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X. Peng et al. / Reactive and Functional Polymers 106 (2016) 93–98
this diamine polymerized with four dianhydrides (i.e., PMDA, BPDA,
BTDA, and ODPA). The as-synthesized polyimides had excellent dielec-
tric constant of up to 7.2, low dielectric loss (b0.04) and high break-
down strength, as well as high glass transition temperature of 275–
320 °C and tensile strengths of 175–221 MPa.
room temperature. The resulting polyamic acid (PAA, 6a) solution
with 9.44 wt% was diluted into 8 wt% with DMAc and casted into PAA
films. The PAA films was imidized into PI (7a) in a high temperature fur-
nace according to the following protocol: (1) heating to 160 °C at a rate
of 10 °C/min and annealing at 160 °C for 2 h to remove the residual sol-
vent, (2) heating at a rate of 10 °C/min and annealing at 250 °C for
15 min, (3) heating at a rate of 5 °C/min and annealing at 350 °C for
2 h to complete the imidization process.
2. Experimental
2.1. Materials
2.5. Measurements
2,2′-bipyridine (99.0%), 4-aminophenol (99.0%), bromine (99.5%),
1,2,4,5-benzenetetracarboxylic anhydride (PMDA, 99%), 3,3′,4,4′-
biphenyltetracarboxylic dianhydride (BPDA, 99.0%), 3,3′,4,4′-
benzophenonetetracarboxylic dianhydride (BTDA, 99.0%), and 4,4′-
oxydiphthalic anhydride (ODPA, 99.0%) were purchased from J&K
Chemical Ltd. All other chemicals and solvents were obtained from
China National Pharmaceutical Group Corporation. 1-methyl-2-
pyrrolidinone (NMP) and N,N-diethylacetamide (DMAc) were purified
using CaH2 and then distilled under reduced pressure and then stored
in the presence of 4 Å molecular sieves.
Proton and carbon (1H- and 13C-NMR) nuclear magnetic resonance
spectra were measured at 400 MHz and 105 Hz, respectively on a Bruker
Avance-400 spectrometers. The intrinsic viscosities were measured
(Ubbelohde viscometer) in DMAc at 25 °C. FTIR spectra were obtained
with Perkin-Elmer SP one FTIR. Elemental analyses were determined
by a Perkin-Elmer model 2400 CHN analyses. Thermal gravimetric anal-
ysis (TGA) was performed with 6–8 mg film samples with a Perkin-
Elmer Pyris 1 TGA in nitrogen at a heating rate of 10 °C/min. The glass
transition temperature was determined by a Perkin-Elmer Diamond dy-
namic thermomechanical analysis (DMA) instrument. During the test, a
load of 10 g (stress, 0.125 MPa) and a heating rate of 10 °C/min in air
were applied. Tensile tests of polymer films were performed using
SANS CMT8012 (Shenzhen, China) instrument at 5 mm/min on strips
(0.5 cm wide, 2 cm long, and about 0.05 mm thick). The dielectric prop-
erties were measured on an Agilent E4980 LCR electrometer at different
frequencies and conductive tapes were served as electrodes to clamp
our samples with 10 mm in diameter and thickness of about 20 μm. Di-
electric breakdown strength was tested using an electric breakdown
strength test machine (DDJ-20 kV, China) under DC high-voltage
power with 200 × 200 mm and thickness of below 3 mm. Scanning elec-
tron microscope (SEM, TESCAN vega3) were employed to obtain the
films thickness.
2.2. Synthesis of 5,5′-dibromo-2,2′-bipyridine
2,2′-bipyridine (1, 4.99 g, 0.032 mol) and bromine (10.24 g,
0.064 mol) were first added in a hydrothermal reaction container and
heated to 150 °C and annealed for 15 h. Then, the mixture was cooled
and the hard solid was powdered and following treated with Na2SO3 so-
lution to remove the unreacted bromine. Finally it was basified with so-
dium hydroxide and filtered. The white solid product 5,5′-dibromo-
2,2′-bipyridine was obtained by the chromatography on silica
(CH2Cl2). 5,5′-dibromo-2,2′-bipyrimidine, 8.64 g (86%). m.p. 221.6–
222.1 °C. 1H-NMR (400 MHz, CDCl3, δ): 7.95 (d, 2H), 8.28 (s, 2H),8.71
(d, 2H); 13C-NMR (100 MHz, CDCl3, δ): 121.47 (C5), 122.25 (C3),
139.64 (C4); 150.28 (C6),153.64 (C2); IR (KBr): 3049 (C\\H,
stretching), 1562, 1453, 1356 (Ar, stretching), 636 (C\\Br, stretching).
