Synthesis and Characterization of Main Chain Liquid Crystalline Polyesters Containing 2,5-Bis(4-hexyloxybenzoyloxy)hydroquinone and Terephthalate Units

The synthesis and characterization of new liquid crystalline polyesters havebeen described containing symmetrically di-substituted hydroquinone monomer.2,5-Bis(4-hexyloxybenzoyloxy) hydroquinone (HQ6) and1,6-bis(4-hydroxybenzoyloxy) hexane (H6) monomers were synthesized via multi-steproutes from 2,5-dihydroxyquinone and in one step route from 4-hydroxybenzoic acid, respectively. Copolyester PHQ6-co-PH6 was synthesized through high temperature solution polymerization of HQ6 and H6 diols with terephthaloyl chloride. The homopolyesters PHQ6 and PH6 were also synthesized from respective monomers with terephthaloyl chloride by the same method. The molecular weights of polyesters containing the disubstituted hydroquinone units were not very high due to the sterichindrance of the bulky substituting groups on the hydroquinone HQ6 monomer. The homopolyester PHQ6 was soluble in common organic solvents, whereas solubility decreased for copolyester due to decreased content of end hexyloxy groups. Liquid crystalline properties were determined by differential scanning calorimetery (DSC) and polarizing optical microscopy (POM) techniques. The effect of the rigid lateral moieties on the thermal properties and liquid crystallinity was investigated on the prepared copolymer and it was compared with that of the analogous homopolyesters. Optical microscopy images showed Schlierene texture characteristic of the nematic phase for both polyester and copolyesters. The DSC experiments were also in accord ance with mesophase formation. The melting points and glass transition temperature values of PHQ6 and PHQ6-co-PH6 were significantly higher than those of the PH6 homopolyester. All of the obtained compounds were characterized by conventional spectroscopic methods.