Highly branched benzoxazine monomer based on cyclotriphosphazene: Synthesis and properties of the monomer and polybenzoxazines

Article abstract of DOI:10.1016/j.polymer.2011.01.003

A hyperbranched organic-inorganic hybrid benzoxazine monomer based on cyclotriphosphazene (CP) has been synthesized, which possesses six organic benzoxazine moieties distributed on the inorganic ring of CP. The high molecular weight (1491 g/mol) monomer s

Full text of DOI:10.1016/j.polymer.2011.01.003

Polymer 52 (2011) 1004e1012
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Highly branched benzoxazine monomer based on cyclotriphosphazene: Synthesis
and properties of the monomer and polybenzoxazines
,
Xiong Wu a, Yun Zhou b, Shu-Zheng Liu a, Ya-Ni Guo a, Jin-Jun Qiu a, Cheng-Mei Liu a
*
a School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
b Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 200135, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A hyperbranched organiceinorganic hybrid benzoxazine monomer based on cyclotriphosphazene (CP)
has been synthesized, which possesses six organic benzoxazine moieties distributed on the inorganic
ring of CP. The high molecular weight (1491 g/mol) monomer showed excellent solubility in common
organic solvents. Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry
(DSC) were used to study the thermal ring-opening polymerization reaction of the novel benzoxazine
monomer. FT-IR spectrum implied that the characteristic absorption peaks of the benzoxazine ring
disappeared completely after curing at 240 ꢀC for 1 h, which illustrated that the completion of poly-
merization reaction. DSC plots indicated that the melting point of the new monomer was 77 ꢀC and an
exothermic peak was 225 ꢀC owing to the ring-opening polymerization of the monomer. Due to its highly
steric crosslinking structure with rigid and thermal stable inorganic CP as the core, the polybenzoxazine
based on the new monomer showed excellent thermal stability and mechanic properties. The char yield
of the polymer at 850 ꢀC was 66.9% in nitrogen, and the Tg of the polybenzoxazine was 152 ꢀC.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 9 September 2010
Received in revised form
22 November 2010
Accepted 1 January 2011
Available online 9 January 2011
Keywords:
Cyclotriphosphazene
Benzoxazine
Ring-opening polymerization
1. Introduction
many methods. Generally, the methodologies can be summarized
in three approaches:
Recently, there has been an increasing interest in design and
preparation of polybenzoxazine because the materials are developed
class of high-performance thermosetting resins instead of common
phenoliceformaldehyde resin in high-tech field. These thermoset-
ting resins not only possess all the advantages of traditional phenolic
resins such as excellent mechanical and thermal properties, but also
have unique advantages of near-zero shrinkage upon curing, thermal
andflameretardant,low waterabsorptionandremarkablemolecular
designflexibility [1]. Furthermore, the polybenzoxazine resinscanbe
easily gained from corresponding monomers under heat polymeri-
zation with or without catalyst, and the corresponding monomers
can be conveniently generated by the Mannich condensation reac-
tion of phenol, formaldehyde, and primary amine [2].
However, there are also several shortcomings that limit the
application of common polybenzoxazine resins, such as high
polymerization temperature, difficulty in processing and poor
mechanic strength (low toughness) [3] and low-level flame retar-
dant. To properly solve these problems and overcome the
encountering disadvantages, lots of researchers have attempted
(i) Blending with other high-performance polymers or filling
with inorganic materials. Rubber [4], polycarbonate [5], poly
(
3-caprolactone) [6], polyurethane [7], epoxy [8] and other
polymers were blended with the benzoxazine resins to
improve the mechanic and thermal properties. Phosphorus-
containing resin [9,10], clay [11] and magnetic Fe3O4 [12] were
added to polybenzoxazine to modify flame retardant and
thermal stability, or to prepare functional materials.
(ii) Synthesis of new polymeric precursors. Therewere three modes:
(a) Main-chain precursors: Liu [13] and Ishida reported the
concept of oligomeric benzoxazine resins where oxazine rings
were in the main chain. Pedro and his copartners [14] synthe-
sized a series of highly fluorinated main chainpolybenzoxazines.
Commonly, high molecular weight polybenzoxazine precursors
were mostly synthesized from aromatic or aliphatic diamine and
bisphenol with paraformaldehyde; (b) Side-chain precursors:
Side-chain polymer strategy was a way to incorporate benzox-
azinemoietiesintoapolymerbackbonetoachieveahighlydense
network, “click reaction” was the most popular routine to ach-
ieve this aim [15e17]. Using this method, benzoxazine groups
were grafted to PVC, polystyrene, polybutadiene; (c) Cross-link-
able telechelic polymers with benzoxazine moieties at the chain
* Corresponding author. Tel.: þ86 27 87544831; fax: þ86 27 87543632.
E-mail address: liukui@mail.hust.edu.cn (C.-M. Liu).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.01.003

X. Wu et al. / Polymer 52 (2011) 1004e1012
1005
end [18,19]. These three kinds of polymeric precursors with
benzoxazine moieties were allowed later crosslinking for
dimensional stability, chemical resistance, and high-tempera-
ture stability.
(ꢁ99%), p-aminobenzoic acid (ꢁ99%), sodium hydroxide (ꢁ96%),
and potassium carbonate were all gained from Sinopharm Chem-
ical Reagent Co., Ltd., China. Hexachlorocyclotriphosphazene
(synthesized as described in the literature [44]) was recrystallized
from dry hexane followed by sublimation (60 ꢀC, 0.05 mmHg) twice
before use (mp 112.5e113.0 ꢀC). Hexaphenoxycyclotriphosphazene
(HPP) was prepared according to the report [45]. All solvents were
purified by standard procedures.