Anal. calcd for C10H6N2Br2: C 38.22, H 1.91, N 8.92, Br 50.95; found: C
38.26, H 1.94, N 8.90, Br 50.97.
3. Results and discussion
3.1. Synthesis of 5,5′-bis [(4-amino)phenoxy]-2,2′-bipyridine (BPBPA)
2.3. Synthesis of 5,5′-bis [(4-amino) phenoxy]-2,2′bipyridine (BPBPA, 4)
Scheme 1 shows the preparation of BPBPA. The synthesis process in-
cludes two steps. 2,2′-bipyridine (1) is brominated using liquid Br2 to
give 5,5′-dibromo-2,2′-bipyridine (2) under high temperature and
pressure. In the second step, the 5,5′-dibromo-2,2′-bipyridine (2) was
reacted with 4-aminophenol (3) in NMP to produce the 5,5′-bis [(4-
amino)phenoxy]-2,2′-bipyridine (4). All the molecules were character-
ized by the elemental analysis, FTIR, 1H-NMR, and 13CNMR spectroscop-
ic techniques and complete data is presented in the experimental
section. The 1H- and 13C-NMR spectra of important compounds (2)
and (4) are given in the supporting information (Figs. S1–S4). Detailed
assignments of protons and carbons are given in the figures, confirming
the molecular structure.
5,5′-dibromo-2,2′-bipyridin (2, 6.28 g, 0.02 mol) and 4-
aminophenol (3, 5.23 g, 0.048 mol) were dissolved in 100 mL NMP. An-
hydrous K2CO3 (2.73 g, 0.02 mol) was added to the solution, and the
mixture was heated at 175 °C for 12 h. After cooling to room tempera-
ture, the mixture was poured into 5 wt% sodium hydroxide solution,
and stirred for 1 h. The solution was filtered, and the filter cake was
washed repeatedly with distilled water. The white crystals of 5,5′-
bis[(4-amino)phenoxy]-2,2′-bipyrimidine (BPBPA, 4) was recrystal-
lized from H2O/ethanol. 6.44 g (87%), m.p. 204.0–204.6 °C. 1H-NMR
(400 MHz, DMSO-d6, δ): 5.06 (s, 4H), 6.61 (d, 4H), 6.85 (d, 4H), 7.29
(d, 2H), 8.21 (s, 2H), 8.29 (d, 2H). 13C-NMR (100 MHz, DMSO-d6, δ):
114.89 (C3′), 120.82 (C2′), 123.92 (C3, C4), 138.43 (C6), 144.75 (C4′),
146.04 (C1′), 148.84 (C2), 155.36 (C5); FTIR (KBr): 3396, 3302 (N\\H,
stretching), 1633, 1558, 1503, 1455 (Ar, stretching). Anal. calcd for
C22H18N4O2: C 71.35, H 4.86, N 15.14, O 8.65; found: C 71.31, H 4.89, N
15.11, O 8.70.
3.2. Polymerization
Scheme 2 shows the synthesis of PIs. The novel PIs (7) were pre-
pared by polymerizing the BPBPA (4) with aromatic dianhydrides (5)
via a two-step method. The polymerization was reacted in the solvent
DMAc for 12 h at room temperature to yield the PI precursors solutions,
polyamic acids (PAA, 6); and the obtained PAA solutions were used to
prepare PI thin films by solution casting and heating process (thermal
imidization). The intrinsic viscosity of the precursors (PAA, 6) was
2.0–2.5 dL/g (2.5, 2.2, 2.0, and 2.3, respectively for PAA-6a, PAA-6b,
PAA-6c, and PAA-6d). The high intrinsic viscosity corresponds to a
high molecular weight, which is helpful for making high performance
PI films with good mechanical properties. Fig. 1 shows the FT-IR spectra
of PAAs and PIs. The characteristic peaks of PAAs (Fig. 1a) appears a big
absorption bands from 2500 to 3600 cm−1 (corresponding to the O\\H
2.4. Preparation of polyimides containing bipyridine units
Four kinds of polyimides containing bipyridine units (PI-7a, PI-7b,
PI-7c, PI-7d) were prepared from the diamine (BPBPA, 4) and aromatic
dianhydrides (BPDA, PMDA, BTDA, OPDA) via a conventional two-step
thermal imidization method. The synthesis of PI-7a is used as an exam-
ple to illustrate the general synthetic route used to produce the
polyimides. BPBPA (3.70 g, 0.01 mol) and BPDA (2.94 g, 0.01 mol)
were added to 63.7 g of DMAc and the mixture was reacted for 12 h at