Thermotropic polyesters with main chain isomeric naphthylene, trans‐1,4‐cyclohexylene, and 2,5‐pyridinediyl units: The importance of the order of ester linkages in the mesogenic units

Two new series of thermotropic polyesters were prepared and their mesomorphic properties were investigated. The polymers consist of triad aromatic ester‐type mesogenic units with decamethylene spacers. The mesogenic units of the first series are composed of isomeric dihydroxynaphthalene moieties as the central structure flanked by two p‐oxybenzoyl groups. In the second series the mesogenic units were of reversed ester linkages: the central moieties, derived from 1,4‐naphthalene dicarboxylic acid, trans‐1,4‐cyclohexanedicarboxylic acid, or 2,5‐pyridinedicarboxylic acid, are connected on both sides to p‐phenylene structures. Two low molecular weight model compounds with 1,4‐naphthylene unit at the center of the mesogenic unit were synthesized and their mesomorphic properties were compared with those of corresponding polymers. It was observed both for the model compounds and the polymers containing 1,4‐naphthylene units that the linking order of the ester group in the mesogenic unit exerted a decisive influence on the capability for the formation of a mesophase. Thermal and mesomorphic properties were investigated by DSC, on a polarizing microscope equipped with a hot‐stage, and by visual observation of stir‐opalescence of the melts.