**2.3 Polymerization**

**PI-1.** Diamine monomer **3** (0.4012 g, 1.253 mmol) and BPDA (0.3689 g, 1.254 mmol) were initially dissolved in 4.8 mL of *m*-cresol to a concentration of 16 wt% in a 25 mL three-necked round-bottom flask equipped with a nitrogen inlet, a Dean-Stark trap, and a mechanical stirrer. After the mixture was stirred at room temperature for 30 min, isoquinoline (ca. 5 drops) was added, and further stirring was conducted at room temperature for 4 h. After the solution was diluted with 4.8 mL of *m*-cresol to a concentration of 8 wt%, the temperature was raised to 190°C slowly and the reaction mixture was stirred for 12 h at this temperature. During this time, the water released during the imidization process was removed by distillation as chlorobenzene/water azeotrope, and small amount of *m*-cresol (total additional volume = 4.8 mL) was added periodically to maintain proper viscosity of the reaction mixture. After cooling to room temperature, the solution was diluted with *m*-cresol and then slowly poured into an excess of vigorously stirred ethanol. The resulting polymer was collected by filtration, washed with ethanol, and then dried in vacuo at 180°C for 12 h (0.7444 g, 100% yield). FTIR (thin film, cm<sup>1</sup> ): 1779 (asym C]O str); 1726 (sym C]O str); 1476 (aromatic C]C); 1373 (CdN str); 1123–1190 (CdF in CF3); 738 (imide ring deformation). <sup>1</sup> H NMR (DMSO-*d*6, 400 MHz, 100°C, ppm): 8.54–8.34 (m, 6H), 8.25–8.04 (m, 2H), 7.66 (d, *J* = 8.1 Hz, 2H), 7.56 (d, *J* = 8.5 Hz, 2H). <sup>1</sup> H NMR (THF-*d*8, 400 MHz, 55°C, ppm): 8.49 (d, *J* = 18.6 Hz, 2H), 8.37 (s, 4H), 8.24–8.08 (m, 2H), 7.73 (d, *J* = 7.9 Hz, 2H), 7.51 (d, *J* = 8.6 Hz, 2H). Anal. calcd for C30H12F6N2O4: C, 62.29; H, 2.09; N, 4.84. Found: C, 61.51; H, 2.06; N, 4.78.

**PI-2.** The same procedure used for **PI-1** was repeated with 0.4017 g (1.254 mmol) of **3**, 0.4043 g of (1.255 mmol) of BTDA, and 5 mL of *m*-cresol. Before heating the reaction mixture to 190°C, the solution was diluted with 5 mL of *m*cresol and there was no injection of the additional solvent (0.7538 g, 99.1% yield). FTIR (thin film, cm<sup>1</sup> ): 1783 (asym C]O str); 1732 (sym C]O str); 1679 (diaryl ketone of BTDA); 1476 (aromatic C]C); 1376 (CdN str); 1137–1191 (CdF in CF3); 723 (imide ring deformation). <sup>1</sup> H NMR (DMSO-*d*6, 400 MHz, 25°C, ppm): 8.40 (s, 2H), 8.35–8.14 (m, 6H), 7.60 (s, 4H). Anal. calcd for C31H12F6N2O5: C, 61.40; H, 1.99; N, 4.62. Found: C, 61.20; H, 2.08; N, 4.44.

**PI-3**. The same procedure used for **PI-1** was repeated with 0.4030 g of (1.258 mmol) of **3**, 0.3907 g (1.259 mmol) of ODPA, and 4.9 mL of *m*-cresol. Before heating the reaction mixture to 190°C, the solution was diluted with 4.9 mL of

*Synthesis and Properties of Fluorinated Polyimides from Rigid and Twisted… DOI: http://dx.doi.org/10.5772/intechopen.92010*

*m*-cresol and there was no injection of the additional solvent (0.7548 g, 100% yield). FTIR (thin film, cm<sup>1</sup> ): 1782 (asym C]O str); 1729 (sym C]O str); 1475 (aromatic C]C); 1375 (CdN str); 1276, 1239 (dOd); 1140–1190 (CdF in CF3); 745 (imide ring deformation). <sup>1</sup> H NMR (DMSO-*d*6, 400 MHz, 25°C, ppm): 8.36 (s, 2H), 8.14 (dt, *J* = 17.8, 6.7 Hz, 2H), 7.77–7.62 (m, 4H), 7.56 (s, 4H). Anal. calcd for C30H12F6N2O5: C, 60.62; H, 2.03; N, 4.71. Found: C, 60.71; H, 2.01; N, 4.66.

