Crystal structure and morphology of syndiotactic polypropylene.

Large, highly regular single crystals of syndiotactic polypropylene (sPP), grown for the first time and examined by electron microscopy and electron diffraction, reveal extra reflections and characteristic streaks that had not been reported in the published X-ray structure analysis of Corradini, Natta, et al.(2) Based upon these findings, we propose that the unit cell is orthorhombic with a=14.50angstrom, b=11.20angstrom, c=7.40angstrom, space group Iba2; this unit cell has a strong subcell with b=5.60angstrom. The new cell symmetry results from the presence of both left-and right-handed helices, while the previously proposed unit cell (2) (having the C222 sub 1 space group) would apply only if all helices were of the same hand. We show that in actual crystals both packing schemes coexist, leading to intermolecular-lattice disorder and thereby to the observed streaking during electron diffraction. The habit of sPP single crystals is rectangular with the b axis of the unit cell parallel to the long dimension of the crystals.