STRUCTURE OF A MESOMORPHIC POLYMER - POLY(HEXAMETHYLENE 4,4'-BIPHENYLDICARBOXYLATE) .1. THE MODEL COMPOUNDS

In order to establish the structure of the title polymer, P6BP, two mo del compounds, di-n-hexyl 4,4'-biphenyldicarboxylate (6BP6) and hexame thylene di-4-biphenylcarboxylate (BP6BP) were synthesized and their cr ystal structures were established by X-ray diffraction. The unit cell of 6BP6 is orthorhombic (a = 7.754 (2), b = 11.428 (5), and c = 54.39 (4) Angstrom; space group Pbca). The crystal structure of 6BP6 was sol ved from single crystal diffraction data (R = 0.067). BP6BP is dimorph ic: the a-form, whose structure was established (R = 0.043), has a mon oclinic unit cell (a = 7.1039 (3), b = 8.1298 (3), c = 22.2308 (4) Ang strom, and a 94.531 (4)degrees; space group P2(1)/c); the beta-form, w hich is obtained upon cooling of the melted alpha-form, is also monocl inic (a = 13.42, b = 7.59, c = 12.95 Angstrom, and beta = 95.4 degrees ). The aliphatic segments in both 6BP6 and alpha-BP6BP are in the all- trans conformation. The biphenyl dihedral angles are 35.5 degrees and 21.1 degrees for 6BP6 and alpha-BP6BP, respectively. The equivalent fi ber repeats, calculated from both model compounds, are 19.568 (12) and 19.599 (6) Angstrom for 6BP6 and BP6BP, respectively. In the crystal, the 6BP6 molecules adopt a parallel disposition and the aromatic grou ps are arranged in a face-to-face pattern. The alpha-BP6BP molecules p ack in layers; the chains in the adjacent layers have different orient ations, while the biphenyl rings adopt the herringbone pattern.