Synthesis of copolybibenzoates with thioether and ether groups in the flexible spacers

Abstract

Main-chain copolybibenzoates with flexible spacers bearing thioether and ether groups were obtained by melt-transesterification of diethyl 1,1 -biphenyl-4,4 -dicarboxylate and β-thioether glycols (thiodiethylene glycol or thiotriethylene glycol), using titanium (IV) isopropoxide as catalyst. A sidereaction of etherification occurred simultaneously with the polycondensation reaction, resulting in the formation of comonomer units with the structure of the dimer of the glycol. This side-reaction was confirmed by the preparation of model compounds. The most probable mechanism for this etherification reaction is the formation of an intermediate three-membered cyclic sulfonium ion by neighbouring sulfur participation. The thermal and X-ray characterization of the synthesised copolymers indicated that a smectic mesophase was rapidly obtained on cooling the copolymers derived from thiodiethylene glycol from the melt and this phase was transformed slowly into a more ordered structure. In contrast, copolymers derived from thiotriethylene glycol did not exhibit liquid-crystalline phases and their crystallisation from the isotropic melt was very fast. It was found that the copolymerization increased the mesophase stability. Finally, the results obtained were compared with those of corresponding homopolybibenzoates with methylene or oxymethylene spacers.