View source for Ziegler-Natta catalyst



A '''Ziegler-Natta catalyst''' is a [[reagent]] or a mixture of reagents used in the production of [[polymers]] of 1-alkenes (α-olefins). Ziegler-Natta catalysts are typically based on titanium compounds and [[organometallic chemistry|organometallic]]  aluminium compounds, for example [[triethylaluminium]], (C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>Al.

Ziegler-Natta catalysts are used to polymerize terminal 1-[[alkene]]s.
:n CH<sub>2</sub>=CHR  →  -[CH<sub>2</sub>-CHR]<sub>n</sub>-

[[Karl Ziegler]], for his discovery of these titanium based catalysts, and [[Giulio Natta]], for using them to prepare stereoregular polymers, were awarded the Nobel Prize in Chemistry in 1963.

==Stereochemistry of poly-1-alkenes==
Karl Ziegler prepared linear [[polyethylene]] with the catalyst he discovered. Giulio Natta used similar catalysts to polymerize  1-alkenes. Poly(1-alkene)s can be [[isotactic]], [[syndiotactic]], or [[atactic]], depending on the relative orientation of the [[alkyl]] groups in polymer chains consisting of units -[CH<sub>2</sub>-CHR]-.  In isotactic polymers, all chiral centers CHR 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 type of catalyst used to prepare it, and once prepared, the polymer's stereochemistry does not change.

The Ziegler-Natta catalysts represented a major breakthrough in polymerization chemistry because they produce a variety of commercially important polymers and can be highly stereoselective.  Previously known radical polymerization reactions result in the formation of atactic polymers.  [[Titanium tetrachloride|TiCl<sub>4</sub>]]-derived catalyst systems <ref>Hill, A.F. ''Organotransition Metal Chemistry'' Wiley-InterScience: New York, 2002: pp. 136-139.</ref>,<ref>Kissin, Y.V. ''Alkene Polymerization Reactions with Transition Metal Catalysts" Elsevier: Amsterdam, 2008; Chapter 4.</ref>,    convert [[propylene]], and many other 1-alkenes, to isotactic polymers such as [[polypropylene]].  Related systems employing [[VCl4|VCl<sub>4</sub>]] yield syndiotactic polymers.

==Preparation of the catalysts==
The first Ziegler-Natta catalyst was produced by treating crystalline α-TiCl<sub>3</sub> with [AlCl(C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>]<sub>2</sub>.  Polymerization reactions of any alkene occur 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 molecule binds at these "vacancies" .  In ways that are still not fully clear, the alkene converts to an alkyl ligand group. The most probable pathway of this reaction is the insertion of the C=C bond of the alkene molecule into the Ti-C bond: 

L<sub>n</sub>Ti-CH<sub>2</sub>-CHR-Polymer + CH<sub>2</sub>=CHR → L<sub>n</sub>Ti-CH<sub>2</sub>-CHR-CH<sub>2</sub>-CHR-Polymer 

The [[coordination sphere]] of the Ti atom restricts the approach of incoming alkene molecules, thereby imposing [[stereoregularity]] on the growing polymer chain.<sup>4</sup>  The [[Cossee-Arlman mechanism]] describes the growth of stereospecific polymers.<ref name="ref6">Elschenbroich, C.; Salzer, A.; ''Organometallics: a concise Introduction'' VCH Verlagsgesellschaft mbH, New York, '''1992''', p. 423-425.</ref>.

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 chains are disengaged from the active centers in the reaction:

L<sub>n</sub>Ti-CH<sub>2</sub>-CHR-Polymer + CH<sub>2</sub>=CHR → L<sub>n</sub>Ti-CH<sub>2</sub>-CH<sub>2</sub>R + CH<sub>2</sub>=CR-Polymer

This reaction occurs quite rarely 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<sub>n</sub>Ti-CH<sub>2</sub>-CHR-Polymer + H<sub>2</sub> → L<sub>n</sub>Ti-H + CH<sub>3</sub>-CHR-Polymer

During the past 40 years, a large number of different supported Ziegler-Natta catalysts were developed which afford a much higher activity in alkene polymerization reactions and much higher contents of crystalline isotactic fractions in the polymers they produce, up to 97-99%.  The principal source of Ti in all these catalysts is TiCl<sub>4</sub>, and the principal support is MgCl<sub>2</sub>.  In order to maintain the high selectivity for an isotactic polymer product, a variety of catalyst modifiers,[[Lewis bases]], must be used.  To form these catalysts, several techniques were developed for combining TiCl<sub>4</sub>, MgCl<sub>2</sub>, and the Lewis base in a single solid pre-catalyst.  The final catalyst system is prepared by combining this solid powder with AlEt<sub>3</sub> and another Lewis base compound. 

