Soluble polyimides based on diaminobenzoic acid alkylester

The synthesis and characterization of a novel series of soluble polyimides, that are based on diaminobenzoic acid alkylester (DBAE) having long-chain alkyl groups with 8-14 carbon atoms, are described. Polyimides obtained from 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and DBAE-8-14, and copolyimides based on BTDA, DBAE-8-14, and 4,4'-diaminodiphenylether (DDE) were insoluble in polar solvents such as N-methyl-2-pyrrolidone (NMP) and the effect of long-chain linear alkyl groups for the enhancement of solubility was not recognized. However, it was found that two methods improve the solubility. One is the use of branched alkyl groups such as nonan-5-yl and 2,6-dimethylheptane-4-yl, and the other is the use of relatively flexible diamine co-monomer, 4,4'-diaminodiphenylmethane (DDM). These polyimides and copolyimides based on DBAE were soluble in various polar solvents and exhibited good thermal stability in air and under nitrogen. The effect of DBAE for the enhancement of solubility was lower than the effects of alkyloxydiaminobenzene (AODB) and alkyldiaminobenzophenone (ADBP), probably due to the rigid ester linkage group in DBAE.