A theoretical investigation of the solid-state synthesis of (SN)x

Abstract

The solid‐state polymerization of S₂N₂ to (SN)_x is investigated as it relates to the molecular, crystallographic, and defect structures of this superconducting polymer. The least motion principle predicts that polymerization should occur by a nonunique reaction to form cis–trans polymer chains normal to the 2₁ axis of the precursor phase. The origin of the disordered structure observed in partially polymerized S₂N₂ is explained by the occurrence of two symmetry equivalent reaction modes, which produce chains related by a center of symmetry. Both monomer and polymer phases have 2/m point symmetry, but the twofold axes are orthogonal. Consequently, the statistical symmetry of (SN)_x is mmm, which is manifest (as a function of thermal history) either by twinning, molecular scale disorder, the statistical nature of the relationship between S₂N₂ and (SN)_x crystal orientations, or a combination of these possibilities. In agreement with prediction, this symmetry is produced by equal population of the twin crystals previously observed in (SN)_x. Coordinates associated with disordered chains in partially polymerized S₂N₂ are successfully predicted by this reaction theory. Fractionally occupied sites are observed in the polymer corresponding to (SN)_x chains with different orientations, but the same chain axis direction. The geometry of one of these chains is substantially different from that of the major chain component.