Low Dielectric Polyetherimides Derived from Bis[4-(4-(4-aminophenoxy)-2-tert-butylphenoxy)phenyl] Sulfone and 4,4'-Bis[4-(4-aminophenoxy)-2-tert-butylphenoxy]perfluorobiphenyl.

Two diamines, bis[4-(4-(4-aminophenoxy)-2-tert-butylphenoxy)phenyl] sulfone (PSNH2) and 4,4'-bis[4-(4-aminophenoxy)-2-tert-butylphenoxy)perfluorobiphenyl (PFNH2), were prepared from 3-tert-butyl-4-hydroxyanisole using a four-step procedure, including two nucleophilic substitutions, demethylation, and catalytic reduction. On the basis of PSNH2 and PFNH2, two series of low dielectric polyetherimides (PEIs) were prepared. Both series of PEIs exhibit moderate-to-high thermal properties, including a glass transition temperature (T g) > 259 °C (depending on the rigidity of dianhydride), a 5 wt % decomposition temperature (T d5%) > 496 °C, and a coefficient of thermal expansion < 66 ppm/°C. Because of the hydrophobic tert-butyl phenylene oxide structure, both series of PEIs show excellent dielectric properties, with a dielectric constant as low as 2.4-2.7. The structure-property relationship of both series of PEIs is discussed in this work.

Introduction

Because of the strong intermolecular interactions and chain rigidity, polyimides exhibit high-temperature stability, oxidative properties, hydrolytic stability, and mechanical properties. These characteristics make them widely applicable in various fields including interlayer dielectrics in flexible printed circuit board,[1] optics,[2] and electronics.[3,4] For high-speed calculation and high-frequency communication, materials with low dielectric characteristics are desired because the propagation velocity of the signal is inversely proportional to the square of the dielectric constant of the interlayer dielectric. At present, the commercially available polyimide Kapton is widely used for interlayer dielectrics in flexible printed circuit boards. However, the dielectric constant of Kapton is around 3.2–3.4, which does not meet the requirement for the next generation of interlayer dielectrics. Therefore, research studies on low dielectric polyimides are significant.[5−8]

Four major strategies have been applied to prepare polyimides with low dielectric constant. The first one is to incorporate microporosity because the dielectric constant of air is as low as 1.[9−11] However, the difficulty of having good control of the pore size and distribution and the reduced mechanical strength might hinder the application of this technique. The second approach incorporates bulky pendants that will increase the free volume.[12−15] For example, Li et al. prepared polyimides derived from 2′,7′-bis(4-aminophenoxy)-spiro(fluorene-9,9′-xanthene).[12] The bulky spirofluorene xanthene structure, which loosens the polymer packing, led to polyetherimide (PEI) with a dielectric constant of 2.65 for (hexafluoroisopropylidene)bis(phthalic anhydride) (6FDA)-based PEI. Chern and Tsai prepared low dielectric polyimides from a di-tert-butyl-containing diamine, 2,4-bis(4-aminophenoxy)-2,6-di-tert-butylbenzene.[13] The resulting polyimides exhibited organosolubility and a low dielectric constant (i.e., 2.74–2.92). The third approach incorporates an aliphatic structure into polyimides.[16] For example, Ward et al. prepared aliphatic polyimides and found that the dielectric constant decreased from 2.76 for BPADA/ODA PEI to 2.55 for BPADA/DCHM PEI, in which BPADA is 4,4′-bisphenol A dianhydride, ODA is 4,4′-oxydianiline, and DCHM is an aliphatic diamine, 4,4′-diaminodicyclohexylmethane. However, compared with BPADA/ODA PEI, a 70 °C decrease in degradation temperature was found for BPADA/DCHM PEI.[16] The fourth approach incorporates a fluorine element because of the low polarizability of the C–F bond.[17−19] Hougham et al. prepared a series of polyimides based on 6FDA and various fluorinated diamines. The resulting fluorinated polyimides exhibit a low dielectric constant of 2.5–2.8.[17] Li et al. prepared PEIs with bulky trifluoromethyl and a chloride pendant group and found that the refractive index, which is related to the dielectric constant according to Maxwell’s relationship, decreases with the fluoro content.[18]

