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

2002; American Chemical Society; Volume: 21; Issue: 12 Linguagem: Inglês

10.1021/om011057c

1520-6041

Zhitao Xu, Kumar Vanka, Timothy K. Firman, Artur Michalak, Eva Zurek, Chuanbao Zhu, Tom Ziegler,

Advanced Polymer Synthesis and Characterization

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.