Deprotection-free preparation of propargyl ether-containing phosphinated benzoxazine and the structure-property relationship of the resulting thermosets

Article abstract of DOI:10.1002/pola.25857

Generally, protection and deprotection procedures of amino groups are required in preparing propargyl ether-containing benzoxazines. In this study, we report a facile, deprotection-free preparation of a propargyl ether-containing phosphinated benzoxazine

Full text of DOI:10.1002/pola.25857

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Deprotection-Free Preparation of Propargyl Ether-Containing
Phosphinated Benzoxazine and the Structure–Property
Relationship of the Resulting Thermosets
Hou Chien Chang, Ching Hsuan Lin, Hung Tse Lin, Shenghong A. Dai
Department of Chemical Engineering, National Chung Hsing University, Taichung, Taiwan
Correspondence to: C. H. Lin (E-mail: linch@nchu.edu.tw)
Received 15 August 2011; accepted 29 October 2011; published online 8 December 2011
DOI: 10.1002/pola.25857
ABSTRACT: Generally, protection and deprotection procedures
of amino groups are required in preparing propargyl ether-
containing benzoxazines. In this study, we report a facile,
deprotection-free preparation of a propargyl ether-containing
phosphinated benzoxazine (2) from the nucleophilic substitu-
tion of a phenolic OH-containing phosphinated benzoxazine (1)
and propargyl bromide in the catalysis of potassium carbonate.
The structure of (2) was characterized and confirmed by a
high-resolution mass spectrum, 1H, 13C, 1H-1H, 1H-13C nuclear
magnetic resonance (NMR) spectra, and X-ray single crystal
diffractogram. infrared (IR) and differential scanning calorime-
try were used to monitor the ring-opening of benzoxazine and
crosslinking of propargyl ether. The microstructure and the
structure–property relationship of the resulting homopolymers
and copolymers are discussed. The Tg of homopolymer of (2)
is 208 ꢀC by dynamic mechanical analysis, the coefficient of
thermal expansion is 43 ppm/ꢀC, and Td 5% (N2) is 393 ꢀC,
respectively, which are higher than those of the homopolymer
of (1). Similar trends were observed in the copolymerization
system. The results demonstrate the beneficial effect of cross-
linking afforded by the propargyl ether group is higher than
C
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that by the phenolic OH group. 2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 50: 1008–1017, 2012
KEYWORDS: benzoxazine; flame retardance; phosphinated;
propargyl ether; protection; synthesis; thermal properties;
thermosets
INTRODUCTION The demand for flame-retardant polymers is
high due to their wide applications in electronic industries,
such as in printed circuit boards or encapsulation. Bromi-
nated compounds have been widely used to reduce flamma-
bility, but they are considered as carcinogens and face
restriction in some area due to health and environmental
concerns.1–6 Some halogen-free, environment-friendly flame-
retardant structure or element, such as alkyne,7–9 deoxyben-
zoin,10–12 triazole,1,13 and phosphorus,14–22 have been incor-
porated into polymers to enhance flame retardancy, and
promising results have been achieved.
flame-retardant polybenzoxazines is interesting for electronic
industries. Recently, flame-retardant benzoxazines based on
phenylphosphine oxide-containing biphenol,24 phosphinated
trisphenol,25 phosphinated triamine,26 aromatic amines,20,27–30
and phosphazine31 have been reported, and the flame retard-
ancy of resulting thermosets has been proven.
