CONFORMATIONAL STUDIES ON MODEL COMPOUNDS OF POLYAMIDES WITH ETHER GROUPS IN THEIR STRUCTURE

The synthesis of 1,5-dibenzamido-3-oxapentane (DEBA) and 1,8-dibenzami do- 3,6-dioxaoctane (TEBA), model compounds of polyamides with repeati ng units [-HNCOC6H4CONH(CH2)2O(CH2)2-] and [-HNCOC6H4-CONH(CH2)20(CH2) 2O(CH2)2-1, respectively, is reported. The mean-square dipole moment, [mu2], of both compounds was measured in dioxane solutions in the inte rval of temperatures 30-60-degrees-C. The values of this quantity lie in the ranges 24.6-24.5 D2 and 25.4-26.0 D2, for DEBA and TEBA, respec tively. The analysis of the spectral patterns of the model compounds i ndicates that the energy E(sigma') of gauche states about the CH2-CH2 bonds adjacent to the amide groups is 0.44 +/- 0.08 kcal mol-1 below t hat of the alternative trans states. Moreover, semiempirical calculati ons show that gauche states about NH-CH2 bonds are also preferred over the alternative trans, in contrast with CH2-O bonds in polyesters, wh ere the opposite occurs. The critical interpretation of the dipole mom ents of both DEBA and TEBA by the rotational isomeric state (RIS) mode l suggests that their polarities are very sensitive to the modulus of the dipole moments associated with the amide groups, but they are near ly insensitive to their orientation. Calculations of the dipolar distr ibution and the corresponding average energies carried out for DEBA an d TEBA suggest that the conformers of higher polarity also have the hi ghest energy.