Isotactic polypropylene. A structure with non parallel chain axes.

The crystal structure of the y form of isotactic polypropylene (y-iPP) is refined with the Rietveld method on X-ray diffraction data collected at low (-120 "C) temperature. The analysis, leading to the proposal of the totally novel crystal architecture with nonparallel chain axes, is discussed in detail, and the reliability of the proposed structure is assessed, also with reference to alternative models. While the overall structure is best represented in terms of the statistical copresence of anticlined isochiral helices at each crystallographic position, as implied by space group Fddd, local packing modes which cannot retain this feature are satisfactorily described in terms of space groups F2dd or Fd2d. Some relevant implications of the yisotactic polypropylene crystal structure on the crystalline morphology of this polymer are presented, while issues concerning the development of this novel architecture remain largely open to future contributions