Chain Distortion of m-Linked Aromatic Polymers:  Poly(m-phenylene) and Poly(m-pyridine)

The crystal structures of poly(m-phenylene) (PMP) and poly(m-pyridine) [poly(pyridine-2,6-diyl)] (PMPy) have been analyzed by the linked-atom Rietveld method for X-ray powder diffraction profiles in order to demonstrate the effect of hydrogen repulsion on the planarity of aromatic polymer chains. Chain conformation of PMP is a slowly winding helix with a five-ring structural repeat, in which the neighboring aromatic rings are rotated owing to the hydrogen repulsion by torsion angles of average value 42°, in contrast to the alternately twisted conformation of poly(p-phenylene). The four helices are packed in a tetragonal unit cell with dimensions a = b = 13.45 Å and c = 11.30 Å (space group: P41212 or P43212), and they engage with one another. The helical structure, isomorphous to that of m-deciphenyl determined by single-crystal X-ray analysis, is stabilized by the interchain interactions. The PMPy chain assumes the anticoplanar conformation relieved from hydrogen repulsion. The two chains are contained in an orthorhombic unit cell with dimensions a = 12.23 Å, b = 3.92 Å, and c = 7.06 Å (space group: Pnam) in a manner similar to orthorhombic polyethylene.