Ziegler-Natta Catalysis of Alkene Polymerization

The product of the following reaction was isolated in 99% yield. What is it  

Ring-opening metathesis is the converse of ring-closing metathesis and holds promise as a polymerization method. It is applied most often when ring opening is accompanied by relief of strain as in, for example, bicyclic alkenes. 

In Section 6.14 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.16 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregular polymer with superior physical properties. Because the catalysts responsible for coordination polymerization are organometallic compounds, we are now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

In the early 1950s, Karl Ziegler, then at the Max Planck Institute for Coal Research in Germany, was studying the use of aluminum compounds as catalysts for the oligomerization of ethylene. 

The earliest Ziegler-Natta catalysts were combinations of titanium tetrachloride (TiCl4) and diethylaluminum chloride [(CH3CH2)2A1C1], but these have given way to more 

Not all ligands use just two electrons to bond to transition metals. Chromium has the electron configuration [Arjd SrC (6 valence electrons) and needs 12 more to satisfy the 18-electron rule. In the compound (benzene)tricarbonylchromium, 6 of these 12 are the IT electrons of the benzene ring the remaining 6 are from the three carbonyl ligands. 

Ferrocene has an even more interesting structure. A central iron is Tr-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes 5 electrons for a total of 10 and iron, with an electron configuration of [Ar]4y 3d contributes 8. Alternatively, ferrocene can be viewed as being derived from Fe (6 valence electrons) and two aromatic cyclopentadienide rings (6 electrons each). Indeed, ferrocene was first prepared by adding iron(II) chloride to cyclopentadienylsodium. Instead of the expected o-bonded species shown in the equation, ferrocene was formed. 

After ferrocene, a large number of related molecules have been prepared—even some in which uranium is the metal. There is now an entire subset of transition-metal organometallic complexes known as metallocenes based on cyclopentadieifide ligands. These compounds are not only structurally interesting, but many of them have useful applications as catalysts for industrial processes. 

The first page of this chapter displayed an electrostatic potential map of ferrocene. You may wish to view a molecular model of it on Learning By Modeling. 

Cyclopentadienylsodium is ionic. Its anion is the cyclopentadienide ion, which contains six n electrons. 

Hundreds of analogs of have been prepared and evaluated as catalysts for eth- 

The metallocene catalyst is used in combination with a promoter, usually methyl-alumoxane (MAO). 

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An Organometallic Com pound That Occurs Naturally Coenzyme B,2 591 Organocopper Reagents 592 Palladium-Catalyzed Cross-Coupling 595 Homogeneous Catalytic Hydrogenation 597 Olefin Metathesis 600 Ziegler-Natta Catalysis of Alkene Polymerization 603 Summary 606 Problems 608... 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

One of the great discoveries of organomctallic chemistry was the catalyzed polymerization of alkenes at atmospheric pressure and ambient temperature. Vast quantities of polyethylene and polypropylene (over 15 million tons annually) are made by Ziegler-Natta catalysis. Ziegler and Natta received the Nobel prize in chemistry in 1963, and the importance of their work w stimulating interest in organometallic chemistry should not be underestimated. 

The discovery of the stereoregular polymerization of alkenes by Ziegler-Natta catalysis opened a possible route to optically active polymers by a suitable modification of the catalyst. Indeed, Natta, in 1961, succeeded in polymerizing ben-zofurane 2 (Scheme 2) under the influence of a catalyst obtained by a combining of AICI3 and phenylalanine. Optical activity was detected for the polymer [15]. This reaction seems the first example of homogeneous asymmetric catalysis by a metal complex, however it is difficult to estimate the efficiency of the process from the specific rotation of the polymer. The asymmetric polymerization of... 

The 1963 the Nobel Prize in Chemistry was awarded to Karl Ziegler and Giulio Natta for their discoveries in the field of the chemistry and technology of high polymers . The polymerization of alkenes by heterogeneous Ziegler-Natta catalysis is of vast importance to the polymer industry. In... 

Alkenes. —Reviews on Ziegler-Natta catalysis and the stereoregular and sequence-regular polymerization of butadiene have been published and the stereoselective oligomerizations of isoprene by lithium and palladium catalysts have been compared. Semi-empirical MO calculations suggest that Ziegler-Natta polymerization proceeds via a bis-alkene complex and a metallacyclo-pentane intermediate. ... 

Ziegler-Natta type catalysis is one of two methods used commercially to produce high density polyethylene, the other being metal oxide catalysis. Ziegler-Natta catalysis is very flexible the variety of catalyst systems that fall into this family is immense. In addition to ethylene, many other alkenes may also be polymerized, to produce either homopolymers when reacted in isolation or copolymers when... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

Applications of HT-type catalysts, prepared by the above methods, have been reported in recent years for basic catalysis (polymerization of alkene oxides, aldol condensation), steam reforming of methane or naphtha, CO hydrogenation as in methanol and higher-alcohol synthesis, conversion of syngas to alkanes and alkenes, hydrogenation of nitrobenzene, oxidation reactions, and as a support for Ziegler-Natta catalysts (Table 2). 

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