X-RAY-DIFFRACTION STUDY OF THE STRUCTURE OF POLY(ETHYLENE 4,4'-BIPHENYLDICARBOXYLATE)

The crystal structure of the mesophase polymer poly(ethylene 4,4'-biph enyldicarboxylate), [-C6H4-COO-CH2CH2-OOC-C6H4-]n. (P2BP), was establi shed using the model compound approach. The model compound, diethyl 4, 4'-biphenyldicarboxylate (2BP2), belongs to the triclinic system (a = 6.0625(2), b = 6.7033(3), c = 9.8917(3) angstrom; alpha = 97.62(1), be ta = 99.82(1), gamma = 101.30(1)degrees), space group P1BAR. The struc ture of 2BP2 was solved by direct methods and refined by a least-squar es procedure to R = 5.60%. The 2BP2 molecule is centrosymmetric with t he side chain in the trans conformation and a disordered aromatic ring system. The structure of the parent polymer, P2BP, was deduced from i ts X-ray fiber pattern. P2BP has a triclinic unit cell (a = 5.75, b = 3.82, c = 14.62 A; alpha = 90.1, beta = 90.3, gamma = 78.1-degrees), a nd the space group is P1BAR. The geometrical and conformational parame ters derived from 2BP2 were used to generate an initial polymer chain which was then packed into its unit cell. The chain position and orien tation were then refined. The final R value reached 12.4%. The P2BP po lymer adopts a nearly fully extended conformation. The biphenyl moiety is planar, while the carboxylate plane is tilted by 5-degrees with re spect to the biphenyl plane. In the packing of both 2BP2 and P2BP, the chains are parallel to one another and the aromatic rings are face-to -face. However, in 2BP2, the molecules are displaced along the main mo lecular axis, while in P2BP, the chains are shifted sideways. It is su ggested that, between 337 and 367-degrees-C, i.e., in the mesophase, P 2BP is nematic with a face-to-face disposition of the rings.