Origin of the rapid trimerization of cyanate ester in a benzoxazine/cyanate ester blend

Article abstract of DOI:10.1021/acs.macromol.5b00334

Blends of cyanate ester and benzoxazine have been independently studied by several researchers, and different reaction mechanisms were reported. Recently, we unexpectedly observe that gelation occurred in a 50 wt % methyl ethyl ketone solution of P-oda/BA

Full text of DOI:10.1021/acs.macromol.5b00334

Article
pubs.acs.org/Macromolecules
Origin of the Rapid Trimerization of Cyanate Ester in a Benzoxazine/
Cyanate Ester Blend
†
,†
,‡
Meng Wei Wang, Ru Jong Jeng,* and Ching Hsuan Lin*
†
Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan
Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan
‡
*
S Supporting Information
ABSTRACT: Blends of cyanate ester and benzoxazine have
been independently studied by several researchers, and
different reaction mechanisms were reported. Recently, we
unexpectedly observe that gelation occurred in a 50 wt %
methyl ethyl ketone solution of P-oda/BACY (1/1 mol/mol)
blend after 24 h at 30 °C, in which P-oda is a 4,4′-oxyaniline/
phenol-based benzoxazine and BACY is a dicyanate ester of bisphenol A. Previous studies suggest that the rapid trimerization of
cyanate ester in the blend is related to the ring-opened structure of benzoxazine. However, the possibility of ring-opening
polymerization for benzoxazine at 30 °C is rare. Therefore, it is highly likely that the catalytic effect results from the benzoxazine
itself, not from the ring-opened structure of benzoxazine. Through IR and DSC analyses, we conclude that the tertiary amine of
benzoxazine catalyzes the trimerization of cyanate ester, and we propose a three-step catalytic mechanism of benzoxazine for the
trimerization of cyanate ester.
INTRODUCTION
ester is not the phenolic hydroxyl but the oxygen anion, which
is the same intermediate in the ring-opening processes of both
benzoxazines.
■
Benzoxazines are generally synthesized from the Mannich
condensation of phenolic compound/primary amine/form-
aldehyde, and can be polymerized to thermosets by thermally
Recently, we unexpectedly observe that gelation occurred in
a 50 wt % methyl ethyl ketone solution of P-oda/BACY (1/1
mol/mol) blend after 24 h at 30 °C, in which P-oda is a 4,4′-
oxyaniline/phenol-based benzoxazine and BACY is a dicyanate
ester of bisphenol A (Scheme 1). In addition, gelation also
occurred in a 50 wt % methyl ethyl ketone solution of PBz-6M/
BACY (1/1 mol/mol) blend after 24 h 30 °C, in which PBz-6
M is an ortho-dimethyl bisphenol A/ortho-tetramethyldiami-
nodiphenylmethane-based polybenzoxazine precursor (Scheme
1
−7
activated ring-opening polymerization.
With the strong
intra- and intermolecular hydrogen bonding interactions, these
thermosets exhibit many advantages, such as low surface
3
,8−10
11,12
energy,
and superior thermal
and electrical proper-
1
3
ties. On the other hand, cyanate esters are known for their
high glass transition temperature, low dielectric properties, and
high thermal stability after being fully cured to form a triazine
14−22
network (or polycyanurate).
Blends of cyanate ester and
23
benzoxazine have been independently studied by Nair et al.,
2
4
25
26
Scheme 1. Structures of P-oda, BACY, and PBz-6M
Kimura et al., Gu et al., and Lin et al. Different reaction
mechanisms have been reported, but they all found that the
blends are miscible, and the trimerization of cyanate ester is
accelerated in the presence of benzoxazine. Hamerton et al. has
reported that the trimerization of cyanate ester can be catalyzed
27
by the phenol group. Therefore, several researchers indicated
that the phenol resulting from the ring-opening polymerization
2
3,24
of benzoxazine catalyzes the trimerization.
