Synthesis and characterization of unsymmetrical benzonitrile‐containing polyimides: Viscosity‐lowering effect and dielectric properties

ABSTRACT A new unsymmetrical diamine, 2‐(3‐aminophenoxy)‐6‐(4‐aminophenoxy)benzonitrile (3,4‐APBN), is synthesized via two consecutive S N Ar reactions and the temperature‐dependent reactivity of the fluorides in 2,6‐difluorobenzonitrile, whose first S N Ar reaction occurs at 70 °C and second, at 100 °C, allowing timing control of reaction sequence and circumventing the transetherification side reaction. Thus, a series of polyimides (PIs) is prepared from the polymerization of 3,4‐APBN with five common dianhydrides (6FDA, DSDA, OPDA, BTDA, and PMDA). For comparison, a second series is also prepared from two symmetrical diamines ([2,6‐bis(3‐aminophenoxy)benzonitrile (3,3‐APBN) and 2,6‐bis(4‐aminophenoxy)benzonitrile (4,4‐APBN)] and 6FDA or PMDA. The processability of the poly(amic acids) (PAAs), for the first series is greatly improved since their solution viscosities are much lower than PAAs based on symmetrical diamines. Besides having high glass‐transition temperatures (249–332 °C), and thermal stability [5% weight loss in the range of 505–542 °C (air) and 512–546 °C (nitrogen)], these PIs form tough, transparent and flexible films that have a tensile‐strength range of 82.1–121.3 MPa, elongations‐at‐break of 5.33–9.81%, and tensile moduli of 2.11–2.97 GPa. Their film dielectric constants are 3.08–3.62 at 10 kHz, moderately higher than that (2.92) of analogous PI (CP2) without nitrile groups. Overall, we found that the reduction of structural symmetry in repeat units can improve the polymer processibility as well as increasing their dielectric constants. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51 , 4998–5011