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, Cp2ZrEt+, and two ion pairs of composition Cp2ZrEt+A- (A- = CH3B(C6F5)3-, B(C6F5) 4-) was carried out. It was shown that the nature of counterion affects mainly the ratio between the most stable β-agostic and nonagostic Cp2ZrEt+A- isomers, the thermodynamic and kinetic characteristics of ethylene addition to the Cp2ZrEt+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 Cp2ZrEt+A-, and the lower the activation energy of chain propagation. All possible pathways of the interaction between Cp2ZrEt+CH3B(C6F5)3- and ethylene molecule were examined and compared. Among all species of composition Cp2ZrEt+CH3B(C6F5)3-, the nonagostic isomer was found to be the most reactive toward ethylene.