More recently,
Gu et al. prepared a phenol/aniline-based benzoxazine (PA)
and 2,4-dimethylphenol/2,4,6-trimethyl aniline-based benzox-
2
8
azine (s-PA). Free ortho positions to the O of oxazine are
available for PA, but the ortho and para positions to O or N of
oxazine of s-PA are blocked by methyl groups (see Figure 4 of
2
8
the above study ). Therefore, a phenolic OH structure was
formed for ring-opened PA, yet only the phenolic arylether
structure was formed for the ring-opened s-PA. Given the fact
that both P-a and s-PA catalyzed the trimerization of cyanate
ester in the benzoxazine/cyanate ester blend, it is concluded
that the fundamental catalyst for the trimerization of cyanate
Received: February 16, 2015
Revised: April 2, 2015
Published: April 15, 2015
©
2015 American Chemical Society
2417
DOI: 10.1021/acs.macromol.5b00334
Macromolecules 2015, 48, 2417−2421

Macromolecules
Article
Figure 1. Pictures of a 50 wt % MEK solution of P-oda/BACY (1/1, mol/mol): (a) freshly prepared solution, and (b−d) the thermally treated
solutions, (b) 30 °C for 24 h, (c) 50 °C for 4 h, and (d) 100 °C for 2 h.
2
9
1
). The possibility of ring-opening polymerization for
benzoxazine at 30 °C is rare. Therefore, it is highly likely that
the catalytic effect results from the benzoxazine itself, not from
the ring-opened structure of benzoxazine. From IR and DSC
analyses, we conclude that the tertiary amine of benzoxazine
catalyzes the trimerization of cyanate ester. Detailed analyses
and a proposed mechanism are reported in this work.
EXPERIMENTAL SECTION
■
Materials. 4,4′-Oxydianiline/phenol-based benzoxazine (P-oda)
with a melting point at 128 °C was prepared, according to a previously
reported procedure. The shape melting and correct 1H NMR
spectrum support the purity of P-oda. The diicyanate ester of
bisphenol A (BACY) was purchased from TCI. N,N-Dimethylaniline
and anisole were purchased from Acros. The o-dimethylbisphenol A/o-
26
tetramethyldiaminodiphenylmethane-based polybenzoxazine precursor
6
(
PBz-6M) was prepared in our previous work. All solvents were
Figure 2. FTIR spectra of (a) BACY (b) P-oda, and P-oda/BACY
blend after thermal treatment at (c) 30 °C for 24 h, (d) 50 °C for 4 h,
and (e) 100 °C for 2 h.
HPLC grade and were purchased from TEDIA.
Characterization. Differential scanning calorimetry (DSC) scans
were obtained using a PerkinElmer DSC 7 in a nitrogen atmosphere at
a heating rate of 10 °C/min. IR spectra were obtained from at least 32
scans in the standard wavenumber range of 650−4000 cm using a
PerkinElmer RX1 infrared spectrophotometer. For IR measurement, a
few drops of MEK solution of benzoxazine/BACY were dropped onto
a KBr pellet, and dried at various temperatures for various periods of
time in a vacuum oven.
solution is homogeneous, whereas the solution turns into gel
after thermal treatment.
Figure 4 shows the IR spectra of the (a) BACY, PBz-6M, and
PBz-6M/BACY blend after thermal treatment at (c) 30 °C for
−1
2
4 h, (d) 50 °C for 4 h, and (e) 100 °C for 2 h. The cyanate
−1
ester absorption peaks at 2273 and 2237 cm disappeared after
4
1
h at 50 °C (Figure 4d). The triazine absorptions at 1573 and
RESULTS AND DISCUSSION
366 cm− emerged gradually as thermal treatment proceeded.
1
■
Figure 1 shows the pictures of a 50 wt % methyl ethyl ketone
solution of P-oda/BACY before and after thermal treatment at
This also suggests the rapid trimerization of cyanate ester.
According to the IR spectrum of MEK (Figure S1), there are
strong absorption at 1714 cm and medium absorption at
−1
3
0−100 °C. The freshly prepared solution is homogeneous,
−1
−1
while the solution turns into gel after thermal treatment. The
gel is also insoluble in tetrahydrofuran and dimethyl sulfoxide,
suggesting that a cross-linking structure is present.
1364 cm . Therefore, the absorption at around 1710 cm in
Figure 4 comes from the residual MEK in the gel. The higher
molecular weight of PBz-6 M (a main-chain type polybenzox-
azine precursor) leads to chain entanglement and hinders the
evaporation of MEK in the gel, so the amount of residual MEK
is more obvious in PBz-6M/BACY system (Figure 4) than that
in P-oda/BACY system (Figure 2). Therefore, some absorption
intensity at 1366 cm− in Figure 4 comes from the absorption
of MEK. However, no absorption at around 1500−1600 cm
was observed for MEK, so the absorption intensity of 1573
cm was not influenced by the residual MEK. This explains the
reason why the intensity increase of the 1573 and 1366 cm−
bands do not coincide in Figure 4.
