Effect of Ligand Structure on Olefin Polymerization by a Metallocene/Borate Catalyst: A Computational Study

We have carried out a systematic
computational study on olefin polymerization by metallocene/borate
catalysts, using three metallocenes: Cp2ZrMe2 (Cp), rac-SiMe2-bis­(1-(2-Me-(4-PhInd))­ZrMe2 (4-PhInd), and rac-SiMe2-bis­(1-(2-Me-(4,5-BenzInd))­ZrMe2 (4,5-BenzInd). Detailed reaction pathways, including the structure of the catalytically
active ion pair, anion displacement, chain propagation, and chain
termination steps, are reported for ethene homopolymerization, alongside
with investigation of ethene–propene copolymerization reactions.
Initially, all catalysts form inner-sphere ion pairs ([L2ZrMe]+–[B­(C6F5)4]−) with a direct Zr–F interaction, which
is weak enough to be displaced by the incoming monomer. In comparison
to Cp, the bulky and electron-rich 4-PhInd and 4,5-BenzInd show higher barriers for anion displacement
but lead to relative stabilization of the resulting π complexes. 4-PhInd enables the most feasible propene uptake, and both
catalysts suppress the chain termination reactions relative to Cp. The borate counterion is shown to have a minor influence
after the catalyst activation step.