(iii) Design and synthesis of new benzoxazine monomers with
additional functionality. One strategy was introducing addi-
tional polymerizable groups into benzoxazine, such as nitrile
[20], acetylene [21], propargyl [22], allyl [23], maleimide [24] or
epoxy [25] functionalities. This approach allowed increasing
crosslinking densityand minimizing dangling side groups, thus
leading to improving toughness and thermal properties.
Another strategy was introducing self-catalyst groups into
benzoxazine. Carboxylic acid [26], oxyalcohol [27] and primary
amine [28] functional groups were connected to benzoxazine
to lower the polymerization temperature. Furthermore, the
effect of different substituted groups on polymerization
temperature and thermal stability of polybenzoxazine were
also investigated in-depth [29].
2.2. Measurements
Thestructureofthecompoundwasverifiedbyproton(1H), carbon
(
13C) and phosphorus (31P) nuclear magnetic resonance spectroscopy
(NMR) using Bruker AV400 NMR spectrometerat proton frequency of
400 MHz as well as the corresponding carbon and phosphorus
frequencies at room temperature using deuterated solvents as the
solvent. Signals were averaged from 256 transients for 1H NMR and
31P NMR, and 1024 transients for 13C NMR to yield spectra with
sufficient signal-to-noise ratio. Thermal transitions were monitored
with a differential scanning calorimeter (DSC), Model 204F1 from
NETZSCH Instruments, and scan rate of 10 ꢀC/min over a temperature
range of 30e300 ꢀC and nitrogen flow rate of 20 mL/minwere used in
DSC experiments. Thermogravimetric analysis (TGA) was performed
with a NETZSCH Instruments’ High Resolution STA 409PC thermog-
ravimetric analyzer that was purged with nitrogen at a flow rate of
70 mL/min. A heating rate of 20 ꢀC/min was used and scanning range
was from 40 ꢀC to 850 ꢀC. Infrared spectra were recorded using
a Bruker VERTEX 70 Fourier transform infrared spectrometer (FT-IR)
with a heating device. Elemental analysis was carried out on
a German Vario Micro cube microanalyzer. Mechanical properties
were measured using a dynamic mechanical thermal analysis (DMA)
apparatus (PerkinElmer, Diamond DMA). Specimens (50 ꢂ 10 ꢂ
1.0mm)weretestedin3pointbendingmode.Thethermaltransitions
were studied in the scope of 20e200 ꢀC at a heating rate of 4 ꢀC/min
and at a fixed frequency of 1 Hz.
Among the mentioned above the three methods to improve the
performance of polybenzoxazine, designing new benzoxazine
monomer is a more attractive approach to overcome the short-
comings of traditional polybenzoxazine. By now, although most of
benzoxazine monomers consisted of organic element only, more
and more attention has been paid to design and synthesis of inor-
ganic heteroatom-containing benzoxazine monomer [30e35].
Cyclotriphosphazene (CP) derivatives are typical classes of organ-
iceinorganic compounds with a planar non-delocalized cyclic ring
consisting of alternating N and P atoms. Six functional groups can
be attached onto a CP ring and the groups are normally projected
above and below the CP plane due to steric hindrance [36e38].
Because of the versatility of cyclotriphosphazene chemistry, the CP
ring of high stability and biocompatibility allows a wide range of
functional groups to be attached onto CP. Highly branched
conductive polyaniline with high electrochromic contrast based on
CP [39], star-branched polymers with CP cores [40], catalysts of
transition metal ionic compounds based on CP [41], CP conjugates
with varied properties in vitro [42] and flame-retardant CP-con-
taining polyurethanes [43] were prepared through different
methods. By introducing the CP units into the polymers or com-
pounds, the corresponding hybrid materials were endowed with
thermal stability and environment-friendly flame-retardant prop-
erties or other functionalities.
2.3. Synthesis of [N3P3(OC6H4{NHC(O)CH3}-4)6] (I)
This compound was synthesized as reported [46]. White crystal,
Yield: 91%, MP. 252e255 ꢀC 1H NMR (DMSO-d6, TMS, ppm): 9.92
(1H, eNH), 6.79e7.45(4H, dd, AreH), 2.04(3H, eCH3). 13C NMR
(DMSO-d6, TMS, ppm): 168.0(C]O), 144.9(CeO), 136.4(CeN), 120.5
(CH), 119.9(CH), 23.8(CH3). 31P NMR (DMSO-d6, ppm): 9.18.
In current study, a star-like benzoxazine monomer based on CP
has been synthesized, and the new benzoxazine monomer
possesses six benzoxazine moieties substituted onto CP ring. The
new hyperbranched benzoxazine monomer underwent ring-
opening polymerization with or without catalysts to produce
highly dimensional crosslinking structure with rigidly inorganic CP
as the core. The structure of the new monomer was confirmed by
1H NMR, 13C NMR, 31P NMR and elemental analysis. Fourier trans-
form infrared spectroscopy (FT-IR) and differential scanning calo-
rimetry (DSC) were used to study the thermal ring-opening
polymerization reaction of the monomer; the thermal property and
mechanic performance of the thermoset polymer were also eval-
uated by thermal gravimetric analyzer and dynamic mechanical
thermal analysis (DMA), respectively.
2.4. Synthesis of [N3P3(OC6H4{NH2}-4)6] (II)
The compound was synthesized as reported [46]. White powder,
yield: 80%, MP. 172e174 ꢀC 1H NMR (DMSO-d6, TMS, ppm):
2. Experimental
2.1. Materials
4-acetamidophenol (98%) was purchased from Alfa Aesar
Reagent Co., Ltd., USA. Salicylaldehyde (ꢁ98%), sodium borohydride
(96%), paraformaldehyde (95%), phenol, p-hydroxylbenzoic acid
Fig. 1. The structures of HPP, BOz, CB1, CB2 and CB3.

