DFT Study of Ethylene Polymerization by Zirconocene−Boron Catalytic Systems. Effect of Counterion on the Kinetics and Mechanism of the Process

Abstract

The effect of counterion on the kinetics and mechanism of ethylene polymerization by the zirconocene−boron catalytic system was examined using the density functional theory approach. A comparative study of three model catalytic species, namely, ethylzirconocene cation, Cp 2 ZrEt +, and two ion pairs of composition Cp 2 ZrEt + A - (A - = CH 3 B(C 6 F 5 ) 3 -, B(C 6 F 5 ) 4 - ) was carried out. It was shown that the nature of counterion affects mainly the ratio between the most stable β-agostic and nonagostic Cp 2 ZrEt + A - isomers, the thermodynamic and kinetic characteristics of ethylene addition to the Cp 2 ZrEt + A - ion pairs, and the chain propagation kinetics. The weaker the nucleophilicity of the counterion, the higher the fraction of the β-agostic isomer, the higher the exotermicity, and the lower the activation barrier to ethylene addition to Cp 2 ZrEt + A -, and the lower the activation energy of chain propagation. All possible pathways of the interaction between Cp 2 ZrEt + CH 3 B(C 6 F 5 ) 3 - and ethylene molecule were examined and compared. Among all species of composition Cp 2 ZrEt + CH 3 B(C 6 F 5 ) 3 -, the nonagostic isomer was found to be the most reactive toward ethylene.