Aryl propargyl ether is known to undergo Claisen type rear-
rangement to benzopyran with thermal treatment, and sub-
sequently polymerize via the curing of the C¼¼C bond in the
benzopyran structure.32–39 Agag and Takeichi prepared a
propargyl ether-containing benzoxazine (P-appe, Fig. 1) from
p-nitrophenol by a three-step procedure including (1) nucle-
ophilic substitution with propargyl bromide, (2) reduction of
nitro group, and (3) formation of benzoxazine.40 Yagci and
coworkers prepared the same compound from 4-aminophe-
nol by a four-step procedure including (1) protection of the
amino group by acetic acid, (2) nucleophilic substitution
with propargyl bromide, (3) deprotection of amino from
acetamide, and (4) formation of benzoxazine.41 Propargyl
ether containing benzoxazines have also been prepared by
Benzoxazines are resins that can be polymerized to thermo-
sets via the thermally activated ring-opening of oxazine link-
age. Epoxy/benzoxazine systems have recently been used in
the field of copper clad laminates due to the characteristics
of higher Tg, better adhesion to glass fiber, and better tough-
ness than the epoxy/phenol novolac system. Although carbon
fiber reinforced polybenzoxazine P(B-a) shows improved
flame retardancy,23 neat polybenzoxazines cannot meet the
flame-retardancy standard, UL-94 V-0. Thus, the research on
Additional Supporting Information may be found in the online version of this article.
C
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EXPERIMENTAL
Materials
Phenolic OH-containing benzoxazine (1) was prepared by
the procedure in our previous work.44 Potassium carbonate
and propargyl bromide were purchased from Acros. Bis(4-
(2H-benzo[e][1,3]oxazin-3(4H)-yl)phenyl)methane (P-d) was
kindly supplied by the Shikoku Corporation, Japan. N,N-Di-
methyl acetamide (DMAc) was purchased from Tedia and
purified by distillation under reduced pressure over calcium
hydride (Acros) and stored over molecular sieves. The other
solvents used are commercial products and were used with-
out further purification.
FIGURE 1 Structure of P-appe.
Ishida and coworkers by a similar approach.42,43 In these
strategies, protection form of amine (in nitro or acetamide
form) is required to prevent side reactions from amino and
propargyl bromide. After the nucleophilic substitution of
phenolic OH and propargyl bromide is complete, regenerat-
ing of amino group, no matter from reduction of nitro group
or deprotection from acetamide, is needed. Therefore, it
would be of interest if propargyl ether-containing benzoxa-
zines could be prepared by an alternative approach without
any reduction or protection–deprotection procedure.
Synthesis of Benzoxazine (2)
A total of 4.41 g (0.01 mol) of (1), 2.07 g (0.015 mol) of po-
tassium carbonate, and 10 mL of DMAc were introduced into
a 100-mL round-bottom glass flask equipped with a con-
denser and a magnetic stirrer. Also 1.78 g (0.012 mol) of
80% propargyl brꢀomide in toluene was added. The mixture
was stirred at 80 C for 24 h. After that, the mixture was fil-
tered and the filtrate was poured into water (250 mL). The
precipitate was filtered and dried (88% yield). As the phos-
phorus and the adjacent aliphatic carbon are both chiral cen-
ters, two diastereomers were formed in the preparation. The
ratio of two diastereomers, calculated from nuclear magnetic
resonance (NMR) spectra, is around 4:1. After crystallization
in methanol, the minor of the two diastereomers was
removed (70% yield). A melting point at 174 ꢀC with en-
Herein, we report a deprotection-free synthesis of a propargyl
ether-containing benzoxazine (2) via the nucleophilic substi-
tution of a phenolic OH-containing benzoxazine (1) with pro-
pargyl bromide in the catalysis of potassium carbonate. In this
work, the amino group is protected in a benzoxazine form, so
no reduction or deprotection procedure is required after the
nucleophilic substitution. To the best of our knowledge, no
propargyl ether containing-benzoxazines have been prepared
via this facile approach. We also demonstrate that copolyben-
zoxzines based on the propargyl ether benzoxazine (2) show
better thermal properties than those based on the phenolic
OH benzoxazine (1). The detailed synthesis, curing behavior,
the microstructure of (2), and the structure–property relation-
ship of the resulting thermosets are reported in this work.
ꢀ
thalpy of 180 J/g and an exothermic peak at 255 C with en-
thalpy of 499 J/g were observed in the differential scanning
calorimetry (DSC) thermogram.
HR-MS (FABþ) m/z: calcd. for C29H22O4NP 479.1286; anal.
