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$14) and N-(3,5-diaminophenyl)-3,4,5-tris(alkoxy)benzamide (DPABA-X, X = 6,12) are described. Polyimides obtained from 3,3 0 ,4,4 0 -benzophenonetetracarboxylic dianhydride (BTDA) and ADBA-9$14 were insoluble, however, copolyimides based on BTDA, ADBA-9$14, and 4,4 0 -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.

ABSTRACT: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 0 ,4,4 0 -benzophenonetetracarboxylic dianhydride (BTDA) and DBAE-8-14, and copolyimides based on BTDA, DBAE-8-14, and 4,4 0 -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 0 -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. [doi:10.1295/polymj.PJ2005257] KEY WORDS Polyimide / Soluble Polyimide / Diaminobenzamide Alkylester / Branched Alkyl Group / Copolymerization / Thermal Stability / Solubility / Polyimides exhibit excellent thermal and mechanical properties, and have extensive engineering and microelectronics applications.1 Aromatic polyimides such as pyromellitic polyimides are prepared from aromatic diamines and aromatic tetracarboxylic dianhydrides via poly(amic acids). Since conventional aromatic polyimides are insoluble, these polymers are usually processed as the corresponding soluble poly-(amic acid) precursors, and then either thermally or chemically imidized. However, owing to the instability of poly(amic acids) and the liberation of water in the imidization process, problems can arise. Extensive research has been recently carried out to improve the solubility of polyimides. We have reported the synthesis and characterization of soluble polyimides and copolyimides based on alicyclic dianhydride such as 2,3,5-tricarboxycyclopentyl acetic dianhydride (TCA-AH), 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (cyclohexene-DA), and 4-(2,5-dioxotetrahydrofuran-3-yl)-tetralin-1,2-dicarboxylic anhydride (tetralin-DA). [25][26][27] Conventional tetracarboxylic dianhydrides such as 3,3 0 ,4,4 0 -benzophenone tetracarboxylic dianhydride (BTDA) and alkyldiaminobenzophenone (ADBP) having long-chain linear alkyl groups with 9-14 carbon atoms (ADBP-9-14), 28 and BTDA and alkyloxydiaminobenzene (AODB) having long-chain linear alkyl groups with 10-14 carbon atoms (AODB-10-14) have also been used (Scheme 1). 29 It was concluded that ADBP with an even number of carbon atoms was effective in enhancing the solubility, while polymers based on ADBP with an odd number of carbon atoms remained insoluble. AODB bearing alkyl groups via an ether linkage are mo...