Synthesis and characterization of novel 3,6-di[3?,5?-bis(trifluoromethyl)phenyl]pyromellitic dianhydride for polyimide synthesis

A novel dianhydride monomer, 3,6-di[3',5'-bis(trifluoromethyl)phenyl]pyromellitic dianhydride (12FPMDA), was synthesized via a three-step process: (1) the preparation of 3,5-bis(trifluoromethyl)benzene boronic acid (6FBB) and 3,6-dibromo-1,2,4,5-tetramethylbenzene (2B4MB) via Grignard and bromination reactions, respectively; (2) the Suzuki cross-coupling reaction of 6FBB and 2B4MB, resulting in 3,6-di[3',5'-bis(trifluoromethyl)phenyl]tetramethylbenzene (12F4MB); and (3) the oxidation and cyclodehydration of 12F4MB to afford 12FPMDA. 12FPMDA was then characterized by Fourier transform infrared (FTIR), 1 H NMR, 19 F NMR, elemental analysis, and a melting-point apparatus, and it was used to prepare polyimides with aromatic diamines such as 1,1-bis(4-aminophenyl)-2,2,2-trifluoroethane and 4,4'-diaminodiphenylether. Polyimides were synthesized via a two-step process: (1) the preparation of poly(amic acid) in p-chlorophenol with isoquinoline and (2) solution imidization at the reflux temperature for 12 h. They were designed to have molecular weights of 20,000 g/mol via off-stoichiometry. The resulting polyimides were characterized by FTIR, NMR, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis, and their solubility, solution viscosity, water absorption, coefficients of thermal expansion (CTEs), and dielectric constants were also evaluated. The polyimides exhibited excellent solubility even in acetone and toluene, high glass-transition temperatures (> 311 °C), good thermal stability (> 518 °C in air), and well-controlled molecular weights (19,000-21,000 g/mol). They also provided low CTEs (35-50 ppm/°C), water absorption (1.26-1.35 wt %), and dielectric constants (2.49-2.52).