s-Triazine-based functional monomers with thermocrosslinkable propargyl units: Synthesis and conversion to the heat-resistant polymers

Article abstract of DOI:10.1016/j.polymer.2016.09.027

Three s-triazine-based functional monomers with thermo-polymerizable propargyl-ether units were synthesized by a facile procedure. These monomers can be thermally cured to form the crosslinked networks, which showed 5-wt% loss temperature of up to 400?°C and the char yields of more than 50% at 1000?°C. Moreover, the crosslinked networks exhibited the coefficients of thermal expansion (CTE) of below 43?ppm?°C^?1 varying from 30 to 300?°C and glass transition temperatures (T_g) of up to 290?°C, respectively. These monomers were also used to improve the thermostability of a commercial bismaleimide (4,4′-bismaleimidodiphenylmethane). The results indicated that blending the bismaleimide and the triazine monomers gave the new resins, which showed higher T_g and lower CTE than the bismaleimide, suggesting the triazine monomers can be considered as the modifiers for enhancement of the thermostability of the commercial bismaleimides.

Full text of DOI:10.1016/j.polymer.2016.09.027

Polymer 102 (2016) 301e307
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
s-Triazine-based functional monomers with thermocrosslinkable
propargyl units: Synthesis and conversion to the heat-resistant
polymers
Junfeng Zhou, Jiajia Wang, Kaikai Jin, Jing Sun, Qiang Fang*
Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2016
Received in revised form
25 August 2016
Accepted 10 September 2016
Available online 12 September 2016
Three s-triazine-based functional monomers with thermo-polymerizable propargyl-ether units were
synthesized by a facile procedure. These monomers can be thermally cured to form the crosslinked
networks, which showed 5-wt% loss temperature of up to 400 ꢀC and the char yields of more than 50% at
1000 ꢀC. Moreoveꢁr,1 the crosslinked networks exhibited the coefficients of thermal expansion (CTE) of
below 43 ppm ꢀC varying from 30 to 300 ꢀC and glass transition temperatures (Tg) of up to 290 ꢀC,
respectively. These monomers were also used to improve the thermostability of a commercial bisma-
leimide (4,40-bismaleimidodiphenylmethane). The results indicated that blending the bismaleimide and
the triazine monomers gave the new resins, which showed higher Tg and lower CTE than the bisma-
leimide, suggesting the triazine monomers can be considered as the modifiers for enhancement of the
thermostability of the commercial bismaleimides.
Keywords:
s-Triazine
Functional polymers
Propargyl
© 2016 Elsevier Ltd. All rights reserved.
Thermostability
Heat-resistant polymers
1. Introduction
for producing the polymers with high molecular weight. From an
application point of view, the residues of the catalysts greatly in-
Of various aromatic heterocyclic polymers, these with 1,3,5-
triazine (also called as s-triazine) rings as the main chains have
attracted interest for many years because of their wide applications
in industry [1e5]. Polymers based on s-triazine rings show higher
thermostability and interesting electric properties [6]. A typical s-
triazine-based polymer, melamine-formaldehyde resin, has been
widely used in many areas as structural adhesives, precursor for
fire-retardant fibers and the matrix resin for the glass-fiber rein-
fluence the physical and chemical properties of the polymers.
Therefore, developing the direct thermo-crosslinkable triazine
polymers is necessary.
It is noted that propargyl-ether is a frequently-used thermo-
crosslinkable group. The propargyl-ether containing polymers can
be easily converted to the crosslinked networks, showing high
thermostability, low moisture absorption, and low dielectric con-
stant [20e22]. In particular, no volatile by-products produce during
the thermo-crosslinking reaction of the polymers [21]. This char-
acteristic of propargyl-ether group inspires us to design the mol-
ecules containing both s-triazine and propargyl-ether units. In this
contribution, we report the synthesis and conversion of three types
of the triazine-propargyl-ether containing monomers, whose
chemical structure is shown in Chart 1. The monomers showed high
reactivity at high temperature, and gave the thermo-crosslinked
resins exhibiting high thermostability. Furthermore, the mono-
mers were used to enhance the thermostability of a commercial
bismaleimide, 4,40-bismaleimidodiphenylmethane (BDM). The re-
sults indicated that the monomers can greatly improve the heat
resistance of BDM such as glass transition temperature and co-
efficients of thermal expansion. Here, we report the details.
forced composites [7e9]. On the other hand,
p-conjugated s-
triazine-based polymers or macromolecules exhibit good opto-
electronic properties, and they can be used as organic light emitting
diode (OLED) and light-guide materials [10e12]. In the recent years,
these polymers have been used for construction of the porous
polymers based on the symmetry of triazine ring. Such s-triazine-
based porous materials may have potential applications as the
materials for separation of gases [13e15].
However, directly thermocrosslinkable triazine polymers
should be developed [16e19]. In many cases, catalysts are required
* Corresponding author.
E-mail address: qiangfang@sioc.ac.cn (Q. Fang).
http://dx.doi.org/10.1016/j.polymer.2016.09.027
0032-3861/© 2016 Elsevier Ltd. All rights reserved.

