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, (C2H5)3Al.
Ziegler-Natta catalysts are used to polymerize terminal 1-alkenes.
- n CH2=CHR → -[CH2-CHR]n-
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 -[CH2-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. 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-TiCl3 with [AlCl(C2H5)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:
LnTi-CH2-CHR-Polymer + CH2=CHR → LnTi-CH2-CHR-CH2-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:
LnTi-CH2-CHR-Polymer + CH2=CHR → LnTi-CH2-CH2R + CH2=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:
LnTi-CH2-CHR-Polymer + H2 → LnTi-H + CH3-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 TiCl4, and the principal support is MgCl2. 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 TiCl4, MgCl2, and the Lewis base in a single solid pre-catalyst. The final catalyst system is prepared by combining this solid powder with AlEt3 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
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 aluminium compound. Most commonly the polymer MAO or methylaluminoxane ([CH3AlO]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.
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 d0 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 β-H elimination. By tuning, one can effectively "dial in" the molecular weight of the polymer product. For example, "half-sandwich" zirconium species, tend to give low molecular weight polymers because of their enhanced tendency to undergo β-hydride elimination.
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 [Cp2Zr(CH3)CH3B(C6F5)3]. 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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