The role of intermediate chain migration in propene polymerization using substituted {Pr-i(CpFlu)}ZrCl2/MAO catalysts

Comparison of pentad distributions obtained from NMR spectra and from a mol ecular mechanics-based modeling approach is performed for the catalysts {Pr -i(3-X-CpFlu)}ZrCl2 (X = H, Me, Et, Pr-i, Bu-t) at a range of different tem peratures. In order to model the temperature dependency of the pentad distr ibutions the variation in steric influence along with the change of the rot ational energy level for catalysts with substituents displaying relatively low barriers to rotation is treated approximately by calculating energy pro files of 360 degrees 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 degrees C) among the five c atalysts. Even in the case of X = Bu-t, back-skip seems to be unimportant f or explaining the formation of isotactic polymer at room temperature.