Theoretical Study of the Interactions between Cations and Anions in Group IV Transition-Metal Catalysts for Single-Site Homogeneous Olefin Polymerization

Abstract

Density functional theory has been used to investigate the interaction between a series of cationic polymerization catalysts and their anionic counterions. The catalyst systems include (NPR3)2TiMe+, (Cp)(NCR2)TiMe+, (CpSiR2NR‘)TiMe+, (Cp)OSiR3TiMe+, and (Cp)NPR3TiMe+. The counterions studied are B(C6F5)4-, MeB(C6F5)3-, TMA-MAOMe-, and MAOMe-, where TMA = trimethylaluminum and MAO = methylalumoxane. Two simplified model structures, which have been proposed as the counterions for the active (TMA-MAOMe-) and dormant (MAOMe-) ion pairs in single-site catalysts activated by MAO, were used for the last two counterions. The interaction between the cation and anion will be discussed in terms of ion-pair formation and separation energies. Full quantum-mechanical (QM) calculations demonstrate that, for the same catalyst system but different anions, the ion-pair separation energies increase in the order B(C6F5)4- < MeB(C6F5)3- < TMA-MAOMe- < MAOMe-. For the same counterion but different cations, the (NPR3)2TiMe+ system has the lowest separation energy. Increasing the size of the R group decreases the ion-pair separation energy. Combined quantum-mechanical (QM) and molecular-mechanical (MM) models (QM/MM) for MeB(C6F5)3- and TMA-MAOMe- have also been developed and examined by comparing the ion-pair formation and separation energies to the full QM results. The QM parts of MeB(C6F5)3- and TMA-MAOMe- are represented by MeBCl3- and MeBMe2Cl-, respectively. The other parts of the anions are replaced by MM atoms. Preliminary studies on olefin insertion reactions for the (NPH3)2TiMe−μMe−A (A = B(C6F5)3 and TMA-MAO) systems suggest that the QM/MM models satisfactorily reproduce the behavior of the ion-pair system in the insertion process.