Well-defined organic-inorganic hybrid benzoxazine monomers based on cyclotriphosphazene: Synthesis, properties of the monomers and polybenzoxazines

Article abstract of DOI:10.1016/j.polymer.2011.07.037

Well-defined organic-inorganic hybrid benzoxazine monomers based on cyclotriphosphazene (CP) cores have been synthesized. Theses monomers possessed controllable number of organic benzoxazine moieties and biphenyl groups distributed on the inorganic ring o

Full text of DOI:10.1016/j.polymer.2011.07.037

Polymer 52 (2011) 4235e4245
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Well-defined organiceinorganic hybrid benzoxazine monomers based
on cyclotriphosphazene: Synthesis, properties of the monomers
and polybenzoxazines
,
,
Xiong Wu a, Shu-Zheng Liu a, Da-Ting Tian a b, Jin-Jun Qiu a, Cheng-Mei Liu a
*
a School of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China
b School of Chemical & Environmental Engineering, Hubei Institute for Nationalities, Enshi 44500, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Well-defined organiceinorganic hybrid benzoxazine monomers based on cyclotriphosphazene (CP)
cores have been synthesized. Theses monomers possessed controllable number of organic benzoxazine
moieties and biphenyl groups distributed on the inorganic ring of CP. Corresponding ring-opening
polymerization under heat led to highly cross-linked polymers with different number of crosslinking
sites. The ring-opening polymerization behaviors of the new benzoxazine monomers were investigated
by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and dynamic
mechanical thermal analysis (DMA). The results indicated that the polymerization activity of the
monomers depended on the number of benzoxazine moiety of corresponding monomers. Those
monomers with more benzoxazine moieties resulted in higher polymerization rate. Due to the high
crosslinking nature with rigid and stable inorganic CP ring and biphenyl groups in the networks, the
resulted polybenzoxazines from the monomers showed excellent thermal stability, mechanic property
and humidity resistance.
Received 15 February 2011
Received in revised form
28 June 2011
Accepted 23 July 2011
Available online 28 July 2011
Keywords:
Benzoxazine
Cyclotriphosphazene
Organiceinorganic
Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
1. Introduction
Generally, different functional moieties attached to benzoxazine
monomers may offer corresponding polybenzoxazines of novel
Benzoxazine is a class of heterocyclic compounds capable of
undergoing heat ring-opening polymerization with or without
catalysts to form the corresponding polymers, which surpass
conventional phenolic resin in mechanical strength [1], thermal
stability [2] and durability under humid environment [3]. The
unique chemistry of benzoxazines is responsible for a number of
inherent processing benefits, including low melting viscosity, no
volatile release upon cure, and near-zero overall shrinkage [3,4].
Thermally initiated ring-opening polymerization results in high
modulus thermosetting materials with high glass transition
temperature, low flammability, stable and low dielectric constant,
high dimensional stability and low cost [3e8]. Benzoxazine
monomer can be easily synthesized from the corresponding
phenolic compound, amine, and formaldehyde [9]. This simple and
versatile synthetic method has supported development of a wide
range of benzoxazine monomers tethered with different substitu-
ents [4,10e17].
properties. So benzoxazine monomers with various reactive groups
have been synthesized and reported recently. For instance, (i)
Benzoxazine monomers involving curable multi-functional moie-
ties, such as nitrile [18], acetylene [19], propargyl [20], allyl [21],
maleimide [22], epoxy [23] and furan [24] functionalities. These
groups allow further curing to increase crosslinking density of final
materials and minimizing dangling side groups, thus leading to
final polybenzoxazine with improved toughness and thermal
properties. (ii) Benzoxazine monomers containing self-catalyst
groups. Carboxylic acid [25], oxyalcohol [26] and primary amine
[6] functional groups were introduced to lower the polymerization
temperature. Polybenzoxazine-based composites [27] are also an
effective way to improve the properties of the polybenzoxazines
and attain high-performance materials.
Up to now, most of benzoxazine monomers were mainly
focused on the ones consisting of mainly organic element. To
further understanding their structureeproperty relationship, close
attention has been paid recently to design and synthesis of inor-
ganic heteroatom-containing benzoxazine monomers, such as
phosphor, silicon, sulfur [28e33]. These new heteroatom-
containing monomers also impacted the properties of the final
* Corresponding author. Tel.: þ86 27 87559627; fax: þ86 27 87543632.
E-mail address: liukui@mail.hust.edu.cn (C.-M. Liu).
0032-3861/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.07.037

4236
X. Wu et al. / Polymer 52 (2011) 4235e4245
polybenzoxazines. It is well known that such organiceinorganic
hybrids have attracted substantial attention because the new
compounds may possess organic materials’ structural flexibility,
convenient processing properties and inorganic compounds’
thermal and mechanical stability [34].
(d, 2P, PCl2). Product B1: white crystal, yield: 80%. 1H NMR (CDCl3,
TMS, ppm): 7.24e7.54 (m,16H, C12H8). 31P NMR (CDCl3, ppm): 19.78
(d, 2P, P(O2C12H8)), 29.52 (dd, 1P, PCl2).
2.2.2. Synthesis of [N3P3(OC6H4{NHCOCH3}-4)4(O2C12H8)] (T2)
Cyclotriphosphazene (CP) derivatives [35e37] are typical class
of organiceinorganic compounds with a planar non-delocalized
cyclic ring consisting of alternating N and P atoms and six func-
tional groups attached onto the CP ring. There were many reports
related to the inorganic cyclotriphosphazene chemistry due to its
versatile properties. Incorporation of the CP ring into organic
compounds or polymers yielded flame- and heat-retardant mate-
rials [38,39], star-branched polymers [40], columnar super-
molecular liquid crystals [41], branched and conducting polyaniline
with high electrochromic contrast [42], CP-based biomaterials [43]
and catalysts [44], and corresponding hybrid materials showed
favorable behaviors in biocompatibility, thermal stability and
dielectric properties.