1006
X. Wu et al. / Polymer 52 (2011) 1004e1012
Scheme 1. Synthesis of highly branched benzoxazine HBOz.
6.41e6.52(4H, dd, AreH), 4.91(2H, AreNH2). 13C NMR (DMSO-d6,
TMS, ppm): 145.3(CeN),140.5(CeO), 120.6(CH),113.9(CH). 31P NMR
(DMSO-d6, ppm): 10.67.
149.6(CeO),146.5(CeN),134.4(CH),133.7(CH),123.8(CH),122.6(CH),
120.3(C), 120.2(CH), 117.7(CH). 31P NMR (DMSO-d6, ppm): 9.12.
Elemental analysis Calcd. (%) for C78H60N9O12P3: C, 66.52; H, 4.29; N,
8.95. Found: C, 65.46; H, 4.33; N, 9.00.
2.5. Synthesis of [N3P3(OC6H4{NCHC6H4(2-OH)}-4)6] (III)
2.6. Synthesis of [N3P3(OC6H4{NHCH2C6H4(2-OH)}-4)6] (IV)
A solution of salicylaldehyde (18.3 g, 149.9 mmol) in ethanol
(50 mL) was added to a solution of II (16.3 g, 20.8 mmol) in ethanol
(200 mL) under argon atmosphere, and the mixture was refluxed
under vigorous stirring for 24 h. The resulting yellow solid
(compound III) was filtered off and washed with a large excess of
water, ethanol (3 ꢂ 5 mL), andhexane (3 ꢂ 5 mL) and driedinvacuum
at 40 ꢀC for 48 h. Yield: 27.5 g (96%), MP.152e154 ꢀC 1H NMR (DMSO-
d6, TMS, ppm): 12.88(1H, CH]N), 8.84(1H, HOeAr), 6.86e7.49(8H,
AreH). 13C NMR (DMSO-d6, TMS, ppm): 164.5(CeOH), 161.3(C]N),
Sodium borohydride (1.8 g, 46.1 mmol) was added to a solution
of III (18 g, 12.8 mmol) in 150 mL tetrahydrofuran (THF) in small
portion while stirring. After reaction at 25 ꢀC for 12 h, 100 mL water
was added to the solution, and the mixture was stirred at 25 ꢀC for
another 12 h. Then the solution was evaporated under reduced
pressure to remove the organic solvent, the resulting solid was
extracted with CH2Cl2, washed with water, dried over anhydrous
Na2SO4, concentrated and precipitated in petroleum ether. Pale

X. Wu et al. / Polymer 52 (2011) 1004e1012
1007
Fig. 2. 1H NMR spectrum of HBOz (solvent: CDCl3).
Fig. 4. 31P NMR spectrum of HBOz (solvent: CDCl3).
powder was gained, and dried in vacuum at 40 ꢀC for 48 h. Yield:
16.3 g (90%), MP. 154e156 ꢀC 1H NMR (Acetone-d6, TMS, ppm): 9.50
(1H, HOeAr), 6.41e7.17(8H, AreH), 5.78(1H, NH), 4.14e4.16(2H,
CH2). 13C NMR (Acetone-d6, TMS, ppm): 155.9(CeOH), 146.8(CeN),
134.9(CeO), 129.2(CeCH2), 128.3(CH), 126.4(CH), 121.8(CH), 119.6
(CH), 115.7(CH), 113.2(CH), 42.8(CH2). 31P NMR (Acetone-d6, ppm):
10.62. Elemental analysis Calcd. (%) for C78H72N9O12P3: C, 65.96; H,
5.11; N, 8.88. Found: C, 65.78; H, 5.06; N, 8.88.
3. Results and discussion
3.1. Synthesis and characterization
The HBOz monomer was synthesized according to Scheme 1.
The overall yield after five steps was about 60%. The purified sample
was satisfactory for further analysis. Herein, the structure of the
novel HBOz monomer was confirmed with 1H, 13C, 31P NMR and
element analysis.
Fig. 2 shows the 1H NMR spectrum of HBOz. Resonances
appearing at 4.52 ppm and 5.25 ppm are assigned to the methylene
protons of AreCH2eN and OeCH2eN of the oxazine ring, respec-
tively. The multiplets at 6.70e6.72, 6.79e6.88, and 6.90e7.11 ppm
are assigned to the aromatic protons.
2.7. Synthesis of [N3P3(OC6H4{NCH2CH2C6H4(2-O)}-4)6] (HBOz)
A solution of IV (10.0 g, 7.0 mmol) and paraformaldehyde (1.9 g,
63.4 mmol) in 150 mL 1,4-dioxane was stirred at 100 ꢀC for 4 days
under an argon atmosphere. Then the solution was evaporated
under reduced pressure to remove the organic solvent, the result-
ing solid was dissolved in CH2Cl2, and washed with diluted solution
of sodium hydroxide. The organic layer was collected, dried over
anhydrous Na2SO4, concentrated and then precipitated in petro-
leum ether. A light yellow powder was collected, dried in vacuum at
40 ꢀC for 48 h. Yield: 9.9 g (95%), MP. 75e77 ꢀC.
