Control of Head/Tail Isomeric Structure in Polyimide and Isomerism‐Derived Difference in Molecular Packing and Properties

Two sequence isomeric poly(amic acid)s (PAAs) are successfully synthesized from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride and unsymmetrical 5(6)‐amino‐2‐(4‐aminobenzene) benzimidazole (PABZ). The syntheses are based on the site‐selective reactivity of head/tail amino groups of PABZ and solubility differences of PABZ in good solvent (dimethyl sulfoxide, DMSO) and poor solvent (N‐methyl‐2‐pyrrolidone, NMP). The proton nuclear magnetic resonance (1H‐NMR) results reveal that the content of head tail–head tail (HTHT) bonding units in PAA‐DMSO (PAA synthesized in DMSO) is 37%, while this content increases to 54% in PAA‐NMP (PAA synthesized in NMP). The wide‐angle X‐ray diffraction (WAXD) results indicate polyimide (PI)‐NMP film with high HTHT content exhibits a semicrystalline structure, while PI‐DMSO film is amorphous. Moreover, PI‐NMP also shows higher in‐plane orientation than PI‐DMSO. The ordered molecular packing and higher in‐plane orientation of PI‐NMP lead to an increase in mechanical properties and a decrease in in‐plane thermal expansion coefficient.