Everything about Ziegler-natta Catalyst totally explained
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 aluminium compounds, for example
triethylaluminium, (C
2H
5)
3Al.
Ziegler-Natta catalysts are used to polymerize terminal 1-
alkenes.
» n CH
2=CHR → -[CH
2-CHR]
n-
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
2-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 doesn't 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.
TiCl4-derived catalyst systems,, convert
propylene, and many other 1-alkenes, to isotactic polymers such as
polypropylene. Related systems employing
VCl4 yield syndiotactic polymers.
Preparation of the catalysts
The first Ziegler-Natta catalyst was produced by treating crystalline α-TiCl
3 with [AlCl(C
2H
5)
2]
2. 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
nTi-CH
2-CHR-Polymer + CH
2=CHR → L
nTi-CH
2-CHR-CH
2-CHR-Polymer
The
coordination sphere of the Ti atom restricts the approach of incoming alkene molecules, thereby imposing
stereoregularity on the growing polymer chain.
4 The
Cossee-Arlman mechanism describes the growth of stereospecific polymers..
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
nTi-CH
2-CHR-Polymer + CH
2=CHR → L
nTi-CH
2-CH
2R + CH
2=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
nTi-CH
2-CHR-Polymer + H
2 → L
nTi-H + CH
3-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
4, and the principal support is MgCl
2. 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
4, MgCl
2, and the Lewis base in a single solid pre-catalyst. The final catalyst system is prepared by combining this solid powder with AlEt
3 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.
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
4 systems are metallocene compounds. One such compound is (Cp)
2TiCl
2; this compound doesn't have a vacant site like the TiCl
3 crystal, and as a result, must also be activated by an alkyl aluminium compound. Most commonly the polymer
MAO or methylaluminoxane ([CH
3AlO]
n) is used as a
cocatalyst. Like AlEt
3, 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.
Activity and chain termination
Activity depends on the nature of the metal. Ti, Zr, and Hf form highly active catalysts. It is theorized that these catalysts feature d
0 species. Without any d-electrons, the titanium-alkene bond isn't 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. For example, "half-
sandwich" zirconium species, tend to give low molecular weight polymers because of their enhanced tendency to undergo β-hydride elimination. 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
Polymers prepared by Ziegler-Natta catalysts
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