Evidence for Mixed-Ion Clusters in Metallocene Catalysts: Influence on Ligand Exchange Dynamics and Catalyst Activity

摘要

Ionic metallocene alkyl complexes [L_2M-R~+···X~-] (M = Ti, Zr, or Hf) are well-known as highly active olefin polymerization catalysts. The solution structure of these catalysts in typical hydrocarbon solvents is, however, complicated by their tendency to form ion aggregates [L_2M-R~+···X~-]_n (n = 2, 3, or higher). These ion aggregates show enhanced dynamic behavior; that is, the rates of "chain swinging" or site epimerization, k_(ex), increase with increasing ion pair concentration. Since alkyl ligand migration is a key step in polymer chain growth, such an enhancement in ligand mobility may have important implications for catalytic activity.