Ziegler-Natta catalyst

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A Ziegler-Natta catalyst is a reagent used in the production of unbranched, stereoregular vinyl polymers. Ziegler-Natta catalysts are typically based on titanium chlorides and organometallic trialkyl aluminium compounds, for example (CH₃)₆Al₂.

Ziegler-Natta catalysts are used to polymerize terminal alkenes.

n RCH=CH₂ → -[RCH-CH₂]_n-

Karl Ziegler, for his discovery of this titanium based catalyst, and Giulio Natta, for using it to prepare stereoregular polymers, were awarded the Nobel Prize in Chemistry in 1963.

Contents 1 Stereochemistry of poly-1-alkenes 2 Preparation of the catalysts 3 Mechanism and the origin of stereospecificity 4 Activity and chain termination 5 Homogeneous Ziegler-Natta catalysts 6 Compounds Created By This Catalyst 7 References

[edit] Stereochemistry of poly-1-alkenes

The main polymer studied by Ziegler was polyethylene. Giulio Natta used this same system to polymerize prochiral 1-alkenes. Poly(1-alkene)s can be isotactic, syndiotactic, or atactic, depending on the relative orientation of the methyl groups. In isotactic polymers all chiral centers share the same stereochemistry. Chiral centers in syndiotactic polymers alternate their relative stereochemistry. Atactic polymers lack regular stereochemistry. The stereoregularity of the polymer depends on the catalyst used to prepare it, and once prepared, the polymer's stereochemistry does not change.

The Ziegler-Natta catalyst represented a major breakthrough in polymerization because can be highly stereoselective. Previously known free radical polymerization results in atactic polymers. TiCl₄-derived systems³ convert propene, to isotactic polypropylene. Related systems employing VCl₄ yield syndiotactic polymers.

[edit] Preparation of the catalysts

The Ziegler-Natta catalyst is synthesized by treating crystalline α-TiCl₃ with [AlCl(C₂H₅)₂]₂. Polymerization occurs at special Ti centers located on the exterior of the crystallites. Most titanium ions in these crystallites are surrounded by six chloride ligands to give an octahedral structure. At the surface, however "defects" occur where some Ti centers lack their full complement of chloride ligands. The alkene binds at these "vacancies" . In ways that are unclear, the alkene converts to an alkyl ligand group. The coordination sphere of the metal restricts the approach of incoming alkenes, thereby imposing stereoregularity on the growing polymer chain.⁴ The Cossee-Arlman mechanism describes the growth of stereospecific polymers.⁶:

R₂AlC₂H₅ + (n-1) CH₂=CH₂ → R₂Al(CH₂CH₂)_nH

Termination occurs by β-hydride elimination, whereby a hydrogen is abstracted by the metal to leave a double bond at the end of the polymer chain, as seen by the following reaction⁶:

L_nTiCH₂CH₂R' → L_nTiH + CH₂=CHR'

An alternative route to the catalyst entails the reaction of TiCl₄ and [AlEt₃]₂. The stereoselectivity of this catalytic system improves when the titanium-aluminium complex is supported on MgCl₂. In order to maintain the high selectivity for an isotactic polymer product, a Lewis base must be used. To form this catalyst, the Lewis base and MgCl₂ are milled together and mixed with a heptane solution containing TiCl₄. The resulting solid is collected by filtration. The catalysis can then be carried out by adding this solid catalyst to a heptane solution saturated with the alkene of interest; the polymerization reaction is activated when AlEt₃ is added, and the heptane solution is mildly heated.

It should be noted that titanium(IV) chloride and alkyl aluminium compounds are unstable in air, the aluminium compounds even being pyrophoric. The catalyst, therefore, must be prepared under an inert atmosphere.

[edit] Mechanism and the origin of stereospecificity

This stereoregularity is believed to follow from a polymer growth mechanism known as the Cossee-Arlman mechanism, in which the polymer grows at vacant Cl sites at the Ti surface.