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J. Zhou et al. / Polymer 102 (2016) 301e307
Chart 1. Chemical structure of the thermocrosslinkable s-triazine monomers and 4,40-bismaleimidodiphenylmethane (BDM).
2. Experimental section
56.31. Anal. Calcd. for [C30H21N3O6]: C, 69.36; H, 4.07; N, 8.09.
Found: C, 68.95; H, 4.12; N, 8.08. LRMS (EI, m/z) Calcd. for (M þ Hþ)
2.1. Materials and instruments
C
30H22N3Oþ6 ¼ 520.1. Found 520.0.
Hydroquinone, 3-bromopropyne, cyanuric chloride, 4-
aminophenol, 4-cyanophenol and trifluoromethanesulfonic acid
were purchased from Admas-Beta Reagents (Shanghai) Co., Ltd, and
used as received. 4,40-bismaleimidodiphenylmethane (BDM) was
purchased from Shuangfeng (Hubei) Chemical Co., Ltd. All starting
chemicals used without further purification unless otherwise
stated.
FT-IR spectra were recorded on a Thermo Scientific Nicolet
spectrometer with KBr pellets. 1H NMR and 13C NMR spectra were
obtained on a JEOL ecz400 or Agilent 500/54/ASP instrument. Mass
spectrometry was performed on Agilent Technologies 5973N and
Thermo Fisher Scientific LTQ FT Ultra. Elemental Analysis was
performed on Elementar vario MicroCube. Differential Scanning
Calorimetry (DSC) was determined with ꢁT1A Instrument of DSC
Q200 at a heating rate of 10 or 20 ꢀC min under nitrogen flow.
Thermogravimetric analysis (TGA) was perfꢁo1rmed on a TG 209F1
apparatus with a heating rate of 10 ꢀC min in nitrogen and air.
Thermo-mechanical analysis (TMA) was carried out on a Mettler
Toledo TMA/SDTS841e, performing with a load of 0.02 N and a
heating rate of 5 ꢀC minꢁ1, respectively.
2.4. Synthesis of A-PET
To a mixture of compound 2 (10.00 g, 24.85 mmol), N, N-
dimethylformamide (200 mL), and anhydrous K2CO3 (41.20 g,
298.21 mmol) was added 3-bromopropyne (9.16 g, 6.64 mL,
77.04 mmol) via a syringe. After being stirred at room temperature
for 2 days, the mixture was poured into water, and the resulting
precipitate was filtered. The filtrate cake was re-dissolved in
acetone, and the obtained solution was poured into n-hexane to
give a precipitate. This procedure was repeated for three times to
yield purer A-PET as brown powder in a yield of 78%. 1H NMR
(400 MHz, DMSO-d6, ppm):
d 9.17e8.90 (br s, 3H, NeH), 7.89e7.46
(br s, 6H, AreH), 6.99e6.86 (d, 6H, AreH), 4.81e4.73 (d, 6H,
OeCH2eC), 3.58e3.53 (t, 3H, CeC^CH). 13C NMR (100 MHz, DMSO-
d6, ppm):
d 164.00, 152.42, 133.84, 121.80, 114.71, 79.62, 78.04,
55.65. Anal. Calcd. for [C30H24N6O3]: C, 69.79; H, 4.68; N, 16.27.
Found: C, 69.79; H, 4.77; N, 16.08. HRMS (MALDI-FT, m/z) Calcd. for
(M þ Hþ) C30H25N6O3þ ¼ 517.19. Found 517.20.
2.5. Synthesis of Ph-PET
2.2. Synthesis of compounds 1, 2 and 3 (see Scheme 1)
A
mixture of compound 3 (8.00 g, 22.39 mmol), 3-
bromopropyne (11.97 g, 8.67 mL, 100.74 mmol), anhydrous K2CO3
(27.84 g, 201.48 mmol) and acetone (200 mL) was heated to reflux
and kept at the temperature for overnight. After being cooled to
room temperature, the mixture was filtered to remove the solid.
The filtrate was concentrated under reduced pressure, and the
resulting residue was re-dissolved in chloroform. The solution was
washed with water, evaporated to remove the solvent. The crude
Ph-PET was thus obtained, which was then purified by recrystal-
lized from chloroform to give pure product as a white solid in a
These compounds were prepared according to the procedures
previously reported [23e25].
2.3. Synthesis of E-PET
To a solution of compound 1 (3.56 g, 24.03 mmol) and cyanuric
chloride (1.48 g, 8.01 mmol) in acetone (100 mL) was added
dropwise an aqueous NaOH solution (2 M) at room temperature.
When the pH value of the solution was constant, the resulting
mixture was filtered. The filtrate cake was successively washed
with deionized water and ethanol to give white powder. E-PET was
thus obtained in a yield of 72%. 1H NMR (400 MHz, CDCl3, ppm):
yield of 74.6%. 1H NMR (400 MHz, CDCl3, ppm)
d 8.76e8.63 (d, 6H,
AreH), 7.17e7.06 (d, 6H, AreH), 4.84e4.76 (d, 6H, OeCH2eC),
2.61e2.55 (t, 3H, CeC^CH). 13C NMR (100 MHz, CDCl3, ppm)
d
170.81, 161.15, 130.75, 130.08, 114.91, 78.26, 75.89, 56.02. Anal.
d
7.10e7.03 (d, 6H, AreH), 6.98e6.91 (d, 6H, AreH), 4.70e4.64 (d,
6H, OeCH2eC), 2.57e2.53 (t, 3H, CeC^CH). 13C NMR (100 MHz,
CDCl3, ppm): 173.95, 155.43, 145.85, 122.37, 115.68, 78.47, 75.94,
Calcd. for [C30H21N3O3]: C, 76.42; H, 4.49; N, 8.91. Found: C, 76.36;
H, 4.56; N, 8.79. LRMS (EI, m/z) Calcd. for (M
þ
Hþ)
d
C
30H22N3Oþ6 ¼ 472.1. Found 472.0.

