Ziegler–Natta catalyst

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:

• Solid and supported catalysts based on titanium compounds. They are used in polymerization reactions in combination with cocatalysts, organoaluminum compounds such as triethylaluminium, Al(C₂H₅)₃.
• Metallocene catalysts, combination of various mono- and bis-metallocene, in particular ansa- (or bridged) metallocene complexes of Ti, Zr or Hf. They are usually used in polymerization reactions in combination with a different organoaluminum cocatalyst, methylaluminoxane (or methylalumoxane, MAO).
• Post-metallocene catalysts based on complexes of various transition metals with multidentate oxygen- and nitrogen-based ligands. These complexes are also activated with MAO.

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.

Stereochemistry of poly-1-alkenes

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.

Short segments of polypropylene, showing examples of isotactic (above) and syndiotactic (below) tacticity.

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.

Classes of Ziegler–Natta catalysts

Catalysts derived from titanium chlorides

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]

Metallocene catalysts

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]

Non-metallocene catalysts

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.

Chemistry of the polymerization reactions

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

L_nTi–CH₂−CHR–Polymer + CH₂=CHR →

L_nTi–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:

L_nTi–CH₂-CHR–Polymer + H₂ → L_nTi-H + CH₃-CHR–Polymer

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.

Commercial polymers prepared with Ziegler–Natta catalysts

• Polyethylene
• Polypropylene
• Copolymers of ethylene and 1-alkenes
• Polybutene-1
• Polymethylpentene
• Polycycloolefins
• Polybutadiene
• Polyisoprene
• Amorphous poly-alpha-olefins (APAO)
• Polyacetylene

References

• R. Hoff, R. T. Mathers, eds. Handbook of Transition Metal Polymerization Catalysts" Wiley, 2010
• B. A. Krentsel, Y. V. Kissin, V. I. Kleiner, S. S. Stotskaya Polymers and Copolymers of Higher a-Olefins, Hanser Publishers, 1997.
• Y. V. Kissin Alkene Polymerization Reactions with Transition Metal Catalysts, Elsevier: Amsterdam, 2008.
• G. Natta, F. Danusso, eds. Stereoregular Polymers and Stereospecific Polymerizations, Pergamon Press, 1967.
• P. Corradini, G. Guerra and L. Cavallo (2004). "Do New Century Catalysts Unravel the Mechanism of Stereocontrol of Old Ziegler-Natta Catalysts?". Acc. Chem. Res. 37 (4): 231–241. doi:10.1021/ar030165n. PMID 15096060. 
• Takahashi, T. "Titanium(IV) Chloride-Triethylaluminum": Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons, Ltd, 2001.
• G. J. P. Britovsek, V. C. Gibson and D. F. Wass (1999). "The Search for New-Generation Olefin Polymerization Catalysts: Life beyond Metallocenes". Angewandte Chemie International Edition 38 (4): 428–447. doi:10.1002/(SICI)1521-3773(19990215)38:4<428::AID-ANIE428>3.0.CO;2–3. 

1. ^ ^{a b c} Hill, A.F. Organotransition Metal Chemistry Wiley-InterScience: New York, 2002: pp. 136–139.
2. ^ ^{a b c d e} Kissin, Y.V. Alkene Polymerization Reactions with Transition Metal Catalysts" Elsevier: Amsterdam, 2008; Chapter 4.
3. ^ Bochmann, M. Organometallics 1, Complexes with Transition Metal-Carbon σ-Bonds Oxford University Press, New York, 1994: pp. 69–71.
4. ^ Bochmann, M. Organometallics 2, Complexes with Transition Metal-Carbon π-Bonds Oxford University Press, New York, 1994: pp. 57–58.
5. ^ H. G. Alt and A. Koppl (2000). "Effect of the Nature of Metallocene Complexes of Group IV Metals on Their Performance in Catalytic Ethylene and Propylene Polymerization". Chem. Rev. 100 (4): 1205–1222. doi:10.1021/cr9804700. PMID 11749264. 
6. ^ Reference 2, Chapter 6.
7. ^ Elschenbroich, C.; Salzer, A.; Organometallics: a Concise Introduction VCH Verlagsgesellschaft mbH, New York, 1992, p. 423-425.