Synthesis and characterization of a thermotropic liquid-crystalline poly[2,5-bis(4′-alkoxycarbonylphenyl) styrene]

We report the synthesis and characterization of a series of novel mesogen-jacketed liquid-crystalline polymers, poly[2,5-bis(4′- alkoxycarbonylphenyl)styrene]s (1-m, where m is the number of carbon atoms in the alkyl tails), along with the corresponding monomers, 2,5-bis(4′- alkoxycarbonylphenyl)styrenes (2-m), and their precursors, 2,5-bis(4′- alkoxycarbonylphenyl)toluenes (3-m). The influence of the tail length on the thermotropic properties of the two types of low-molecular-mass compounds and macromolecules was investigated with a combination of differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray diffraction techniques. Except for compound 3-3, which exhibited a monotropic nematic phase, all members of the low-molar-mass molecules developed no mesophase during both heating and cooling processes. The glass-transition temperatures of the polymers decreased as the tail lengths increased. The 5% weight loss temperatures of all the polymers under a nitrogen atmosphere were above 360 °C, indicating quite high thermal stability. Although polymers 1-1 and 1-2 were non-liquid-crystalline, columnar nematic phases were observed for the remaining homopolymers with longer alkyl tails. The mesophases of 1-3 to 1-9 that developed at high temperatures remained upon cooling to room temperature, whereas those of 1-10 to 1-12 disappeared during the cooling process.