C. Wang et al.
Polymer 224 (2021) 123723
◦
1
000 C under N
2
atmosphere. Dynamic mechanical analysis (DMA) was
3. Results and discussion
◦
ꢀ 1
recorded on a DMA Q800 instrument with a heating rate of 5 C min in
air.
3.1. Synthesis and characterization
The monomer S1 was synthesized from vanillin via traditional Wit-
ting reaction to convert the aldehyde group into vinyl. By treating S1
with 4-nitrophthalonitrile, phthalonitrile monomer S2 was prepared by
a facile one-step reaction. The chemical structure of S2 was character-
2
.2. Synthesis of S1
This monomer was synthesized according to the procedure previ-
ously reported [32]. To
a
stirring solution of methyl-
1
13
ized by H NMR, C NMR, HRMS, FT-IR spectra and elemental analysis.
As can be seen from 1H NMR spectrum of S2 (Fig. 1), the peaks in the
triphenylphosphonium bromide (50.10 g, 1.0 mol) in THF (200 mL) was
added NaH (1.92 g, 0.5 mol) and potassium tert-butoxide (35.90 g, 2.0
mol) at room temperature. After addition, the mixture was stirred for an
additional 10 min. Then a solution of vanillin (20.00 g, 1.0 mol) in THF
region of 7.0–8.0 can be ascribed to the aromatic hydrogens, and the
characteristic peaks of vinyl group appear at 6.73 ppm, 5.87 ppm and
2
.28 ppm. All characteristic carbon signal peaks are consistent with the
proposed structures (Fig. 1).
series of PN resins (V-PN) were prepared by thermal-
polymerization of the monomer S1 and S2 in different molar ratios
(
80 mL) was added dropwise to the mixture. After stirred at room
temperature for 2 h, the reaction mixture was neutralized with dilute
HCl and extracted with ethyl acetate. The organic layer was combined,
A
2
washed with saturated brine, dried over anhydrous Na SO4, and filtered.
(
from 1:1–1:50). At high temperature, the V-PN resins can form cross-
After removal of the solvent under reduced pressure, the obtained res-
idue was purified by column chromatography on silica gel using a
linked networks due to the addition polymerization of phthalonitrile
groups in the presence of the phenol groups as catalysts.
mixture of petroleum ether and ethyl acetate (9:1, v/v) as the eluent to
1
give S1 as a colorless liquid in a yield of 97%. H NMR (400 MHz, CDCl
3
,
3
.2. Curing behaviors of V-PN resins
δ): 6.93 (s, 1 H), 6.91 (s, 1 H), 6.87 (d, 1H), 6.64 (d, 1 H), 5.67 (s, 1 H),
5
1
.58 (d, 1 H), 5.12 (d, 1 H), 3.90 (s, 3 H). 13C NMR (126 MHz, CDCl
46.55, 145.54, 136.51, 130.16, 120.03, 114.32, 111.42, 107.99.
3
, δ):
The curing behaviors of the series of V-PN resins were monitored by
DSC and FT-IR. As shown from the results of DSC (Fig. 2 and Table 1),
during the first scan, the melting points of V-PN resins are measured in
the range of 114–129 ◦C, and there are two exothermic peaks can be
observed for each sample. The first peak is attributed to the polymeri-
2
.3. Synthesis of S2
–
zation of the C C bond. The second peak can be ascribed to the poly-
–
To a stirring solution of S1 (5.00 g, 1.0 mol) and 4-nitrophthaloni-
trile (5.77 g, 1.0 mol) in DMSO (100 mL) was added anhydrous K CO
5.53 g, 1.2 mol) at room temperature. After stirred for an additional 5 h
merization of phthalonitrile groups. Due to the different reactivity of
2
3
monomer S1 and S2, with the molar ratio of S2 increasing, the peak
(
–
–
temperature of polymerization of C C bond for each V-PN decreases,
under nitrogen, the reaction mixture was poured into water (near 500
mL). The formed solid was filtered, washed with water and dried at air.