J. Zhou et al. / Polymer 102 (2016) 301e307
303
Scheme 1. Procedures for the synthesis of the triazine-based monomers.
2.6. Preparation of the cured sample for thermo-mechanical
analysis
chloride in the presence of an inorganic base at ambient temper-
ature. We tried to synthesize A-PET by using a route similar to the
synthesis of E-PET, whereas the intermediate, 4-(prop-2-yn-1-
yloxy) aniline, was difficultly obtained in high purity. Fortunately,
when an efficient route was employed (see Scheme 1), A-PET was
prepared in a high yield [24]. For Ph-PET, it was obtained through a
A triazine monomer was added into a flat bottomed glass tube
(
f
1.0 cm) filled with argon. The tube was heated to 150 ꢀC and
maintained at the temperature for 1 h. The temperature was then
elevated, and the tube was kept at 180 ꢀC for 2 h, 240 ꢀC for 4 h,
290 ꢀC for 2 h, respectively. Finally, the tube was slowly cooled to
room temperature. A fully cured sample was thus obtained.
The procedure for curing the blends of the triazine monomers
with 4,40-bismaleimidodiphenylmethane (BDM) is as following: a
mixture of a triazine monomer, BDM and chloroform was heated to
reflux with stirring and maintained at the temperature for several
hours until a homogenous solution was formed. The solution was
evaporated under vacuum to remove the solvent, and the obtained
solid was treated according to a route similar to curing triazine
monomers, as described above.
3. Results and discussion
3.1. Synthesis and characterization
The procedure for the preparation of the triazine monomers was
depicted in Scheme 1. As described in Scheme 1, three triazine
monomers were prepared by using three different routes. E.g., E-
PET was prepared by a nucleophilic substitution reaction between
4-(prop-2-yn-1-yloxy) phenol (compound 1) with cyanuric
Fig. 1. DSC traces of the triazine monomers at a heating rate of 10 ꢀC minꢁ1 in N2.

