Polyimides Based on Aromatic Diisocyanates Containing Pendent Flexible Alkoxy Chains and Aromatic Dianhydrides: Synthesis, Characterization, and Liquid-Crystal Alignment Properties Anjana Sarkar, Mahadeo R. Halhalli, Arun D. Kulkarni, Prakash P. Wadgaonkar Polymer Science and Engineering Division, National Chemical Laboratory, Pune 410008, India Received 17 April 2008; accepted 21 August 2008 DOI 10.1002/app.29388 Published online 2 January 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Four new aromatic diisocyanates containing pendent flexible alkoxy chains, namely, 5-butyloxy-1,3- phenylenediisocyanate, 5-octyloxy-1,3-phenylenediisocya- nate, 5-dodecyloxy-1,3-phenylenediisocyanate, and 5-hexa- decyloxy-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 0 -oxydiphthalic anhydride, 3,3 0 ,4,4 0 -(hexa- fluoroisopropylidene) diphthalic anhydride, and 3,3 0 ,4,4 0 - benzophenonetetracarboxylic dianhydride—in benzonitrile in the presence of 1,4-diazabicyclo[2.2.2]octane as a cata- lyst to obtain a series of polyimides with inherent viscos- ities in the range 0.16–0.43 dL/g. Except for 3,3 0 ,4, 4 0 -benzophenonetetracarboxylic dianhydride based poly- imides, all of the polyimides were found to be soluble in dichloromethane, m-cresol, 1-methyl-2-pyrrolidinone, tetra- chloroethane, and pyridine either at room temperature or on heating and could be cast into tough, flexible, transpar- ent 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 pend- ent flexible alkoxy chains were in the range 159–246  C as measured by differential scanning calorimetry. The tem- perature for the 10% weight loss of the polyimides was in the range 410–460  C in a nitrogen atmosphere, which indi- cated good thermal stability. A pretilt angle of 2.97  was observed for the polyimide derived from 5-octyloxy-1, 3-phenylenediisocyanate and 4,4 0 -oxydiphthalic anhydride. V V C 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 461–472, 2009 Key words: high performance polymers; polyimides; thermal properties INTRODUCTION Polyimides exhibit excellent thermal and mechanical properties and have extensive engineering and micro- electronics applications. 1 Because conventional aro- matic polyimides are insoluble, these polymers are usually processed as the corresponding soluble pol- y(amic acid) precursors and then either thermally or chemically imidized. However, there are some prob- lems because of the instability of poly(amic acid)s and the liberation of water in the imidization process. Therefore, soluble polyimides that can be easily proc- essed are desired. There are several approaches that have been tried with varying degrees of success to improve the solubility/processability of aromatic pol- yimides. 2–6 The attachment of flexible side chains has drawn particular interest in polyimide synthesis because it increases not only the processability of polyimides but also the pretilt angle of liquid-crystal molecules on rubbed polymer surfaces. 7–9 Consider- ing the two most routinely practiced routes for the synthesis of polyimides, namely, the diamine–dianhy- dride reaction and the diisocyanate–dianhydride reac- tion, three options are available for attaching flexible side chains to aromatic polyimides. These are (1) dia- mine modification, 10–12 (2) dianhydride modification, 9 and (3) diisocyanate modification. 13–18 The purpose of this study was to synthesize aro- matic polyimides based on meta-oriented aromatic diisocyanates containing pendent flexible alkoxy chains of various carbon lengths by polycondensation with commercially available aromatic dianhydrides and to study the effect of pendent flexible alkoxy chains on the properties of the resulting polyimides. 5-Alkoxy- 1,3-phenylenediisocyanates, namely, 5-butyloxy-1,3- phenylenediisocyanate (BPDC), 5-octyloxy-1,3-phenyl- enediisocyanate (OPDC), 5-dodecyloxy-1, 3-phenylene- diisocyanate (DDPDC), and 5-hexadecyloxy-1,3- phenylenediisocyanate (HDPDC), were synthesized and polycondensed with aromatic dianhydrides, namely, 4,4 0 -oxydiphthalic anhydride (ODPA), 3,3 0 ,4,4 0 - (hexafluoroisopropylidene) diphthalic anhydride (FDA), and 3,3 0 ,4,4 0 -benzophenonetetracarboxylic Journal of Applied Polymer Science, Vol. 112, 461–472 (2009) V V C 2009 Wiley Periodicals, Inc. Correspondence to: P. P. Wadgaonkar (pp.wadgaonkar@ ncl.res.in).