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Polyimides based on aromatic diisocyanates containing pendent flexible alkoxy chains and aromatic dianhydrides: Synthesis, characterization, and liquid-crystal alignment properties



Four new aromatic diisocyanates containing pendent flexible alkoxy chains, namely, 5-butyloxy-1,3-phenylenediisocyanate, 5-octyloxy-1,3-phenylenediisocyanate, 5-dodecyloxy-1,3-phenylenediisocyanate, and 5-hexadecyloxy-1,3-phenylenediisocyanate, were synthesized by Curtius rearrangement of the corresponding 5-alkoxy-1,3-phenylene diacylazides. These diisocyanates were each polycondensed with three commercially available aromatic dianhydrides: 4,4′-oxydiphthalic anhydride, 3,3′,4,4′-(hexafluoroisopropylidene) diphthalic anhydride, and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride—in benzonitrile in the presence of 1,4-diazabicyclo[2.2.2]octane as a catalyst to obtain a series of polyimides with inherent viscosities in the range 0.16–0.43 dL/g. Except for 3,3′,4,4′-benzophenonetetracarboxylic dianhydride based polyimides, all of the polyimides were found to be soluble in dichloromethane, m-cresol, 1-methyl-2-pyrrolidinone, tetrachloroethane, and pyridine either at room temperature or on heating and could be cast into tough, flexible, transparent films from a solution in m-cresol or tetrachloroethane. X-ray diffractograms revealed that the polyimides with longer alkoxy chains had layered structures. The glass-transition temperature of the polyimides containing pendent flexible alkoxy chains were in the range 159–246°C as measured by differential scanning calorimetry. The temperature for the 10% weight loss of the polyimides was in the range 410–460°C in a nitrogen atmosphere, which indicated good thermal stability. A pretilt angle of 2.97° was observed for the polyimide derived from 5-octyloxy-1,3-phenylenediisocyanate and 4,4′-oxydiphthalic anhydride. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009