479.1288, (M)þ C29, H22, O4, N, P. Fourier transform
TABLE 1 Thermal Properties of P(2), (1)/P-d, and (2)/P-d Copolymers
Thermal
Stability
Flammability
Phosphorus
Char
Sample
ID
Content
(wt %)
CTE
(ppm/ꢀC)c
Yield
(wt %)e
UL-94
Grade
Tg (ꢀC)a
Tg (ꢀC)b
Td (ꢀC)d
t1
t2
P(P-d)
0
208
208
220
224
225
230
237
194
187
187
180
180
194
201
204
206
208
211
163
156
162
160
50
43
40
39
39
38
37
49
48
49
47
c
412
393
440
435
420
418
425
382
378
368
367
45
48
56
56
57
53
57
46
47
47
45
>60
1.2
–
Burning
V-0
P(2)
6.4
0.5
1.8
0.6
1.2
1.0
0.8
1.7
1.1
1.1
0.5
P(2)-0.5
P(2)-0.75
P(2)-1.0
P(2)-1.25
P(2)-1.5
P(1)-0.75
P(1)-1.0
P(1)-1.25
P(1)-1.5
a
0.5
11.1
6.1
3.4
2.0
1.2
6.3
3.7
2.8
2.1
V-1
0.75
1.00
1.25
1.50
0.75
1.00
1.25
1.50
V-0
V-0
V-0
V-0
V-0
V-0
V-0
V-0
Peak temperature of tan d, measured by DMA at a heating rate of
ꢀC/min.
Measured by TMA at a heating rate of 5 ꢀC/min.
Coefficient of thermal expansion before Tg.
5% decomposition temperature in nitrogen.
Residual weight percentage at 800 ꢀC in nitrogen.
d
e
5
b
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SCHEME 1 Synthesis of benzoxazine (2).
infrared spectroscopy (FTIR) absorption (KBr): sp-hybridized
CAH at 3295 cmꢁ1, symmetric ArAOAC at 1041 cmꢁ1, a
characteristic mode of benzene with an attached oxazine
Characterization
DSC was performed with a Perkin-Elmer DSC 7 in a nitrogen
atmosphere at a heating rate of 10 min/ꢀC. Thermal gravi-
metric analysis (TGA) was performed with a Perkin-Elmer
Pyris1 at a heating rate of 20 ꢀC/min in a nitrogen atmos-
phere. Dynamic mechanical analysis (DMA) was performed
with a Perkin-Elmer Pyris Diamond DMA with a sample size
of 5.0 ꢂ 1.0 ꢂ 0.2 cm3. The storage modulus E0 and tan d
were determined as the sample was subjected to the temper-
ature scan mode at a programmed heating rate of 5 ꢀC/min
at a frequency of 1 Hz. The test was performed by a bending
mode with an amplitude of 5 lm. Thermal mechanical anal-
ysis (TMA) was performed with a Perkin-Elmer Pyris Dia-
1
ring at 956 cmꢁ1. H NMR (ppm, CDCl3), d ¼ 2.43 (1H, H1),
4.51 (2H, H3), 4.72 (1H, H8), 4.91–5.33 (2H, H27), 6.61 (1H,
H6), 6.67–6.69 (1H, H5), 6.85 (1H, H11), 6.98 (1H, H13), 7.18
(1H, H12), 7.23–7.26 (2H, H23and H25), 7.38 (1H, H24), 7.40–
7.43 (2H, H14 and H18), 7.69 (1H, H17), 7.86–7.89 (1H, H19),
7.96 (1H, H22), 7.99–8.01 (1H, H16). 13C NMR (ppm, CDCl3),
d ¼ 55.8 (C3), 60.1 and 60.8 (C8,1JPAC ¼ 111.8 Hz), 75.4
(C1), 77.7 (C27), 78.4 (C2), 114.8 (C9), 115.3 (C5), 117.5
(C11), 119.9 (C25), 121.1 (C13), 121.4 and 121.7 (C15, 1JPAC
¼
39 Hz), 121.8 (C6), 122.4 (C20), 123.4 (C16), 124.5 (C23), 125.0
(C22), 128.1 (C14), 128.3 (C18), 128.8 (C12), 130.7 (C24), 132.6
(C19), 133.8 (C17), 136.2 (C21), 143.6 (C7), 149.8 (C4), 159.6
(C26), 154.7 (C10). Single crystal data: C29H22O4NP, 0.48 ꢂ
0.44 ꢂ 0.42 mm3, monoclinic with a ¼ 9.8242 (8) Å, b ¼
13.0353 (9) Å, c ¼ 17.9252 (15) Å, a ¼ 90ꢀ, b ¼ 93.002 (10)ꢀ,
c ¼ 90ꢀ with Dc ¼ 1.389 mg/m3 for Z ¼ 4, V ¼ 2292.38 (4) Å3,
T ¼ 297 (2) K, k ¼ 0.71073 Å, F(000) ¼ 1000, final R indices:
R1 ¼ 0.0378, WR2 ¼ 0.1104.
