A DENSITY-FUNCTIONAL STUDY ON THE INSERTION MECHANISM AND CHAIN TERMINATION IN KAMINSKY-TYPE CATALYSTS - COMPARISON OF FRONTSIDE AND BACKSIDE ATTACK

Non-local density functional (DF) calculations have been carried out o n the reaction of ethylene with Cp(2)Zr(+)-Et, which serves as a model for the resting state between two insertions. The beta-agostic Cp(2)Z r(+)-Et is 47.0 kJ mol(-1) more stable than the alpha-agostic conforme r. Frontside insertion of the olefin can take place after rotation aro und the Zr-C-alpha-bond forming the alpha-agostic Cp(2)Zr(+)-Et. An al pha-agostic pi-complex is formed with a complexation energy of 81.1 kJ mol(-1) and the frontside transition state has an activation energy o f 2 kJ mol(-1) relative to the pi-complex. The reaction is exothermic by 118.9 kJ mol(-1). Without rotation around the Zr-C-alpha bond a bet a-agostic pi-complex is formed and H-transfer from the polymer chain e nd to the olefin takes place. This reaction leads to chain termination with an activation barrier of 29.8 kJ mol(-1). An alternative path fo r the olefin insertion starts with a backside attack of the olefin. Th e 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 pi-comp lex. Backside insertion does not involve inversion at the metal centre . The formation of syndiotactic polypropene in the case of the backsid e insertion can only be explained with chain-end control. Comparison o f three chain termination processes (beta-hydride elimination, C-H act ivation and H-exchange) indicates that H-exchange is the most probable reaction. beta-Elimination is strongly endothermic and frontside C-H- activation makes a rotation around Zr-C-alpha necessary.