In the corresponding 13C NMR spectrum in Fig. 3, resonances
appearing at 50.5 ppm and 79.7 ppm are assigned to the methylene
carbons of AreCH2eN and OeCH2eN of the oxazine ring, respec-
tively. Other chemical shifts (ppm) are assigned to the aromatic
carbon resonances: 154.3(CeO), 145.3(CeN), 144.9(CeO), 127.9
(CH), 126.8(CH), 121.7(C), 120.9(CH), 120.7(CH), 119.1(CH), 116.9
(CH).
The 31P NMR is shown in Fig. 4, and there is a single peak at
9.67 ppm, which indicates that the substituted reaction onto CP
ring was complete.
2.8. Synthesis of C6H4NCH2CH2C6H4(2-O) (BOz), (4-COOH)
C6H4NCH2CH2C6H4(2-O) (CB1), C6H4NCH2CH2C6H3(2-O)(4-COOH)
(CB2), and (4-COOH)C6H4NCH2CH2C6H3(2-O)(4-COOH) (CB3)
The structures of BOz, CB1, CB2 and CB3 were showed in Fig. 1
and synthesized as reported methods [26].
Fig. 5. The FT-IR spectra of HBOz and HBOz curing at 240 ꢀC for different time in the
Fig. 3. 13C NMR spectrum of HBOz (solvent: CDCl3).
region between 2000 and 500 cmꢃ1
.

1008
X. Wu et al. / Polymer 52 (2011) 1004e1012
Elemental analysis for HBOz is following: Calcd. (%): C, 67.60; H,
curing behavior of the new monomer HBOz was examined by DSC.
Herein, several catalysts were used to estimate the possibility of
decreasing the polymerization temperature of HBOz monomer by
mixing the catalysts with HBOz powder. Three compounds whose
boiling point temperatures are higher than 180 ꢀC were utilized as
the catalyst because of the high polymerization temperature of
benzoxazines. These catalysts were imidazole (Ci), N, N-dime-
thylbenzylamine (Cii) and a neutral salt [48] obtained from the
reaction of diethanolamine and p-toluenesulfonic acid with the
molar ratio 1:1 (Ciii). The results of the heat curing behaviors using
the catalysts to reduce the polymerization temperatures of the
monomer were shown in Fig. 6, a sharp exothermic peak corre-
sponding to the ring-opening polymerization was observed for
HBOz, the onset and maximum temperatures of the exotherm were
177 ꢀC and 225 ꢀC, respectively. The amount of exotherm for HBOz
was 215.5 J/g. In addition, there was a small endothermic peak
corresponding to melting point at 77 ꢀC for HBOz. By adding curing
catalysts with the content of 3 wt% to HBOz monomer, the
exothermic peaks changed obviously from the DSC results. For
HBOz/Ci-3 wt%, the onset and maximum temperatures of the exo-
therm were reduced to 132 ꢀC and 215 ꢀC, respectively. The corre-
sponding amount of exotherm was lowered to 202.4 J/g; for HBOz/
Cii-3 wt% and HBOz/Ciii-3 wt%, the onset temperatures of the exo-
therm were both reduced to 135 ꢀC, the maximum temperatures of
the exotherm were separately lowered to 222 ꢀC and 212 ꢀC. And
the corresponding amounts of exotherm were correspondingly
diminished to 204.5 J/g and 70.8 J/g, respectively. Comparing to
HBOz, the polymerization temperatures and enthalpies were
obviously reduced. Besides, the HBOz/catalysts mixtures with
catalysts’ contents of 1 wt% and 5 wt% were also investigated and
the results were summarized in Table 1. It was found that the onset
exothermic temperatures, maximum exothermic temperatures and
exothermic enthalpies were all decreased with the increase of the
catalysts’ content.
4.86; N, 8.45. Found: C, 66.86; H, 4.91; N, 8.57. All analysis results
indicate that the desired hyperbranched benzoxazine monomer
has been prepared successfully.
3.2. Curing behavior of HBOz monomer monitored by FT-IR
The ring-opening polymerization characteristic was monitored
by FT-IR spectrometer, the results were shown in Fig. 5. The KBr pill
containing HBOz powder was heated at 240 ꢀC for different time
and the FT-IR spectra were recorded. The characteristic absorption
bands due to benzoxazine structure are situated at 972 cmꢃ1 (out of
plane-bending vibrations of CeH), 1227 cmꢃ1 (asymmetric
stretching of CeOeC), 1184 cmꢃ1 (asymmetric stretching of
CeNeC), 1033 cmꢃ1 (symmetric stretching of CeOeC), and
1337 cmꢃ1 (CH2 wagging), respectively [47]. The wave-numbers
from 700 cmꢃ1 to 900 cmꢃ1 and at 1458 cmꢃ1 are assigned to
disubstituted benzene ring; two very strong absorption peaks
located at 950 cmꢃ1 and 1165 cmꢃ1 are corresponding to stretching
of PeOeAr and N]P, respectively. The spectrum of Fig. 5(b) was
recorded immediately after the sample pill was quickly heated to
240 ꢀC. It was found that the intensity of characteristic absorption
of benzoxazine structure and disubstituted benzene ring obviously
decreased, the characteristic peaks located at 972 cmꢃ1 and
1337 cmꢃ1 were nearly to disappear. When the curing time was
prolonged to 0.5 h and 1 h at 240 ꢀC, the intensity of benzoxazine
characteristic peaks continued to decrease, then disappeared after
1 h. These results indicate that the HBOz monomer has been
polymerized completely. At the same time, the intensity of disub-
stituted benzene ring’s characteristic peaks located in the range of
700 cmꢃ1e900 cmꢃ1 and 1458 cmꢃ1 also began to decrease grad-
ually, this indicated a high substitution pattern of the aromatic
rings as a consequence of the crosslinking reactions, and the ring-
opening reaction model of HBOz was displayed in Scheme 2.
