Search

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.

With six different metallocenes, namely iPr[CpFlu]ZrCl2I, iPr[3‐MeCpFlu]ZrCl2II, iPr[3‐EtCpFlu]ZrCl2III, iPr[3‐iPrCpFlu]ZrCl2IV, iPr[IndFlu]ZrCl2Vand iPr[3‐tBuCpFlu]ZrCl2VI propene polymerizations were carried out at different polymerization temperatures. MAO was used as a cocatalyst for all polymerizations. In case of metallocenes II, III and IV an increase in isotacticity with increasing polymerization temperature was observed. This is due to the increased rotation and, as a consequence, to the increased steric demand of the substituent at the cyclopentadienyl residue. With metallocene V a catalyst of in principle the same type was synthesized, but rotation of the substituent is not possible. Here, in the contrary, the assumed effect was observed, that the stereospecificity of the metallocene decreases, while raising the polymerization temperature. In metallocene I there is no rotatory substituent at the cyclopentadienyl residue and therefore a more stereoirregular polymer is formed at higher polymerization temperatures. Metallocene VI produces poly(propylene) with slightly increased isotacticity at higher polymerization temperature.

Kinetic and polymer analytical results obtained with the systems Me2Si(Ind)2ZrMe2/[B(C6F4Si(i‐Pr)3)4]−[(C6H5)3C]+(2)and Me2Si(Ind)2ZrMe2/[B(C6F5)4]−[(C6H5)3C]+(3)in olefin polymerizations are compared (Me: methyl; Ind: indenyl). Both systems show that the polymerization rate increases with increasing metallocene concentration in a surplus due to the formation of binuclear species. The expected influence of increasing cation‐anion distance on the stereo‐errors of the poly(propylene)s when changing from system 3into system 2could not be detected.