Poly(2,6-dimethyl phenylene oxide) (PPO), with hydrophobic dimethyl and low polar phenylene oxide structure, possesses a low dielectric constant of 2.5. Oligo PPOs with allyl, methyl methacrylate, and styrene curable linkage have been widely used for printed circuit boards.[20−22] Taking advantage of the low dielectric characteristic of PPO, Ueda et al. prepared fluorinated polyimides containing phenylene oxide and a perfluorobiphenyl structure.[23] They concluded that the incorporation of the perfluorobiphenyl unit accompanied with a phenylene ether unit effectively decreases the dielectric constant while maintaining the thermal stability of polyimides. In this work, combining the structural advantage of PPO and the second and fourth approaches, two tert-butyl phenylene oxide-containing diamines, bis[4-(4-(4-aminophenoxy)-2-tert-butylphenoxy)phenyl] sulfone (PSNH2) and 4,4′-bis[4-(4-aminophenoxy)-2-tert-butylphenoxy]perfluorobiphenyl (PFNH2), were prepared. On the basis of PSNH2 and PFNH2, two series of low dielectric PEIs were prepared. The structure–property relationship of the PEIs was discussed.

Results and Discussion

Monomer Synthesis

Diamines, PSNH2 and PFNH2, were synthesized from 3-tert-butyl-4-hydroxyanisole through a four-step procedure (Scheme ). The first step is the nucleophilic substitution of 3-tert-butyl-4-hydroxyanisole with bis(4-fluorophenyl)sulfone and decafluorobiphenyl, forming dimethoxy bis[4-(4-methoxyl-2-tert-butylphenoxy)phenyl] sulfone (PSOCH3) and 4,4′-bis[4-(4-methoxyl-2-tert-butylphenoxy)perfluorobiphenyl] (PFOCH3), respectively. The second step is the demethylation of PSOCH3 and PFOCH3, leading to bisphenol bis[4-(4-hydroxyl-2-tert-butylphenoxy)phenyl] sulfone (PSOH) and 4,4′-bis[4-(4-hydroxyl-2-tert-butylphenoxy)]perfluorobiphenyl (PFOH), respectively. Diamines were prepared through the nucleophilic substitution of bisphenol with 4-fluoro-nitrobenzene, followed by catalytic reduction. 1H and 13C NMR spectra were applied to identify the structure of dimethoxy (Figures S1 and S2), bisphenol (Figures S3 and S4), and dinitro (Figures S5 and S6). Figure S7 shows the 1H NMR spectra of (a) PSNH2 and (b) PFNH2. An amino peak was clearly observed at 4.9 ppm in both spectra. 1H–1H correlation spectroscopic spectrum confirms the splitting patterns of both diamines (Figure S8).

Scheme 1

Synthesis of Diamines, PSNH2 and PFNH2

Synthesis of PEIs

The PEIs were prepared by thermal imidization of poly(amic acid) (PAA) that was prepared by low-temperature polymerization of three commercially available dianhydrides with the two diamines. As shown in Table S1, gelation occurred for the PFNH2-based PAA at a normal solid content (15–20 wt %). A homogeneous solution can only be obtained at 5 wt % for pyromellitic dianhydride (PMDA)/PFNH2 PAA. By contrast, a homogeneous solution can be obtained at 20 wt % for all PSNH2-based PAA. Therefore, because of the poor solubility of PFNH2-based PAA, the solubility of PFNH2-based PEIs is expected to be poor and will be discussed later. The molecular weight of PEIs is described in Table S2. The number-average molecular weight (Mn) ranges from 8.2 to 9.2 × 104 g/mol. The weight-average molecular weight (Mw) of PSNH2-based PEI ranges from 22.6 to 23.4 × 104 g/mol. However, PFNH2-based PEI is insoluble in N-methyl-2-pyrrolidinone (NMP) for the gel permeation chromatography (GPC) measurement (see Scheme ).

