T.-Q. Liu et al.
Polymer 209 (2020) 122963
in molecular design of quinoxaline monomers remains urgent to prepare
high performance polyimides or other heterocyclic polymers.
product, then recrystallized with MeOH to provide the corresponding
1
◦
brighter yellow diamine QHDA (5.14 g, 75%, m. p. 206–207 C). H
NMR (DMSO‑d6, 500 MHz): δ 9.14–8.97 (1H), 8.02–7.94 (2H),
7.73–7.68 (1H), 7.24–7.11 (1H), 6.96–6.91 (1H), 6.73–6.71 (2H),
5.97–5.93 (2H), 5.67–5.56 (2H) (Fig. 1). 13C NMR (DMSO‑d6, 125 MHz):
δ 151.35–151.27, 151.00–150.56, 149.79–147.06, 144.48–143.03,
Because steric hindrance effects can increase the Tg of polymers by
restricting molecular-segment rotation, in this study, a series of qui-
noxaline diamines with various pendant groups, including 3/2-(4-ami-
nophenyl)-2/3-quinoxalin-6-amine (QHDA), 3/2-(4-aminophenyl)-2/3-
phenyl quinoxalin-6-amine (QBDA), and 3/2-(4-aminophenyl)-2/3-
methylquinoxalin-6-amine (QMDA), were synthesized and poly-
condensated with four conventional dianhydrides, which was expected
to provide PIs with enhanced thermal properties. The relationship be-
tween monomer structures and polymer properties was also discussed to
design and manufacture intrinsic thermostable PI films.
142.74–137.51,
135.86–134.86,
129.68–127.92,
128.68,
124.57–124.25, 122.64–120.91, 114.37–114.26, 106.03–105.72
(Fig. S3). Fourier transform infrared spectroscopy (FTIR) (KBr, cmꢀ 1):
3326, 3208, 3024, 1615, 1525, 1504, 831, 669. High resolution mass
spectrometer (HRMS) (ESI) m/z calculated for C14H12N4 [M + H+]
237.1140, found 237.1135.
2. Experimental section
2.3. Synthesis of QBDA
2.1. Materials
Scheme 2 shows the synthetic route of the QBDA diamine monomer.
According to a modified literature procedure [26–30], a solution of
1-iodo-4-nitrobenzene (12.50 g, 50 mmol), ethynylbenzene (6.50 mL,
60 mmol), Pd (OAc)2 (112.3 mg, 0.5 mmol), CuI (95.3 mg, 0.5 mmol),
Xantphos (289.3 mg, 0.5 mmol CAS: 161265-03-8), and Cs2CO3 (32.60
g, 100 mmol) in anhydrous DMF (200 mL) was heated to 60 ◦C for 16 h.
After cooling to room temperature, water (200 mL) was added, and the
mixture was extracted with ethyl acetate (200 mL × 2). The combined
organic layer was washed with brine (200 mL × 2) and dried over
MgSO4. The organic solvent was evaporated, the resulting residue was
purified by flash column chromatography to give the 1-nitro-4- (phe-
nylethynyl)benzene (a) (10.47 g, 94%). 1H NMR (CDCl3, 500 MHz): δ
8.23–8.21 (m, 2H), 7.68–7.66 (m, 2H), 7.57–7.55 (m, 2H), 7.40–7.39
(m, 3H) (Fig. S4). 13C NMR (CDCl3, 125 MHz): δ 146.98, 132.28, 131.85,
130.27, 129.29, 128.55, 123.65, 122.11, 94.72, 87.56 (Fig. S5).
