Editing Ziegler–Natta catalyst

{{Short description|Catalyst for synthesis of polymers of 1-alkenes}}
A '''Ziegler–Natta catalyst''', named after [[Karl Ziegler]] and [[Giulio Natta]], is a [[catalyst]] used in the synthesis of [[polymers]] of 1-alkenes ([[alpha-olefin]]s). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility:
* Heterogeneous [[Catalyst support|supported catalysts]] based on titanium compounds are used in polymerization reactions in combination with cocatalysts, [[organoaluminum]] compounds such as [[triethylaluminium]], Al(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>. This class of catalyst dominates the industry.<ref>{{cite encyclopedia|encyclopedia=Kirk-Othmer Encyclopedia of Chemical Technology|publisher=Wiley-VCH
|author1=Giuliano Cecchin |author2=Giampiero Morini |author3=Fabrizio Piemontesi |title=Ziegler–Natta Catalysts |year=2003 |doi=10.1002/0471238961.2609050703050303.a01 |isbn=0471238961}}</ref>
* Homogeneous catalysts usually based on complexes of the [[group 4 elements|group 4 metals]] [[titanium]], [[zirconium]] or [[hafnium]]. They are usually used in combination with a different organoaluminum cocatalyst, [[methylaluminoxane]] (or methylalumoxane, MAO). These catalysts traditionally contain [[metallocene]]s but also feature [[multidentate]] oxygen- and nitrogen-based [[ligands]].<ref name=Handbook>{{cite book|title=Handbook of Transition Metal Polymerization Catalysts|editor1-first=Ray|editor1-last=Hoff|editor2-first=Robert T.|editor2-last=Mathers|date=2010|publisher=John Wiley & Sons |edition=Online |isbn=9780470504437|doi=10.1002/9780470504437}}</ref>

Ziegler–Natta catalysts are used to polymerize terminal [[alkene]]s (ethylene and alkenes with the [[vinyl group|vinyl]] double bond):
:''n''&nbsp;CH<sub>2</sub>=CHR → −[CH<sub>2</sub>−CHR]<sub>''n''</sub>−;

==History==
The 1963 [[Nobel Prize in Chemistry]] was awarded to German [[Karl Ziegler]], for his discovery of first titanium-based catalysts, and Italian [[Giulio Natta]], for using them to prepare stereoregular polymers from [[propylene]]. Ziegler–Natta catalysts have been used in the commercial manufacture of various polyolefins since 1956. As of 2010, the total volume of plastics, elastomers, and rubbers produced from alkenes with these and related (especially Phillips) catalysts worldwide exceeds 100 million tonnes. Together, these polymers represent the largest-volume commodity plastics as well as the largest-volume commodity chemicals in the world.

In the early 1950s workers at [[Phillips Petroleum]] discovered that chromium catalysts are highly effective for the low-temperature polymerization of ethylene, which launched major industrial technologies culminating in the [[Phillips catalyst]]. A few years later, Ziegler discovered that a combination of [[titanium tetrachloride]] (TiCl<sub>4</sub>) and [[diethylaluminium chloride]] (Al(C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>Cl) gave comparable activities for the production of polyethylene. Natta used crystalline [[Titanium trichloride|α-TiCl<sub>3</sub>]] in combination with [[triethylaluminium|Al(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>]] to produce first [[isotactic]] [[polypropylene]].<ref name=nattadanusso>{{cite book|editor1-first=G. |editor1-last=Natta|editor2-first= F.|editor2-last= Danusso |title=Stereoregular Polymers and Stereospecific Polymerizations|publisher= Pergamon Press|date= 1967}}</ref> Usually Ziegler catalysts refer to [[titanium]]-based systems for conversions of [[ethylene]] and Ziegler–Natta catalysts refer to systems for conversions of [[propylene]]. 

Also, in the 1960s, [[BASF]] developed a gas-phase, mechanically-stirred [[polymerization]] process for making [[polypropylene]]. In that process, the particle bed in the reactor was either not fluidized or not fully fluidized. In 1968, the first gas-phase fluidized-bed polymerization process, the Unipol process, was commercialized by [[Union Carbide]] to produce polyethylene. In the mid-1980s, the Unipol process was further extended to produce [[polypropylene]].