Scheme 2

Synthesis of PSNH2-Based and PFNH2-Based PEIs

Solubility of PEIs

Table S3 lists the organosolubility data of PEIs. PSNH2-based PEIs display the solubility in NMP, dimethyl sulfoxide (DMSO), and m-cresol upon heating. 6FDA/PSNH2 is even soluble in dichloromethane, chloroform, and tetrahydrofuran (THF) without heating. The conformation of PSNH2-based and PFNH2-based PEIs might be explained by the single-crystal diffractogram of PSOCH3 (Figure a, the detailed diffraction data of PSOCH3 are listed in Tables S4–S8. Note that the single-crystal diffractogram of PSNH2 is not available) and PFOCH3 (Figure b, the detailed diffraction data of PFOCH3 are listed in Tables S9–S13. Note that the single-crystal diffractogram of PFNH2 is not available). As shown in Figure , PSOCH3 displays a non-coplanar conformation of the diphenyl sulfone (ph–SO2–ph) linkage that retards the stacking of PEI chains. This could be responsible for the solubility of PSNH2-based PEIs. By contrast, PFOCH3 exhibits a more coplanar conformation that leads to close stacking of polymer chains. This explains the insolubility of PFNH2-based PEIs.

Figure 1

Single-crystal diffractogram of (a) PSOCH3 and (b) PFOCH3.

XRD of PEIs

Figure S9 shows X-ray diffraction (XRD) patterns for both series of PEIs. According to Bragg’s law, the d-spacing values are 5.40, 5.31, and 5.67 Å for PMDA/PSNH2, 4,4′-oxydiphthalic anhydride (ODPA)/PSNH2, and 6FDA/PSNH2, respectively, and are 4.75, 4.96, and 5.23 Å for PMDA/PFNH2, ODPA/PFNH2, and 6FDA/PFNH2 PEIs, respectively. With the same dianhydride, PSNH2-based PEIs show larger d-spacings than PFNH2-based PEIs. The non-coplanar, bend conformation of diphenyl sulfone causes PSNH2-based PEIs to exhibit larger d-spacings than PFNH2-based PEIs. The XRD d-spacing data are consistent with the solubility data.

Film Quality and Thermal Properties

Figure S10 shows images of both series of PEIs. All of the films are transparent and foldable. Because they are foldable for thermal mechanical analysis, dynamic mechanical analyzer (DMA) and thermomechanical analyzer (TMA) are used to measure the thermal properties. Figure a shows the DMA thermograms of the synthesized PEIs. Depending on the rigidity of dianhydride, the Tg values of the PEIs range from 258 to 274 °C for PSNH2-based PEIs and from 272 to 291 °C for PFNH2-based PEIs (Table ). The coplanar structure of perfluorophenyl, which explains the poor solubility of PFNH2-based PEIs, also explains the higher Tg values of PFNH2-based PEIs than that of PSNH2-based PEIs. For the same diamine, the order of Tg is PMDA-based > 6FDA-based > ODPA-based PEIs. The trend is consistent with the rigidity of the dianhydride moiety. Figure b shows the TMA thermograms of the synthesized PEIs. The Tg values are in the range of 237–265 °C for PSNH2-based PEIs and 244–265 °C for PFNH2-based PEIs (Table ). The trend of the structure–Tg relationship observed in the TMA measurement is the same as that of the DMA measurement. The coefficients of thermal expansion (CTE) are in the range of 60–66 ppm/°C, demonstrating moderate dimensional stability. The tert-butyl, which hinders the packing of PEIs, may explain the CTE value that is higher than common polyimides. The thermal stability of the PEIs was evaluated by TGA in an N2 atmosphere (Figure S11). The 5% decomposition temperatures ranged from 496 to 515 °C for PSNH2-based PEIs and from 508 to 530 °C for PFNH2-based PEIs. The char yields at 800 °C are in the range of 38–50% for PSNH2-based PEIs and 53–57% for PFNH2-based PEIs. Both series of PEIs show moderate-to-high thermal stability.

Figure 2

(a) DMA thermograms and (b) TMA thermograms of the synthesized PEIs.

Table 1

Thermal Properties of the Synthesized PEIs

PEIs ID	E′ (GPa)a	Tg (°C)b	Tg (°C)c	CTE (ppm/°C)d	Td5% (°C)e	char yield (%)f
PMDA/PSNH2	2.3	274	256	60	514	45
ODPA/PSNH2	0.9	259	237	64	496	50
6FDA/PSNH2	0.6	258	240	63	515	49
PMDA/PFNH2	3.0	291	265	65	508	57
ODPA/PFNH2	0.7	272	244	66	527	57
6FDA/PFNH2	0.9	288	261	66	530	53

Measured by DMA at a heating rate of 5 °C/min; the storage modulus (E′) is recorded at 50 °C.

