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 (T
g) of 390–479 °C,
reasonable coefficients of thermal expansion of 36–61 ppm K–1, and excellent optical transparency (T
400 > 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.
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