A density functional study on the insertion mechanism and chain termination in Kaminsky-type catalysts; comparison of frontside and backside attack

Abstract

Non-local density functional (DF) calculations have been carried out on the reaction of ethylene with Cp 2Zr +, which serves as a model for the resting state between two insertions. The β-agostic Cp 2Zr +Et is 47.0 kJ mol −1 more stable than the α-agostic conformer. Frontside insertion of the olefin can take place after rotation around the ZrC α-bond forming the α-agostic Cp 2Zr +Et. An α-agostic π-complex is formed with a complexation energy of 81.1 kJ mol −1 and the frontside transition state has an activation energy of 2 kJ mol −1 relative to the π-complex. The reaction is exothermic by 118.9 kJ mol −1. Without rotation around the ZrC α bond a β-agostic π-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 −1. An alternative path for the olefin insertion starts with a backside attack of the olefin. The activation barrier for the backside insertion is 28.9 kJ mol −1 and the reaction is exothermic by 24.9 kJ mol −1 relative to the π-complex. Backside insertion does not involve inversion at the metal centre. The formation of syndiotactic polypropene in the case of the backside insertion can only be explained with chain-end control. Comparison of three chain termination processes (β-hydride elimination, C-H activation and H-exchange) indicates that H-exchange is the most probable reaction. β-Elimination is strongly endothermic and frontside C-H-activation makes a rotation around ZrC α necessary.