New polymer syntheses, 17. Cis/trans isomerism of 1,4‐cyclohexanedicarboxylic acid in crystalline, liquid‐crystalline and amorphous polyesters

Abstract Dimethyl 1,4‐cyclohexanedicarboxylate was treated with various potential isomerization catalysts under reflux (ca. 280°C). Lithium cloride and sulfonic acids caused rapid isomerization. Also the diphenyl ester and 1,4‐cyclohexandicarbonyl dichloride were isomerized, and in all cases a thermodynamically controlled equilibrium with 66 ± 2% trans ‐isomer was obtained. The same cis/trans ratio was found for the molten polyesters prepared from 1,4‐butanediole, 1,6‐hexanediole or phenylhydroquinone and 1,4‐cyclohexanedicarboxylic acid. The polyester, prepared at 20°C from phenylhydroquinone and trans ‐1,4‐cyclohexanedicarbonyl dichlorid, exhibits a mesogenic phase above 250°C; yet, this LC‐phase is thermodynamically unstable and isomerizes to an isotropic melt containing 66 ± 2% trans ‐isomer. However, when the free diacid or the crystalline polyester from hydroquinone and 1,4‐cyclohexanedicarboxylic acid are heated or synthesized at ca. 300°C, a content of 100% trans ‐isomer is the thermodynamical optimum. This is also true for the LC‐phase of polyesters prepared from methyl‐ or chlorophydroquinone. The trans content of LC‐copolyesters, containing variable mole ratios of monomeric units of hydroquinone and phenylhydroquinone, varies with these mole ratios between 66 and 100%. Obviously, the coexistence of LC‐domains containing 100% trans ‐isomer and isotropic domains containing 66% trans ‐isomer are responsible for this result.