mond TMA at
a heating rate of 5
ꢀC/min. NMR
Curing Procedure
For copolymerization systems, (1)/P-d and (2)/P-d with
various weight ratios were melted, stirredꢀhomogeneously in
a temperature-controlled hot plate at 160 C for 20 min, and
then transferred to an aluminum mold. The samples were
cured at 180 ꢀC (2 h), 200 ꢀC (2 h), 220 ꢀC (2 h), and
240 ꢀC (1 h) in an air-circulating oven (The reason for
ꢀ
240 C curing will be explained in ‘‘Microstructure’’ section.).
Thereafter, samples were allowed to cool slowly to room
temperature to prevent cracking. For (1)/P-d and (2)/P-d
copolymers, the relation between the sample ID and the con-
tent of phosphorus is listed in Table 1. The number after P is
the phosphorus content in the copolymer. For example, P(1)-
1.25 means the (1)/P-d copolymer with a phosphorus content
of 1.25 wt %, and P(2)-1.5 means the (2)/P-d copolymer with
a phosphorus content of 1.5 wt %. Homopolymer of (2), P(2),
was cured by the same curing program as copolymers.
FIGURE 2 (a) 1H NMR (as prepared), (b) 1H NMR (methanol-
recrystallized), (c) 31P NMR (as prepared), and (d) 31P NMR
(methanol-recrystallized) spectra of (2).
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FIGURE 3 (a) 1H NMR and (b) 13C NMR spectra of (2).
measurements were performed using a Varian Inova 600
NMR in DMSO-d6, and the chemical shift was calibrated by
setting the chemical shift of DMSO-d6 as 2.49 ppm. Infra-
red (IR) spectra were measured by a Perkin-Elmer RX1
infrared spectrophotometer in KBr powder form. High-re-
solution mass spectra were obtained by a Finnigan/
Thermo Quest MAT 95XL mass spectrometer. The UL-94
vertical test was performed according to the testing proce-
dure of FMVSS 302/ZSO 3975 with a test specimen bar of
127 mm in length, 12.7 mm in width, and about 1.27 mm
in thickness. The height of the burner flame was 25 mm
and the height from the top of the burner to the bottom of
the test bar was 10 mm. During the test, the polymer
specimen was subjected to two 10 s ignitions. After the
first ignition, the flame was removed and the time for the
polymer to self-extinguish (t1) was recorded. Cotton igni-
tion was noted if polymer dripping occurred during the
test. After cooling, the second ignition was performed on
the same sample. The self-extinguishing time (t2) and
dripping characteristics were again recorded. Five speci-
mens were measured and the average burning time was
recorded. If t1 þ t2 was less than 10 s with no dripping, it
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propargyl bromide in the catalysis of potassium carbonate,
we prepared a propargyl ether-containing benzoxazine (2)
(88% yield, Scheme 1). In this work, the amino group is pro-
tected in a benzoxazine form, so no deprotection procedure
is required after the nucleophilic substitution.
Figure 2 shows the NMR spectra of as-prepared (2); there-
fore, the crude product before crystallization. Because the
phosphorus and the adjacent aliphatic carbon are both
chiral centers, two diastereomers were formed in the
preparation. This can be supported by the complicated
peak patterns at 4.5–5.5 ppm in Figure 2(a), and the
two 31P NMR peaks in Figure 2(c). The ratio of two dia-
stereomers, calculated from NMR spectra, is around 4:1.