A mixture of T1 (10.0 g, 21.69 mmol), 4-acetamidophenol
(15.82 g, 104.12 mmol), and K2CO3 (17.24 g, 124.94 mmol) in
acetone (300 mL) was heated under reflux for 2 days under argon.
Reaction was precipitated in hot water, the resulting white solid
(compound T2) was filtered off, washed successively with large
amount of hot water, a little ethanol and hexane, and dried in
vacuum at 50 ꢁC for 48 h. Yield: 19.34 g (97%), MP.298e299 ꢁC.
1H NMR (DMSO-d6, TMS, ppm): 10.02(4H, NH), 6.65e7.64(24H, Ar-
H), 2.05(12H, eCH3). 13C NMR (DMSO-d6, TMS, ppm): 169.5(CaO),
148.5(CeO), 146.4(CeO), 138.1(CeN), 131.4(CH), 131.1(CH), 129.2(C),
127.8(CH), 122.8(CH), 122.3(CH), 121.5(CH), 25.3(CH3). 31P NMR
(DMSO-d6, ppm): 10.39(d, 2P, P(OC6H4NHCOCH3)), 25.36(t, 1P,
P(O2C12H8)). Elemental analysis Calcd. (%) for C44H40N7O10P3: C,
57.46; H, 4.38; N, 10.66. Found: C, 57.14; H, 4.33; N, 10.65.
Previous results [45] have proved that combining the CP with
benzoxazine moiety through chemical bonding is an effective route
to obtain high-performance polybenzoxazine. The new polymer
curing from the high-branched benzoxazine monomer exhibits
excellent thermal stability and mechanic property compared to
common polybenzoxazines based on phenol and biphenol A. In
order to explore the structureeproperty relationship of the newly
designed monomers and corresponding polybenzoxazines, here we
are reporting that well-defined organiceinorganic benzoxazine
monomers possessing controllable number of benzoxazine moie-
ties groups substituted on CP ring have been synthesized. The
structures of the monomers were confirmed by 1H NMR, 13C NMR,
31P NMR and elemental analysis. All monomers underwent ring-
opening polymerization with or without catalysts to produce
highly cross-linked structures. Fourier transform infrared spec-
troscopy (FT-IR) and differential scanning calorimetry (DSC) were
used to study the thermal ring-opening polymerization behaviors
of the monomers; dynamic mechanical thermal analysis (DMA)
was used to observe the variation of storage modulus for these
monomers under curing; the thermal property and mechanic
performance of the thermoset polymer were also evaluated by
thermal gravimetric analyzer (TGA) and DMA, respectively.
2.2.3. Synthesis of [N3P3(OC6H4{NHCOCH3}-4)2(O2C12H8)2] (B2)
Compound B2 was synthesized and purified according to the
synthetic method of T2. Yield: 13.71 g (98%), MP.168e171 ꢁC.
1H NMR (DMSO-d6, TMS, ppm): 10.08(2H, NH), 7.16e7.71(24H, Ar-
H), 2.05(6H, eCH3). 13C NMR (DMSO-d6, TMS, ppm): 169.6(CaO),
148.6(CeO), 146.4(CeO), 138.2(CeN), 131.6(CH), 131.2(CH),
129.2(C), 127.9(CH), 123.0(CH), 122.5(CH), 121.7(CH), 25.2(CH3).
31P NMR (DMSO-d6, ppm): 9.92(t, 1P, P(OC6H4NHCOCH3)), 25.50(d,
2P, P(O2C12H8)). Elemental analysis Calcd. (%) for C40H32N5O8P3: C,
59.78; H, 4.01; N, 8.71. Found: C, 59.68; H, 3.96; N, 8.78.
2.2.4. Synthesis of [N3P3(OC6H4{NH2}-4)4(O2C12H8)] (T3)
A mixture of T2 (16.0 g, 17.41 mmol) and 4 mol/L sulfuric acid
(140 mL) in EtOH (200 mL) was heated under reflux for 24 h under
argon. After the solution was cooled to room temperature,
ammonia was added dropwise under ice bath until the pH reached
8. Then the gray solid (compound T3) was filtered off and washed
with excess water, and dried in vacuum at 50 ꢁC for 48 h. Yield:
11.64 g (89%), MP.115e117 ꢁC. 1H NMR (DMSO-d6, TMS, ppm):
6.52e7.52(24H, Ar-H), 5.02(8H, eNH2). 13C NMR (DMSO-d6, TMS,
ppm): 148.7(CeO), 146.6(CeO), 142.3(CeN), 131.2(CH), 130.9(CH),
129.3(C), 127.6(CH), 123.0(CH), 122.5(CH), 116.1(CH). 31P NMR
(DMSO-d6, ppm): 10.95(d, 2P, P(OC6H4NH2)), 26.04(t, 1P,
P(O2C12H8)). Elemental analysis Calcd. (%) for C36H32N7O6P3$H2O:
C, 56.18; H, 4.45; N, 12.74. Found: C, 56.34; H, 4.13; N, 12.46.
2. Experimental
2.1. Materials
2,20-dihydroxybiphenyl (99%) and 4-acetamidophenol (98%)
were purchased from Alfa Aesar Reagent Co., Ltd., USA. Salicy-
laldehyde (ꢀ98%), sodium borohydride (96%), paraformaldehyde
(95%), sodium hydroxide (ꢀ96%), sulfuric acid (95e98%), ammonia
2.2.5. Synthesis of [N3P3(OC6H4{NH2}-4)2(O2C12H8)2] (B3)
Compound B3 was synthesized and purified according to the
synthetic methodology of T3. Yield: 10.83 g (93%), MP.294e296 ꢁC.
1H NMR (DMSO-d6, TMS, ppm): 6.62e7.68(24H, Ar-H), 5.06(4H,
eNH2). 13C NMR (DMSO-d6, TMS, ppm): 147.8(CeO), 146.8(CeO),
140.7(CeN), 130.6(CH), 130.3(CH), 128.5(C), 126.9(CH), 122.2(CH),
121.7(CH), 114.8(CH). 31P NMR (DMSO-d6, ppm): 11.04(t, 1P,
P(OC6H4NH2)), 26.01(dd, 2P, P(O2C12H8)). Elemental analysis Calcd.
