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.

Site epimerization in ansa-zirconocene polymerization catalysts

“…In addition, the relative energies are altered upon complexation: the frontside b-agostic complex is most stable, but the two a-agostic complexes are only about 4 kcal/mol higher in energy; hence, in the case of the frontside complexes, the a-agostic conformer is stabilized by 8 kcal/mol relative to the b-agostic conformer. A similar relative stabilization of the a-agostic conformer by a coordinating anion has been noted previously [92]. One may therefore expect that the presence of coordinating counterions may affect the relative propensity of the back-skip and insertion processes studied presently.…”

“…[16,87,89,92,94,-97]. To check the influence of this typical anion on our results, we have added it to 2a3 and 2b2, placing it in similar positions as in the published work by Nifant'ev et al [92], and performed geometry optimizations on the resulting catalyst-counterion complexes (see Figs. S1-S4 in the Supplementary Information).…”

Aspects of coordination polymerization in heterogeneous and homogeneous catalysis. A computational survey

“…It can approach cis to the l-methyl bridge, an approach we will henceforth call the ''cis'' approach. Nifant'ev et al [52] have studied ethylene insertion via this approach for the Cp 2 ZrEt + A )…”

Ultra Accelerated Quantum Chemical Molecular Dynamics Study on Ethylene Polymerization Reaction Using CpSiH2NHTiCl2—Constrained Geometry Catalyst

“…Some theories suggest that ionic pair of cationic catalyst and counter ion is completely separated from each other during monomer insertion and therefore the effect of counter ion on polymerization calculations is neglected [2,[8][9][10][11]. On the other hand, ionic pair model assumes that the catalytic active species can be stabilized by the strong electrostatic interaction with the counter ion and monomer inserts into the ionic pair of two ionic species [12][13][14][15][16][17][18].…”