Pure S2 was obtained by column chromatography on silica gel using a
whereas the peak temperature of polymerization of phthalonitrile
groups increases. For V-PN-5 (the molar ratio of S1:S2 = 1:50), the first
peak temperature is closer to the homopolymerization temperature of
S2 (shown in Fig. S2, Supporting Information), and the temperature of
mixture of petroleum ether and ethyl acetate (9:1, v/v) as the eluent in a
1
yield of 81%. H NMR (400 MHz, DMSO‑d
6
, δ): 7.99 (d, 1 H), 7.63 (d, 1
◦
the second exothermic peak reaches to 372 C. At the second scans in the
H), 7.31 (s, 1 H), 7.21 (q, 1 H), 7.15 (d, 1 H), 7.10 (d, 1 H), 6.73 (d, 1 H),
.87 (d, 1 H), 5.27 (d, 1 H), 3.73 (s, 3 H). 13C NMR (126 MHz, DMSO‑d
,
6
DSC curves of V-PN resins, no obvious exothermic peaks can be
observed, suggesting that the monomers were completely cured.
The curing behavior of V-PN resins can also be confirmed by the FT-
IR spectrum (Fig. 3 and Fig. S3). Taking V-PN-5 resin as an example,
5
δ): 161.75, 151.54, 141.04, 137.11, 136.57, 136.40, 122.92, 121.57,
1
20.80, 119.94, 116.94, 116.43, 115.90, 115.57, 111.52, 108.00, 56.27.
+
HRMS-ESI (m/z): calcd [M + Na] , 299.0789; found, 299.0791. Anal.
◦
when heating the mixture of S1 and S2 to 300 C, the characteristic
Calcd: C, 73.90; H, 4.38; N, 10.14. Found: C, 73.925; H, 4.60; N, 10.11.
absorption peak attributed to the stretching vibration of vinyl group at
ꢀ ꢀ 1
1
9
86 cm and 3074 cm disappears, whereas the characteristic ab-
–
ꢀ 1
sorption peak of C
–
N at 2221 cm still exists. It demonstrates that the
–
–
2
.4. Preparation of V-PN resins
polymerization of C C bond in monomers can proceed at the lower
temperature, which is consistent with the results of DSC. After heating to
To a mixture of S1 and S2 in a different molar ratio added
◦
3
2
70 C, the characteristic absorption peak belonging to cyano group at
dichloromethane under stirring. After addition, the formed solution was
further treated with ultrasound for 5 min. After removal of the solvent
under reduced pressure at room temperature, the obtained residue was
cured according to the DSC curve. Taking V-PN-5 as an example, put 1
equivalent of S1 and 50 equivalents of S2 into a glass tube with a hor-
ꢀ 1
ꢀ 1
221 cm
disappeared, and the absorption peaks at 1025 cm
attributed to phthalocyanine groups and the absorption peaks at 1360
ꢀ 1
cm ascribed to triazine groups appear, indicating the V-PN resins was
further cured by the additional polymerization of phthalonitrile groups
in the catalysis of hydroxyl groups.
◦
izontal bottom surface, slowly raised the temperature to 130 C and kept
◦
it for 2 h under vacuum conditions, then heated up to 150 C in a ni-
◦
◦
◦
3.3. Thermostability and mechanical properties of cured V-PN resins
trogen atmosphere and kept at 150 C for 1 h, 200 C for 2 h, 250 C for
◦
◦
2
h, 340 C for 3 h, 370 C for 4 h, and decreased to room temperature at
◦
ꢀ 1
The thermostability of V-PN resins were studied by TGA. As shown in
the results of TGA (Fig. 4, Fig. S4 and Table 2), the cured V-PN resins
a rate of 10 C min . The above processes were all carried out in a
quartz tube furnace.
◦
exhibit the high 5% weight loss temperatures (T5d) of 419–486 C.
Especially, they also give high char yield up to 76% at 1000 C under N
◦
2
2
.5. Water uptake test
atmosphere. With the molar ratio of S2 increased, the T5d and char yield
of V-PN resins elevate. The V-PN-5 resin display the best heat-resistant
◦
The water uptake test of V-PN-5 was carried out by immersing the
properties with the T5d of 486 C and the high char yield of 76%. The
samples in boiling water. Two samples were tested parallelly with the
rectangle shapes having the mass of 1.6631 g and 0.1015 g, respectively.
The water uptake values were calculated by weighing the samples three
times and averaging.
weight loss temperatures and char yields of other reported PN resins are
summarized in Table 2. As shown from Table 2, the thermostabilities of
bio-based V-PNs are comparable to those of the PN resins derived from
petroleum-based materials. The excellent properties can meet the
3