Tg corresponding to the peak temperature of the tan δ curve.

Measured at a heating rate of 5 °C/min by TMA.

CTE recorded from 100 to 200 °C.

Temperature corresponding to 5% weight loss at a heating rate of 20 °C/min by thermogravimetry.

Residual weight % at 800 °C.

Contact Angle

Figure describes the advancing contact angle of water of the synthesized PEIs. The contact angles are 83.11, 81.11, and 85.30° for PSNH2-based PEIs and 87.89, 84.79, and 89.61° for PFNH2-based PEIs. Because of the hydrophobic perfluorobiphenyl moiety, PFNH2-based PEIs display a higher contact angle. In addition, all their values are much higher than that of the ODPA/OPA polyimide (53°), suggesting a hydrophobic characteristic for the synthesized PEIs. We think that the hydrophobic tert-butyl phenylene oxide structure increases the hydrophobicity of the synthesized PEIs, and a low dielectric constant is expected (will be discussed later).

Figure 3

Contact angle of the synthesized PEIs.

Dielectric Properties

Hougham et al. proposed three strategies to prepare polyimides with low dielectric properties: (i) incorporating diamine and dianhydride which minimizes polarizability, (ii) incorporating diamine and dianhydride which imparts a high degree of free volume, and (iii) changing the hydrophobicity which eliminates water.[17] The dielectric constant (Dk) and dielectric loss tangent (Df) of both series of PEIs are listed in Table . The Dk value is 2.6–2.7 for PSNH2-based PEI and is 2.4 for PFNH2-based PEI. The Df value is 0.015–0.016 for PSNH2-based PEI and is 0.009–0.010 for PFNH2-based PEI. The Dk value is much lower than that of Kapton (PMDA/ODA, around 3.2–3.4),[8] showing the low dielectric characteristic of the synthesized PEIs. The Dk value is also lower than that (around 2.68) of ODPA-based PEIs with 2,6-dimethylphenylene oxide and perfluorobiphenyl units.[23] It is known that PPO-based thermoset, such as SA9000 (oligo PPO with the methacrylate chain end)[24] and OPE-2st (oligo PPO with the styrene chain end),[25] exhibits very good dielectric characteristic. Therefore, the bulky tert-butyl group,[26−28] the non-coplanar conformation of diphenyl sulfone (ph–SO2–ph), and phenylene oxide[24,25] explain the low dielectric characteristics of PSNH2-based PEIs, whereas the bulky tert-butyl group, the symmetric perfluorobiphenyl,[23] and phenylene oxide explain the low dielectric characteristics of PSNH2-based PEIs.

Table 2

Contact Angle and Dielectric Properties of the Synthesized PEIs

PEI ID	contact angle (deg)	thickness (μm)	Dk (1 MHz)	Df (1 MHz)
PMDA/PSNH2	83.11	43	2.7	0.015
ODPA/PSNH2	81.11	46	2.6	0.016
6FDA/PSNH2	85.30	51	2.6	0.015
PMDA/PFNH2	87.89	10	NAa	NAa
ODPA/PFNH2	84.79	41	2.4	0.010
6FDA/PFNH2	89.61	46	2.4	0.009

The film is too thin for measurement.

Conclusions

We have successfully prepared two diamines, PSNH2 and PFNH2, with tert-butyl and phenylene oxide structures. The structure of the diamines was successfully analyzed by IR, NMR, and mass spectra. Two series of PEIs, PSNH2-based and PFNH2-based PEIs, were successfully prepared based on the condensation of the two diamines with three commercially available dianhydrides. PSNH2-based PEIs are soluble in most solvents, whereas PFNH2-based PEIs are insoluble in the same solvent. The single-crystal structure of PSOCH3 shows a non-coplanar conformation of the diphenyl sulfone (ph–SO2–ph) linkage, whereas the single-crystal structure of PFOCH3 exhibits a more coplanar conformation that leads to close stacking of polymer chains. X-ray diffraction shows that PSNH2-based PEIs exhibit larger d-spacings than PFNH2-based PEIs. Both results explain the result of solubility. All the PEI films are transparent and foldable. DMA thermograms show that the Tg values of the PEIs range from 258 to 274 °C for PSNH2-based PEIs and from 272 to 291 °C for PFNH2-based PEIs. Contact angle data show hydrophobic characteristics for the synthesized PEIs. Dielectric data show a very low dielectric constant for both PEIs. The bulky tert-butyl group, the non-coplanar conformation of diphenyl sulfone (ph–SO2–ph), and phenylene oxide explain the low dielectric characteristics of PSNH2-based PEIs, whereas the bulky tert-butyl group, the symmetric perfluorobiphenyl, and phenylene oxide explain the low dielectric characteristics of PSNH2-based PEIs.

