STRUCTURE OF BETA-ISOTACTIC POLYPROPYLENE - A LONG-STANDING STRUCTURAL PUZZLE

The structure of beta-isotactic polypropylene (beta-iPP) has been inve stigated taking into consideration X-ray and electron-diffraction data supplemented by packing energy calculations. The structure is charact erized by extensive disorder, and two structural models can be envisag ed: both are based on domains of helices all of the same chirality, ar ranged on a pseudohexagonal lattice. The simplest satisfactory model i s trigonal (P3(1)21 and the enantiomorphic P3(2)21) with a = b = 11.03 angstrom and c = 6.49 angstrom. Three monomers form the asymmetric un it, and six chains, each with a 0.5 occupancy factor, coexist in the u nit cell. This is a consequence of statistical directional disorder, j ust like in the alpha- and gamma-crystalline modifications of isotacti c 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 l attice with a = 11.03 angstrom, b = 19.10 angstrom, and c = 6.49 angst rom. The two models, which reasonably account for all the available ex perimental evidence concerning beta-iPP, are only marginally different and can probably coexist. Preliminary packing-energy calculations con firm that both arrangements are acceptable. It is further suggested th at similar structures based on incoherently or coherently coupled enan tiomorphic domains of modest size are very likely in chiral crystallin e polymorphs of achiral macromolecules.