Polymerization, stereospecific catalysis

The occurrence of stereospecific polymerization in solution has been explained by the stetic restrictions of ligands bonded to the metal center. For example, the following stmcture has been postulated as an intermediate in solution catalysis (68) ... 

Moreover, the molecular catalysts have provided systematic opportunities to study the mechanisms of the initiation, propagation, and termination steps of coordination polymerization and the mechanisms of stereospecific polymerization. This has significantly contributed to advances in the rational design of catalysts for the controlled (co)polymerization of olefinic monomers. Altogether, the development of high performance molecular catalysts has made a dramatic impact on polymer synthesis and catalysis chemistry. There is thus great interest in the development of new molecular catalysts for olefin polymerization with a view to achieving unique catalysis and distinctive polymer synthesis. 

Considerable interest has been shown in the new processes of stereospecific polymerization, not only so far as they concern the production of new classes of polymers, having unusual characteristics and improved properties, but also because they are representative of a peculiar new type of heterogeneous catalysis, of great interest from the practical and the theoretical points of view 1-5). 

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). 

Zhiquan Shen and Jun Ouyang, Rare earth coordination catalysis in stereospecific polymerization 395 Errata 429... 

Shen Z, Ouyang J Coordination catalysis in stereospecific polymerization 1987 [42]... 

In research with Ziegler catalysts, Cossee (11) and Arlmann and Cossee (12) hypothesized that the insertion of propylene monomer takes place in a cis conformation into a titanium-carbon bond. Natta et al. (8) postulated that in the stereospecific polymerization, chiral centers on the surface are needed to produce isotactic polymers. These and other issues regarding the nature of the active sites have helped to increase the interest in investigations of homogeneous metallocene catalysis. 

Arlman EJ, Coose P, Ziegler-Natta Catalysis III Stereospecific Polymerization of Propene with the Catalyst System TiCl3-AlEt3, J Catal, 3, 99 (1964)... 

Stereospecific Polymerization. It was found quite early in the development of Ziegler-Natta catalysts that stereospecific polymerization usually requires the presence of a crystalline catalyst surface—i.e, heterogeneous catalysis. Most recently, this has been confirmed in the careful examination of catalyst surfaces by Rodriguez, van Looy, and Gabant (35). It must be remembered, however, that this work dealt largely with a-olefins. 

The catalysis of the stereospecific polymerization of conjugated dienes is of considerable interest from both the scientific and the industrial points of view [1,2]. From butadiene and isoprene, as the industrially most important 1,3-dienes, in comparison with the polymerization of olefins many more structurally different stereoregular polymers can be derived cf the structures of the stereoregular polybutadienes and polyisoprenes given in Scheme 1 [106]. 

Rioka, M. Tsutsui, T. Ueda, T. Rashiwa, N. Stereospecific Polymerization of a-Olefin with an Ethylene Bis(l-indenyl)hafnium Dichloride and Methyl-aluminoxane Catalyst System. In Catalytic Olefin Polymerization, Studies in Surface Science and Catalysis-, Reii, T., Soga, R., Eds. Elsevier New York, 1990, p 483. 

Arlman, E.J. Cossee, P. Ziegler-Natta catalysis III. Stereospecific polymerization of propene with the catalyst system TiCls-AlEts. J. Catal. 1964, 3, 99-104. 

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