Ziegler-Natta catalyst

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 alkyl aluminum compounds.

Ziegler-Natta catalysts are used to polymerize terminal alkenes.

Polymerization Reaction:

n 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.

Stereochemistry of poly-1-alkenes

The first polymer mainly studied by Ziegler was polyethylene, wherein the product lacks chiral centers. Giulio Natta used this catalytic system to polymerize prochiral 1-alkenes. Polymerization of 1-alkenes can proceed with various stereoregularity depending on the alkene and the catalyst. Poly(1-alkene)s can be isotactic, syndiotactic, or atactic. Isotactic polymers occur when all chiral centers share the same stereochemistry. Chiral centers in syndiotactic polymers alternate their relative stereochemistry. Atactic polymers lack regular stereochemistry.

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

Preparation of the catalysts

The Ziegler-Natta catalyst is synthesized by treating crystalline α-TiCl₃ with AlCl(Et)₂. The titanium metal forms a crystal structure where all titanium ions are surrounded by 6 chloride ions in an octahedral structure. At all edges of the crystal, however, there are Cl vacancies where the metal has open bonding positions. One of the vacancies can be filled by donation of electrons of one of the CH₂ groups in AlCl(Et)₂. The last vacant site can be filled by a pi electron donor (an alkene). The transition metal's ligands restrict the incoming alkene's position with respect to the growing polymer chain to impose stereoregularity.⁴ Through a series of steps that include electron shifting and migration, the Cossee-Arlman mechanism describes the growth of stereospecific polymers.

The insertion of new alkyl groups into the polymer chain occurs at the transition metal, and the polymer grows and stays bonded to the alkyl aluminum cocatalyst as seen by this growth reaction⁶:

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

Termination occurs by β-H elimination, where a hydrogen is abstracted by the metal from the terminal carbon in the polymer to leave a double bond at the end of the polymer chain, as seen by the following reaction⁶:

R₂AlCH₂CH₂R' → R₂AlH + CH₂=CHR'

An alternative route to form this catalyst uses TiCl₄ and AlEt₃. Titanium(IV) chloride and triethylaluminium are supplied in solution and are pyrophoric in air and sensitive to water. The catalyst, therefore, must be prepared under an inert atmosphere. This catalytic system has proven more stereoselective when the titanium-aluminum 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 purified simply 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 to the heptane solution and mildly heated.

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 aluminum compound. Most commonly the polymer MAO or methylaluminoxane (^*_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

Activity and chain termination

Activity depends on the nature of the metal. Of titanium's column in the periodic table, titanium is the most active catalyst, followed far behind by hafnium and the zirconium. Titanium, though, is highly active in polymerization only in its Ti(IV) oxidation state⁵, which is a d⁰ state. Without any d-electrons, the titanium-alkene bond is not stabilized by backbonding, so the barrier to react and add to the growing polymer chain is decreased, increasing the activity of the catalyst.

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 These two rate constants dictate the effectiveness of a catalytic system at creating long polymers. Since the discovery to Ziegler-Natta catalysts, researchers have made effors to tune these two rate constants by studying related systems to obtain high and low molecular weight polymers.⁸ For example, Zirconium species, particularly "half-sandwich" metallocene catalysts, are known to form low molecular weight species because of the lower activity of zirconium-based catalysts and the promotion of β-H elimination by the labile C-Zr bond.⁸ High molecular weight polymers form when bulky ligands surround the transition metal.

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 aluminum cocatalyst); these species are cationic and become active in solution by losing a labile ligand. One such catalyst is ^*.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.

References

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.
Takahashi, T. "Titanium(IV) Chloride-Triethylaluminum": Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons, Ltd, 2001.
Hill, A.F. Organotransition Metal Chemistry Wiley-InterScience: New York, 2002: pp. 136-139.
Bochmann, M. Organometallics 1, Complexes with Transition Metal-Carbon σ-Bonds Oxford University Press, New York, 1994: pp. 69-71.
Bochmann, M. Organometallics 2, Complexes with Transition Metal-Carbon π-Bonds Oxford University Press, New York, 1994: pp. 57-58.
Elschenbroich, C.; Salzer, A.; Organometallics: a concise Introduction VCH Verlagsgesellschaft mbH, New York, 1992, p. 423-425.
Fink, G.; Brintzinger, H.H.; Ziegler Catalysts Springer-Verlag, 1995, p. 161-164.
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.

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