STRUCTURE OF BETA-ISOTACTIC POLYPROPYLENE - A LONG-STANDINGSTRUCTURAL PUZZLE

The structure of beta-isotactipc olypropylene (beta-iPP) as been investigated taking into consideration X-ray and electron-diffraction data supplemented by packing energy calculations. The structure is characterized by extensive disorder, and two structural models can be envisaged both are based on domains of helices all of the same chirality, arranged on a pseudohexagonal lattice. The simplest satisfactory model is trigonal (P3121 and the enantiomorphic P3221) with a = b = 11.03 A and c = 6.49 A. Three monomers form the asymmetric unit, and six chains, each with a 0.5 occupancy factor, coexist in the unit cell. This is a consequence of statistical directional disorder, just like in the and ycrystalline modifications of isotactic polypropylene. Coupling of the chiral domains across (200) or (220) glide planes of the trigonal cell can result either in enantiomorphic twins or, if the domains are coherent, in a new achiral orthorhombic lattice with a = 11.03 A, b = 19-10 A, and c = 6.49 A. The two models, which reasonably account for all the available experimental evidence concerning beta-iPP are only marginally different and can probably coexist. Preliminary packing-energy calculations confirm that both arrangements are acceptable. It is further suggested that similar structures based on incoherently or coherently coupled enantiomorphic domains of modest size are very likely in chiral crystalline polymorphs of achiral macromolecules