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A Ziegler-Natta catalyst is a reagent used in the production of unbranched, stereoregular vinyl polymers. Ziegler-Natta catalysts are typically based on titanium chlorides and organometallic alkyl aluminum compounds. Ziegler-Natta catalysts are used to polymerize terminal alkenes. Polymerization Reaction: n RCH=CH2 → -RCH-CH2n- Karl Ziegler, for his discovery of this titanium based catalyst, and Giulio Natta, for using it to prepare stereoregular polymers, were awarded the Nobel Prize in Chemistry in 1963. Stereochemistry of poly-1-alkenes The main polymer studied by Ziegler was polyethylene. Giulio Natta used this same system to polymerize prochiral 1-alkenes. Poly(1-alkene)s can be isotactic, syndiotactic, or atactic. In isotactic polymers all chiral centers share the same stereochemistry. Chiral centers in syndiotactic polymers alternate their relative stereochemistry. Atactic polymers lack regular stereochemistry. The stereoregularity depends on the catalyst. The Ziegler-Natta catalyst represented a major breakthrough in polymerization because it is highly stereoselective. Previously known free radical polymerization results in atactic polymers. TiCl4-derived systems3 convert propene, to isotactic polypropylene. Related systems employing VCl4 yield syndiotactic polymers. Preparation of the catalysts The Ziegler-Natta catalyst is synthesized by treating crystalline α-TiCl3 with AlCl(C2H5)22. Polymerization occurs 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 binds at these "vacancies" . In ways that are unclear, the alkene converts to an alkyl ligand group. The coordination sphere of the metal restricts the approach of incoming alkenes, thereby imposing stereoregularity on the growing polymer chain.4 The Cossee-Arlman mechanism describes the growth of stereospecific polymers.6: R2AlC2H5 + (n-1) CH2=CH2 → R2Al(CH2CH2)nH Termination occurs by β-hydride elimination, whereby a hydrogen is abstracted by the metal to leave a double bond at the end of the polymer chain, as seen by the following reaction6: LnTiCH2CH2R' → LnTiH + CH2=CHR' An alternative route to the catalyst entails the reaction of TiCl4 and AlEt32. The stereoselectivity of this catalytic system improves when the titanium-aluminum complex is supported on MgCl2. In order to maintain the high selectivity for an isotactic polymer product, a Lewis base must be used. To form this catalyst, the Lewis base and MgCl2 are milled together and mixed with a heptane solution containing TiCl4. The resulting solid is collected by filtration. The catalysis can then be carried out by adding this solid catalyst to a heptane solution saturated with the alkene of interest; the polymerization reaction is activated when AlEt3 is added, and the heptane solution is mildly heated. It should be noted that titanium(IV) chloride and alkyl aluminium compounds are unstable in air, the aluminium compounds even being pyrophoric. The catalyst, therefore, must be prepared 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 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 aluminum compound. Most commonly the polymer MAO or methylaluminoxane (CH3AlOn) 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.4 Activity and chain termination Activity depends on the nature of the metal. Ti, Zr, and Hf form highly active catalysts.5 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.5 By tuning, one can obtain "dial in" the molecular weight of the polymer product.8 For example, "half-sandich" zirconium species, tend to give low molecular weight polymers because of their enhanced tendency to undergo β-hydride elimination.8 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 aluminum 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)3.7 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 Compounds Created By This Catalyst | |||||||
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