NMR AND DIPOLE-MOMENT INVESTIGATION OF THE CONFORMATIONAL DISTRIBUTION IN A LIQUID-CRYSTAL FORMING POLYMER CONTAINING AN ASYMMETRIC CARBON IN THE FLEXIBLE SPACER

The conformational distribution of the main chain liquid crystal polym er, poly(3-((methyltrimethylene)oxy)trimethylene p,p'-bibenzoate) has been studied in solution by analysis of NMR vicinal coupling constants and the dipole moment. Analysis of the temperature dependence of the coupling constants for the methylene groups adjacent to the asymmetric carbon in the glycol spacer, measured between -45 degrees C and 45 de grees C, yields values for the conformational energies for each of the bonds flanking the asymmetric carbon. Spectra recorded in deuterated chloroform and 1,4-dioxane-d(8) yield similar values for the coupling constants, indicating that solvent polarity does not significantly inf luence the conformational distribution. Because of uncertainty in the assignment of NMR signals, two sets of energies are obtained for each bond. This ambiguity is partly resolved by analysis of the dipole mome nt. An experimental value of 0.73 is obtained for the dipolar correlat ion coefficient, g, in dioxane at 30 degrees C, which is consistent wi th energies E-sigma beta = 0.5 +/- 0.3 kJ mol(-1) and E-sigma alpha = -1.3 +/- 0.4 kJ mol(-1) for the bond adjacent to the ether linkage. Te ntative values of E-sigma alpha p = 0.0 +/- 0.30 and E-sigma beta p = 1.1 +/- 0.4 kJ mol(-1) are obtained for the neighboring bond attached to the eater group, based on the NMR analysis and comparison with conf ormational energies in poly(3-methyloxetane), which possesses a simila r structural unit.