PdCl2 was added to a solution of a (8.90 g, 40 mmol) in anhydrous
DMSO (250 mL) (709.3 mg, 4 mmol). The mixture was heated to 145 ◦C
for 4 h and cooled to ambient temperature. Water (250 mL) was added to
the reaction mixture, and the mixture was extracted with ethyl acetate
(150 mL × 2). The combined organic phase was washed with brine (100
mL × 2) and dried over MgSO4. The organic solvent was removed under
reduced pressure. The crude products were purified by column chro-
matography to afford the 1-(4-nitrophenyl)-2-phenylethane-1,2-dione
(b) (8.78 g, 86%). 1H NMR (CDCl3, 500 MHz): δ 8.37–8.34 (m, 2H),
8.18–8.16 (m, 2H), 8.00–7.98 (m, 2H), 7.73–7.69 (m, 1H), 7.57–7.54
(m, 2H) (Fig. S6). 13C NMR (CDCl3, 125 MHz): δ 192.86, 192.08, 151.16,
137.30, 135.48, 130.97, 130.07, 129.24, 124.14 (Fig. S7).
All commercial materials were used as received from Energy
Chemical or Adamas-beta, Alfa Aesar, TCI, and Acros unless otherwise
noted. The 1,2,4,5-Benzenetetracarboxylic dianhydride (PMDA),
3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 3,3′,4,4′-benzo-
phenonetetracarboxylic dianhydride (BTDA), and 3,3′,4,4′-oxy-
diphthalic dianhydride (ODPA) were from ChinaTech (Tianjin)
Chemical Co., Ltd, and were used without further purification. H2 was
from Wetry (99.999% purity).
2.2. Synthesis of QHDA
Scheme 1 shows the synthesis route of the QHDA diamine monomer.
A solution of 4-nitrobenzene-1,2-diamine (7.66 g, 50 mmol) and 2-
bromo-1-(4-nitrophenyl)ethanone (12.20 g, 50 mmol) in anhydrous
dimethyl sulfoxide (DMSO) (100 mL) was stirred vigorously at room
temperature for 5 h. The reaction was quenched by water addition (100
mL), and then the mixture was poured into excess water (1 L). During
vacuum suction filtration, excess DMSO was removed by water rinsing
and the precipitate was filtered. The solid was dissolved in dichloro-
methane followed by washed with brine (200 mL × 3), and dried over
MgSO4. The organic solvent was evaporated and the resulting residue
was purified by flash-column chromatography to give the corresponding
precursor QHDN (8.50 g, 57%). 1H nuclear magnetic resonance (NMR)
(DMSO‑d6, 500 MHz): δ 9.92–9.91 (1H), 9.00–8.98 (1H), 8.70–8.67
(2H), 8.65–8.61 (1H), 8.50–8.48 (2H), 8.45–8.42 (1H) (Fig. S1). 13C
NMR (DMSO‑d6, 125 MHz): δ 151.50, 149.33, 148.59, 147.68, 146.90,
The mixture of b (6.70 g, 26 mmol), 4-nitrobenzene-1,2-diamine
(4.00 g, 26 mmol), and saccharin (241.0 mg, 1.3 mmol) in acetic acid
(250 mL) was stirred at room temperature for 1 h. The reaction was
monitored by TLC and quenched by water addition (250 mL). The pre-
cipitate was filtered, washed with brine three times, and dried in a
vacuum oven to deliver the corresponding precursor QBDN (9.49 g,
98%). 1H NMR (DMSO‑d6, 500 MHz): δ 8.99–8.96 (1H), 8.63–8.58 (1H),
8.43–8.38 (1H), 8.27–8.24 (2H), 7.81–7.79 (2H), 7.55–7.53 (2H),
144.26,
141.47–141.46,
140.72–140.67,
131.87–131.42,
129.80–129.62, 125.84–125.47, 124.67–124.40 (Fig. S2).
According to the literature [24,25], 5% Pd/C (0.90 g) was added to a
solution of dinitro compound (QHDN) (8.50 g, 29 mmol) in anhydrous
MeOH (200 mL). The reaction atmosphere was replaced with H2 for
three times. The reaction mixture was stirred overnight and reduced by
H2 at room temperature and atmospheric pressure to obtain dark yellow
Scheme 1. Synthetic route of QHDA.
2