Poly(phenylene-carborane) for boron-carbide/carbon ceramic precursor synthesized via nickel catalysis

Article abstract of DOI:10.1016/j.polymer.2017.03.036

High thermally and thermo-oxidatively stable poly(phenylene-carborane) (PPB) was synthesized via Ni(0)-catalyzed polymerization of bis(aryl chloride) monomer containing carborane. This polymer was soluble in THF and NMP at room temperature, thus allowing

Full text of DOI:10.1016/j.polymer.2017.03.036

Polymer 115 (2017) 224e231
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Poly(phenylene-carborane) for boron-carbide/carbon ceramic
precursor synthesized via nickel catalysis
a
a
a
a
b
b, *
Shengli Cheng , Kuanyu Yuan , Xin Wang , Jianhua Han , Xigao Jian , Jinyan Wang
a
Polymer Science & Materials, Chemical Engineering College, Dalian University of Technology, Dalian, 116024, China
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116024, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 December 2016
Received in revised form
High thermally and thermo-oxidatively stable poly(phenylene-carborane) (PPB) was synthesized via
Ni(0)-catalyzed polymerization of bis(aryl chloride) monomer containing carborane. This polymer was
soluble in THF and NMP at room temperature, thus allowing molecular weight and spectroscopic
20 February 2017
analysis. Number-average molecular weight determined by 1H-NMR spectroscopy was 5.4 ꢀ 103 g/mol.
Accepted 16 March 2017
The crystalline character of PPB was verified by XRD analysis. This polymer exhibited excellent thermal
ꢁ
ꢁ
and thermo-oxidative stability with 5% weight loss temperature to be 707 C and >1000 C in nitrogen
ꢁ
and air, respectively. High char yields of 93.2% and 97.9% were obtained at 1000 C in nitrogen and air,
Keywords:
respectively. Additionally, PPB has proven to be excellent sing-source precursor to boron-carbide/carbon
Ni(0)-catalyzed polymerization
Poly(phenylene-carborane)
Boron-carbide
ceramic materials with high ceramic yields in the range of 91.1%e92.6%. According to XRD analysis, boron
ꢁ
carbide crystallization for PPB occurred between 1000 and 1200 C.
©
2017 Published by Elsevier Ltd.
1
. Introduction
ceramics also leads to more flexible shaping/forming processes
for manufacturing ceramics compared with conventional ceramics
fabricating processes. Fabrication of porous ceramics, ceramic fi-
bers, composites, and ceramic films has been reported [23,26e30].
Ni(0)-catalyzed polycondensation of difunctional monomers is a
very attractive approach toward high-performance polymers,
because this process leads to formation of aromatic carbon-carbon
bonds under mild conditions and tolerates a large variety of func-
tional groups [31]. A number of functionalized polyphenylenes
have been synthesized via nickel catalysis, such as poly(-
Since first reported at the end of 1963 in both the United States
and the USSR, carborane has received considerable attention for its
unique properties, such as high thermal and chemical stability,
three-dimensional structure, aromaticity and ease of functionali-
zation [1]. Among of them, the high thermal stability of carborane
has attracted the interest of polymer chemists seeking for high-
temperature applications such as ceramic precursors and
oxidation-resistant coatings [2e6]. Numerous polymers containing
carborane cage structures have been reported [7e17], and poly-
mers containing the carborane within the backbone exhibited high
ceramic yields [6,18,19]. Our current research is focused on the
synthesis and characterization of polymer materials containing
carborane for high-temperature polymers and polymeric pre-
cursors for ceramics. The polymer-derived ceramics method offers
many important advantages compared with the conventional
powder-based ceramic fabricating process. For example, the
structures, compositions, and properties of ceramics can be tailored
at atomic, molecular, and nanoscales by changing the chemistries of
precursors [20e25]. Additionally, this method of polymer-derived
benzophenone)s (PBPs) [32e35]. In this work,
a
novel
poly(phenylene-carborane) (PPB) with excellent thermal and
thermo-oxidative stability was synthesized via Ni(0)-catalyzed
polycondensation of 1,2-bis(4-chloro-phenyl)-carborane. PPB
proved to be excellent precursor to boron-carbide/carbon ceramic
materials with high ceramic yield. Malenfant et al. have presented a
powerful route for the preparation decaborane-containing poly-
mers, which have proven to be excellent ceramic precursors
[29,36,37]. Polymers reported by Malenfant's group were prepared
via ruthenium-catalyzed ring-opening metathesis polymerization
(ROMP). These polymers with high molecular weights and narrow
molecular weight distributions are capable of forming ordered
nanoscale structures via self-assembly, and self-assembly is a
promising approach for achieving controlled nanoscale architec-
tures in ceramics. However, monomers and polymers developed by
*
Corresponding author.
E-mail address: wangjinyan@dlut.edu.cn (J. Wang).
http://dx.doi.org/10.1016/j.polymer.2017.03.036
032-3861/© 2017 Published by Elsevier Ltd.
0