After recrystallization in methanol, the minor of the two
diastereomers was removed (70% yield, note that remov-
ing the minor diastereomer is not necessary for practical
application). This can be supported by the simplified pat-
terns in Figure 2(b) and the single 31P peak in Figure 2(d),
respectively.
FIGURE 4 X-ray single crystal diffractogram of (2).
was considered to be a V-0 grade, an industrial standard
for flame-retardancy. If t1 þ t2 was in the range of 10–30
s without any dripping, the polymer was considered to be
a V-1 material. If t1 þ t2 was in the range of 10–30 s with
dripping that ignited a cotton indicator located below the
sample, the polymer was considered to be a V-2 material.
1
Figure 3 shows the (a) H and (b) 13C NMR spectra of meth-
anol-recrystallized (2). The signals of propargyl ether were
observed at 4.51 (H3) and 2.43 (H1) in Figure 3(a), and at
78.4 (C2), 75.4 (C1), and 55.8 (C3) ppm in Figure 3(b),
respectively. The signals of oxazine linkage were observed at
4.72 (H8) and 4.91 and 5.33 (H27) in Figure 3(a), and at 60.1
RESULTS AND DISCUSSION
1
and 60.8 (C8, JPAC ¼ 111.8 Hz) and 77.7 (C27) ppm in Fig-
Synthesis and Characterization of (2)
ure 3(b), respectively. The intact of oxazine linkage indicates
the stability of oxazine linkage in the alkaline-catalyzed reac-
tion condition. It should be noted that carbon C8 is a chiral
center, so the two hydrogens of OCH2N (H27) are not mag-
netic equivalent. The detailed assignment of each peak,
In our previous work, we reported a facile synthesis of a
phenolic OH-containing benzoxazine (1) by the reaction of
4-aminophenol, 2-hydroxybenzaldehyde, and 9,10-dihydro-9-
oxa-10-phosphaphenanthrene-10-oxide (DOPO).44 In this
work, based on the nucleophilic substitution of (1) with
FIGURE 5 DSC thermogram of benzoxazine (2).
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DSC Thermogram
Figure 5 shows the DSC thermogram of a single crystal of
benzoxazine (2). A melting point at 174 ꢀC, an exothermic
peak at 255 ꢀC, and a shoulder at around 260 ꢀC were
observed. When compared with the value (230 ꢀC and 237
ꢀC) of (1),44 the melting point of (2) is reduced remarkably
due to the disappearance of hydrogen bonding resulting
from phenolic OH. The reduction in melting point leads to a
reasonable processing window for (2). The enthalpy of the
exothermic peak is as high as 499 J/g, which is much higher
than that of benzoxazine without propargyl ether, in which
propargyl ether is replaced by a methyl group.29 Surprisingly,
in units of kJ/mol, the exothermic enthalpy (239 kJ/mol) of
(2) is even higher than the value (188 kJ/mol) of P-appe,40
probably because the DSC thermogram of (2) was measured
by a high-purity single crystal.
Microstructure
Supporting Information Figure S3 shows the IR spectra of
(2) after accumulative curing at 180 ꢀC for various periods
of time. The absorptions of the characteristic mode of ben-
zene with an attached oxazine ring45 at 956 cmꢁ1 and pro-
pargyl ether at 2113 cmꢁ1 decreased gradually, and almost
disappeared after curing for 60 min. A C¼¼C absorption,
which resulted from the benzopyran (the product of Claisen
type rearrangement of propargyl ether), appeared after cur-
ing for 15 min. Supporting Information Figure S4 shows DSC
thermograms of (2) after accumulative curing at 180 ꢀC for
various periods of time (the same thermal history as in Sup-
porting Information Fig. S3). The first exotherm decreased
apparently after each curing cycle. The second exotherm
seemed not to be as obviously affected by the curing time as
FIGURE 6 IR spectra of (2) after accumulative curing. The cur-
ing temperature and curing time are marked in the Figure 5.
assisted by the correlation in 1HA1H, and 1HA13C spectra
(Supporting Information Figs. S1 and S2), confirms the struc-
ture of (2). The structure of (2) can further be confirmed by
the X-ray single crystal diffractogram, as shown in Figure 4.
