| 초록 |
MAO (methylalumoxane) is an activator essential to initiation process in the metallocene-based polymerization system and hence have been recognized as a cocatalyst. However, despite its prominent effect on the monomer initiation the understanding of the exact role of the MAO for the metallocene-based polymerization have been far less completed partly due to lack of information about the exact 3D structure of MAO. In the present study, we have investigated the effect of MAO on the reaction mechanism by using a density functional theory (DFT). To take into account the effect of MAO, we used olefin separated ion pair (OSIP) model in which MAO is treatd as a counterion and analyzed the structures and energetics of each stationary point using a software DMol3 from Accerlys Inc.. The results such as energy profiles are then compared to that from the cationic model system in the absence of MAO. One of our primary results shows that the π-complex is less stable for the OSIP model than for the cationic model. This might be due to the fact that the MAO in the OSIP model forms an ion pair with catalyst and shields electron-deficient metal of catalyst. The shielding reduces the coordination ability of metal and conclusively destabilizes the π-complex. The presence of MAO also increases the energy barrier between π-complex and transition state. Higher energy barrier in OSIP model than cationic model is probably due to the steric hindrance between the MAO and an inserting monomer.
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