Synthesis and properties of new fluorene-based polyimides containing trifluoromethyl and isopropyl substituents

Abstract

A new fluorene-based diamine monomer containing pendant trifluoromethyl and isopropyl substituents, 9,9-bis[4-(4-amino-2-trifluoromethylphenoxy)-3-isopropylphenyl]fluorene (3), was successfully synthesized through a three-steps reaction using 2-isopropylphenol and 9-fluorenone as starting materials. Then it was employed to prepare a series of fluorinated polyimides (5a–5d) with various commercial aromatic dianhydrides via a one-step high-temperature polycondensation. These polymers could be quickly dissolved in some strong polar organic solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and N,N-dimethylformamide, chloroform and dichloromethane at room temperature. Flexible and transparent films can be obtained conveniently by solution-casting. Their films exhibited high thermal stability with glass transition temperatures in the range of 239–287 °C, and good optical transparency with the cutoff wavelength in the range of 326–363 nm. They also revealed low dielectric constants with the values in the range of 2.65–2.84 at 1 MHz measured for their capacitance. In comparison with some trifluoromethyl-substituted polyimides without the isopropyl substituents, these polyimides showed better solubility and lower dielectric constants and cutoff wavelength.