Editing Ziegler–Natta catalyst (section)

==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.