The structural characterization of samples of syndiotactic polypropylene (s-PP) isothermally crystallized from the melt is presented. Form I of s-PP, with variable amount of disorder, is obtained at every crystallization temperature. The limit ordered modification, characterized by a fully antichiral packing along both axes of the unit cell and by a monoclinic symmetry P21/a, is obtained only at high values of the crystallization temperature (Tc > 135 °C). At crystallization temperatures in the range 120-135 °C, modifications characterized by disorder in the regular alternation of left-and right-handed helices along both axes of the unit cell are formed. At Tc < 120 °C, disorder in the stacking of bc layers of helices piled along a, implying a shift of b/4 among consecutive layers (b/4 shifts disorder), can also arise. The amount of both kinds of disorder depends on the crystallization temperature and increases in the as-prepared samples and in samples rapidly crystallized by quenching the melt. These samples are crystallized in a disordered modification which can be described by the statistical space group Bmcm. This model is characterized by a statistical disorder in the substitution of enantiomorphous helices in each site of the lattice, although local correlation about the chirality of neighboring chains may still be preserved. This correlation is lost in the long range because of the occurrence of the b/4 shifts disorder. A "continuum" of intermediate disordered modifications of form I exists among the limit statistical disordered form Bmcm and the limit ordered monoclinic form P21/a. In these modifications, the chains are already rotated around their axes by nearly 5°and displaced along c according to the packing pattern of the limit ordered modification P21/a.

The conditions for the crystallization of form II of syndiotactic polypropylene are described. As-prepared, melt-crystallized, and oriented-fiber specimens of different samples of syndiotactic polypropylene having different degrees of stereoregularity, prepared with a metallocene-based catalyst as well as with the traditional Ziegler−Natta vanadium-based catalyst, are analyzed by X-ray diffraction and solid-state 13C NMR CPMAS spectroscopy. According to this analysis, the crystallization of form II is favored in samples having a low degree of stereoregularity. Indeed, as-prepared and melt-crystallized samples of highly stereoregular syndiotactic polypropylene always crystallize in form I, whereas less stereoregular samples present small amounts of crystals of form II and/or a mode of packing of form II occurs as a defect in a prevailing mode of packing of form I. The pure form II has been obtained by stretching compression-molded specimens of samples having low stereoregularity, prepared with the metallocene- or vanadium-based catalysts. By stretching the most stereoregular samples obtained with the metallocene catalyst, fibers in the trans planar form III are obtained. However, also for the higher stereoregular sample, the pure form II can be obtained upon release of the tension in fibers initially in the trans planar form III. Under these conditions a transition from form III into the isochiral helical form II occurs. Annealing fiber samples of form II or form III at high temperatures (100 °C) gives fibers with a mixture of crystals of forms I and II.

Isothermal crystallizations from the melt of samples of syndiotactic polypropylene having different degrees of syndiotacticity have been performed. The thermal behavior and the structural features of the melt-crystallized samples have been analyzed by differential scanning calorimetry and X-ray diffraction. According to this analysis, less stereoregular samples, crystallized in disordered modifications of form I with defects implying local arrangements of the chains such as in form II, present recrystallization phenomena during heating. For most stereoregular samples relevant recrystallization phenomena are absent. Extrapolation to the equilibrium melting temperature of syndiotactic polypropylene was attempted by using the Hoffmann−Weeks method, taking into account the occurrence of recrystallization and superheating phenomena. A value of nearly 182 °C has been obtained for the equilibrium melting temperature of the fully syndiotactic polypropylene.

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