Figure 2 shows IR spectra of (a) BACY, (b) P-oda, and P-
oda/BACY blend after thermal treatment at (c) 30 °C for 24 h,
d) 50 °C for 4 h, and (e) 100 °C for 2 h. The cyanate ester
absorption peaks at 2273 and 2237 cm disappeared after 24 h
(
−1
1
at 30 °C (Figure 2c). The triazine absorptions at 1576 and
1
−1
1
368 cm− appeared gradually with increasing annealing
temperature (Figure 2c−e). The results indicate the rapid
trimerization of cyanate ester in the blend.
To further investigate the origin of rapid trimerization of
cyante ester, a 50 wt % methyl ethyl ketone solution of PBz-
−1
1
6
M/BACY (1/1 mol/mol) was prepared, in which PBz-6 M is
It is highly unlikely that ring-opening polymerization for P-
oda would occur at 30 °C. In addition, the ortho positions to O
or N of oxazine of PBz-6 M are all blocked by the methyl
group. As a result, the ring-opening polymerization for PBz-6 M
is very difficult to proceed at 30 °C. This can be supported by
the intact signal of the out-of-plane mode of the benzene to
an o-dimethyl bisphenol A/o-tetramethyldiaminodiphenylme-
29
thane-based polybenzoxazine precursor (Scheme 1). Figure 3
shows pictures of a 50 wt % solution of PBz-6M/BACY before
and after thermal treatments at 30−100 °C. The results are the
same as those in Figure 1. Likewise, the freshly prepared
2
418
DOI: 10.1021/acs.macromol.5b00334
Macromolecules 2015, 48, 2417−2421

Macromolecules
Article
Figure 3. Pictures of a 50 wt % MEK solution of PBz-6M/BACY (1/1, mol/mol): (a) freshly prepared solution and (b−d) the thermally treated
solutions, (b) 30 °C for 24 h, (c) 50 °C for 4 h, and (d) 100 °C for 2 h.
Figure 4. FTIR spectra of (a) BACY, (b) PBz-6M, and PBz-6M/
BACY blend after thermal treatment at (c) 30 °C for 24 h, (d) 50 °C
for 4 h, and (e) 100 °C for 2 h.
which oxazine is attached at 943 and 936 cm−1 for P-oda and
PBz-6M. Therefore, the information from IR analysis in Figures
2
and 4 suggests that the catalytic effect results from the
benzoxazine itself, not from the ring-opened structure of
benzoxazine. Furthermore, Figures S2 and S3 show pictures of
a 50 wt % MEK solution of phenol/BACY and bisphenol A/
BACY (2/1 and 1/1 mol/mol), respectively, before and after
thermal treatments. Homogeneous solutions were observed for
all the phenol/BACY system after thermal treatment. This
results exclude the catalytic effect from phenol structure. We
divided the benzoxazine structure of P-oda into two parts to
discuss the catalytic mechanism. The first one is an N,N-
dimethylaniline-containing section (shown as red color in
Scheme 1), whereas the second one is the anisole-containing
section (shown as blue color in Scheme 1). Figure S4 shows
pictures of a 50 wt % solution of (a) anisole/BACY and (b)
N,N-dimethylaniline/BACY (2/1, mol/mol) before and after
thermal treatments. A homogeneous solution was observed for
the anisole/BACY system after thermal treatment, while
gelation occurred for the N,N-dimethylaniline/BACY system
after thermal treatment at 80 °C for 48 h. Figure 5 shows IR
spectra of (a) anisole/BACY and (b) N,N-dimethylaniline/
BACY blend after thermal treatment at 80 °C for various
periods of time. No obvious change in the cyanate ester
Figure 5. FTIR spectra of (a) anisole/BACY and (b) N,N-
dimethylaniline/BACY blend after thermal treatment at 80 °C for
various periods of time.
