High organosolubility and optical transparency of novel polyimides derived from 2′,7′-bis(4-amino-2-trifluoromethylphenoxy)-spiro (fluorene-9,9′-xanthene)

A novel spiro(fluorene-9,9′-xanthene) skeleton bis(ether amine) monomer, 2′,7′-bis(4-amino-2-trifluoromethylphenoxy)-spiro(fluorene-9,9′-xanthene), was prepared through a simple acid-catalyzed condensation reaction of 9-fluorenone with resorcinol to form the spiro framework through an sp3 carbon atom. Subsequent nucleophilic substitution reaction of spiro[fluorene-9,9′-(2′,7′-dihydroxyxanthene)] with 2-chloro-5-nitrobenzotrifluoride in the presence of potassium carbonate in N,N-dimethylacetamide, was followed by catalytic reduction with hydrazine and Pd/C in ethanol. A series of new polyimides were synthesized from the diamine with various commercially available aromatic tetracarboxylic dianhydrides via a conventional two-stage process with the thermal or chemical imidization of the poly(amic acid) precursors. Most of the polyimides obtained from both routes were soluble in many organic solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide and m-cresol. All the polyimides could afford transparent, flexible, and strong films with low moisture absorptions of 0.35–0.64% and low dielectric constants of 2.63–3.09 at 1 kHz. Thin films of these polyimides showed an UV–vis absorption cutoff wavelength at 356–384 nm, and those of polyimides from 4,4′-oxydiphthalic dianhydride and 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) were essentially colorless. The polyimides exhibited excellent thermal stability, with decomposition temperatures (at 10% weight loss) above 540 °C in both air and nitrogen atmospheres and glass transition temperatures (Tg) in the range of 274–323 °C.