Computational Studies
on Isospecific Polymerization of 1-Hexene Catalyzed by Cationic Rare
Earth Metal Alkyl Complex Bearing a <i>C</i><sub>3</sub> <i>i</i>Pr-trisox Ligand
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Abstract
1-Hexene polymerization catalyzed by dicationic rare
earth metal alkyl species [Ln(<i>i</i>Pr-trisox)(CH<sub>2</sub>SiMe<sub>3</sub>)]<sup>2+</sup> (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 barrier 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