(%) for C36H32N7O6P3: C, 60.09; H, 3.92; N, 9.73. Found: C, 60.20; H,
3.93; N, 9.70.
H
N
(25e28%), imidazole ( ) and potassium carbonate were all
N
obtained from Sinopharm Chemical Reagent Co., Ltd., China. Hex-
achlorocyclotriphosphazene (synthesized according to literature
[46]) was recrystallized from dry hexane followed by sublimation
(60 ꢁC, 0.05 mmHg) twice before use (MP. 112.5e113.0 ꢁC). All
solvents were purified by standard procedures.
2.2.6. Synthesis of [N3P3(OC6H4{NCHC6H4(2-OH)}-4)4(O2C12H8)]
(T4)
2.2. Synthetic section
A solution of salicylaldehyde (7.81 g, 63.91 mmol) in 50 mL
tetrahydrofuran (THF) was added to a solution of T3 (10.0 g,
13.32 mmol) in THF (200 mL) under argon atmosphere, and the
mixture was refluxed under vigorous stirring for 24 h. The resulting
yellow solution was concentrated and precipitated in petroleum
ether. The resulting yellow powder (compound T4) was gained and
2.2.1. Synthesis of [N3P3Cl4(O2C12H8)] (T1) and [N3P3Cl2(O2C12H8)2]
(B1)
These compounds were synthesized as report [47]. Product T1:
white crystal, yield: 87%. 1H NMR (CDCl3, TMS, ppm): 7.25e7.56 (m,
8H, C12H8). 31P NMR (CDCl3, ppm): 12.85 (t, 1P, P(O2C12H8)), 24.76

X. Wu et al. / Polymer 52 (2011) 4235e4245
4237
washed with petroleum ether, dried in vacuum at 40 ꢁC for 48 h.
Yield: 15.08 g (97%), MP. 272e275 ꢁC. 1H NMR (DMSO-d6, TMS,
ppm): 12.89(4H, CHaN), 8.92(4H, HO-Ar), 6.85e7.68(40H, Ar-H).
13C NMR (DMSO-d6, TMS, ppm): 164.8(CeOH), 161.5(CaN),
149.9(CeO), 148.5(CeO), 146.9(CeN), 134.7(CH), 133.9(CH),
131.6(CH), 131.3(CH), 129.2(C), 128.1(CH), 124.2(CH), 123.0(CH),
122.8(C), 120.6(CH), 120.4(CH), 117.9(CH). 31P NMR (DMSO-d6,
ppm): 11.03(d, 2P, P(OC6H4NCHC6H4OH)), 25.21(t, 1P, P(O2C12H8)).
Elemental analysis Calcd. (%) for C64H48N7O10P3$H2O: C, 64.81; H,
4.25; N, 8.27. Found: C, 64.90; H, 4.15; N, 8.58.
resulting solid was dissolved in CH2Cl2, and washed with diluted
solution of sodium hydroxide. The organic layer was collected,
dried over anhydrous Na2SO4, concentrated and then precipitated
in petroleum ether. A light yellow powder was collected, dried in
vacuum at 40 ꢁC for 48 h. Yield: 9.89 g (95%), MP. 80e82 ꢁC. 1H NMR
(Acetone-d6, TMS, ppm): 4.70 (8H, Ar-CH2eN), 5.42 (8H,
OeCH2eN), 6.51e7.54 (40H, Ar-H). 13C NMR (Acetone-d6, TMS,
ppm): 50.7(Ar-CH2eN), 80.3(OeCH2eN), 155.3(CeO), 148.8(CeO),
146.7(CeN), 145.5(CH), 130.7(CH), 130.2(CH), 129.3(CH), 128.6(CH),
127.9(C), 126.8(C), 122.7(CH), 122.5(CH), 122.1(CH), 121.5(CH),
119.7(CH), 117.4(CH). 31P NMR (Acetone-d6, ppm): 10.9(d, 2P,
P(OC6H4NCH2CH2OC6H4)), 25.4(t, 1P, P(O2C12H8)). Elemental anal-
ysis for TBOz$H2O, Calcd. (%): C, 65.75; H, 4.71; N, 7.89. Found: C,
65.79; H, 4.65; N, 7.88.
2.2.7. Synthesis of [N3P3(OC6H4{NCHC6H4(2-OH)}-4)2(O2C12H8)2]
(B4)
Compound B4 was synthesized and purified according to the
synthetic methodology of T4. Yield: 12.63 g (98%), MP. 240e241 ꢁC.
1H NMR (DMSO-d6, TMS, ppm): 12.97(2H, CHaN), 9.02(2H, HO-Ar),
6.97e7.69(32H, Ar-H). 13C NMR (DMSO-d6, TMS, ppm):
165.1(CeOH), 161.5(CaN), 150.0(CeO), 148.5(CeO), 147.1(CeN),
134.8(CH), 133.9(CH), 131.6(CH), 131.3(CH), 129.2(C), 128.1(CH),
124.3(CH), 123.1(CH), 123.0(C), 120.7(CH), 120.6(CH), 117.9(CH). 31P
NMR (DMSO-d6, ppm): 10.18(t, 1P, P(OC6H4NCHC6H4OH)), 25.25(d,
2P, P(O2C12H8)). Elemental analysis Calcd. (%) for C50H36N5O8P3: C,
64.73; H, 3.91; N, 7.55. Found: C, 64.58; H, 3.92; N, 7.60.