Besides, comparing the absorption peaks located at 950 cmꢃ1 and
1165 cmꢃ1 from Fig. 5(aed), there were no significant changes even
under high temperature for 1 h, which illustrated that the inorganic
CP ring was thermally stable at such condition.
On the other hand, carboxylic acid-containing benzoxazines
(Fig. 1) used as the functional catalysts were also studied. Andreu
[26] reported that CB1, CB2 and CB3 produced significant decrease in
the polymerization temperature of BOz monomer. Herein, the
carboxylic acid-containing benzoxazines were also added to HBOz
monomer as curing catalysts. To carry out this study, several
samples of HBOz/CB1, HBOz/CB2 and HBOz/CB3 were prepared, and
scanned by DSC. The results obtained from heating DSC plots were
collected in Fig. 7. When small amount (3 wt%) of carboxylic acid-
3.3. Curing behavior of HBOz monomer scanned by DSC
It is known that the high polymerization temperature is one
important factor that limits the application of polybenzoxazine. The
Scheme 2. Polymerization of HBOz monomer under heating.

X. Wu et al. / Polymer 52 (2011) 1004e1012
1009
Fig. 7. DSC plots of HBOz monomer using functional catalysts with the content of 3 wt
%. a. HBOz, b. HBOz/CB1-3 wt%, c. HBOz/CB2-3 wt%, d. HBOz/CB3-3 wt%.
Fig. 6. DSC plots of HBOz monomer using common catalysts with the content of 3 wt%.
a. HBOz, b. HBOz/Ci-3 wt%, c. HBOz/Cii-3 wt%, d. HBOz/Ciii-3 wt%.
containing benzoxazines show little effect on decrease polymeri-
zation temperature, but the exothermic enthalpies are diminished
obviously with increasing the catalysts’ content and it will let the
monomer mixture polymerize more mildly than that of pure
monomer.
containing benzoxazines were added, the maximum exothermic
peaks had little change, the onset temperatures of the exothermic
peaks (To) just reduced a little comparing with that of pure HBOz:
the To of HBOz/CB1-3 wt%, HBOz/CB2-3 wt% and HBOz/CB3-3 wt%
were lowered to 172 ꢀC, 168 ꢀC and 172 ꢀC, respectively; and it was
177 ꢀC for HBOz. However, the exothermic enthalpies had signifi-
cant decrease: the enthalpies of HBOz/CB1-3 wt%, HBOz/CB2-3 wt%
and HBOz/CB3-3 wt% were reduced to 191.6 J/g, 197.7 J/g and
191.3 J/g, respectively; while it was 215.5 J/g for HBOz. Changing the
amounts of carboxylic acid-containing benzoxazines at 1 wt% and
5 wt% level resulted in similar effect and the results were also
summarized in Table 1. We can conclude that carboxylic acid-
Comparing the two series of catalysts, it is clear to find that the
common curing catalysts (Ci, Cii and Ciii) are more suitable for the
curing of HBOz than the carboxylic acid-containing benzoxazines,
they effectively decrease the polymerization temperature. The
carboxylic acid-containing benzoxazines reduced the polymeriza-
tion temperature of BOz effectively in report [47], but not obviously
in HBOz system. Possible reason is that the common curing cata-
lysts can dilute the reaction mixture, low viscosity is a benefit to the
polymerization rate; but functional carboxylic acid-containing
catalysts can copolymerize with the monomer, the high viscosity
system owing to the highly crosslinking polymerization reaction
between HBOz and functional catalysts restrains the catalytic
activity of the carboxylic acid groups located on the carboxylic acid-
containing benzoxazines (Fig. 7).
Table 1
Thermal property of HBOz/Catalysts series samples.
Hd
T5%
T10%
Tmax
(ꢀC)
Yc
a
b
c
e
f
g
h
Samples
Tm
To
Tmax
D
(J/g)
(ꢀC) (ꢀC) (ꢀC)
(ꢀC) (ꢀC)
(%)
HBOz
77
177 225
215.5
403
449
453
66.9
3.4. Thermal properties of the new polybenzoxazines
HBOz/Ci-1 wt%
HBOz/Ci-3 wt%
HBOz/Ci-5 wt%
80
e
140 224
132 215
115 208
213.6
202.4
193.5
374
378
373
431
419
405
445
441
459
60.4
59.4
57.8
88
A series of samples forming by HBOz powder with different
curing catalysts (see in Table 1) were added to test tubes, and the
HBOz/Cii-1 wt%
HBOz/Cii-3 wt%
HBOz/Cii-5 wt%
70
e
167 228
135 222
130 225
207.2
204.5
202.7
341
379
301
401
422
402
440
435
471
56.7
57.2
30.3
78
HBOz/Ciii-1 wt%
HBOz/Ciii-3 wt%
HBOz/Ciii-5 wt%
78
68
e
138 220
135 212
130 203
135.2
70.88 329
60.1
363
401
374
384
420
434
423
60.5
51.2
43.5
331
HBOz/CB1-1 wt% 78
HBOz/CB1-3 wt% 79
HBOz/CB1-5 wt% 76
176 225
172 225
168 222
199.6
191.6
153.1
395
409
415
435
450
457
492
478
461
61.4
62.4
59.0
HBOz/CB2-1 wt% 78
HBOz/CB2-3 wt% 78
HBOz/CB2-5 wt% 78
170 225
168 225
160 225
203.7
197.7
189.0
398
400
393
436
441
436
450
474
470
58.7
60.3
60.1
HBOz/CB3-1 wt% 77
HBOz/CB3-3 wt% 79
HBOz/CB3-5 wt% 75
175 225
172 225
168 221
202.1
191.3
187.5
390
395
387
429
437
429
445
468
462
57.7
59.3
58.5
a
Melt point temperature determined by DSC (10 ꢀC/min).
