Metallocene catalysis was employed to prepare model polypropylenes
(PP) with very high
molecular weights and narrow molecular weight distributions (MWDs) of
M
w/M
n ≈ 2, typical for
“single
site” metallocene catalysts. The viscoelastic properties of PP
melts were investigated by means of
oscillatory rheometry. Arrhenius and WLF equations were applied to
model the influence of temperature
on PP rheology. With very high molecular weight polypropylene
samples it became possible to determine
the onset of the plateau zone, reflecting the rubber-like properties of
PP. For the first time plateau moduli
and entanglement molecular weights M
es were
determined and found to be depend upon stereoregularity. The entanglement molecular weights were found to be
6900 g/mol for isotactic PP, 7050 g/mol
for atactic PP, and 2170 g/mol for syndiotactic PP. This influence
of stereoregularity was attributed to
the different conformations of syndiotactic PP with respect to
isotactic and atactic PP in PP melts.
The influence of silylene-bridged bis(indenyl) ligand substitution, especially benzannelation and 2-methyl substitution, on methylaluminoxane-activated metallocene-catalyzed ethene/1-octene copolymerization in toluene at 40 °C was investigated. 2-Methyl substitution gave significantly higher molecular masses at the expense of catalyst activity, whereas benzannelation promoted 1-octene incorporation and randomness of the resulting poly(ethene-co-1-octene) copolymers. Force field calculations based on steric arguments were used to explain experimental copolymerization results. Activation energy differences between ethene and 1-octene insertion accounted for improved 1-octene incorporation in the case of benzannelated metallocenes. According to 13 C-NMR microstructure analysis, copolymerization followed first-order Markov statistics.
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