2.2.11. Synthesis of [N3P3(OC6H4{NCH2CH2C6H4(2-O)}-4)2(O2C12H8)2]
(BBOz)
BBOz was synthesized and purified according to the synthetic
methodology of TBOz. Yield: 9.64 g (94%), MP. 118e121 ꢁC. 1H NMR
(Acetone-d6, TMS, ppm): 4.70 (4H, Ar-CH2eN), 5.42 (4H,
OeCH2eN), 6.72e7.39 (32H, Ar-H). 13C NMR (Acetone-d6, TMS,
ppm): 49.9(Ar-CH2eN), 79.3(OeCH2eN), 154.5(CeO), 148.0(CeO),
146.1(CeN), 144.6(CH), 129.9(CH), 129.6(CH), 128.5(CH), 127.7(CH),
127.0(C), 126.2(C), 121.9(CH), 121.6(CH), 121.1(CH), 120.6(CH),
119.0(CH), 116.5(CH). 31P NMR (Acetone-d6, ppm): 11.0(dd, 1P,
P(OC6H4NCH2CH2OC6H4)), 25.6(d, 2P, P(O2C12H8)). Elemental
analysis for BBOz, Calcd. (%): C, 65.34; H, 4.22; N, 7.33. Found: C,
65.19; H, 4.60; N, 7.35.
2.2.8. Synthesis of [N3P3(OC6H4{NHCH2C6H4(2-OH)}-4)4(O2C12H8)]
(T5)
Sodium borohydride (1.18 g, 30.85 mmol) was added to a solu-
tion of T4 (15 g, 12.85 mmol) in 150 mL THF in small portion with
stirring. Reaction was carried out at 25 ꢁC for 12 h, 100 mL of 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. The resulting solid was dissolved in CH2Cl2, and washed
with water. The organic layer was collected, dried over anhydrous
Na2SO4, concentrated and precipitated in petroleum ether. Pale
powder was obtained, and dried in vacuum at 40 ꢁC for 48 h. Yield:
13.60 g (90%), MP. 100e102 ꢁC. 1H NMR (Acetone-d6, TMS, ppm):
8.57(4H, HO-Ar), 6.67e7.57(40H, Ar-H), 5.22(4H, NH), 4.36(8H,
CH2). 13C NMR (Acetone-d6, TMS, ppm): 156.4(CeOH), 148.8(CeO),
146.8(CeO), 142.9(CeN), 130.5(CH), 130.1(CH), 129.7(CH),
129.5(CH), 128.8(C), 126.7(CH), 126.3(CH), 122.9(CH), 122.5(C),
120.4(CH), 116.1(CH), 114.6(CH), 44.7(CH2eN). 31P NMR (Acetone-
d6, ppm): 11.12(d, 2P, P(OC6H4NHCH2C6H4OH)), 26.15(t, 1P,
P(O2C12H8)). Elemental analysis Calcd. (%) for C64H56N7O10P3: C,
65.36; H, 4.80; N, 8.34. Found: C, 65.18; H, 4.69; N, 8.40.
2.3. Measurements
The structures of the compounds were verified by proton (1H),
carbon (13C) and phosphorus (31P) nuclear magnetic resonance
spectroscopy (NMR) using Bruker AV400 NMR spectrometer at
proton frequency of 400 MHz at room temperature. Infrared spectra
were recorded on a Bruker VERTEX 70 Fourier transform infrared
spectrometer (FT-IR) with a heating device using a resolution of
4 cmꢂ1. 32 scans were taken to the KBr pellets mixed with samples
in transmittance mode. Elemental analysis was carried out on
a German Vario Micro cube microanalyzer. 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/min were used in DSC experiments. Dynamic mechanical
thermal analysis (DMA) was also used to observe the curing process
from DMA 242, NETZSCH. Samples were added into aluminum cap,
and a probe pin was plunged into it. During curing reaction,
a sinusoidal stress was applied to the samples by moving the probe
pin up and down in the sample at a frequency of 1 Hz with ampli-
tude of 0.05 mm under 1 N dynamic load. Thermal gravimetric
analysis was performed with a NETZSCH Instruments’ High Reso-
lution STA 409PC thermogravimetric 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 to 850 ꢁC. Mechanical
properties were measured using a dynamic mechanical thermal
analysis (DMA) apparatus (PerkinElmer, Diamond DMA). Speci-
mens (50 ꢃ 10 ꢃ 1.0 mm) were tested in 3 point bending mode. The
thermal transitions were studied in the range of 30e300 ꢁC at
a heating rate of 4 ꢁC/min and at a fixed frequency of 1 Hz.
2.2.9. Synthesis of [N3P3(OC6H4{NHCH2C6H4(2-OH)}-4)2(O2C12H8)2]
(B5)
Compound B5 was synthesized and purified according to the
synthetic methodology of T5. Yield: 11.45 g (95%), MP. 141e143 ꢁC.
1H NMR (Acetone-d6, TMS, ppm): 8.54(2H, HO-Ar), 6.76e7.64(32H,
Ar-H), 5.36(2H, NH), 4.38(4H, CH2). 13C NMR (Acetone-d6, TMS,
ppm): 156.4(CeOH), 149.1(CeO), 147.5(CeO), 142.9(CeN),
130.7(CH), 130.4(CH), 129.6(CH), 128.8(CH), 127.0(C), 126.3(CH),
122.9(CH), 122.6(CH), 122.5(C), 120.4(CH), 116.1(CH), 114.4(CH),
44.6(CH2eN). 31P NMR (Acetone-d6, ppm): 11.42(t, 1P,
P(OC6H4NHCH2C6H4OH)), 26.06(d, 2P, P(O2C12H8)). Elemental
analysis Calcd. (%) for C50H40N5O8P3: C, 64.45; H, 4.33; N, 7.52.
Found: C, 64.44; H, 4.33; N, 7.53.
2.2.10. Synthesisof[N3P3(OC6H4{NCH2CH2C6H4(2-O)}-4)4(O2C12H8)]
(TBOz)
2.4. Sample preparation
A solution of T5 (10.0 g, 8.51 mmol) and paraformaldehyde
(1.53 g, 51.06 mmol) in 150 mL of 1,4-dioxane was stirred at 100 ꢁC
for 6 days under argon atmosphere. Then the solution was evapo-
rated under reduced pressure to remove the organic solvent, the
Samples for dynamic mechanical analysis and humidity
absorption test were prepared as the following manner: the
monomers were separately added to the mould of Teflon, and

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X. Wu et al. / Polymer 52 (2011) 4235e4245
melted to remove the air at 160 ꢁC and 180 ꢁC each for 1 h under
vacuum, then step cured in an oven at 180 ꢁC and 200 ꢁC each for
8 h, then post-cured at 220 ꢁC and 240 ꢁC each for 2 h.
