Determination of plateau moduli and entanglement molecular weights of isotactic, syndiotactic, and atactic polypropylenes synthesized with metallocene catalysts

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 approximate to 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 theology. 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 G(N) degrees S and entanglement molecular weights M(e)s 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.