Experimental Section

Materials

3-tert-Butyl-4-hydroxyanisole was purchased from TCI. Potassium carbonate (K2CO3) was purchased from Showa. 4-Fluoro-1-nitrobenzene and pyridine hydrochloride were purchased from Alfa. Decafluorobiphenyl was purchased from Matrix. Boron tribromide (BBr3) was purchased from Sigma-Aldrich. Bis(4-fluorophenyl)sulfone and palladium on active carbon (Pd/C) were purchased from Acros. PMDA, ODPA, and 4,4′-hexafluoroisopropylidenediphthalic anhydride (6FDA) were purchased from Chriskev and recrystallized from acetic anhydride before being used.

N,N-Dimethylacetamide (DMAc) was purchased from Tedia, purified by distillation under reduced pressure over calcium hydride, and stored over molecular sieves. NMP, ethanol, acetone, chloroform (CHCl3), dichloromethane (CH2Cl2), and THF were of high-performance liquid chromatography (HPLC) grade, purchased from Teida, and used without further purification. Dimethylformamide and m-cresol were of HPLC grade, purchased from Acros, and used without further purification.

Characterization

NMR measurements were performed in DMSO-d6 or CDCl3 using a Varian Inova 600 NMR. The chemical shift of 1H NMR was calibrated by setting the chemical shift of trace amounts of DMSO-d5 in DMSO-d6 as 2.49 ppm and trace amounts of CHCl3 in CDCl3 as 7.20 ppm. IR spectra were performed by a PerkinElmer RX1 infrared spectrophotometer, and the spectra were obtained from at least 64 scans in the standard wavenumber range of 650–4000 cm–1. Melting point measurements were performed using a Fargo Melting Point Apparatus MP-2D and a PerkinElmer DSC 7 in a nitrogen atmosphere at a heating rate of 20 °C/min. The molecular weights of PEIs were measured by GPC, Hitachi LaChrom Elite. The data were measured with NMP as the eluent at 60 °C with a flow rate of 0.6 mL/min, and the molecular weights were calibrated with polystyrene standards. Thermogravimetric analysis (TGA) was measured at a heating rate of 20 °C/min in flowing nitrogen (20 mL/min) using a PerkinElmer Pyris1. TMA was measured with a sample size of 10 mm × 5 mm using an SII TMA/SS6100. The analysis was performed at a heating rate of 5 °C/min from 40 to 350 °C in an ambient atmosphere. The coefficient of thermal expansion (CTE) in the temperature range of 50–150 °C was recorded from the second run. The X-ray crystallographic data were collected on a Bruker D8 VENTURE. High-resolution mass spectrometry (HRMS) measurements were performed using a Finnigan/ThermoQuest MAT 95XL mass spectrometer by electron impact ionization. The dielectric constant of the PEIs was measured at a frequency of 1 MHz, using an HP 4291B rf impedance/material analyzer. Experimental data were obtained at least five times for each test.

