A Density Functional Study On The Insertion Mechanism And Chain Termination In Kaminsky-type Catalysts; Comparison Of Frontside And Backside Attack

John C.W. Lohrenz(a), Tom K. Woo(a), Liangyou Fan(b), Tom Ziegler(a)*

(a)Department of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada, T2N 1N4
(b)Novacor Research & Technology Corporation, 2928-16 Street, N.E. Calgary, Alberta, Canada, T2E 7K7

Abstract:

Nonlocal density functional (DF) calculations have been carried out on the reaction of ethylene with Cp2Zr+-Et, which serves as a model for the resting state between two insertions. The b-agostic Cp2Zr+-Et is 47.0 kJ/mol more stable than the a-agostic conformer. Frontside insertion of the olefin can take place after rotation around the Zr-Ca bond forming the a-agostic Cp2Zr+-Et.
An a-agostic p-complex is formed with a complexation energy of 81.7 kJ/mol and the frontside transition state has an activation energy of 2 kJ/mol relative to the p-complex. The reaction is exothermic by 118.9 kJ/mol. Without rotation around the Zr-Ca bond a b-agostic p-complex is formed and H-transfer from the polymer chain end to the olefin takes place. This reaction leads to chain termination with an activation barrier of 29.8 kJ/mol. An alternative path for the olefin insertion starts with a backside attack of the olefin. The activation barrier for the backside insertion is 32.2 kJ/mol and the reaction is exothermic by 24.9 kJ/mol relative to the p-complex. Backside insertion does not involve inversion at the metal centre. The formation of syndiotactic polypropene in case of the backside insertion can only be explained with chain-end control. Comparison of three chain termination processes (b-hydride elimination, C-H activation and H-exchange) gives indication for the H-exchange to be the most probable reaction. b-Elimination is strongly endothermic and frontside C-H-activation makes a rotation around Zr-Ca necessary.