62.3% and 63.8% respectively. HBOz monomer was synthesized
based on reported method [45] and its structure was shown in
Scheme 3. The molecular weights of the monomers were high
(HBOz, 1492 g/mol; TBOz, 1224 g/mol and BBOz, 956 g/mol),
however all three organiceinorganic hybrid monomers exhibited
good solubility. They can be easily dissolved in many organic
solvents at room temperature in 20 s with a concentration of 0.1 g/
mL (solvents: acetone, dioxane, THF, CHCl3, CH2Cl2, DMF, DMSO,
et al). Products purity has been confirmed by 1H, 13C, 31P NMR and
elemental analysis.
2.5. Gel content
The gel fraction was determined by standard extraction method
[48] using chloroform after constant weight obtained with drying
for 20 h at 90 ꢁC under vacuum. The process involved in continuous
extraction with chloroform in a 250 mL round bottom flask for
more than 48 h until constant weight obtained. After the extrac-
tion, the samples were dried and the gel content was calculated as
the ratio between the dry polymer remaining after the extraction
and the initial amount of dry polymer.
3.2. Curing behaviors of the monomers monitored by FT-IR
The ring-opening polymerization process was initially moni-
tored by FI-IR spectrometer. The KBr pellets separately containing
HBOz, TBOz and BBOz powders were cured at 220 ꢁC and corre-
sponding FT-IR spectra were recorded. As shown in Fig. 1, the
characteristic absorption bands of HBOz benzoxazine structures
were shown at 972 cmꢂ1 (out of plane-bending vibrations of CeH),
1227 cmꢂ1 (asymmetric stretching of CeOeC), 1035 cmꢂ1
(symmetric stretching of CeOeC), 1081 cmꢂ1 (asymmetric
stretching of CeNeC) and 1340 cmꢂ1 (CH2 wagging), respectively
[50]. The peaks at 1480 and 1453 cmꢂ1 were attributed to meth-
ylene scissoring in the oxazine ring. The region from 748 to
884 cmꢂ1 and 1613, 1581 cmꢂ1 were assigned to ortho- and para-
disubstituted benzene rings. Two very strong absorption peaks
located at 950 cmꢂ1 and 1165 cmꢂ1 were corresponding to the
characteristic stretching of PeO-Ar and NaP of CP derivatives.
These characteristic absorption peaks were clearly seen for HBOz
monomer at room temperature. Upon HBOz sample was quickly
heated to 220 ꢁC (Fig. 1(b)), it was found that the intensity of
characteristic absorption of benzoxazine structure and disubsti-
tuted benzene ring decreased, the characteristic peaks at 972, 1340
and 1480 cmꢂ1 nearly disappeared. When the curing time was
prolonged to 1 h at 220 ꢁC, all the characteristic peaks attributed
to oxazine ring disappeared. Meanwhile, the intensity of the peak
at 1613 cmꢂ1 gradually increased as a result of ortho-substituted
phenols which were accompanied with the peaks of weakening
intensity in the range of 748 and 884 cmꢂ1 and 1581 cmꢂ1, and
2.6. Humidity absorption
The cured samples were conditioned under vacuum at 90 ꢁC for
20 h before placed in air (75% and 33% RH). All these experiments
were conducted at room temperature. Then, the weight percent-
ages of humidity absorption of the cured samples were calculated
according to the formula as follows, where the Wt and W0 repre-
sented sample weights after and before (dry sample) humidity
absorption respectively.
Wt ꢂ W
Humidity Absorption Contentð%Þ ¼
0 ꢃ 100%
W0
3. Results and discussion
3.1. Synthesis and characterization
Initial experimental results showed that directly using amine
(T3, B3), phenol and formaldehyde to prepare the corresponding
benzoxazine monomers furnished product as a mixture of mono-
mer and cross-linked polymer with separation difficulties. Next,
TBOz and BBOz monomers were synthesized according to Schemes
1 and 2, respectively [49]. The overall yields of TBOz and BBOz were
Scheme 1. Synthesis of organiceinorganic benzoxazine monomer of TBOz.

X. Wu et al. / Polymer 52 (2011) 4235e4245
4239
Scheme 2. Synthesis of organiceinorganic benzoxazine monomer of BBOz.
such change continued when the curing time was increased. These
variations were due to the ring-opening polymerization of ben-
zoxazine moieties in HBOz, which led to formation of higher
substitution pattern of the aromatic rings thanks to Mannich-type
linkage polymers after the crosslinking reaction [50] (see in
Scheme 3).
Fig. 2 showed the FT-IR spectra of TBOz under curing at 220 ꢁC.
The bands assigned to oxazine were at 835 cmꢂ1 (symmetric
stretching of CeNeC), 972 cmꢂ1 (out of plane-bending vibrations of
CeH), 1227 cmꢂ1 (asymmetric stretching of CeOeC), 1339 cmꢂ1
(CH2 wagging) and 1483, 1454 cmꢂ1 (scissoring of CH2), respec-
tively [49]. The intensity of these characteristic peaks gradually
Scheme 3. Polymerization of HBOz, TBOz and BBOz monomers under heating.

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X. Wu et al. / Polymer 52 (2011) 4235e4245
Fig. 3. The FT-IR spectra of BBOz in the region between 1900 and 500 cmꢂ1 under
curing.
Fig. 1. The FT-IR spectra of HBOz in the region between 1900 and 500 cmꢂ1 under
curing.
3.3. Curing behaviors of the monomers scanned by DSC
decreased when the curing time was prolonged, and corresponding
peaks nearly disappeared at 220 ꢁC for 2 h. In the mean time, the
intensity of the peak at 1613 cmꢂ1 increased along with the
decreasing intensity of peaks in the range of 532e780 cmꢂ1, and
880, 1581 cmꢂ1, as a result of ring-opening polymerization of
benzoxazine moieties in TBOz. In addition, a peak at 1093 cmꢂ1 was
attributed to the biphenyl and existed during the entire curing
period, which were not found on the spectra of HBOz (see in Fig. 1),
and it existed during the whole curing period.
