Packing of molecules of poly-p-phenylenebenzo-bis-oxazole in crystalline structures: Calculation by the molecular mechanics method

Abstract

The mutual packing of trans- and cis-stereoisomeric molecules of poly-p-phenylenebenzo-bisoxazole is calculated by the molecular mechanics method. By varying all intramolecular and intermolecular parameters, energetically favorable structures are found. Calculation is performed both for molecules with uniform rotational isomeric composition (only TRANS or CIS mutual orientation of heterocycles along the polymer chain) and for molecules with random alternation of rotational isomers. In ordered structures, all molecules are shown to have a flat conformation and they are packed so that their planes are shifted along and perpendicular to the direction of molecular axes. The shifts can be similar [towards one (Δ) or another (−Δ) side] and alternating (±Δ). For trans-TRANS molecules with a homogeneous rotational isomeric composition, longitudinal shifts Δ and −Δ are not equivalent because, in one case, similar heteroatoms of neighboring molecules appear to be the most closely positioned, whereas in the other case, this is true of different heteroatoms. As a result, different types of molecular packing develop: in the first case, structures with parallel mutual orientation of molecular planes form and, in the second case, the structures are characterized by parquet arrangement. When trans-TRANS molecules are packed with an alternating longitudinal shift, the mutual packing of molecules shows the parquet pattern. At the same time, for cis-stereoisomeric molecules with homogeneous or arbitrary rotational isomeric composition, only the parquet pattern in their mutual arrangement is observed. This conclusion disagrees with experimental evidence according to which, in crystalline structures, the planes of molecules are parallel to each other. For the above structures, the packing energy calculated with allowance for the experimental values of cell parameters appears higher than that for the structures under study. The difference in energy exceeds 80 kJ/mol (per one monomer unit). The experimentally observed type of crystalline structure is assumed to be conceived even at the stage of nucleation of crystal-solvate phases.