New polyimides containing methyl benzamidobenzoate or dimethyl benzamidoisophthalate as bulky pendant groups. Effects on solubility, thermal and gas transport properties

This work reports the synthesis of six new polyimides obtained by the reaction of two novel diamine monomers: methyl 4-(3,5-diaminobenzamido)benzoate and dimethyl 5-(3,5-diaminobenzamido)isophthalate, bearing one or two methyl ester groups in their phenyl rings, respectively, with three known dianhydrides, 4,4′-(hexafluoroisopropylidene) diphatalic anhydride (6FDA), 4,4′-(dimethylsilanediyl) dipthalic anhydride (SiDA) and 4,4′-oxydiphthalic anhydride (ODPA). The polyimides were synthesized using the “two-step” method, which first involves forming a poly(amic acid) precursor that is subsequently cyclodehydrated to form the imide linkage. The yields were higher than 95% in all cases, and the structural characterization was performed by spectroscopic techniques like FT-IR, ¹H, ¹³C, and ²⁹Si NMR. In addition, the effect on the solubility in common organic solvents and thermal and gas transport properties of these polyimides were studied as a function of the nature of the respective side group and the dianhydride used.