Thermotropic polyesters, copolyesters and model compounds based on terephthalic acid and 4,4′‐alkylenediphenols

Main chain thermotropic polyesters 1a and 1b and random copolyesters 1ab, based on terephthalic acid and 4,4′‐hexamethylenediphenol (5a) or 4,4′‐decamethylenediphenol (5b) were synthesized in good yields by a solution reaction in presence of benzenesulfonyl chloride. A mechanism is discussed. Low molar mass models 4a and 4b and oligomers 2a, 2b, 3a and 3b, with the same mesogenic unit and end groups comparable to the spacers used in polymers 1, were synthesized and their liquid‐crystalline properties compared to those of polymers. Models with at least two mesogenic units and polymers exhibit both smectic and nematic mesophases. Increasing spacer length decreases dramatically the stability of the nematic mesophases, but lowers only slightly the melting points. The phase diagram for random copolyesters 1ab were determined by DSC. Broad endotherms with overlapping multiple peaks were found for most of the copolymers. Monomeric units of 1a and 1b cocrystallize in 1ab, which leads only to slight melting point depression. The variations of nematic‐isotropic transition versus composition of copolymers are linear, while the smectic phase stability appreciably decreases.