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J. Zhou et al. / Polymer 102 (2016) 301e307
Table 1
TGA data of the cured triazine monomers.
Polymers
Thermostabilitya
Td5 (ꢀC)
Td10 (ꢀC)
Tpeak (ꢀC)
R1000 (%)
Cured E-PET
Cured A-PET
Cured Ph-PET
410
443
481
413
464
535
412
52.6
64.5
68.7
458
489/681b
a
T5%, T10%, Tpeak and R1000 are 5-wt% loss temperature, 10-wt% loss temperature,
thermal degradation peak temperature, and char residue at 1000 ꢀC, respectively.
b
Two degradation peaks were observed.
Fig. 2. FT-IR spectra of E-PET before and after thermo-crosslinking.
two-step route. 4-cyanophenol was trimerized in the presence of
CF3SO3H to produce compound 3, which was followed by the
standard Williamson etherification reaction with 3-bromopropyne.
The chemical structures of the triazine monomers were char-
acterized by FT-IR, 1H NMR, 13C NMR, mass spectrometry, and
elemental analysis (EA). The detailed data were summarized in
Experimental section and Supporting Information. As can be seen
from Experimental section and Supporting Information, all mono-
mers showed a characteristic absorption peak of the terminal
acetylene group at about 3200 cmꢁ1 and 2100 cmꢁ1 in their FT-IR
spectra. Moreover, the 13C NMR spectra of the monomers exhibi-
ted the signal at around 170 ppm, which is attributed to the triazine
ring. Thus, all data are consistent with the proposed structure.
Fig. 3. Relative length change (dL/L) versus temperature of cured triazine monomers.
Fig. 1. As shown in Fig. 1, monomer E-PET showed the lowest
meting point (138 ꢀC) among the tree triazine the monomers. This
phenomenon may be attributed to the existence of rotable ester
linkage in E-PET molecule. The cross-linking reaction of the
monomers occurred at about 190 ꢀC and gave the maximum
exothermic peaks of low 285 ꢀC. The curing exothermic enthalpy
3.2. Thermo-crosslinking reaction of the triazine monomers
The thermo-crosslinking reaction of the three triazine mono-
mers was monitored by DSC traces, and the results were shown in
(DH) of the monomers had an order of E-PET > A-PET > Ph-PET,
Scheme 2. Proposed thermo-crosslinking reaction mechanism of the triazine monomers with propargyl-ether group.

