Structure and isothermal crystallization behavior of polypropylene prepared at high polymerization temperature

The structure, morphology, and isothermal crystallization behaviors of polypropylene (PP) prepared with heterogeneous Ziegler-Natta catalyst at high temperature (100°C) were investigated with differential scanning calorimetry, wide-angle X-ray diffraction, temperature-rising elution fractionation, gel permeation chromatography, and ¹³C NMR. The results reveal that the crystalline structure changes with variation of the composition of the PP. The isotactic PP (iPP)1 prepared with Et₃Al and "TMA-depleted" methylaluminoxane crystallizes from the melt in the mixtures of the α and β forms, whereas each fraction obtained from pure PP1 does not show β-PP crystal at the same crystallization condition. In addition, the γ-PP crystal is appeared for the fractions of low mmmm%-[mmmm] (mmmm pentad content) values and molecular weight. Moreover, it was found that the iPP2 or iPP3 prepared with Hex₃Al crystallizes from the melt in mixtures of the α and γ forms, even at atmospheric pressure and for high molecular weight. The microstructure showed in the PP samples obtained at high temperature could be well explained with the shift in the alkylaluminium-donor equilibrium reactions at high polymerization temperature.