Fig. 3 showed the FT-IR spectra of BBOz monomer under curing
at 220 and 240 ꢁC. Similarly, the oxazine assignments were at
835 cmꢂ1 (symmetric stretching of CeNeC), 972 cmꢂ1 (out of
plane-bending vibrations of CeH), 1227 cmꢂ1 (asymmetric
stretching of CeOeC), 1340 cmꢂ1 (CH2 wagging) and 1480,
1453 cmꢂ1 (scissoring of CH2), respectively [49]. These peaks didn’t
disappear under curing at 220 ꢁC for 2 h or at 240 ꢁC for 1 h more,
but they almost disappeared at 240 ꢁC for 2 h. Meanwhile, the
change of peaks at the fingerprint region in 1613e1581 cmꢂ1 and
780e532 cmꢂ1, similar to that of HBOz and TBOz, was due to the
ring-opening polymerization of benzoxazine moieties.
The polymerization behaviors of the new monomers (HBOz,
TBOz and BBOz) were analyzed by DSC. As seen in Fig. 4, an
endothermic peak at 77 ꢁC and an exothermic peak at 226 ꢁC are
corresponding to the melting point and ring-opening polymeriza-
tion temperature of HBOz. The onset exothermic temperature is
177 ꢁC and exothermic enthalpy of HBOz is 211.8 J/g (316.0 kJ/mol).
Comparing to HBOz, the temperatures of melting points, onset
polymerization and polymerization exothermic peaks all rose for
TBOz and BBOz monomers. The melting points of TBOz and BBOz
monomers are 81 ꢁC and 118 ꢁC, respectively; onset exothermic
temperatures of them are at 181 ꢁC and 185 ꢁC, and the corre-
sponding exothermic peaks’ temperatures of TBOz and BBOz are
229 ꢁC and 257 ꢁC, respectively. The higher melting and polymer-
ization temperatures of TBOz and BBOz are due to the steric
hindrance of rigid biphenyl groups substituted onto CP ring, which
hinders free movement of molecules for the monomers and
constraints the stretch of polymer chains under heating. The higher
number of the biphenyl groups the monomer has, the more difficult
to polymerize of the monomer under heat. In addition the
exothermic enthalpies of TBOz and BBOz are 173.5 J/g (212.4 kJ/
mol) and 119.7 J/g (114.4 kJ/mol), respectively. Based on the poly-
merization heat per mole of benzoxazine moiety in the monomer,
HBOz, TBOz and BBOz will respectively have 52.7 kJ/mol, 53.1 kJ/
mol, and 57.2 kJ/mol, indicating the increasing difficulty of ben-
zoxazine polymerization as the number of oxazine group per
From the FT-IR results of the three monomers under curing, it
can be concluded that the disappearance of the oxazine assign-
ments for the monomers became more difficult under heat in the
order of HBOz, TBOz and BBOz, which implied that HBOz monomer
exhibited the highest rate of polymerization while BBOz was the
lowest one.
Fig. 2. The FT-IR spectra of TBOz in the region between 1900 and 500 cmꢂ1 under
curing.
Fig. 4. DSC plots of the monomers in the first heating scan. a. HBOz, b. TBOz, c. BBOz.

X. Wu et al. / Polymer 52 (2011) 4235e4245
4241
molecule decreases. From the DSC results, it is concluded that HBOz
is the easiest cured monomer while BBOz monomer is the most
difficult one under heat. Furthermore, after the first heating scan of
the pure monomers, the residual samples were also subjected to
the second heating scan under DSC. Testing results implied that the
monomers didn’t undergo residual crosslinking reaction after the
first heating even with their complex multi-functional structures.
The polymerization temperatures of the benzoxazine mono-
mers can be lowered with addition of small portion of catalyst.
Imidazole (C) was proved to be a suitable curing agent for lowering
the polymerization temperature [28,45]. Imidazole was separately
added to the three monomers. The mixtures were scanned by DSC,
and the results are shown in Fig. 5. Significant changes to onset
exothermic temperatures and exothermic peaks’ temperatures
have been observed after adding 3 wt% catalysts to the monomers.
The onset exothermic temperatures of TBOz and BBOz have
decreased to 159 ꢁC and 174 ꢁC, respectively, and it was reduced to
132 ꢁC for HBOz. Corresponding exothermic peaks’ temperatures of
TBOz and BBOz decreased to 206 ꢁC and 232 ꢁC, and it was lowered
to 215 ꢁC for HBOz. Besides, the enthalpies of the monomers have
dropped significantly after adding catalyst. The enthalpies of HBOz,
TBOz and BBOz were reduced to 202.4 J/g, 163.8 J/g and 109.9 J/g,
respectively. The results show that imidazole is an effective catalyst
to decrease the polymerization temperatures of the monomers, and
results in a milder polymerization exothermic process.
Fig. 6. The plots of the G0 of the monomers under curing at 180 ꢁC after pre-curing at
180 ꢁC for 1 h.
polymerization induction time was long for BBOz monomer, it
needed more time at 180 ꢁC to start gelation. On other hand, the G0
value of TBOz molten mixtures increased rapidly after 10 min and
then reached a plateau, which implied that TBOz could be gelation
earlier than BBOz. As a result, the gelation times were different for
the three monomers at 180 ꢁC, with HBOz having the shortest
gelation time while BBOz was the longest one among the three
monomers. Based on afore-mentioned results, it can be concluded
that the reactive activity of the monomers is greatly affected by
quantity of the benzoxazine moieties and the biphenyl groups.
Rigid biphenyl groups let the induction time of polymerization
become longer for their steric hindrance [53]; while more ben-
zoxazine moieties can improve the polymerization activity of the
monomers. This conclusion was also consistent with the data from
FT-IR and DSC.
