NMR-based structural characterization of common aromatic poly(amic acid) polymers

Polyamic acid (PAA) has been widely synthesized as the precursor of polyimide for over 50 years. PAA has had recent attention when used as a final polymer in a variety of applications including sensors, flexible electronics, drug delivery and food packaging. In all cases, structural modifications are sought to advance usability of PAA polymers, where chemistry of the polymers plays a key role for further modifications. However, there is a gap in literature that would discuss structural and cis-/trans-isometric characterizations of the PAA polymers. In this study, common PAA polymers synthesized from the couples of (1) pyromellitic dianhydride and 4,4′-oxydianiline, (2) pyromellitic dianhydride-4-sulfonyldianiline, (3) 4,4′-carbonyldiphtalic dianhydride and 4,4′-oxydianiline and (4) pyromellitic dianhydride and 4,4′-ethylenedianiline were characterized with 1D and 2D NMR and supported with ATR-FTIR techniques. The findings revealed that both diamine and dianhydride sources along with the solvent choice are crucial for cis-/trans-binding of diamine to dianhydride monomers, average molecular weight and intramolecular interactions. The current study, to the best of our knowledge, is the first report on structural annotation of common PAA polymers along with the effect of the solvent on the isomers and average molecular weight.