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 alpha and gamma forms. The content of the gamma 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 gamma form is favored by the presence of stereodefects. The thermal analysis has indicated that the crystals of gamma form melt at lower temperature than the crystals of alpha 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 gamma form obtained in these samples always presents structural disorder, which produces local situation of packing typical of the alpha 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 gamma form accounts for their lower melting temperature. This analysis indicates that metallocene isotactic polypropylene crystallizes in a continuum of disordered modifications intermediate between the alpha and gamma forms, and the amount of disorder present in the crystals depends on the crystallization conditions and on the stereoregularity of the sample.