest-volume commodity plastics as well as the largest-volume commodity chemicals in the world.
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Giulio Natta used first 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 above 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. The stereoregularity of the polymer is determined by the catalyst used to prepare it.
The first class of modern Ti-based catalysts (and some V-based catalysts) for alkene polymerization can be roughly subdivided into two subclasses, (a) catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene copolymerization reactions leading to copolymers with a low 1-alkene content, 2–4 mol.% (LLDPE resins), and (b) catalysts suitable for the synthesis of isotactic 1-alkenes. The overlap between these two subclasses is relatively small because the requirements to the respective catalysts differ widely.
Ziegler discovered the first catalyst suitable for polymerization of ethylene into a linear, highly crystalline polymer. This catalyst was a combination of TiCl4 and Al(C2H5)2Cl. Modern commercial catalysts of this type are nearly all supported, i.e. bound to a solid with a high surface area. Both TiCl4 and TiCl3 give active catalysts.[1][2] The support in the majority of the catalysts is MgCl2. A third component of most catalysts is a carrier, a material that determines the size and the shape of catalyst particles. The preferred carrier is microporous spheres of amorphous silica with a diameter of 30–40 mm. During the catalyst synthesis, both the Ti compounds and MgCl2 are packed into the silica pores. All these catalysts are activated with organoaluminum compounds such as Al(C2H5)3.[2]
Natta used crystalline α-TiCl3 in combination with Al(C2H5)3 to produce first isotactic polypropylene. All modern supported Ziegler–Natta catalysts designed for polymerization of propylene and higher 1-alkenes are prepared with TiCl4 as the active ingredient and MgCl2 as a support. The third component of all such catalysts is an organic modifier, usually an ester of an aromatic diacid or a diether. The modifiers react both with inorganic ingredients of the solid catalysts as well as with organoaluminum cocatalysts.[2] These catalysts polymerize propylene and other 1-alkenes to highly crystalline isotactic polymers.,[1][2]
The second class of Ziegler–Natta catalysts are metallocenes. They are soluble in aromatic hydrocarbons and usually contain two components, a metallocene complex and a special organometallic cocatalyst, MAO, [−O–Al–CH(CH3-)n. The idealized metallocene catalysts have the composition Cp2MCl2 (M = Ti, Zr, Hf) such as titanocene dichloride. Typically, the organic ligands are derivatives of cyclopentadienyl. In some complexes, the two Cp rings are linked with bridges, like −CH2−CH2− or >SiPh2.,,[1][2][3]
Depending of the type of their cyclopentadienyl ligands, metallocene catalysts can produce either isotactic or syndiotactic polymers of propylene and other 1-alkenes.,[4][5]
Ziegler–Natta catalysts of the third class, non-metallocene catalysts, use a variety of complexes of various metals, ranging from scandium to lanthanoid and actinoid metals, and a large variety of ligands containing oxygen, nitrogen, phosphorus, and sulfur. The complexes are activated using MAO, as is done for metallocene catalysts.
Most Ziegler–Natta catalysts and all the alkylaluminium cocatalysts are unstable in air, and the alkylaluminium compounds are pyrophoric. The catalysts, therefore, are always prepared and handled under an inert atmosphere.
The structure of active centers in Ziegler–Natta catalysts is firmly established only for metallocene catalysts. A metallocene complex Cp2ZrCl2 reacts with MAO and is transformed into a metallocenium ion Cp2Zr+-CH3. A polymer molecule grows in length by numerous insertion reactions of C=C bonds of 1-alkene molecules into the Zr–C bond in the ion:
Cp2Zr+−CH3 + n CH2=CHR → Cp2Zr+−(CH2−CHR)n−CH3
Many thousands of alkene insertion reactions occur at each active center resulting in the formation of long polymer chains attached to the center. On occasion, the polymer chain is disengaged from the active centers in the chain termination reaction:
Cp2Zr+−(CH2−CHR)n−CH3 + CH2=CHR → Cp2Zr+−CH2−CH2R + CH2=CR–Polymer
Another type of chain termination reaction called β-hydrogen elimination reaction also occurs periodically:
Cp2Zr+−(CH2−CHR)n−CH3 → Cp2Zr+−H + CH2=CR–Polymer
Polymerization reactions of alkene with solid Ti-based catalysts occur at special Ti centers located on the exterior of the catalyst crystallites. Some titanium atoms in these crystallites react with organoaluminum cocatalysts with the formation of Ti–C bonds. The polymerization reaction of alkenes occurs similarly to the reactions in metallocene catalysts
LnTi–CH2-CHR–CH2−CHR–Polymer
The two chain termination reactions occurs quite rarely in Ziegler–Natta catalysis and the formed polymers have a too high molecular weight to be of commercial use. To reduce the molecular weight, hydrogen is added to the polymerization reaction:
The Cossee-Arlman mechanism describes the growth of stereospecific polymers,.[6][7] This mechanism states that the polymer grows through alkene coordination at a vacant site at the Ti atom, which is followed by insertion of the C=C bond into the Ti-C bond at the active center.
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