On the form IV of syndiotactic polypropylene

The packing of the chains in (T(6)G(2)T(2)G(2))(n) conformation of the form TV of s-PP is revisited on the basis of packing energy and structure factor calculations. According to this analysis, an alternative mode of packing has been suggested. A monoclinic structural model, with the unit cell centered on the C face, is obtained, after small changes of the atomic coordinates in the triclinic structural model as proposed by Chatani et al. The monoclinic model presents a lower packing energy than the triclinic model and a good agreement between the calculated and observed structure factors. The triclinic structural model implies that all the chains are rotated by the same amount around the chain suds with respect to the monoclinic structural model. Since clockwise and counter clockwise rotations are equivalent, the monoclinic structural model may be taken as descriptive of the order in the long range, for the form IV of s-PP, or in other terms, descriptive of an average structure (space group C2, unit cell constants equal to a(m), = 14.17 Angstrom, b(m), = 5.72 Angstrom c(m), = 11.6 Angstrom, and beta(m), = 108.8 degrees). The triclinic structural model for this polymorph, instead (space group P1, unit cell constants equal to a(t), = 5.72 Angstrom, b(t) = 7.64 Angstrom, c(t), = 11.60 Angstrom, alpha(t), = 73.1 degrees, beta(t), = 88.8 degrees, gamma(t), = 112.0 degrees) is probably more properly descriptive of local situation of order (the symmetry, locally, is broken). Analogies between the monoclinic limit ordered structural model for the form IV and the orthorhombic limit ordered structural model for the form II (with chains in the more stable (TTGG)(n) conformation) of s-PP are also provided. (C) 1998 John Wiley & Sons, Inc.