It should be noted that titanium(IV) chloride, all solid Ziegler-Natta catalysts and alkyl aluminium compounds are unstable in air, and the alkylaluminium compounds are [[pyrophoric]].  The catalysts, therefore, must be prepared and handled under an inert atmosphere.

==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. 

[[Image:ZNCosseeArlman.PNG|500px|Tebbe Resonance Structure]]  

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 TiCl<sub>4</sub> systems are metallocene compounds.  One such compound is (Cp)<sub>2</sub>TiCl<sub>2</sub>; this compound does not have a vacant site like the TiCl<sub>3</sub> crystal, and as a result, must also be activated by an alkyl aluminium compound.  Most commonly the polymer [[Methylaluminoxane|MAO]] or methylaluminoxane ([CH<sub>3</sub>AlO]<sub>n</sub>) is used as a [[cocatalyst]].  Like AlEt<sub>3</sub>, 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.<ref>Bochmann, M. ''Organometallics 1, Complexes with Transition Metal-Carbon σ-Bonds'' Oxford University Press, New York, 1994: pp. 69-71.</ref>

==Activity and chain termination==
Activity depends on the nature of the metal.  Ti, Zr, and Hf form highly active catalysts.<ref name="ref5">Bochmann, M. ''Organometallics 2, Complexes with Transition Metal-Carbon π-Bonds'' Oxford University Press, New York, 1994: pp. 57-58.</ref> It is theorized that these catalysts feature d<sup>0</sup> 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 [[Beta-hydride elimination|β-H elimination]].<ref name="ref5" /> By tuning, one can effectively "dial in" the molecular weight of the polymer product.<ref name="ref8">{{cite journal | author = H. G. Alt and A. Koppl | title = Effect of the Nature of Metallocene Complexes of Group IV Metals on Their Performance in Catalytic Ethylene and Propylene Polymerization | year = 2000 | journal = [[Chem. Rev.]] | volume = 100 | issue = 4 | pages = 1205-1222 | doi=10.1021/cr9804700}}</ref> For example, "half-[[sandwich compound|sandwich]]" zirconium species, tend to give low molecular weight polymers because of their enhanced tendency to undergo β-hydride elimination.<ref name="ref8" />

==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]] [Cp<sub>2</sub>Zr(CH<sub>3</sub>)CH<sub>3</sub>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>].<ref>Fink, G.; Brintzinger, H.H.; ''Ziegler Catalysts'' Springer-Verlag, '''1995''', p. 161-164.</ref> 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.<sup>9</sup>

==Polymers prepared by Ziegler-Natta catalysts==
*[[Polypropylene]]
*Amorphous Poly-alpha-olefins ([[APAO]])
*[[Polyvinyl alcohol]]

==References==
{{refbegin}}
* {{cite journal | author = P. Corradini, G. Guerra and L. Cavallo | title = Do New Century Catalysts Unravel the Mechanism of Stereocontrol of Old Ziegler-Natta Catalysts? | year = 2004 | journal = [[Acc. Chem. Res.]] | volume = 37 | issue = 4 | pages = 231-241 | doi = 10.1021/ar030165n }}
* Takahashi, T. "Titanium(IV) Chloride-Triethylaluminum": ''Encyclopedia of Reagents for Organic Synthesis.'' John Wiley & Sons, Ltd, 2001.
* {{cite journal | author = G. J. P. Britovsek, V. C. Gibson and D. F. Wass | title = The Search for New-Generation Olefin Polymerization Catalysts: Life beyond Metallocenes | year = 1999 | journal = [[Angewandte Chemie International Edition]] | volume = 38 | issue = 4 | pages = 428-447 | doi = 10.1002/(SICI)1521-3773(19990215)38:4<428::AID-ANIE428>3.0.CO;2-3 | doilabel = 10.1002/(SICI)1521-3773(19990215)38:4%3C428::AID-ANIE428%3E3.0.CO;2-3}}
{{refend}}
{{reflist|2}}

{{Organometallics}}

<!-- Categories -->
[[Category:Industrial processes]]
[[Category:polymer chemistry]]
[[Category:Catalysts]]
[[Category:Coordination compounds]]

[[de:Ziegler-Natta-Katalysator]]
[[es:Catalizador Ziegler-Natta]]
[[fa:کاتالیزگر زیگلر-ناتا]]
[[it:Catalizzatori di Ziegler-Natta]]
[[nl:Ziegler-Natta-katalysator]]
[[ja:チーグラー・ナッタ触媒]]
[[pl:Katalizatory Zieglera-Natty]]
[[pt:Catalisador Ziegler-Natta]]
[[zh:齐格勒-纳塔催化剂]]

{{WH}}
{{WS}}