Ziegler-Natta polymerization of alkenes

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

One such process is the Cossee-Arlman mechanism,proposed for the Ziegler-Natta polymerization of alkenes (also discussed in Section 14-4-1). According to this mechanism, a polymer chain can grow as a consequence of repeated 1,2 insertions into a vacant coordination site, as follows ... 

Ziegler-Natta polymerization of alkenes is an important industrial process for the manufacture of polyolefins. Although it originally involved the use of the triethylaluminum-TiCft complex as the catalysts, many other transition metal complexes and /-block compounds (lanthanides) also catalyze the polymerization of alkenes. Group IV metallocenes exhibit particularly outstanding properties. 

As with other transition metal-catalyzed reactions (Ziegler-Natta polymerization of alkenes, olefin metathesis), the mechanism of the Heck reaction is complicated. In brief, the species that reacts with the aryl halide is I Pd, where L is a ligand such as tiiphenylphosphine. By a process known as oxidative addition, palladium inserts into the carbon-halogen bond of the aryl halide. 

The Cossee-Arlman mechanism proposed for the Ziegler-Natta polymerization of alkenes. 

The four membered titanacyclobutane ring is nearly planar. Studies of Tebbe s compound are also of interest in connection with Ziegler-Natta polymerization of alkenes. Commercial catalysts incorporate TiCl and aluminium alkyls (p. 371). 

Ziegler-Natta polymerization of alkenes nCH2=CHR -[CH2-CHR] - Stereoregular polymer Complex of TiClj and AKC2HO,... 

For Ziegler-Natta polymerization of alkenes, second-order deactivation decay explains much better experimental data than the first-order decay. Such behavior in supported systems was attributed to simultaneous deactivation of adjacent catalytic species on the surface. In homogeneous polymerization with metallocene catalysts (Fig. 9.53), a metallocene complex Cp2ZrCl2 reacts first (kj) with a cocatalyst MAO ([—O—Al—CH (CH3-) ). Subsequently, alkenes molecules are inserted (kpi) into the Zr-C bond of the formed metaUocenium ion Cp2Zr -CH3. A polymer molecule grows in length by numerous insertion reactions (kpi). 

The mechanistic similarity between Ziegler-Natta polymerization of olefins and the alkene cyclization reactions described above suggested that early transition metal catalysts would be effective catalysts for the coupling of... 

TRADITIONAL ZIEGLER-NATTA POLYMERIZATION OF NONPOLAR ALKENE MONOMERS... 

The Ziegler-Natta polymerization of ethylene can be adapted to make molecules of only modest size (C6--C20) and containing certain functional groups. If, for example, the mctal-alkyls initially obtained are heated (in the presence of ethylene and a nickel catalyst), the hydrocarbon groups are displaced as straight-chain 1-alkenes of even carbon number. Large quantities of such alkenes in the C12-C20 range are... 

Most unsaturated substances such as alkenes, alkynes, aldehydes, acrylonitrile, epoxides, isocyanates, etc., can be converted into polymeric materials of some sort—either very high polymers, or low-molecular-weight polymers, or oligomers such as linear or cyclic dimers, trimers, etc. In addition, copolymerization of several components, e.g., styrene-butadiene-dicyclo-pentadiene, is very important in the synthesis of rubbers. Not all such polymerizations, of course, require transition-metal catalysts and we consider here only a few examples that do. The most important is Ziegler-Natta polymerization of ethylene and propene. 

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