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| - Crystallization of Metallocene-Made Isotactic Polypropylene: Disordered Modifications Intermediate between the α and γ Forms
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| - A detailed analysis of the structure and the thermal behavior of samples of isotacticpolypropylene (iPP) prepared with a highly regiospecific metallocene catalysts is presented. The samplespresent 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 γforms. The content of the γ form increases with increasing the crystallization temperature, reaches amaximum value, and then decreases for a further increase of the temperature. The crystallization of theγ form is favored by the presence of stereodefects. The thermal analysis has indicated that the crystalsof γ form melt at lower temperature than the crystals of α form obtained at the same crystallizationtemperature. A comparison between the experimental X-ray diffraction profiles of the melt-crystallizedsamples and the calculated profiles for models of structure has shown that the γ form obtained in thesesamples always presents structural disorder, which produces local situation of packing typical of the αform. The amount of structural disorder decreases with increasing the crystallization temperature andthe degree of stereoregularity of the samples. The presence of disorder in the crystals of γ form accountsfor their lower melting temperature. This analysis indicates that metallocene isotactic polypropylenecrystallizes in a continuum of disordered modifications intermediate between the α and γ forms, and theamount of disorder present in the crystals depends on the crystallization conditions and on thestereoregularity of the sample.
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