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

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 (NPR 3 ) 2 TiMe +, (Cp)(NCR 2 )TiMe +, (CpSiR 2 NR‘)TiMe +, (Cp)OSiR 3 TiMe +, and (Cp)NPR 3 TiMe + . The counterions studied are B(C 6 F 5 ) 4 -, MeB(C 6 F 5 ) 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(C 6 F 5 ) 4 - < MeB(C 6 F 5 ) 3 - < TMA-MAOMe - < MAOMe - . For the same counterion but different cations, the (NPR 3 ) 2 TiMe + 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(C 6 F 5 ) 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(C 6 F 5 ) 3 - and TMA-MAOMe - are represented by MeBCl 3 - and MeBMe 2 Cl -, respectively. The other parts of the anions are replaced by MM atoms. Preliminary studies on olefin insertion reactions for the (NPH 3 ) 2 TiMe−μMe−A (A = B(C 6 F 5 ) 3 and TMA-MAO) systems suggest that the QM/MM models satisfactorily reproduce the behavior of the ion-pair system in the insertion process.