Stereospecific polymerization of alkenes

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

Group 4 metallocenes are important as catalysts in the stereospecific polymerization of alkenes. Consequently, alkenes have been the most extensively studied substrates in reactions with metallocenes. 

Towards the end of the second millennium, studies of the transition elements continued to make major contributions to chemical science and technology. The development of new catalysts and reagents represents one area of activity. Examples are provided by the activation of saturated hydrocarbons by rhodium or lutetium complexes, new syntheses of optically active products in reactions which employ chiral metal compounds, and transition metal compounds which catalyse the stereospecific polymerization of alkenes. The ability of transition metal centres to bind to several organic molecules has been exploited in the construction of new two- and three-dimensional molecular architectures (Figure 1.4). New materials containing transition elements are being developed, one... 

Ciardelli, F. and P. Giusti (eds.), Structural Order in Polymers , Pergamon Press, New York, 1981. Proceedings of the International Symposium on Macromolecules, Florence, Italy, September 1980 dedicated to the memory of Natta with an introduction by Mark. The symposium outlines the main scientific and industrial achievements in the stereospecific polymerization of alkenes and dialkenes, and the relationship between physical properties and molecular structure. 

However, this concept cannot be transferred to the polymerization of 1-alkenes. Recently, critical remarks on the carbene mechanism of ethylene insertion into the Co—CHj bond were published Zambelli et al. performed an elegant study on the discrimination of the carbene mechanism for the stereospecific polymerization of propylene. According to the carbene mechanism the insertion of the first C3H6 molecule into the Mt—bond have to result in the formation of chain with an isobutyl end group i) enriched by in the methyl and methylene groups (scheme (17 a) >) ii) enriched by in the methyl group with threo- or erithro-configuration (see structures (12a) and (12b)). 

Reactions of lithium alkyls are generally considered to be carbanionic in nature, but in reactions with alkyl halides free radicals have been detected by electron spin resonance.32 Lithium alkyls are widely employed as stereospecific catalysts for the polymerization of alkenes, notably isoprene, which gives up to 90% of 1,4-cA-polyisoprene numerous other reactions with alkenes have been studied.33 The TMED complexes again are especially active not only will they polymerize ethylene but they will even metallate benzene and aromatic compounds, as well as reacting with hydrogen at 1 atm to give LiH and alkane. 

Finally, actinide complexes such as [MCp 2R2] (M = Th or U, R = alkyl or H) are very active catalysts for the hydrogenation and polymerization of olefins. The complexes [U(allyl)3X] (X = Cl, Br, I) are excellent initiators for the stereospecific polymerization of butadiene, which produces rubbers that have remarkable mechanical properties. Some other complexes are active for the heterogeneous CO reduction and alkene metathesis. The field of catalysis using organoactinide complexes should considerably expand in the near future. 

Also in the 1980s, the discovery of homogeneous stereospecific catalysts for the polymerization of 1-alkenes has opened up new prospects for research on stereospecific polymerization and stereoregular polyolefins. Ewen and coworkers79 achieved this discovery on the basis of earlier research on metallocenes in combination with alkyl-Al-oxanes by Sinn and Kaminsky.10... 

In a number of classes of systems, the catalytic and other chemical effects of metal ions on reactions of organic and inorganic molecules are generally recognized the catalysis of nucleophilic reactions such as ester hydrolysis the reactions of alkenes and alkynes in the presence of metal carbonyls (8, 9, 69) stereospecific polymerization in the presence of Ziegler catalysts (20, 55, 56) the activation of such small molecules as H2 (37), 02 (13), H202 (13), and possibly N2 (58) and aromatic substitution reactions of metal-cyclopentadienyl compounds (59, 63). 

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