Soluble polyimides based on long-chain alkyl groups via amide linkages

The synthesis and characterization of a novel series of soluble polyimides bearing long-chain alkyl groups on their side chains using aromatic diamine monomers such as N-alkyl-3,5-diaminobenzamide (ADBA-X, X = carbon numbers of alkyl chain, 9 similar to 14) and N-(3,5-diaminophenyl)-3,4,5-tris(alkoxy)benzamide (DPABA-X, X = 6,12) are described. Polyimides obtained from 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA) and ADBA-9 similar to 14 were insoluble, however, copolyimides based on BTDA, ADBA-9 similar to 14, and 4,4'-diaminodiphenylether (DDE) were soluble. The solubility of these copolyimides may be improved by the entropy effect of long-chain linear alkyl groups as well as the randomizing effect based on copolymerization. The effect of ADBA for the enhancement of solubility was compared with a series of functional diamines bearing long-chain alkyl groups via various linkage groups reported from our laboratory such as alkoxydiaminobenzene (AODB, ether linkage), alkyldiaminobenzophenone (ADBP, benzoyl linkage) and diaminobenzoic acid alkylester (DBAE, ester linkage), and it is concluded that the effect of functional diamines are increased as AODB (ether linkage) > ADBP (benzoyl linkage) > ADBA (amide linkage) > DBAE (ester linkage). The polyimides and copolyimides based on BTDA, DPABA-6 or DPABA-12, and DDE containing 50 mol % or more DPABA were soluble, showing that the effect of DPABA for the enhancement of solubility was larger than ADBA. It is speculated that the three long-chain alkyl groups in DPABA enhance the solubility of polyimides. Above polyimides and copolyimides were soluble in various polar solvents and exhibited good heat-resistance.