On the form II of syndiotactic polypropylene

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 C-13 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 sample's 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 degrees C) gives fibers with a mixture of crystals of forms I and II.