Onset temperature of exothermic peak.
Maximum of the polymerization exotherm.
Polymerization enthalpy by DSC.
b
c
d
e
T5%: The temperature for which the weight loss is 5% by TGA scan (20 ꢀC/min)
under nitrogen.
f
T10%: The temperature for which the weight loss is 10%.
Tmax: Maximum weight loss temperature.
Yc: Char yields at 850 ꢀC.
Fig. 8. TG curves of PHBOz by curing HBOz monomer using common curing catalysts
with the content of 3 wt%. a. PHBOz, b. PHBOz/Ci-3 wt%, c. PHBOz/Cii-3 wt%, d. PHBOz/
Ciii-3 wt%.
g
h

1010
X. Wu et al. / Polymer 52 (2011) 1004e1012
Fig. 11. TG curves of PHBOz and PBOz/HPP.
Fig. 9. TG curves of PHBOz by curing HBOz monomer using functional curing catalysts
with the content of 3 wt%. a. PHBOz, b. PHBOz/CB1-3 wt%, c. PHBOz/CB2-3 wt%, d.
PHBOz/CB3-3 wt%.
sample tubes were cured stepwise at 120 ꢀC for 1 h and 200 ꢀC for
2 h, and then post-cured at 220 and 240 ꢀC for 1 h each in salt-bath
under an argon atmosphere. All the cured samples were dark red
color. Thermal stability of the novel polybenzoxazines was inves-
tigated by TGA. The TGA profiles of polybenzoxazines from the
polymerization of HBOz monomer, HBOz/Ci-3 wt%, HBOz/Cii-3 wt%,
and HBOz/Ciii-3 wt% were shown in Fig. 8. The 5% and 10% weight
loss temperatures (T5% and T10%) for PHBOz were 403 and 449 ꢀC,
respectively. While with 3 wt% catalysts, the T5% and T10% were all
lower than that of PHBOz: for PHBOz/Ci-3 wt%, they were 378 and
419 ꢀC, respectively; for PHBOz/Cii-3 wt%, they were 379 and
422 ꢀC; meanwhile they were 329 and 374 ꢀC for PHBOz/Ciii-3 wt%.
Similarly, the maximum weight loss temperature (Tm) of PHBOz
was also higher than those of the others with catalysts, and the
details were shown in Table 1. The Char yields at 850 ꢀC (Yc) of
polybenzoxazines were also recorded in Table 1. The Yc of PHBOz
was as high as 66.9%, and the others with 3 wt% these common
catalysts were much lower than that of PHBOz, PHBOz/Ciii-3 wt%
sample held the lowest yield, only 51.2%. It is well known that the
common catalysts do not copolymerize with the monomer for lack
of reactive groups; they remain in the polybenzoxazine systems
after polymerization and act as unstable impurities. The remaining
catalyst will accelerate the degradation of polybenzoxazine under
high temperature and result in reducing the heat resistance (Fig. 9).
The thermal stability of the novel polybenzoxazines using
functional curing catalysts (CB1, CB2 and CB3) was also investigated
by TGA, the results were shown in Fig. 9 and Table 1. It was excited
to find that the thermal stability of the polybenzoxazines with acid-
containing benzoxazines was as good as that of PHBOz, some of
them were even higher than that of PHBOz. For example, the 5% and
Tm for PHBOz/CB1 were higher than that of PHBOz, they were 409
and 478 ꢀC, respectively (PHBOz were separately 401 and 451 ꢀC).
The T5% and T10% of PHBOz with different contents of functional
catalysts were separately located in the range of 390e410 ꢀC and
430e460 ꢀC. And the Tm and Yc were mainly distributed in the
range of 460e490 ꢀC and 58%e62%, respectively. These factors
reflecting to the thermal stability were better than that of PHBOz
with common curing catalysts (Ci, Cii and Ciii). The reasons maybe
result from the reactive benzoxazines groups on the acid-contain-
ing benzoxazines, which can copolymerize with HBOz monomers
and get into the skeleton of crosslinking system.
3.5. Thermal properties of HBOz and PHBOz comparing to
monofunctional benzoxazine and its polymers
To compare the thermal properties with common poly-
benzoxazines, another benzoxazine sample was introduced, which
was prepared from the mixtures of BOz monomer and HPP at molar
ratio of 6:1 (Its theoretical elemental composition is similar to
HBOz). BOz/HPP was polymerized under the same curing condition
of PHBOz to obtain PBOz/HPP polymer. The DSC curves of HBOz
monomer and BOz/HPP are shown in Fig. 10. A sharp exothermic
Fig. 10. DSC curves of HBOz and BOz/HPP (molar ratio 6:1).
Fig. 12. Storage moduli of PHBOz and PBOz/HPP polybenzoxazines.

X. Wu et al. / Polymer 52 (2011) 1004e1012
1011
4. Conclusions
We have synthesized
a
novel star-branched benzoxazine
monomer based on CP ring, the new organiceinorganic hybrid
benzoxazine monomer possesses low melting point (77 ꢀC) and
good solubility in common solvents though with high molecular
weight. The new benzoxazine monomer could be ring-opening
polymerized completely at 240 ꢀC for 1 h, according to the disap-
pearance of benzoxazine characteristic peaks located at 1227 cmꢃ1
and 972 cmꢃ1. A sharp exothermic peak at 225 ꢀC for ring-opening
polymerization of the benzoxazine monomer was observed at DSC
plots, and the onset polymerized temperature was about 180 ꢀC.
