Determination of the crystal structure of isotactic cis-1,4-poly(1,3-hexadiene) by X-ray diffraction and molecular mechanics

The crystal structure of isotactic cis-1,4-poly(1,3-hexadiene) has been determined through a combination of X-ray diffraction analysis and molecular mechanics. Two reasonable values for unit cell parameters were obtained from the fiber diffraction pattern of a well-oriented sample. Energy maps performed on a molecular model of the polymer chain yielded an absolute minimum corresponding to a conformation having the internal parameters similar to those of an s(2/1) helical polymer chain. Packing energy minimizations performed for all the orthorhombic and monoclinic space groups having crystallographic s(2/1) chain symmetry indicate unequivocally that the best mode of packing is obtained for a crystal structure characterized by a unit cell with parameters a = 8.98 angstrom, b=7.82 angstrom, and c=8.10 angstrom in the P2(1)2(1)2(1) space group. The calculated powder and fiber diffraction patterns of this structural model are, in very good agreement with the experimental diffraction patterns. Similarities and differences with the crystal structures of isotactic cis-1,4-poly(1,3-pentadiene) and isotactic cis-1,4-poly(2-methyl-1,3-pentadiene) are discussed.