The crystal is monoclinic, with a ¼ 9.8242 (8) Å, b ¼
13.0353 (9) Å, c ¼ 17.9252 (15) Å, a ¼ 90ꢀ, b ¼ 93.002
(10)ꢀ, and c ¼ 90ꢀ.
FIGURE 7 DSC thermograms of (2) after accumulative curing (the same thermal history as in Fig. 6).
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SCHEME 2 Polymerization mechanism of (2).
the first exotherm. IR and DSC data suggest that the ring
opening of oxazine and the first-stage crosslinking of propar-
gyl ether (the formation of benzopyran) are correlated with
the first DSC exotherm.
enthalpy corresponding to the ring opening of oxazine and
crosslinking of propargyl group decreased after each curing
cycle. The second exothermic peak decrease obviously after
curing at 240 ꢀC for 30 min, and disappeared after curing
for 60 min. From IR and DSC data, the second exotherm is
related with the polymerization of C¼¼C bond in the benzo-
pyran structure, and the final curing temperature for homo-
polymer and copolymers are set to 240 ꢀC, 1 h. According
to the data in Supporting Information Figures S3–S4 and
Figures 5 and 6, a two-stage polymerization mechanism of
(2) was proposed in Scheme 2. This first stage is related
Figure 6 shows the IR spectra of (2) after accumulative cur-
ing, and the curing temperature and curing time are marked
in the figure. The absorption of C¼¼C of benzopyran at
1,673 cmꢁ1 decreased gradually after curing at 220 ꢀC for
30 min, and disappeared after curing at 240 ꢀC for 60 min.
Figure 7 shows the DSC profiles of (2) after accumulative
curing (the same thermal history as in Fig. 6). The exothermic
FIGURE 8 (a) DSC, (b) DMA, (c) TMA, and (d) TGA thermograms of P(2).
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studies,49,50 the release of aniline fragments is responsible
for the low thermal stability of polybenzoxazines, so the
5% degradation temperatures of common polybenzoxazines
are not high. Therefore, introduction of a crosslinkable site
into aniline moiety, such as allyl,51 nitrile group,52,53 and
maleimide,54,55 has been proved as an effective way to
enhance the thermal stability of polybenzoxazines. In this
case, the aniline linkage in P(2) is bonded to the other
repeating unit via the crosslinking of propargyl ether, which
makes P(2) thermally stable.
Thermal and Flame-Retardant Properties of Copolymers
Figure 9 shows the DMA thermograms of (a) (1)/P-d and
(b) (2)/P-d copolymers, and the detailed data are listed in
Table 1. For the (1)/P-d copolymers, Tgs decrease with the
content of (1). In contrast, Tgs increase with the content of
(2) in the (2)/P-d copolymerꢀs. For example, Tg decreases
ꢀ
from 208 C for P(P-d) to 180 C for P(1)-1.5), but increases
to 237 ꢀC for P(2)-1.5. The height of tan d peak increases
with the content of (1) in the (1)/P-d copolymers, suggest-
ing the crosslinking density decreases with the content of
(1). However, opposite trend was observed in the (2)/P-d
copolymers, suggesting the crosslinking density increases
with the content of (2). The observation is consistent with
the Tg data. A very interesting phenomenon in Tg value was
observed. In Figure 8, we report the Tg of P(2) is 208 ꢀC.