In the 1970s, [[magnesium chloride]] (MgCl<sub>2</sub>) was discovered to greatly enhance the activity of the titanium-based catalysts. These catalysts were so active that the removal of unwanted amorphous polymer and residual titanium from the product (so-called deashing) was no longer necessary, enabling the commercialization of [[linear low-density polyethylene]] (LLDPE) resins and allowed the development of fully amorphous copolymers.<ref>{{cite encyclopedia |encyclopedia=Handbook of Transition Metal Polymerization Catalysts |publisher=John Wiley & Sons |date=2010 |editor1-last=Hoff |editor1-first=Ray |edition=Online |pages=131–155 |doi=10.1002/9780470504437.ch6 |isbn=9780470504437 |last2=Mink |first2=R. I. |last3=Kissin |first3=Y. V. |first1=T. E. |last1=Nowlin |editor2-first=Robert T. |editor2-last=Mathers |chapter=Supported Magnesium/Titanium-Based Ziegler Catalysts for Production of Polyethylene}}</ref>

The fluidized-bed process remains one of the two most widely used processes for producing [[polypropylene]].<ref name="Technology Economics Program">{{cite book |url=http://www.magcloud.com/browse/issue/411008 |title=Polypropylene Production via Gas Phase Process, Technology Economics Program |date=2012 |publisher=Intratec |isbn=978-0-615-66694-5}}</ref>

==Stereochemistry of poly-1-alkenes==
Natta first used 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]]. 
[[Image:Polypropylene tacticity.svg|thumb|upright=1.8|left|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 on the left 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 −[CH<sub>2</sub>−CHR]−, like the CH<sub>3</sub> 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==
===Heterogeneous catalysts===
{{main|Heterogeneous catalysis}}
The first and dominant class of titanium-based catalysts (and some [[vanadium]]-based catalysts) for alkene polymerization can be roughly subdivided into two subclasses:
* catalysts suitable for homopolymerization of ethylene and for ethylene/1-alkene [[copolymer]]ization reactions leading to copolymers with a low 1-alkene content, 2–4&nbsp;mol% ([[LLDPE]] resins), and
* 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.

Commercial catalysts are supported by being bound to a solid with a high surface area. Both [[titanium tetrachloride|TiCl<sub>4</sub>]] and [[titanium trichloride|TiCl<sub>3</sub>]] give active catalysts.<ref name="ref1">{{cite book|last=Hill|first= A. F. |title=Organotransition Metal Chemistry |publisher=Wiley-InterScience|location= New York|date= 2002| pages= 136–139}}</ref><ref name="ref2">{{cite book|last=Kissin|first= Y. V. |title=Alkene Polymerization Reactions with Transition Metal Catalysts |publisher=Elsevier|location= Amsterdam|date= 2008 |chapter=Chapter 4}}</ref> The support in the majority of the catalysts is [[magnesium chloride|MgCl<sub>2</sub>]]. 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&nbsp;mm. During the catalyst synthesis, both the titanium compounds and MgCl<sub>2</sub> are packed into the silica pores. All these catalysts are activated with organoaluminum compounds such as [[Triethylaluminium|Al(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>]].<ref name="ref2"/>

All modern supported Ziegler–Natta catalysts designed for polymerization of propylene and higher 1-alkenes are prepared with [[titanium tetrachloride|TiCl<sub>4</sub>]] as the active ingredient and [[magnesium chloride|MgCl<sub>2</sub>]] as a support. Another component of all such catalysts is an organic modifier, usually an [[ester]] of an [[Aromatic acid|aromatic diacid]] or a [[ether|diether]]. The modifiers react both with inorganic ingredients of the solid catalysts as well as with organoaluminum cocatalysts.<ref name="ref2"/> These catalysts polymerize propylene and other 1-alkenes to highly crystalline isotactic polymers.<ref name="ref1"/><ref name="ref2"/>

===Homogeneous catalysts===
A second class of Ziegler–Natta catalysts are soluble in the reaction medium. Traditionally such homogeneous catalysts were derived from [[metallocene]]s, but the structures of active catalysts have been significantly broadened to include nitrogen-based ligands.
[[File:VersifyCats.png|thumb|A post-metallocene catalyst developed at [[Dow Chemical]].<ref name=Klosin>{{cite journal |author1=Klosin, J.|author2=Fontaine, P. P.|author3=Figueroa, R. |title=Development of Group Iv Molecular Catalysts for High Temperature Ethylene-Α-Olefin Copolymerization Reactions|journal=Accounts of Chemical Research|year=2015|volume=48|issue=7 |pages=2004–2016|doi=10.1021/acs.accounts.5b00065|pmid=26151395|doi-access=free}}</ref>]]

====Metallocene catalysts====
{{main|Kaminsky catalyst}}
These catalysts are metallocenes together with a cocatalyst, typically MAO, −[O−Al(CH<sub>3</sub>)]<sub>''n''</sub>−. The idealized metallocene catalysts have the composition [[Cyclopentadienyl|Cp]]<sub>2</sub>MCl<sub>2</sub> (M = Ti, [[zirconium|Zr]], [[hafnium|Hf]]) such as [[titanocene dichloride]]. Typically, the organic ligands are derivatives of [[cyclopentadienyl]]. In some complexes, the two [[cyclopentadiene]] (Cp) rings are linked with bridges, like −CH<sub>2</sub>−CH<sub>2</sub>− or >SiPh<sub>2</sub>. Depending on the type of their cyclopentadienyl ligands, for example by using an [[Ansa-metallocene|''ansa''-bridge]], metallocene catalysts can produce either isotactic or syndiotactic polymers of propylene and other 1-alkenes.<ref name="ref1"/><ref name="ref2"/><ref>{{cite book|last=Bochmann|first=M. |title=Organometallics 1, Complexes with Transition Metal-Carbon σ-Bonds |publisher=Oxford University Press|location=New York |date=1994| pages=69–71|isbn=9780198558132}}</ref><ref>{{cite journal |first1=H. G. |last1=Alt |first2=A. |last2=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 |pmid=11749264}}</ref>