In the search for a deeper understanding and control of Ziegler-Natta polymerisation at the molecular level, a number of metallocene catalysts have been developed, often offering fine control over the composition and tacticity of the polymer chain so produced. Other organometallic compounds that are capable of forming the same stereoregular polymers as the Ziegler-Natta TiCl₄ systems are metallocene compounds. One such compound is (Cp)₂TiCl₂; this compound does not have a vacant site like the TiCl₃ crystal, and as a result, must also be activated by an alkyl aluminium compound. Most commonly the polymer MAO or methylaluminoxane ([CH₃AlO]_n) is used as a cocatalyst. Like AlEt₃, it activates the transition metal complex by behaving as a Lewis Acid and abstracting one of the halides to create a vacancy where the alkene can be introduced to the complex.⁴

[edit] Activity and chain termination

Activity depends on the nature of the metal. Ti, Zr, and Hf form highly active catalysts.⁵ It is theorized that these catalysts feature d⁰ species. Without any d-electrons, the titanium-alkene bond is not stabilized by [pi backbonding]], so the barrier for alkene binding is decreased.

The length of a polymer chain is determined by two competing rate constants, the rate of chain propagation (transferring the alkene to the growing polymer chain) versus the rate of termination. Termination usually occurs by β-H elimination.⁵ By tuning, one can obtain "dial in" the molecular weight of the polymer product.⁸ For example, "half-sandwich" zirconium species, tend to give low molecular weight polymers because of their enhanced tendency to undergo β-hydride elimination.⁸

[edit] Homogeneous Ziegler-Natta catalysts

Significant effort has been dedicated to developing other catalysts that effectively polymerize a number of branched alkenes. In addition, there has been an interest in developing homogeneous Ziegler-Natta catalysts (that don't require the aluminium cocatalyst); these species are cationic and become active in solution by losing a labile ligand. One such catalyst is the agostic complex [Cp₂Zr(CH₃)CH₃B(C₆F₅)₃].⁷ The borate anion dissociates, leaving a vacant active site to bind alkene, allowing polymerization to commence. Developments have built upon advances in non-coordinating anions. In addition to those based on cyclopentadienyl ligands, catalysts are increasingly designed using nitrogen-based ligands.⁹

[edit] Compounds Created By This Catalyst

• Amorphous Poly-alpha-olefins (APAO)

[edit] References

1. ↑  Corradini, P.; Guerra, G.; Cavallo, L. "Do New Century Catalysts Unravel the Mechanism of Stereocontrol of Old Ziegler-Natta Catalysts?" Accounts of Chemical Research Vol. 37 (2004) pp. 231-241.
2. ↑  Takahashi, T. "Titanium(IV) Chloride-Triethylaluminum": Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons, Ltd, 2001.
3. ↑  Hill, A.F. Organotransition Metal Chemistry Wiley-InterScience: New York, 2002: pp. 136-139.
4. ↑  Bochmann, M. Organometallics 1, Complexes with Transition Metal-Carbon σ-Bonds Oxford University Press, New York, 1994: pp. 69-71.
5. ↑  Bochmann, M. Organometallics 2, Complexes with Transition Metal-Carbon π-Bonds Oxford University Press, New York, 1994: pp. 57-58.
6. ↑  Elschenbroich, C.; Salzer, A.; Organometallics: a concise Introduction VCH Verlagsgesellschaft mbH, New York, 1992, p. 423-425.
7. ↑  Fink, G.; Brintzinger, H.H.; Ziegler Catalysts Springer-Verlag, 1995, p. 161-164.
8. ↑  Alt, H.G.; Koppl, A.; "Effect of the Nature of Metallocene Complexes of Group IV Metals on Their Performance in Catalytic Ethylene and Propylene Polymerization" Chemical Reviews Vol. 100 (2000) pp. 1205-1221.
9. ↑  Britovsek, G. J. P.; Gibson V. C.; Wass, D. F. "The Search for New-Generation Olefin Polymerization Catalysts: Life Beyond Metallocenes" Angewandte Chemie, International Edition, 1999, volume 38, pages 428-447.