Crystallization of Metallocene-Made Isotactic Polypropylene:  Disordered Modifications Intermediate between the α and γ Forms

A detailed analysis of the structure and the thermal behavior of samples of isotactic polypropylene (iPP) prepared with a highly regiospecific metallocene catalysts is presented. The samples present variable amounts of stereodefects (isolated rr triads) and provide the first example of metallocene-made iPP not including regioirregularities. They crystallize from the melt in mixtures of the α and γ forms. The content of the γ form increases with increasing the crystallization temperature, reaches a maximum value, and then decreases for a further increase of the temperature. The crystallization of the γ form is favored by the presence of stereodefects. The thermal analysis has indicated that the crystals of γ form melt at lower temperature than the crystals of α form obtained at the same crystallization temperature. A comparison between the experimental X-ray diffraction profiles of the melt-crystallized samples and the calculated profiles for models of structure has shown that the γ form obtained in these samples always presents structural disorder, which produces local situation of packing typical of the α form. The amount of structural disorder decreases with increasing the crystallization temperature and the degree of stereoregularity of the samples. The presence of disorder in the crystals of γ form accounts for their lower melting temperature. This analysis indicates that metallocene isotactic polypropylene crystallizes in a continuum of disordered modifications intermediate between the α and γ forms, and the amount of disorder present in the crystals depends on the crystallization conditions and on the stereoregularity of the sample.