**PI-4**. The same procedure used for **PI-1** was repeated with 0.4003 g (1.250 mmol) of **3**, 0.5560 g (1.252 mmol) of 6-FDA, and 6 mL of *m*-cresol. Before heating the reaction mixture to 190°C, the solution was diluted with 6 mL of *m*cresol without injection of any additional solvent (0.8961 g, 98.4% yield). FTIR (thin film, cm<sup>1</sup> ): 1789 (asym C]O str); 1733 (sym C]O str); 1476 (aromatic C]C); 1375 (CdN str); 1145–1193 (CdF in CF3); 722 (imide ring deformation). <sup>1</sup> H NMR (DMSO-*d*6, 400 MHz, 25°C, ppm): 8.34 (s, 2H), 8.28 (t, *J* = 7.8 Hz, 1H), 8.23 (t, *J* = 8.0 Hz, 1H), 8.04 (t, *J* = 6.6 Hz, 1H), 7.98 (t, *J* = 6.2 Hz, 1H), 7.80 (dd, *J* = 18.6, 15.8 Hz, 2H), 7.56 (s, 4H). Anal. calcd for C33H12F12N2O4: C, 54.41; H, 1.66; N, 3.85. Found: C, 55.43; H, 1.56; N, 3.77.

**PI-5**. The same procedure used for **PI-1** was repeated with 0.4062 g (1.268 mmol) of **3**, 0.2768 g (1.269 mmol) of PMDA, and 4.2 mL of *m*-cresol. After the solution was diluted with 4.2 mL of *m*-cresol to a concentration of 8 wt%, the temperature was raised to 190°C slowly. The solution became turbid and heterogeneous as soon as the temperature reached 190°C. The heterogeneous reaction mixture was further stirred for 12 h at this temperature. After cooling to room temperature, the solution was poured into an excess of vigorously stirred ethanol. The solid polymer powder was collected by filtration, washed with ethanol, and then dried in vacuo at 180°C for 12 h (0.6392 g, 100% yield). FTIR (KBr, cm<sup>1</sup> ): 1783 (asym C]O str); 1728 (sym C]O str); 1477 (aromatic C]C); 1367 (CdN str); 1125–1192 (CdF in CF3); 724 (imide ring deformation). Anal. calcd for C24H8F6N2O4: C, 57.38; H, 1.61; N, 5.58. Found: C, 56.81; H, 1.70; N, 5.46.

### **2.4 Preparation of polyimide films**

An *N*,*N*-dimethylacetamide (DMAc) solution of the polymers (7.5 wt%) was prepared at room temperature. The DMAc solution was filtered and cast onto a glass plate. The solvent was evaporated in a vacuum oven at room temperature for 5 h and then heated to 180°C for 10 h to remove the residual solvent. To measure the refractive indices of the polyimides, the DMAc solution of the polymers (2.5 wt%) was filtered and cast onto a silicon substrate and then dried in the same manner described above.

## **3. Results and discussion**

### **3.1 Monomer syntheses**

The diamine monomer, 2,6-bis(trifluoromethyl)benzidine (**3**), was prepared in two steps, as reported previously (**Figure 1**) [48]. In the first step, 2-bromo-5-nitro-1,3-bis(trifluoromethyl)benzene (**1**) was reacted with 4-aminophenylboronic acid through a Suzuki coupling reaction in the presence of Pd as a catalyst to produce **2**. The mono-nitro compound was quantitatively converted to the corresponding diamine monomer **3** by hydrogenation with hydrogen in the presence of Pd/C catalyst.

The chemical structures of **2** and **3** were confirmed by <sup>1</sup> H and 13C NMR, FTIR, and an elemental analysis. **Figure 2** shows the NMR spectra of **2** and **3**. Through a reduction reaction, the chemical shifts of proton *H*<sup>1</sup> and carbon *C*<sup>1</sup> moved upfield because they were more shielded by the change of the substituents from the NO2

**Figure 1.** *Synthesis of the unsymmetrical diamine monomer.*

**Figure 2.** *(a) <sup>1</sup> H and (b) 13C NMR spectra of 2 and 3 (DMSO-*d*6, 25°C) [48].*

group to the NH2 group. Meanwhile, there were obvious differences in the chemical shifts between the two amine groups in monomer **3**. The chemical shifts of proton *H*<sup>5</sup> and carbon *C*<sup>1</sup> in **3** appear further downfield compared to those of *H*<sup>4</sup> and *C*<sup>9</sup> due to the deshielding effect of the electron withdrawing CF3 groups, indicating that the amine group located far away from CF3 groups in monomer **3** has a higher electron density and greater nucleophilic reactivity than in the opposite case. The FTIR

*Synthesis and Properties of Fluorinated Polyimides from Rigid and Twisted… DOI: http://dx.doi.org/10.5772/intechopen.92010*

spectra of **2** and **3** are shown in **Figure 3**. The compound **2** gave characteristic bands at 3499, 3401 cm<sup>1</sup> (NdH stretching), at 1620 cm<sup>1</sup> (NdH bending), and at 1333– 1533 cm<sup>1</sup> (NO2 asymmetric and symmetric stretching). After the reduction, the characteristic absorptions of the nitro group disappeared, and the amino group exhibited a pair of NdH stretching bands in the region of 3221–3486 cm<sup>1</sup> and an NdH bending band at 1640 cm<sup>1</sup> . All spectroscopic data obtained were in good agreement with the predicted structures.