Two species of curing catalysts were used to reduce the polymer-
ization temperature, the common catalysts (Ci, Cii and Ciii) could
effectively decrease the onset polymerization temperature and
exothermic enthalpy with small amount of them (ꢄ5 wt%), but they
led to reduction in corresponding polymers’ thermal stability;
while using the functional catalysts (carboxylic acid-containing
benzoxazines: CB1, CB2 and CB3), the onset polymerization
temperatures had little decrease, but final polymers held excellent
thermal stability from TGA results, some of them were even higher
than that of the pure polybenzoxazine. Comparing to the blending
resin of BOz/HPP, the polymerization temperature of the benzox-
azine monomer was about 40 ꢀC lower than that of BOz/HPP
mixture. Besides, the hyperbranched polybenzoxazine also showed
outstanding thermal stability and good mechanic performance
resulting from the highly dimensional crosslinking structure with
the core of rigid and stable inorganic CP ring. It had higher thermal
stability (T5% at 403 ꢀC) and higher char yield (66.9% char yield at
850 ꢀC) than that of PBOz/HPP blend. And the Tg of PHBOz was as
high as 152 ꢀC, which was about 50 ꢀC higher than that of PBOz/HPP
resin. The new polybenzoxazine may be an applicable material in
the application of high technology for its excellent thermal stability
and mechanic properties.
Fig. 13. Tand of PHBOz and PBOz/HPP polybenzoxazines.
peak is observed for BOz/HPP, and the maximum temperature of
the exotherm is 262 ꢀC, which is nearly 40 ꢀC higher than that of
HBOz monomer. An endothermic peak at 58 ꢀC is assigned to the
melting point of BOz/HPP mixture, which is a little lower than that
of HBOz. The enthalpy of BOz/HPP is 211.2 J/g, which is near to
215.5 J/g of HBOz.
The thermal stability of PBOz/HPP and PHBOz was also carried
out on TGA, the results were shown in Fig. 11. As expected, the
thermal stability of PHBOz was much higher than that of PBOz/HPP,
T5% of them were 403 and 285 ꢀC, respectively; and Yc at 850 ꢀC of
them were 66.9% and 33.4%, respectively. The results illustrate that
PHBOz polymer shows much higher thermal stability than that of
common monofunctional polybenzoxazine composite. The reason
is deemed that the inorganic cyclotriphosphazene ring possesses
high thermal stability for its unique structure and can act as an ideal
core to prepare high-performance organiceinorganic hybrid mat-
erials. At the same time the branched benzoxazine moieties are
spatially distributed above and below the CP ring, and bring the
new polybenzoxazine a highly spatial crosslinking structure which
minimizes dangling side groups and improves dimensionally
thermal stability.
Acknowledgment
This work was supported by the National Natural Science
Foundation of China (No. 50703013) and Independent Innovative
Position of Hubei Province. All authors give sincerely thanks to
Analysis and Testing Center of HUST for NMR and E.A. test.
3.6. Mechanic property of the new polybenzoxazine
The dynamic mechanical behavior of the cured benzoxazine
resins was obtained as a function of the temperature beginning in
the glassy state of each sample to the rubbery plateau of each
material. As seen in Fig. 12, the storage modulus (E0) of PHBOz at
room temperature is 4.14 GPa, which is a little higher than that of
PBOz/HPP (3.64 GPa). The E0 value of PHBOz starts to decrease at
about 120 ꢀC, while the E0 value of PBOz/HPP begins to decrease at
about 80 ꢀC. Comparing the decreasing slope of the E0 value from
glassy state to rubbery plateau for PHBOz and PBOz/HPP, it is
found that the decreasing slope of PHBOz is obviously lower than
that of PBOz/HPP. The glass transition temperatures (Tg) of the
polybenzoxazines can be deduced in Fig. 13 from the corre-
Appendix. Supplementary material
Figures showing 1H, 13C, 31P NMR, DSC plots and TGA curves can
be found online at doi:10.1016/j.polymer.2011.01.003.
References
[1] (a) Chernykh A, Agag T, Ishida H. Polymer 2009;50:3153e7;
(b) Ning X, Ishida H. J Polym Sci Part A Polym Chem 1994;32:1121e9;
(c) Ishida H, Allen DJ. J Polym Sci Part B Polym Phys 1996;34:1019e130;
(d) Ghosh NN, Kiskan B, Yagci Y. Prog Polym Sci 2007;32:1344e91;
(e) Shen SB, Ishida H. J Polym Sci Part B Polym Phys 1999;37:3257e68;
(f) Kim HD, Ishida H. J Appl Polym Sci 2001;79:1207e19;
(g) Liu X, Gu Y. J Appl Polym Sci 2002;84:1107e13;
(h) Takeichi T, Agag T. High Perform Polym 2006;18:777e97.
[2] (a) Riess G, Schwob JM, Guth G, Roche M, Lande B. In: Culbertson BM,
McGrath JE, editors. Advances in polymer synthesis. New York: Plenum; 1985.
p. 27e40;
(b) Burke W. J Org Chem 1949;71:109e14.
[3] Yagci Y, Kiskan B, Ghosh N. J Polym Sci Part A Polym Chem 2009;47:5565e76.
[4] Jang J, Seo D. J Appl Polym Sci 1998;67:1e10.
[5] Ishida H, Lee YH. J Appl Polym Sci 2001;81:1021e34.
[6] Zheng SX, Lu H, Guo QP. Macromol Chem Phys 2004;205:1547e58.
[7] Takeichi T, Guo Y, Agag T. J Polym Sci Part A Polym Chem 2000;38:4165e76.