(Figure 5a). However, the cyanate ester absorptions decreased
gradually when the thermal treatment started and disappeared
after 60 h of thermal treatment for the N,N-dimethylaniline/
BACY blend (Figure 5b). In addition, the triazine absorptions
at 1563 and 1364 cm− appeared gradually. These results
suggest that the tertiary amine linkage, not the ether linkage,
catalyzes the trimerization of cyanate ester. The catalytic effect
of N,N-dimethylaniline on the trimerization of cyanate ester
can also be confirmed by DSC thermograms. Figure 6 shows
the DSC heating scans of an N,N-dimethylaniline/BACY blend
with molar ratios of 0.2/1, 0.4/1, and 0.6/1. The exothermic
peak temperature decreased with increasing amount of N,N-
dimethylaniline, while the exothermic enthalpy increased with
1
−1
absorption peaks at 2273 and 2237 cm was found after 60 h
of thermal treatment at 80 °C for the anisole/BACY blend
2
419
DOI: 10.1021/acs.macromol.5b00334
Macromolecules 2015, 48, 2417−2421

Macromolecules
Article
Figure 6. DSC thermogram of (a) BACY, and N,N-dimethylaniline/BACY blend with molar ratios of (b) 0.2/1, (c) 0.4/1, and (d) 0.6/1.
Scheme 2. Proposed Catalytic Mechanism of Benzoxazine to the Trimerization of Cyanate Ester
2
3,24
28
increasing amount of N,N-dimethylaniline, further pointing to
the catalytic effect of N,N-dimethylaniline on the trimerization
of cyanate ester.
2. Indeed, the phenolic OH
and the oxygen anion also
catalyze the trimerization of cyanate ester, but the catalysis
occurs after the ring-opening polymerization of benzoxazine.
More recently, Endo et al. reported that triphenylphosphine
reacts with cyanate ester, and forms phosphonium salt
ASSOCIATED CONTENT
■
1
30
according to H NMR analysis. The reaction occurs through
the nucleophilic addition of unpaired electrons of triphenyl-
phosphine on the electron-deficient cyanate ester (see Scheme
*
S
Supporting Information
Figures S1−S4 showing the IR spectrum of MEKand pictures
of a 50 wt % MEK solution with various compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
30
3
of the above study ). Based on Endo’s result along with the
IR and DSC analyses mentioned above, we propose a three-
step catalytic mechanism of benzoxazine for the trimerization of
cyanate ester in Scheme 2. First, a nitrogen anion is formed as
the unpaired electron of nitrogen attacks the electron-deficient
carbon of cyanate ester. Second, the trimerization reaction
occurs through the attack of the nitrogen anion on the other
two cyanate esters. Third, triazine structure is formed as the
benzoxazine is released.
AUTHOR INFORMATION
■
*
*
Corresponding Authors
(R.J.J.) E-mail: rujong@ntu.edu.tw.
(C.H.L.) E-mail: linch@nchu.edu.tw.
Notes
The authors declare no competing financial interest.
CONCLUSIONS
■
ACKNOWLEDGMENTS
Gelation occurred in a 50 wt % methyl ethyl ketone solution of
P-oda/BACY and PBz-6M/BACY blend after 24 h at 30 °C. IR
spectra revealed the rapid trimerization of cyanate ester in both
blends at 30 °C. Since the possibility of ring-opening
polymerization for benzoxazine at 30 °C is rare, we think
that the catalytic effect results from the benzoxazine itself, not
from the ring-opened structure of benzoxazine. Through IR and
DSC analyses, we conclude that the tertiary amine of
benzoxazine catalyzes the trimerization of cyanate ester.
■
Financial support of this work from the Ministry of Science and
Technology (MOST 103-2622-E-007-025), Taiwan is highly
appreciated.
REFERENCES
■
(
1) Ghosh, N. N.; Kiskan, B.; Yagci, Y. Prog. Polym. Sci. 2007, 32
11), 1344−1391.
2) Ning, X.; Ishida, H. J. Polym. Sci., Part A: Polym. Chem. 1994, 32
(
(
30
Based on Endo’s result along with the IR and DSC analyses
mentioned above, we propose a three-step catalytic mechanism
of benzoxazine for the trimerization of cyanate ester in Scheme
(
6), 1121−1129.
(3) Ning, X.; Ishida, H. J. Polym. Sci., Part B: Polym. Phys. 1994, 32
(5), 921−927.
2
420
DOI: 10.1021/acs.macromol.5b00334
Macromolecules 2015, 48, 2417−2421

Macromolecules
Article
(
4) Kim, S.-K.; Choi, S.-W.; Jeon, W. S.; Park, J. O.; Ko, T.; Chang,
H.; Lee, J.-C. Macromolecules 2012, 45 (3), 1438−1446.
5) Liu, C.; Shen, D.; Sebastian, R. M. a.; Marquet, J.; Scho
Macromolecules 2011, 44 (12), 4616−4622.