Synthesis of Bis[4-(4-methoxyl-2-tert-butylphenoxy)phenyl] Sulfone

Bis(4-fluorophenyl) sulfone (20 g, 78.66 mmol), 3-tert-butyl-4-hydroxyanisole (29.78 g, 78.66 × 2.1 mmol), K2CO3 (21.75 g, 78.66 × 2 mmol), and DMAc (200 mL) were added into a 500 mL round-bottom glass flask equipped with a condenser, a nitrogen inlet, and a magnetic stirrer. The solution was stirred at 120 °C for 12 h. After cooling to 40 °C, the obtained mixture was poured into distilled water. The precipitate was filtered. The crude product was recrystallized from ethanol and dried in a vacuum oven at 100 °C for 8 h. White crystals (95% yield) were obtained. Melting point: 160.7–165.9 °C (onset-end temperature in a differential scanning calorimetry (DSC) thermogram). HRMS (FAB+) m/z: calcd for C34H37O6S 573.23; Anal., 573.2300. 1H NMR (ppm, CDCl3): δ 1.31 (9H, H14), 3.80 (3H, H15), 6.70 (1H, H9), 6.77 (1H, H8), 6.97 (3H, H3, H5, H11), 7.85 (2H, H2, H6). 13C NMR (ppm, CDCl3): δ 30.01 (C14), 34.77 (C13), 55.48 (C15), 111.14 (C9), 111.14 (C11), 117.37 (C3, C5), 122.50 (C8), 129.69 (C2, C6), 134.74 (C12), 143.24 (C7), 147.14 (C4), 156.33 (C10), 162.68 (C1).

Synthesis of Bis[4-(4-hydroxyl-2-tert-butylphenoxy)phenyl] Sulfone

PSOCH3 (10 g, 17.40 mmol) and pyridine hydrochloride (70 g) were added into a 500 mL round-bottom glass flask equipped with a condenser, a nitrogen inlet, and a magnetic stirrer. The solution was stirred at 200 °C for 6 h and was poured into distilled water. The precipitate was filtered. The crude product was recrystallized from toluene and dried in a vacuum oven at 100 °C for 8 h. White crystals (90% yield) were obtained. Melting point: 160.9–165.8 °C (onset-end temperature in a DSC thermogram). HRMS (EI-MS) m/z: [M+] calcd for C32H33O6S, 545.20; Anal., 545.1994. 1H NMR (ppm, DMSO-d6): δ 1.22 (9H, H14), 6.61 (1H, H9), 6.73 (1H, H8), 6.83 (1H, H11), 7.03 (2H, H3, H5), 7.89 (2H, H2, H6), 9.38 (1H, OH). 13C NMR (ppm, DMSO-d6): δ 29.95 (C14), 34.20 (C13), 113.83 (C9), 114.08 (C11), 117.12 (C3, C5), 123.00 (C8), 129.75 (C2, C6), 134.31 (C12), 142.21 (C7), 144.88 (C4), 154.34 (C10), 162.44 (C1).

Synthesis of Bis[4-(4-(4-nitrophenoxy)-2-tert-butylphenoxy)phenyl] Sulfone

PSOH (10 g, 18.29 mmol), 4-fluoro-1-nitrobenzene (5.68 g, 18.29 × 2.2 mmol), K2CO3 (5.06 g, 18.29 × 2 mmol), and DMAc (100 mL) were added into a 250 mL round-bottom glass flask equipped with a condenser, a nitrogen inlet, and a magnetic stirrer. The solution was stirred at 120 °C for 12 h. After cooling to 40 °C, the obtained mixture was poured into ethanol. The precipitate was filtered. The crude product was recrystallized from acetone and dried in a vacuum oven at 100 °C for 8 h. Yellow crystals (90% yield) were obtained. Melting point: 219.2–223.4 °C (onset-end temperature in a DSC thermogram). HRMS (EI-MS) m/z: [M+] calcd for C44H40O10N2S, 788.24; Anal., 788.2399. 1H NMR (ppm, CDCl3): δ 1.32 (9H, H14), 6.88 (2H, H8, H9), 7.00 (2H, H16, H20), 7.04 (2H, H3, H5), 7.15 (1H, H11), 7.90 (2H, H2, H6), 8.19 (2H, H17, H19). 13C NMR (ppm, CDCl3): δ 29.96 (C14), 34.99 (C13), 116.83 (C16, C20), 117.98 (C3, C5), 119.04 (C8), 120.13 (C11), 122.76 (C9), 125.99 (C17, C19), 135.48 (C12), 142.63 (C7), 144.41 (C10), 150.99 (C4, C15), 161.81 (C1), 163.28 (C18).

