Abstract
Comparison of pentad distributions obtained from NMR spectra and from a molecular mechanics-based modeling approach is performed for the catalysts {iPr(3-X-CpFlu)}ZrCl2 (X = H, Me, Et, iPr, tBu) at a range of different temperatures. In order to model the temperature dependency of the pentad distributions the variation in steric influence along with the change of the rotational energy level for catalysts with substituents displaying relatively low barriers to rotation is treated approximately by calculating energy profiles of 360° rotation of the alkyl groups. The temperature at which intermediate chain migration (back-skip) or chain epimerization starts to be important seems to be rather constant (30–50°C) among the five catalysts. Even in the case of X = tBu, back-skip seems to be unimportant for explaining the formation of isotactic polymer at room temperature.
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