Colorless Polyimides from Fluorinated Ladder Diamines Containing Norbornyl Benzocyclobutene Segments

Three fluorinated ladder diamines with norbornyl benzocyclobutene (N2BC) segments were prepared by catalytic norbornene-arene annulation (CANAL) reactions of norbornadiene with fluorinated bromoanilines. NMR results revealed that two regioisomers were obtained for each diamine, with syn-to-anti ratios of ∼2:1. High-temperature solution polycondensation of these diamines with commercial dianhydrides produced a series of colorless polyimides (CPIs). These polymers exhibited extremely high glass-transition temperatures (Tg) of 390–479 °C, reasonable coefficients of thermal expansion of 36–61 ppm K–1, and excellent optical transparency (T400 > 78% and b* < 2.2). These outstanding properties were attributed to the incorporation of fluorinated substituents as well as the rigid but contorted N2BC segments. For certain dianhydrides, the CANAL-F/CF3-derived CPIs exhibited the best optical properties due to their highest fluorine contents, while the CANAL-2F-derived CPIs exhibited the best thermal and mechanical properties, despite the lowest optical transmittance as a result of their most pronounced inter- and intramolecular interactions.