Synthesis of PSNH2

Bis[4-(4-(4-nitrophenoxy)-2-tert-butylphenoxy)phenyl] sulfone (PSNO2) (5.00 g, 6.34 mmol), Pd/C (0.4 g, 4 wt % S–NO2), and DMAc (50 mL) were added into a 250 mL Parr stirred reactor at 25 °C. The system was purged with nitrogen for half an hour before introducing hydrogen. The pressure of the reactor was maintained at about 140 psi under the hydrogen atmosphere for 24 h. Then, the obtained mixture was filtered to remove Pd/C, and the filtrate was poured into distilled water. The crude product was isolated by filtration and dried in a vacuum oven at 100 °C for 8 h. A white powder (70% yield) was obtained. The melting point was not measured by DSC. HRMS (EI-MS) m/z: [M+] calcd for C44H43O6N2S, 727.28; Anal., 727.2839. 1H NMR (ppm, DMSO-d6): δ 1.24 (9H, H14), 4.99 (2H, NH2), 6.59 (2H, H17, H19), 6.61 (1H, H9), 6.79 (2H, H16, H20), 6.86 (1H, H8), 6.97 (1H, H11), 7.07 (2H, H3, H5) 7.90 (2H, H2, H6). 13C NMR (ppm, CDCl3): δ 29.80 (C14), 34.37 (C13), 114.55 (C9), 114.84 (C17, C19), 115.66 (C11), 117.43 (C3, C5), 120.94 (C16, C20), 123.04 (C8), 129.84 (C2, C6), 134.60 (C15), 142.65 (C12), 145.40 (C7, C10), 147.17 (C4), 155.74 (C18), 162.00 (C1).

Synthesis of 4,4′-Bis[4-(4-methoxyl-2-tert-butylphenoxy)perfluorobiphenyl]

Decafluorobiphenyl (10 g, 29.33 mmol), 3-tert-butyl-4-hydroxyanisole (10.79 g, 29.33 × 2.2 mmol), K2CO3 (8.272 g, 29.33 × 2 mmol), and DMAc (100 mL) were introduced into a 250 mL round-bottom glass flask equipped with a condenser, a nitrogen inlet, and a magnetic stirrer. The solution was stirred at 75 °C for 24 h. After cooling to 40 °C, the obtained mixture was poured into distilled water. The precipitate was filtered. The crude product was recrystallized from DMAc and dried in a vacuum oven at 100 °C for 8 h. White crystals (96% yield) were obtained. Melting point: 185.6–188.9 °C (onset-end temperature in a DSC thermogram). HRMS (EI-MS) m/z: [M+] calcd for C34H30O4F8, 654.20; Anal., 654.2020. 1H NMR (ppm, CDCl3): δ 1.48 (9H, H14), 3.79 (3H, H15), 6.65 (2H, H8, H9), 6.99 (1H, H9). 13C NMR (ppm, CDCl3): δ 29.75 (C14), 35.04 (C13), 55.51 (C15), 101.73 (C1), 110.25 (C11), 114.61 (C9), 115.26 (C8), 135.72 (C4), 139.92 (C12), 140.60 and 142.28 (C2, C6), 144.07 and 145.75 (C3, C5), 150.00 (C7), 155.65 (C10).

Synthesis of 4,4′-Bis[4-(4-hydroxyl-2-tert-butylphenoxy)]perfluorobiphenyl

A solution of PFOCH3 (6 g, 9.17 mmol) in 90 mL of dry dichloromethane was cooled to 0 °C. Next, BBr3 (5.733 g, 9.27 × 2.5 mmol) was introduced, and the reaction mixture was stirred for 6 h. The crude product was isolated by filtration and washed with distilled water. The crude product was dried in a vacuum oven at 100 °C for 8 h. A white powder (72% yield) was obtained. Melting point: 238.6–242.2 °C (onset-end temperature in a DSC thermogram). HRMS (EI-MS) m/z: [M+]: calcd for C32H26O4F8, 626.17; Anal., 626.1698. 1H NMR (ppm, CDCl3): δ 1.40 (9H, H14), 6.55 (1H, H9), 6.80 (2H, H8, H11), 9.26 (1H, OH). 13C NMR (ppm, CDCl3): δ 29.59 (C14), 34.41 (C13), 100.79 (C1), 113.075 (C11), 114.17 (C9), 116.28 (C8), 135.41 (C4), 138.65 (C12), 140.10 and 141.80 (C2, C6), 143.65 and 145.22 (C3, C5), 148.21 (C7), 153.52 (C10).