3.4. Curing behaviors of the monomers monitored by DMA
The evolution of storage modulus of each pure organ-
iceinorganic monomer sample during the polymerization was
continuously monitored by DMA. Before test, all samples were pre-
cured at 180 ꢁC for 1 h, and then were tested on DMA. From the
results in Fig. 6, the evolving dynamic storage moduli (G0) of the
monomers were quite different during the isothermal polymeri-
zation experiments at 180 ꢁC. The G0 value of HBOz was about
20 MPa at beginning, while they were about 0 MPa for TBOz and
BBOz samples. These indicated that HBOz sample has begun gela-
tion during the pre-curing process, while TBOz and BBOz were still
at molten state and didn’t begin gelation at 180 ꢁC. Such result was
also proved by observing the pre-curing monomers at 180 ꢁC after
1 h. The G0 value of BBOz molten mixture did not increase notice-
ably in initial 39 min because the curing reaction was not adequate
at 180 ꢁC during this time, and then it increased sharply. Gelation
time was defined as the onset of the storage modulus, where the
material began to develop mechanical properties characteristic of
the elastic solids [51,52]. Thereby it can be deduced that the
3.5. Thermal properties of the novel polybenzoxazines
As seen in Scheme 3 three new polybenzoxazines resulting from
the corresponding monomers possess high crosslinking structures,
which are different compared to original polybenzoxazines. For
single HBOz molecule under heat curing, it can provide at least
twelve reactive crosslinking sites (ꢂOH groups could be reactive
sites) under heat curing, while there are at least eight and four
reactive crosslinking sites for TBOz and BBOz monomers, respec-
tively. These different polymers’ structures with variable reactive
crosslinking sites [54] and different number of rigid biphenyl
groups
may affect
the
properties
of
corresponding
polybenzoxazines.
The CP-based polybenzoxazines samples were prepared by
curing corresponding monomers according to condition A (see in
Table 1) in the salt-bath under argon. All the cured samples (see in
Table 2) were dark red color, and the color increased in the order of
PHBOz, PTBOz, PBBOz. Thermal stability of the novel poly-
benzoxazines was investigated by TGA, and the results were shown
in Fig. 7 and Table 2. It was found that the thermal stability of PTBOz
series was higher than that of the other two polybenzoxazines.
Table 1
Curing condition for the monomers.
Curing condition A
Curing condition B
160 ꢁC, 1 h
200 ꢁC, 4 h
220 ꢁC, 1 h
240 ꢁC, 1 h
160 ꢁC, 1 h
200 ꢁC, 4 h
220 ꢁC, 2 h
240 ꢁC, 2 h
Fig. 5. DSC plots of the monomers using catalyst with the content of 3 wt%. a. HBOz/C-
3 wt%, b. TBOz/C-3 wt%, c. BBOz/C-3 wt%.

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X. Wu et al. / Polymer 52 (2011) 4235e4245
Table 2
Thermal stability of polybenzoxazines samples.
a
b
c
Polymers
T5% (ꢁC)
T10% (ꢁC)
Tmax. (ꢁC)
Ycd (%)
PHBOze
403
378
442
455
303
398
412
449
419
471
484
346
439
468
453
441
499
489
433
475
492
66.9
59.4
69.7
67.7
44.7
59.3
69.6
PHBOz/C-3 wt%e
PTBOze
PTBOz/C-3 wt%e
PBBOze
PBBOz/C-3 wt%e
PBBOzf
a
T5%: The temperature for which the weight loss is 5% by TGA scan (20 ꢁC/min)
under nitrogen.
b
c
d
e
f
T10%: The temperature for which the weight loss is 10%.
Tmax: Maximum weight loss temperature.
Yc: Char yields at 850 ꢁC.
Cured under curing condition A.
Cured under curing condition B.
Fig. 8. DSC plots of BBOz under step-wise curing. (a) 220 ꢁC, 2 h (b) 220 ꢁC, 2 h; 240 ꢁC,
1 h (c) 220 ꢁC, 2 h; 240 ꢁC 2 h.
Meanwhile the thermal stability of PTBOz/C-3 wt% was slightly
higher than that of PTBOz (see in Table 2). The thermal stability next
to PTBOz series is PHBOz series, and corresponding thermal
stability of PHBOz/C-3 wt% was slightly lower than that of their
pure polybenzoxazines. PBBOz series showed the lowest thermal
stability. Surprisingly the values of T5%, T10%, Tmax and Yc for PBBOz
were even much lower than that of PBBOz/C-3 wt% sample. The
possible reason was that BBOz monomer has not been cured
completely under condition A, and this conjecture was verified by
later post-curing test.
Fig. 8 displayed the DSC plots of step-wise cured BBOz samples
under different heat treatment. An obvious exothermic peak exis-
ted at about 259 ꢁC after curing at 220 ꢁC for 2 h, and the
exothermic peak significantly decreased after one more hour curing
at 240 ꢁC (see Fig. 8(b)). Meanwhile, the exothermic polymerization
enthalpy reduced accompanying with the increasing of onset
exothermic temperature and peaks’ temperatures. When the
sample was cured at 220 ꢁC and 240 ꢁC each for 2 h, the exothermic
peaks disappeared, which implied that the sample was already
cured completely. From the results of step-wise DSC monitoring, it
suggested that BBOz were not easy to polymerize completely under
condition A, while it could be cured well under condition B.
Corresponding thermal stability of PBBOz was obviously
improved when BBOz was cured under condition B as shown in
Fig. 9. It can be found that the thermal stability of PBBOz which held
longer post-cured time (condition B) was much higher than the one
cured under condition A. Yc of the PBBOz polymer under condition
B reached up to 69.6%, T5% and T10% rose to 412 ꢁC and 468 ꢁC
respectively.
From the TGA results, we can deduce that organiceinorganic
hybrid polybenzoxazines polymerized from the new monomers
show outstanding thermal stability. It is considered that the
rigidly inorganic CP rings enhanced the thermal stability of the
polybenzoxazines which provide thermally stable sites
comparing to traditional polybenzoxazines. At the same time, the
numbers of benzoxazine moieties on CP ring also played an
important role in the thermal stability of polybenzoxazines by
improving cross-linked density and minimizing dangling side
groups. Besides, like other bulky aromatic substituents the rigid
biphenyl groups substituted onto CP ring also helped to promote
the thermal stability [17], while excessive rigid groups will
constrain the polymerization of the monomers for high steric
hindrance, which will make it more difficult for the ring-opening
polymerization.