====Non-metallocene catalysts====
{{main|Post-metallocene catalyst}}
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]] (O<sub>2</sub>), [[nitrogen]] (N<sub>2</sub>), [[phosphorus]] (P), and [[sulfur]] (S). 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.

==Mechanism of Ziegler–Natta polymerization==
The structure of active centers in Ziegler–Natta catalysts is well established only for metallocene catalysts. An idealized and simplified metallocene complex Cp<sub>2</sub>ZrCl<sub>2</sub> represents a typical precatalyst.  It is unreactive toward alkenes.  The dihalide reacts with MAO and is transformed into a metallocenium ion Cp<sub>2</sub>{{overset|+|Zr}}CH<sub>3</sub>, which is ion-paired to some derivative(s) of MAO. A polymer molecule grows by numerous insertion reactions of C=C bonds of 1-alkene molecules into the Zr–C bond in the ion:
[[File:ZNonSingleSite.png|thumb|480 px|center|Simplified mechanism for Zr-catalyzed ethylene polymerization.]]
Many thousands of alkene insertion reactions occur at each active center resulting in the formation of long polymer chains attached to the center.  The [[Cossee–Arlman mechanism]] describes the growth of stereospecific polymers.<ref name=nattadanusso/><ref>{{cite book|last1=Elschenbroich|first1= C.|last2= Salzer|first2= A.|title=Organometallics: a Concise Introduction|publisher= VCH Verlag|location= New York |date=1992|pages=423–425}}</ref> This mechanism states that the polymer grows through alkene coordination at a vacant site at the titanium atom, which is followed by insertion of the C=C bond into the Ti−C bond at the active center.

===Termination processes===
On occasion, the polymer chain is disengaged from the active centers in the chain termination reaction.  Several pathways exist for termination:
:Cp<sub>2</sub>{{overset|+|Zr}}−(CH<sub>2</sub>−CHR)<sub>''n''</sub>−CH<sub>3</sub> + CH<sub>2</sub>=CHR → Cp<sub>2</sub>{{overset|+|Zr}}−CH<sub>2</sub>−CH<sub>2</sub>R + CH<sub>2</sub>=CR–polymer

Another type of chain termination reaction called a β-hydride elimination reaction also occurs periodically: 
:Cp<sub>2</sub>{{overset|+|Zr}}−(CH<sub>2</sub>−CHR)<sub>n</sub>−CH<sub>3</sub> → Cp<sub>2</sub>{{overset|+|Zr}}−H + CH<sub>2</sub>=CR–polymer

Polymerization reactions of alkenes with solid titanium-based catalysts occur at special titanium 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<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 two chain termination reactions occur 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<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

Another termination process involves the action of protic (acidic) reagents, which can be intentionally added or adventitious.

==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==
{{reflist|30em}}

==Further reading==<!-- these refs should be brought in-line as time permits-->
* {{cite book | first = Y. V. | last = Kissin | title = Alkene Polymerization Reactions with Transition Metal Catalysts | publisher = Elsevier | location = Amsterdam | date = 2008}}
* {{cite journal | first1 = P. | last1 = Corradini | first2 = G. | last2 = Guerra | first3 = L. | last3 = 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 | pmid = 15096060}}
* {{cite encyclopedia | last = Takahashi | first = T. | title = Titanium(IV) Chloride-Triethylaluminum | encyclopedia = Encyclopedia of Reagents for Organic Synthesis | publisher = John Wiley & Sons | date = 2001}}
* {{cite journal | first1 = G. J. P. | last1 = Britovsek | first2 = V. C. | last2 = Gibson | first3 = D. F. | last3 = Wass | title = The Search for New-Generation Olefin Polymerization Catalysts: Life beyond Metallocenes | year = 1999 | journal = [[Angewandte Chemie|Angew. Chem. Int. Ed.]] | volume = 38 | issue = 4 | pages = 428–447 | doi = 10.1002/(SICI)1521-3773(19990215)38:4<428::AID-ANIE428>3.0.CO;2-3| pmid = 29711786}}

{{Organometallics}}

{{DEFAULTSORT:Ziegler-Natta catalyst}}
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