However, the Tg of (2)/P-d copolymers increases with the
content of (2). A Tg value as high as 237 ꢀC, which is much
higher than both P(P-d) and P(2), can be achieved for P(2)-
1.5). A similar result was observed in our previous work, in
which a benzoxazine with bulky trifluoromethyl pendants
showed low Tg.56 However, Tg increased significantly when it
was copolymerized with bis(3,4-dihydro-2H-3-phenyl-1,3-
benzoxazinyl)methane (F-a, a benzoxazine based on bisphe-
nol F/aniline/formaldehyde). It was thought that incorporat-
ing a benzoxazine without bulky pendant into a benzoxazine
with bulky pendants can dilute the steric hindrance caused
by the bulky pendants, leading to thermosets with a higher
crosslinking density and consequently a higher Tg. In this
case, the bulky phosphinate pendant might hinder the curing
of P(2), so the Tg of P(2) is limited to 208 ꢀC. When P-d
was copolymerized with (2), the steric hindrance caused by
the bulky pendants was diluted, leading to copolymers with
higher Tg than neat P(2). The dimensional stability of the
copolymers was measured by TMA, and the data are listed
in Table 1. A similar Tg trend is observed in the TMA mea-
surement. The dimensional stability of (2)/P-d is improved
slightly with the content of (2). The coefficient of thermal
expansion (CTE) of (2)/P-d copolymers range from 37 to 40
ppm/ꢀC, which are smaller than those (47–49 ppm/ꢀC) of
(1)/P-d copolymers. The results further demonstrate the
beneficial effect of crosslinking afforded by the propargyl
ether group is higher than that by the phenolic OH group.
The 5 wt % degradation temperatures of (2)/P-d copoly-
FIGURE 9 DMA thermograms of (a) (1)/P-d and (b) (2)/P-d
copolymers.
with the ring opening of the oxazine and Claisen rearrange-
ment of propargyl ether, and the second stage is the curing
of benzopyran. This observation is consistent with data
reported in previous studies.32–40
Properties of Homopolymers
Figure 8(a) shows the DSC thermogram of P(2). The DSC
scan shows the Tg value of P(2) is 192 ꢀC, which is higher
than the value (161 ꢀC) of P(1), and much higher than the
value (124 ꢀC) of polybenzoxazine in which propargyl ether
is replaced by a methyl linkage.29 It is known that the free
ortho position of a phenolic OH can react with oxazine link-
age,29,46–48 making P(1) a crosslinking polymer. From the Tg
data, the beneficial effect of crosslinking afforded by the
incorporation of the propargyl ether group is higher than
that by the phenolic OH group. Figure 8(b) shows the DMA
thermogram of P(2). Tg is 208 ꢀC from the tan d peak.
ꢀ
Although the value is lower than the value (251 C) of P(P-
appe),40 probably due to the steric hindrance induced by
the bulky phosphinate pendant, the value is much higher
for other thermosets of mono-benzoxazines.ꢀ TMA measure-
ment shows P(2) exhibits a Tg value of 194 C, and a coeffi-
cient of thermal expansion of 43 ppm/ꢀC [Fig. 8(c)]. TGA
measurement shows the 5% decomposition temperature of
P(2) is 393 ꢀC [Fig. 8(d)], which is comparable with the
value (362 ꢀC) of P(P-appe). According to the previous
ꢀ
mers range from 418 to 440 C. The values are relative high
when compared with other polybenzoxazines. As to the
flame retardancy, a thermoset with an UL-94 V-0 grade can
be achieved with a phosphorus content as low as 0.75 wt %
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11 Ranganathan, T.; Zilberman, J.; Farris, R. J.; Coughlin, E. B.;
Emrick, T. Macromolecules 2006, 39, 5974–5975.
for both (1)/P-d and (2)/P-d systems. This result demon-
strates the good flame retardancy of the phosphinate pend-
ant. Interesting, with the same phosphorus content, (2)/P-d
copolymers show shorter burning time than (1)/P-d copoly-
mers. The phenomenon might be probably related with the
difference in microstructure and crosslinking density.
12 Ryu, B. Y.; Moon, S.; Kosif, I.; Ranganathan, T.; Farris, R. J.
Emrick, T. Polymer 2009, 50, 767–774.
13 Ryu, B. Y.; Emrick, T. Angew. Chem. Int. Ed. Engl. 2010, 49,
9644–9647.
14 Allcock, H. R.; Hartle, T. J.; Taylor, J. P.; Sunderland, N. J.
Macromolecules 2001, 34, 3896–3904.
CONCLUSIONS
15 Wang, X.; Hu, Y.; Song, L.; Xing, W.; Lu, H. D.; Lv, P.; Jie, G.
Polymer 2010, 51, 2435–2445.
