Synthesis and characterization of main-chain liquid crystalline copolyesters containing phosphaphenanthrene side-groups

A series of thermotropic main-chain liquid crystalline copolyesters P1–P5 containing 2-(6-oxide-6H-dibenz〈c,e〉〈1,2〉oxaphosphorin-6-yl)-1,4-dihydroxyphenylene (ODOPB) as non-mesogenic unit and 4,4′-dihydroxybiphenyl (DB) as mesogenic unit were prepared by polycondensation with sebacoyl chloride. The thermal and mesogenic properties were characterized by differential scanning calorimetry (DSC), polarizing optical micrography (POM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). It was found that introducing of ODOPB into polyester decreased the melting temperature (Tm) and the isotropic temperature (Ti). The copolyesters P1–P5 exhibited mesomorphic properties even when the molar ratio of the non-mesogenic monomer ODOPB to mesogenic monomer DB reached 6:4 in feed. For explaining the effect of DOPO group on the mesomorphic properties, the X-ray diffraction analysis of a low molecular weight modal compound containing ODOPB was carried out and the results show that the DOPO side-group is parallel with the most-adjacent benzene ring in the main-chain, which reveals the small effect of ODOPB on the mesogenic structure of polyesters. The thermal analysis showed that the incorporation of phosphorus-containing moieties increase the residual char of the formed liquid crystalline polymers. The thermal stability, however, decreases because the OP–O bond is less stable than the common C–C bond.