LIQUID-CRYSTALLINE POLYMERS WITH A BICYCLO[1.1.1]PENTANE CAGE IN THE BACKBONE

Two series of liquid-crystalline polymers were synthesized, both with mesogenic triads connected by aliphatic spacers varying in length from 4 to 10 carbons. The triads consisted of two residues of p-hydroxybenzoic acid attached to the central diacid, which was bicyclo[1.1.1]pentane-1,3-dicarboxylic acid in one series and terephthalic acid in the other. Properties of these polymers were studied by differential scanning calorimetry, thermogravimetric analysis, and polarizing microscopy. The replacement of the aromatic ring of the terephthalic acid by a bicyclopentane cage led to a very substantial depression of the liquid crystalline to isotropic transition and to a less substantive depression of the melting temperature. Thus, the temperature range of the mesogenic state was much narrower for the bicyclopentane polymers than for the terephthalic ones. Apparently, the bulky bicyclopentane cage may fit well into a crystalline lattice but has lesser tendency for forming parallel bands of molecules in the mesogenic state. This is probably a result of the free rotation around the backbone bonds attached to the bridgehead carbons of the bicyclopentane cage as compared to the rotation of the terephthalic residue that is restricted by extensive conjugation within the mesogenic groups.