Thermotropic liquid-crystalline poly(oxadiazole)s with poly(methylene) spacers: Preparation and extraordinary odd-even effect

A novel synthetic procedure for the preparation of poly(oxadiazole)s was developed with nucleophilic substitution of α,ω-alkanediols with oxadiazole-activated bisfluoride. Seven poly(oxadiazole)s were successfully prepared by the solution polymerization of 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole and various α,ω-alkanediols [HO-(CH₂)_n-OH, n = 5-10 or 12] in diphenyl sulfone at temperature greater than 230°C with K₂CO₃ as a catalyst. The reduced viscosities of the poly(oxadiazole)s were 0.14-0.51 dL/g, and the decomposition temperatures were greater than 350°C and decreased from 436 to 379°C with increasing spacer length (n). Corresponding model compounds, consisting of two terminal mesogenic 2,5-bisphenyl-1,3,4-oxadiazole units and central poly(methylene) spacers, were also prepared for comparison. Both the polymers and model compounds exhibited an extraordinary odd-even effect: odd ones showed higher transition temperatures (melting and clearning temperatures). With differential scanning calorimetry, polarized optical microscopy (POM), and X-ray diffraction, we found that the nematic mesophase was the only texture in the melts except for the polymers with longer methylene units (n = 9), in which smectic mesophases were observed.