6) Wang, M. W.; Jeng, R. J.; Lin, C. H. Macromolecules 2015, 48 (3),
30−535.
7) Taskin, O. S.; Kiskan, B.; Yagci, Y. Macromolecules 2013, 46 (22),
773−8778.
8) Wang, C.-F.; Su, Y.-C.; Kuo, S.-W.; Huang, C.-F.; Sheen, Y.-C.;
Chang, F.-C. Angew. Chem., Int. Ed. 2006, 45 (14), 2248−2251.
9) Lin, C. H.; Shih, Y. S.; Wang, M. W.; Tseng, C. Y.; Juang, T. Y.;
Wang, C. F. RSC Adv. 2014, 4 (17), 8692−8698.
10) Mohamed, M. G.; Hsiao, C.-H.; Hsu, K.-C.; Lu, F.-H.; Shih, H.-
K.; Kuo, S.-W. RSC Adv. 2015, 5 (17), 12763−12772.
11) Chernykh, A.; Agag, T.; Ishida, H. Macromolecules 2009, 42
14), 5121−5127.
12) Dogan Demir, K.; Kiskan, B.; Yagci, Y. Macromolecules 2011, 44
7), 1801−1807.
13) Zhang, K.; Zhuang, Q.; Liu, X.; Cai, R.; Yang, G.; Han, Z. RSC
(
́
̈
nfeld, R.
(
5
(
8
(
(
(
(
(
(
(
(
Adv. 2013, 3 (15), 5261−5270.
(14) Fang, T.; Shimp, D. A. Prog. Polym. Sci. 1995, 20 (1), 61−118.
(15) Hamerton, I.; Hay, J. N. Polym. Int. 1998, 47 (4), 465−473.
(16) Chaplin, A.; Hamerton, I.; Howlin, B. J.; Barton, J. M.
Macromolecules 1994, 27 (18), 4927−4935.
17) Guenthner, A. J.; Yandek, G. R.; Wright, M. E.; Petteys, B. J.;
Quintana, R.; Connor, D.; Gilardi, R. D.; Marchant, D. Macromolecules
006, 39 (18), 6046−6053.
18) Maya, E. M.; Snow, A. W.; Buckley, L. J. Macromolecules 2001,
5 (2), 460−466.
19) Snow, A. W.; Buckley, L. J. Macromolecules 1997, 30 (3), 394−
05.
20) Lin, C. H.; Hsiao, C. N.; Li, C. H.; Wang, C. S. J. Polym. Sci.,
Part A: Polym. Chem. 2004, 42 (16), 3986−3995.
21) Ryu, B.-Y.; Emrick, T. Macromolecules 2011, 44 (14), 5693−
700.
22) Cash, J. J.; Davis, M. C.; Ford, M. D.; Groshens, T. J.;
(
2
(
3
(
4
(
(
5
(
Guenthner, A. J.; Harvey, B. G.; Lamison, K. R.; Mabry, J. M.;
Meylemans, H. A.; Reams, J. T.; Sahagun, C. M. Polym. Chem. 2013, 4
(
13), 3859−3865.
23) Sudo, A.; Kudoh, R.; Nakayama, H.; Arima, K.; Endo, T.
Macromolecules 2008, 41 (23), 9030−9034.
24) Kimura, H.; Ohtsuka, K.; Matsumoto, A. Expr. Polym. Lett.
(
(
2
(
(
011, 5 (12), 1113−1122.
25) Li, X.; Gu, Y. Polym. Chem. 2011, 2 (12), 2778−2781.
26) Lin, C. H.; Huang, S. J.; Wang, P. J.; Lin, H. T.; Dai, S. A.
Macromolecules 2012, 45 (18), 7461−7466.
27) Hamerton, I. Chemistry and Technology of Cyanate Ester Resins.
Springer: Berlin, 1994.
28) Li, X.; Luo, X.; Liu, M.; Ran, Q.; Gu, Y. Mater. Chem. Phys.
014, 148 (1−2), 328−334.
29) Chernykh, A.; Liu, J.; Ishida, H. Polymer 2006, 47 (22), 7664−
669.
30) Tsuchiya, K.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 2014,
2 (5), 699−706.
(
(
2
(
7
(
5
2
421
DOI: 10.1021/acs.macromol.5b00334
Macromolecules 2015, 48, 2417−2421