We have provided an alternative approach to prepare a pro-
pargyl ether-containing benzoxazine by the nucleophilic sub-
stitution of a phenolic OH-containing precursor with propar-
gyl ether in the catalysis of potassium carbonate. Four
findings are reported in this work. First, we report the ben-
zoxazine structure is stable in an alkaline condition for
nucleophilic substitution. Second, because the amino group
is protected in a benzoxazine form, no procedure of reduc-
tion or deprotection of the amino group is required. This
approach demonstrates a facile synthesis of propargyl ether-
containing benzoxazines. To the best of our knowledge, no
propargyl ether containing-benzoxazines have been prepared
by this approach. This finding will increase the design flexi-
bility of benzoxazine-containing derivatives. Third, homopoly-
mer of (2) shows better thermal properties than homopoly-
mer of (1). In addition, (2)/P-d copolymers exhibit better
thermal properties than (1)/P-d copolymers. The data sug-
gest that the beneficial effect of crosslinking afforded by the
propargyl ether group is higher than that of the phenolic OH.
Fourth, the flame retardancy and the thermal properties of
the resulting copolymers increase simultaneously with the
content of (2). Generally, incorporating phosphorus element
usually leads to decreased thermal properties, so the result
of this work is rarely seen in the literature. These properties
make (2) attractive for industrial application, especially in
the field of copper clad laminates.
16 Liu, W. H.; Wang, Z. G.; Xiong, L.; Zhao, L. Polymer 2010,
51, 4776–4783.
17 Wang, D. Y.; Song, Y. P.; Lin, L.; Wang, X. L.; Wang, Y. Z.
Polymer 2011, 52, 233–238.
18 Bian, X. C.; Chen, L.; Wang, J. S.; Wang, Y. Z. J. Polym. Sci.
Part A: Polym. Chem. 2010, 48, 1182–1189.
19 Espinosa, L. M. D.; Meier, M. A. R.; Ronda, J. C.; Galia`, M.;
Ca´diz, V. J. Polym. Sci. Part A: Polym. Chem. 2010, 48,
1649–1660.
20 Lin, C. H.; Lin, H. T.; Sie, J. W.; Hwang, K. Y.; Tu, A. P.
J. Polym. Sci. Part A: Polym. Chem. 2010, 48, 4555–4566.
21 Serbezeanu, D.; Vlad-Bubulac, T.; Hamciuc, C.; Aflori, M.
J. Polym. Sci. Part A: Polym. Chem. 2010, 48, 5391–5403.
22 Negrell-Guirao, C.; David, G.; Boutevin, B.; Chougrani, K.
J. Polym. Sci. Part A: Polym. Chem. 2011, 49, 3905–3910.
23 Tamotsu, O. (to Sumitomo Bakelite Co. Ltd.). Jpn. Pat.
106,813A, April 17, 2001.
24 Choi, S. W.; Ohba, S.; Brunovska, Z.; Hemvichian, K.; Ishida,
H. Polym. Degrad. Stab. 2006, 91, 1166–1178.
25 Lin, C. H.; Cai, S. X.; Leu, T. S.; Hwang, T. Y.; Lee, H. H.
J. Polym. Sci. Part A: Polym. Chem. 2006, 44, 3454–3468.
26 Chang, C. W.; Lin, C. H.; Lin, H. T.; Huang, H. J.; Hwang, K.
Y.; Tu, A. P. Eur. Polym. J. 2009, 45, 680–689.
27 Lin, C. H.; Cai, S. X.; Lin, C. H. J. Polym. Sci. Part A: Polym.
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28 Ulrich, W.; Franck, M. WO 057,279 A1, 2002.
The authors thank the National Science Council of the Republic
of China for financial support. Partial sponsorship by the Green
Chemistry Project (NCHU), as funded by the Ministry of Educa-
tion, is also gratefully acknowledged.
29 Lin, C. H.; Lin, H. T.; Chang, S. L.; Huang, H. J.; Hu, Y. M.;
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30 Sponto´ n, M.; Lligadas, G.; Ronda, J. C.; Galia`, M.; Ca´diz, V.
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31 Wu, X.; Zhou, Y.; Liu, S. Z.; Guo, Y. N.; Qiu, J. J.; Liu, C. M.
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