Deuterium Isotope Effects as Evidence for α-Agostic Assistance in Ziegler-Natta Catalysts. Design, Synthesis, and Reactivity of a New Class of Highly Syndiospecific Ziegler-Natta Polymerization Catalysts

Citation

Herzog, Timothy Allan (1997) Deuterium Isotope Effects as Evidence for α-Agostic Assistance in Ziegler-Natta Catalysts. Design, Synthesis, and Reactivity of a New Class of Highly Syndiospecific Ziegler-Natta Polymerization Catalysts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tv4y-w681. https://resolver.caltech.edu/CaltechTHESIS:11192019-173846004

Abstract

By applying the concept of "isotopic perturbation of stereochemistry" to a number of Group III metallocene catalysts, further evidence for an α-agostic interaction in the chain propagation step of Ziegler-Natta polymerization has been obtained. These results are in accord with the "modified Green-Rooney" mechanism. An α-agostic interaction in the transition state of olefin insertion may contribute to the remarkable stereoselectivities of many Ziegler-Natta catalyst systems since it may restrict the possible orientations of the polymer chain such that it has a more significant impact on the orientation of the inserting olefin.

A new class of Group IV metallocene catalysts is presented for the syndiospedfic polymerization of propylene. These catalysts incorporate what are thought to be the key characteristics of syndiospedfic metallocene catalysts: Cs symmetry and rigidly linked cyclopentadienyls of greatly differing size. However, preliminary attempts to develop new syndioselective catalysts have suggested another important characteristic: a pocket in the larger moiety to avoid undesirable steric interactions between the ligand framework and coordinated olefin. In order to accommodate this constraint, a ligand system with a 1,3-dialkylcyclopentadienyl doubly linked to a singly substititued cydopentadienyl was chosen. Group IV metallocenes with these new ligands, in the presence of a cocatalyst (MAO), react rapidly with neat propylene to form highly syndiotactic polypropylene. This is the first example of a stereospecific doubly bridged olefin polymerization catalyst and is the first example of a highly syndiospedfic polymerization catalyst not based on a fluorenyl like ligand. This catalyst system is also very versatile since straightforward changes in the alkyl group of the singly substituted cydopentadienyl and in the reaction conditions lead to dramatic changes in the polymer microstructure. These catalysts should provide an excellent platform for mechanistic study and may be important industrially.

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