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Published February 13, 1996 | public
Journal Article

α-Agostic Interactions and Olefin Insertion in Metallocene Polymerization Catalysts


In the 40 years since the discovery of transition metal catalysts for olefin polymerization by Ziegler and Natta a substantial amount of research has been directed toward understanding the basic mechanistic steps of this important industrial process. Attention has focused in particular on the process of monomer enchainment, which is generally agreed to occur through olefin coordination followed by insertion into a metal-carbon bond. Four general mechanistic proposals for the nature of this insertion have emerged (Scheme 1). The first is alkyl migration to the coordinated olefin and is known as the Cossee-Arlman mechanism. The second model, proposed by Rooney and Green, involves an oxidative 1,2-hydrogen shift from the R-carbon of the polymer chain, generating a metal-alkylidene hydride. This species then reacts with an olefin to generate a metallacyclobutane, and reductive elimination completes the propagation sequence. Somewhat intermediate to the first two proposals is a mechanism proposed by Green, Rooney, and Brookhart where a hydrogen atom on the R-carbon of the growing polymer chain interacts with the metal center throughout the catalytic cycle. This three-center, two-electron covalent bond, termed an "agostic interaction", occurs when the hydrogen atom is simultaneously bonded to both a carbon and a metal atom. The fourth mechanistic possibility is olefin insertion where an R-hydrogen interacts with the metal center only during the transition state of the C-C bond forming step. This mechanism is a hybrid of the Cossee-Arlman and modified Green-Rooney mechanisms.

Additional Information

© 1996 American Chemical Society. Received September 25, 1995. Publication Date (Web): February 13, 1996. This work was generously supported by a grant from the NSF. We thank Prof. J. E. Bercaw, Prof. H. H. Brintzinger, Dr. S. N. Brown, Prof. M. L. H. Green, Prof. W. E. Piers, and Prof. J. J. Rooney for helpful discussions and comments. G.W.C. gratefully acknowledges a Noyes Postdoctoral Fellowship from Caltech and a NSF Postdoctoral Fellowship.

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