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[摘要]：1-Hexene polymerization catalyzed by dicationic rare earth metal alkyl species [Ln(iPr-trisox)(CH(2)SiMe(3))](2+) (Ln = Sc and Y; trisox = trisoxazoline) has been computationally studied by using QM/MM approach. It has been found that the initiation of 1-hexene polymerization kinetically prefers 1,2-insertion (free energy barrier of 17.23 kcal/mol) to 2,1-insertion (free energy bather of 20.05 kcal/mol). Such a preference of 1,2-insertion has been also found for chain propagation stage. The isotactic polymerization was computed to be more kinetically preferable in comparison with syndiotactic manner, and the dicationic system resulted in lower insertion free energy barrier and more stable insertion product in comparison with the monocationic system. The stereoselectivity was found to follow chain-end mechanism, and the isospecific insertion of 1-hexene is mainly controlled by kinetics. In addition, the current computational results, for the first time, indicate that the higher activity of Sc species toward 1-hexene polymerization in comparison with the Y analogue could be ascribed to lower insertion barrier, easier generation of the active species, and its larger chemical hardness. |
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