Synthesis of 4,4′-Bis[4-(4-nitrophenoxy)-2-tert-butylphenoxy]perfluorobiphenyl

PFOH (10 g, 10.59 mmol), 4-fluoro-1-nitrobenzene (3.19 g, 18.29 × 2.2 mmol), K2CO3 (4.41 g, 18.29 × 2 mmol), and DMAc (100 mL) were added into a 250 mL round-bottom glass flask equipped with a condenser, a nitrogen inlet, and a magnetic stirrer. The solution was stirred at 75 °C for 12 h. After cooling to 40 °C, the obtained mixture was poured into ethanol. The precipitate was filtered, washed with ethanol, and dried in a vacuum oven at 100 °C for 8 h. A bright yellow powder (75% yield) was obtained. Melting point: 246.4–249.6 °C (onset-end temperature in a DSC thermogram). HRMS (EI-MS) m/z: [Na+] calcd for C44H32O6N2F8Na, 891.19; Anal., 891.1919. 1H NMR (ppm, CDCl3): δ 1.48 (9H, H14), 6.75 (1H, H8), 6.86 (1H, H9), 6.99 (2H, H16, H20), 7.16 (1H, H11) 8.20 (2H, H17, H19). 13C NMR (ppm, CDCl3): δ 29.69 (C14), 35.27 (C13), 102.38 (C1), 115.800 (C8), 116.70 (C16, C20), 118.80 (C9), 120.41 (C11), 126.01 (C17, C19), 140.60 and 142.26 (C2, C6), 141.25 (C12), 142.62 (C7), 140.09 and 145.85 (C3, C5), 150.28 (C15), 163.51 (C18).

Synthesis of PFNH2

4,4′-Bis[4-(4-nitrophenoxy)-2-tert-butylphenoxy]perfluorobiphenyl (PFNO2) (5.00 g, 5.76 mmol), Pd/C (0.4 g, 4 wt % F–NO2), and DMAc (50 mL) were added into a 250 mL Parr stirred reactor at 25 °C. The system was purged with nitrogen for half an hour before introducing hydrogen. The pressure of reactor was maintained at about 140 psi under the hydrogen atmosphere for 24 h. Then, the obtained mixture was filtered to remove Pd/C, and the filtrate was poured into distilled water. The crude product was isolated by filtration and dried in a vacuum oven at 100 °C for 8 h, and a white powder (63% yield) was obtained. Melting point: 216.2–222.1 °C (onset-end temperature in a DSC thermogram). HRMS (EI-MS) m/z: [M+] calcd for C44H36O4N2F8, 808.26; Anal., 808.2572. 1H NMR (ppm, DMSO-d6): δ 1.40 (9H, H14), 4.98 (2H, NH2), 6.58 (3H, H9, H17, H19), 6.77 (1H, H16, H20), 6.96 (2H, H8, H11). 13C NMR (ppm, CDCl3): δ 29.25 (C14), 34.61 (C13), 101.22 (C1), 114.27 (C9), 114.84 (C17, C19), 115.96 (C11), 116.28 (C8), 120.68 (C16, C20), 134.98 (C4), 139.122 (C12), 140.23 and 141.86 (C2, C6), 143.62 and 145.02 (C3, C5), 145.39 (C7), 145.66 (C15), 150.07 (C10).

Preparation of Polyimide Films

The synthesized diamines, PSNH2 and PFNH2, were reacted with three commercial aromatic dianhydrides, PMDA, ODPA, and 6FDA. A typical synthetic procedure for PEI PMDA/PSNH2 is shown in the following: diamine PSNH2 0.5 g (0.686 mmol) and 2.60 mL of dry DMAc (20 wt %) were added into a 100 mL round-bottom glass flask equipped with a condenser and a magnetic stirrer. The system was purged with nitrogen, and the solution was stirred for an hour. Dianhydride PMDA 0.15 g (0.686 mmol) was added into the solution and stirred for 12 h to form PAA. PAA was cast on a glass substrate and dried at 60 °C (12 h), 100 °C (1 h), 200 °C (1 h), and 300 °C (1 h). The other PEIs were prepared in a similar process. The detailed conditions are described in Table S1.