J. Zhou et al. / Polymer 102 (2016) 301e307
305
crosslinking reaction of the triazine monomers.
In order to well understand the thermo-polymerization process
of the three triazine monomers, a proposed reaction mechanism
was provided, as described in Scheme 2 [26e29].
3.3. Thermostability
The thermostability of the thermo-crosslinked triazine mono-
mers was evaluated by TGA, and the detailed data were summa-
rized in Table 1. As listed in Table 1, thermo-crosslinked triazine
monomers show 5-wt% loss temperatures of higher than 410 ꢀC.
Moreover, their char yields were up to 50% at 1000 ꢀC in nitrogen.
These data are better to that of the commercial phenolic and epoxy
resins [30,31]. Since amido linkage is more stable than ester link-
age, cured A-PET exhibits higher heat resistance than cured E-PET.
Apparently, cured Ph-PET exhibits the highest thermostability
among the three triazine monomers. This result is contributed to
the direct linking of the phenyl units with a triazine ring. Such a
special connection type makes the phenyl groups difficultly rotate
and rearrange, resulting in the highest thermostability of cured Ph-
PET.
The thermostability of the cured triazine monomers was also
investigated through measuring their coefficients of thermal
expansion (CTE). Generally, low CTE is a significant characteristic
for the application of the engineering materials. Fig. 3 exhibited the
relative length change (dL/L) of a cured triazine monomer with the
temperature. Here, the CTE values are equal to the slope according
to the equation dL/L ¼ aLdT, where aL represents linear CTE of a
material, and dT is the differential coefficient of the temperature. As
described in Fig. 3, cured triazine monomers showed the linear CTE
of below 45 ppm ꢀCꢁ1 ranging from 30 to 300 ꢀC. Moreover, they
Fig. 4. DSC traces of the blends at a heating rate of 10 ꢀC minꢁ1 in N2.
which was in accord with the order of the molecular weights of the
monomers. At the second scan, no obvious exothermic peak was
observed, suggesting the cross-linking reaction of the triazine
monomers was closed to completion.
The thermo-crosslinking degree of the triazine monomers was
evaluated by FT-IR spectra. Taking an example, the FT-IR spectra of
E-PET before and after thermo-polymerization were depicted in
Fig. 2. As shown in Fig. 2, the characteristic absorption peaks of
alkyne groups both at 3293 cmꢁ1 and 2121 cmꢁ1 disappeared after
thermo-polymerization, indicating the completion of the thermo-
Scheme 3. Proposed copolymerization mechanism of bismaleimide with the triazine monomers.

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J. Zhou et al. / Polymer 102 (2016) 301e307
100 ꢀC in Tg. This result indicates that introducing triazine mono-
mers to BDM can also greatly improve the heat-resistance of BDM.
4. Conclusion
In summary, we have successfully synthesized three functional
triazine-based monomers. After treated at high temperature, the
monomers transformed to the crosslinked networks, which
showed good thermostability. Blending these monomers with a
frequently used commercial 4,40-bismaleimidodiphenylmethane
(BDM) can decrease the curing exothermic enthalpy of the mono-
mers, as well as greatly improve the thermostability of BDM.
Acknowledgements
This work was supported by Ministry of Science and Technology
of China (2011ZX02703 and 2015CB931900), the Natural Science
Foundation of China (NSFC No. 21374131, 21574146 and 21504103)
and the Science and Technology Commission of Shanghai Munici-
pality (15ZR1449200 and 16JC1403800). The authors are grateful
for the financial support by the Strategic Priority Research Program
of the Chinese Academy of Sciences, Grant No. XDB 20020000).
Fig. 5. Relative length change (dL/L) versus temperature of the cured blends. Here, the
Tg of cured BDM is cited from Ref.[32].
showed glass transition temperatures (Tg) of up to 290 ꢀC. Of the
three cured monomers, cured A-PET showed the lowest linear CTE
(37.3 ppm ꢀCꢁ1) and the highest Tg (323 ꢀC). This result may be stem
from the interaction of NeH in the cured A-PET. These linear CTEs
are better than those of the commercial epoxy resins and phenolic
resins [21].
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2016.09.027.
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