A systematic computational study of electronic effects on hydrogen sensitivity of olefin polymerization catalysts (abstract only)

One of the important product parameters of polyolefins is their molecular weight (distribution). A common way to control this parameter is to add molecular hydrogen during the polymerization, which then acts as a chain transfer agent. The factors governing the hydrogen sensitivity of olefin polymerization catalysts are poorly understood and have attracted little attention from computational chemists. To explore the electronic factors determining hydrogen sensitivity we performed density functional calculations on a wide range of simple model systems including some metallocenes and a few basic models of heterogeneous catalysts. As a quantitative measure for hydrogen sensitivity we used the ratio of (i) the rate constant for chain transfer to hydrogen to (ii) the rate constant for ethene insertion, k(h)/k(p) (see the scheme below), and as a measure of electrophilicity we used the energy of complexation to the probe molecule ammonia. [Formula: see text] For isolated species in the gas phase, complexation energies appear to dominate the chemistry. Ethene complexes more strongly than hydrogen and with increasing electrophilicity of the metal centre this difference grows; the hydrogen sensitivity decreases accordingly. Although many factors (like catalyst dormancy and deactivation issues) complicate the comparison with experiment, this result seems to agree both in broad terms with the experimental lower hydrogen sensitivity of heterogeneous catalysts, and more specifically with the increased hydrogen sensitivity of highly alkylated or fused metallocenes. The opposite conclusion reached by Blom (see Blom et al 2002 Macromol. Chem. Phys. 203 381-7) is due to the use of a very different measure of electrophilicity, rather than to different experimental data

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 (<b>Cp</b>), <i>rac</i>-SiMe₂-bis­(1-(2-Me-(4-PhInd))­ZrMe₂ (<b>4-PhInd</b>), and <i>rac</i>-SiMe₂-bis­(1-(2-Me-(4,5-BenzInd))­ZrMe₂ (<b>4,5-BenzInd</b>). 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 <b>Cp</b>, the bulky and electron-rich <b>4-PhInd</b> and <b>4,5-BenzInd</b> show higher barriers for anion displacement but lead to relative stabilization of the resulting π complexes. <b>4-PhInd</b> enables the most feasible propene uptake, and both catalysts suppress the chain termination reactions relative to <b>Cp</b>. The borate counterion is shown to have a minor influence after the catalyst activation step