3.6. Gel contents of the polybenzoxazines
The gel contents of three polybenzoxazines sample obtained
under condition A were determined using the methodology
described in the experimental part, and the results were shown in
Fig. 10. The gel contents of PHBOz and PTBOz were 98.9% and
99.5% respectively, while the gel content of PBBOz-a (see in Fig. 10
PBBOz-a) was much lower. When PBBOz was cured under curing
Fig. 7. TG curves of polybenzoxazines, corresponding monomers with or without
catalysts were cured under curing condition A. a. PHBOz, b. PHBOz/C-3 wt%, c. PTBOz,
d. PTBOz/C-3 wt%, e. PBBOz, f. PBBOz/C-3 wt%.
Fig. 9. TG curves of PBBOz under different curing conditions. a. Cured under curing
condition A. b. Cured under curing condition B.

X. Wu et al. / Polymer 52 (2011) 4235e4245
4243
condition B, the gel content of PBBOz was increased from 60.7 to
89.7% (see in Fig. 10 PBBOz-b). This result was expected because of
the difficult curing with BBOz monomers and it illustrated the
reason of unusual thermal stability of PBBOz in Fig. 9. It also
explained that the properties of all polybenzoxazines depended on
the different chemical structure of the monomers. More number
of biphenyl groups in the monomer will bring high steric
hindrance to heat curing, which makes the complete curing of
monomer become difficult and thus more severe curing condition
would be needed.
3.7. Dynamic mechanical analysis of the polybenzoxazine
The dynamic mechanical properties of the cyclotriphosphazene-
based polybenzoxazines were measured from room temperature to
the rubbery plateau of each material. The storage moduli (E0) of
these polybenzoxazines were presented in Fig. 11. All the samples
were cured under condition B. The E0 value of PTBOz at room
temperature was 7.2 GPa, while they were 5.4 GPa and 4.1 GPa for
PBBOz and PHBOz, respectively. It is obvious that the E0 values of
the polybenzoxazines were influenced by the number of the ben-
zoxazine moieties and biphenyl groups. The biphenyl groups
improved the E0 value significantly, so the E0 values of PTBOz and
PBBOz were much higher than that of PHBOz. But the number of
the benzoxazine moieties in their monomers also played an
important role in provoking the E0 value of polybenzoxazines,
which led to more spatial crosslinking sites. Because of its equi-
librium to the numbers of benzoxazine moieties and rigid biphenyl
groups, the E0 value of PTBOz was the highest among the three
polybenzoxazines. However, the E0 values of all the three poly-
benzoxazines are higher than that of common polybenzoxazine
(E0 w 3.5 GPa) [45].
Fig. 11. Storage moduli of the polybenzoxazines.
PBBOz between the transition zones of Tg, because there are more
cross-linked sites for single PHBOz and PTBOz molecule, with cor-
responding monomers of more benzoxazine moieties for ring-
opening polymerization than that of PBBOz.
3.8. Humidity absorption of the polybenzoxazines
The strip specimens of the three polybenzoxazines were placed
in the environment of air at different relative humidity (RH) using
the methodology described in the experimental part, and the
sample size used was 20 mm ꢃ 10 mm ꢃ 1 mm (average thickness).
As seen in Fig. 13, regardless of the RH value of the air is high (see in
Fig.13(a)) or low (see in Fig.13(b)), all the polybenzoxazine samples
exhibited a low water uptake value (<0.9 wt%) at room tempera-
ture after 8 days. Besides, the final water absorption values of the
three polybenzoxazines were quite different, the water uptake
values decreased in the order from PHBOz, PTBOz to PBBOz. It was
about 0.80 wt% for PHBOz, and 0.55 wt% for PTBOz, and the value of
PBBOz was about 0.45 wt%, which was a little higher than that of
bisphenol-A based polybenzoxazine [3] (<0.4 wt%) and a little
lower than that of epoxy [3] (<0.62 wt%). The lowest water
absorption of PBBOz was considered from the hydrophobic
biphenyl groups in the network. The more numbers of the biphenyl
groups in the polybenzoxazines, the lower water absorption they
might have. In contrast the more benzoxazine moieties in their
monomers, the higher water absorption value they have in corre-
sponding polybenzoxazines because of the increased hydrophilic
hydroxyl groups resulting from the ring-opening polymerization of
benzoxazine moieties.
The glass transition temperature (Tg) is also an important
property of the polybenzoxazines specimens, and the Tg of the
polybenzoxazines can be deduced in Fig.12 from the corresponding
peak temperature of tand
value. The Tg of PHBOz was 152 ꢁC, while
they were 254 ꢁC and 210 ꢁC for PTBOz and PBBOz. Although PHBOz
had the highest cross-linked degree, there existed lots of soft N,N-
dimethylene (eCH2eNeCH2e) chains (see in Scheme 3), which
maybe start to move under lower temperature. However PTBOz
and PBBOz had lower cross-linked degree, it possessed bulky
biphenyl groups, which possibly constrained the cross-linked chain
to move under heat and let it start to move at higher temperature.
So the Tg of PTBOz and PBBOz is much higher than that of PHBOz.
The tand values of PHBOz and PTBOz were much lower than that of
Fig. 10. Gel contents of the polybenzoxazines. PHBOz, PTBOz and PBBOz-a were cured
under condition A; PBBOz-b was cured under curing condition B.
Fig. 12. Tand of the polybenzoxazines.

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X. Wu et al. / Polymer 52 (2011) 4235e4245
Fig. 13. Humidity absorption of the polybenzoxazines at different relative humidity (RH) at room temperature. a. RH ¼ 75%; b. RH ¼ 33%.
4. Conclusions
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Appendix. Supplementary material
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