The α" "superstructure" of syndiotactic polystyrene:: A frustrated structure

The crystal structure of syndiotactic polystyrene (sPS) in its stable alpha form (a so-called "superstructure" named alpha " is reevaluated. As shown by all previous investigations, the trigonal unit cell contains three triplets of extended sPS chains. The azimuthal orientation of the three triplets is shown to differ from those assumed so far, which generates a clear-cut example of frustrated packing. The azimuthal settings and relative shifts of the triplets indicate that two triplets maximize their interactions, while the third one interacts less favorably with its neighbors. This frustrated packing scheme fully justifies the three-triplets unit cell and constitutes an interesting variant of related frustrated packings previously described for isochiral three-fold helices.