**3.2 Model reaction**

A model reaction was conducted to investigate the suitability of a nucleophilic addition and cyclodehydration of the diamine monomer in the polymerization reaction condition as well as to obtain a model compound as a reference material for a structural analysis. The diamine monomer was reacted with two-equivalent of phthalic anhydride in *m*-cresol in the presence of a catalytic amount of isoquinoline

**Figure 3.** *FTIR spectra of mono-nitro (2) and diamine (3) compounds [48].*

(**Figure 4**), and all reaction steps were examined by thin layer chromatography. Despite the difference in the electron density between the two amines resulting from the unsymmetrical structure, the two amines had sufficient nucleophilicity to react with the phthalic anhydride. As a result, the corresponding diamic acid form was generated quantitatively within 0.5 h at room temperature, and cyclodehydration was completed within 0.5 h at 190°C. Finally, diimide model compound (**4**) was obtained quantitatively. The structure of **4** was confirmed by FTIR, <sup>1</sup> H NMR and 13C NMR spectroscopy (**Figure 5**). The FTIR spectrum of the model compound shows absorption bands at 1789, 1380, and 722 cm�<sup>1</sup> corresponding to the C]O imide stretching, CdN imide stretching, and imide ring deformation, respectively, without the characteristic absorptions of the amino groups. The <sup>1</sup> H and 13C NMR spectra also supported the formation of the diimide model compound. Owing to the unsymmetrical presence of CF3 groups on the product, proton and carbon peaks on both phthalimide units appeared with different chemical shifts in the NMR spectra, in which the peaks of phthalimide connected to a trifluoromethylated phenyl ring appeared further downfield due to the electron-withdrawing characteristic of the CF3 groups. All spectroscopic data obtained were in good agreement with the predicted structure.

The structure features of **4** were further detailed by single-crystal X-ray diffraction, and its X-ray quality crystals were obtained by the slow evaporation of saturated tetrahydrofuran/water solution at room temperature [48]. As shown in **Figure 6**, the solid-state structure of **4** also ensures the coupling reaction of **3** to the phthalic anhydrides. It is noteworthy that the plane of a phenyl moiety is almost orthogonally located relative to the adjacent *m*-(CF3)2Ph plane such that the biphenyl of **4** had a rigid but twisted structure with a dihedral angle (*θ*) of 76°. Compared with 2,2<sup>0</sup> -bis(trifluoromethyl)benzidine (*θ* = 59°) [27], the benzidine unit of **4** is more distorted. Therefore, it was expected that the obtained polyimides would exhibit good solubility and transparency with high mechanical and thermal properties, as they have a rigid but twisted structure with bulky CF3 groups, thereby reducing the interchain interactions.