[8] Ishida H, Allen DJ. Polymer 1996;37:4487e95.
[9] Espinosa MA, Cádiz V, Galià M. J Polym Sci Part A Polym Chem 2004;42:
279e89.
sponding temperature of peak of tan
d
value. Tg of PHBOz and
value of
PBOz/HPP are 152 ꢀC and 101 ꢀC, respectively. The tan
d
PHBOz is much lower than that of PBOz/HPP after 80 ꢀC because
of higher dimensional crosslinking structures of PHBOz. It is
known to us that PBOz/HPP is low crosslinking structure, while it
is highly steric crosslinking structure for PHBOz curing from the
star-branched organiceinorganic hybrid benzoxazine. Besides,
there are lots of rigid and stable CP in the PHBOz as crosslinking
cores, which also contribute to improving the mechanic properties
of PHBOz to some extent. For these reasons mentioned above,
PHBOz shows higher mechanic performance than that of PBOz/
HPP under heat.
[10] Espinosa MA, Galià M, Cádiz V. Polymer 2004;45:6103e9.

1012
X. Wu et al. / Polymer 52 (2011) 1004e1012
[11] Agag T, Takeichi T. Polymer 2000;41:7083e90.
[12] Kiskan B, Demirel AL, Kamer O, Yagci Y. J Polym Sci Part A Polym Chem
2008;46:6780e8.
[13] Liu J. Synthesis, Characterization, Reaction Mechanism and Kinetics of 3,4-
Dihydro-2H-1,3- benzoxazine and its Polymers. Ph.D. Thesis. Cleveland, OH:
Case Western Reserve University; May 1995.
[14] Velez-Herrera P, Doyama K, Abe H. Macromolecules 2008;41:9704e14.
[15] Kiskan B, Demiray G, Yagci Y. J Polym Sci Part A Polym Chem 2008;46:
3512e8.
[29] Sudo A, Hirayama S, Endo T. J Polym Sci Part A Polym Chem 2010;48:
2777e82.
[30] Lin CH, Cai SX, Leu TS. J Polym Sci Part A Polym Chem 2006;44:3454e68.
[31] Liu Y, Hsu C, Chou C. J Polym Sci Part A Polym Chem 2007;45:1007e15.
[32] Huang J, Kuo S, Huang H. J Appl Polym Sci 2009;111:628e34.
[33] Spontón M, Ligadas G, Ronda JC, Galià M, Cádiz V. Polym Degrad Stab
2009;94:1693e9.
[34] Ye YS, Yen YC, Chang FC. Polymer 2009;50:3196e203.
[35] Liu YF, Yue ZQ, Gao JG. Polymer 2010;51:3722e9.
[16] Ergin M, Kiskan B, Gacal B, Yagci Y. Macromolecules 2007;40:4724e7.
[17] Kukut M, Kiskan B, Yagci Y. Des Monomers Polym 2009;12:167e76.
[18] Kiskan B, Colak D, Muftuoglu AE, Cianga I, Yagci Y. Macromol Rapid Commun
2005;26:819e24.
[36] Wisian-Neilson P. In: Glerià M, Jaeger RD, editors. Phosphazenes: a worldwide
insight. New York: Nova Science; 2004. p. 109e25.
[37] Allcock HR. Chemistry and applications of polyphosphazenes. Hoboken:
Wiley-Interscience; 2003 [part II].
[19] Tasdelen MA, Kiskan B, Yagci Y. Macromol Rapid Commun 2006;27:1539e44.
[20] Brunovska Z, Ishida H. J Appl Polym Sci 1999;73:2937e49.
[21] Kim HJ, Brunovska Z, Ishida H. Polymer 1999;40:6565e73.
[22] Agag T, Takeichi T. Macromolecules 2001;34:7257e63.
[23] Agag T, Takeichi T. Macromolecules 2003;36:6010e7.
[24] Ishida H, Ohba S. Polymer 2005;46:5588e95.
[25] Andreu R, Espinosa MA, Galià M, Cádiz V, Ronda JC, Reina JA. J Polym Sci Part A
Polym Chem 2006;44:1529e40.
[26] Andreu R, Reina JA, Ronda JC. J Polym Sci Part A Polym Chem 2008;46:
3353e66.
[38] Sudhakar S, Sellinger A. Macromol Rapid Commun 2006;27:247e54.
[39] Jia P, Xu J, Ma J. Eur Polym J 2009;45:772e8.
[40] Chang JY, Ji HJ, Han MJ. Macromolecules 1994;27:1376e80.
[41] Harmjanz M, Scoot BL, Burns CJ. Chem Commun; 2002:1386e7.
[42] Cho YW, Lee J, Song S. Bioconjugate Chem 2005;16:1529e35.
[43] Yuan CY, Chen SY, Tsai CH. Polym Adv Technol 2005;16:393e9.
[44] Jaeger RD, Glerià M. Prog Polym Sci 1998;23:179e276.
[45] Ye C, Zhang Z, Liu W. Synth Commun 2002;32:203e9.
[46] Barberá J, Jiménez J, Laguna A. Chem Mater 2006;18:5437e45.
[47] Andreu R, Reina JA, Ronda JC. J Polym Sci Part A Polym Chem 2008;46:
6091e101.
[27] Kiskan B, Koz B, Yagci Y. J Polym Sci Part A Polym Chem 2009;47:6955e61.
[28] Agag T, Arza CR, Maurer FHJ, Ishida H. Macromolecules 2010;43:2748e58.
[48] Kimura H, Matsumoto A, Ohtsuka K. J Appl Polym Sci 2008;107:710e8.