S. Cheng et al. / Polymer 115 (2017) 224e231
225
Malenfant's group contain decaborane groups rather than carbor-
ane groups, so the monomers and polymers reported by Mal-
enfant's group are not stable in air [37]. In this work, PPB with
excellent thermal and thermo-oxidative stability has proven to be
excellent precursor for boron-carbide/carbon ceramics with
ceramic yields in the range of 91.1e92.6%, and these ceramics were
well characterized by XRD, SEM, TEM, Raman and elemental
analysis.
acetonitrile (250 mL) were then added by syringe. The reaction
flask was covered in aluminum foil and was stirred at 60 C for 48 h.
The reaction mixture was poured into 500 mL of distilled water. The
precipitate was filtered and washed with distilled water. The pure
product was obtained by recrystallization from n-hexane in 79%
ꢁ
yield (15.1 g). Purity: 99.93% (HPLC). Found: C, 79.06%; H, 4.04%.
ꢂ1
Anal. Calcd for C14
3047.9 (Ar-H), 1495.2 (C¼C), 1090.9, 755.7, 686.7.
500 MHz, CDCl ): 7.54e7.49 (m, 2H), 7.47e7.42 (m, 2H),
7.36e7.32 (m, 3H), 7.33e7.29 (m, 2H). C NMR (126 MHz, CDCl
134.29 (s), 132.84 (s), 131.63 (s), 128.72 (s), 128.51 (s), 128.43 (s),
H
9
Cl: C, 79.07%; H, 4.26%. FT-IR (KBr, cm ):
1
H
NMR
(
3
d
13
2
. Experimental
3
):
d
2.1. Materials
122.97 (s), 121.82 (s), 90.36 (s), 88.28 (s).
B
10
H12(CH
3
CN)
2
was synthesized according to literature pro-
2.4. Synthesis of 1-(4-chlorophenyl)-2-phenyl-carborane
cedures [38]. Decaborane was purchased from Zhengzhou Sigma
Chemical Co.,Ltd. (China). Trimethylsilylacetylene, 1,8-diazabicyclo
A 1000 mL three-necked round bottom flask equipped with a
magnetic stirrer, a nitrogen inlet, and a reflux condensing tube was
charged with 12.8 g (60.0 mmol) of 1-chloro-4-(2-phenylethynyl)
[
5.4.0]undec-7-ene (DBU), copper(I) iodide (CuI), bis(-
triphenylphosphine)palladium(II) dichloride (PdCl (Ph ), bis(-
triphenylphosphine)nickel(II) dichloride (NiCl (Ph ), 1-
ethynylbenzene, 1-chloro-4-iodobenzene, and powdered zinc
99.99%) were purchased from J&K Chemical Co and used without
2
3 2
)
2
3
)
2
3 2
benzene, 14.6 g (72.0 mmol) of B10H12(CH CN) , and 400 mL of
toluene. The reaction mixture was magnetically stirred under ni-
ꢁ
ꢁ
(
trogen for 2 h at 100 C and then stirred for 12 h at 115 C. When the
reaction was completed, the reaction solution was cooled to room
temperature and 50 mL of methanol was added to resolve the
further purification. Triphenylphosphine and bipyridine were pur-
chased from aladdin and used as received. Acetonitrile, toluene,
and triethylamine were purchased from Tianjin Fuyu Fine Chemical
Co., Ltd. (China). Acetonitrile was distilled from phosphorus pent-
3 2
unreacted B10H12(CH CN) . The toluene was evaporated under
vacuum to afford crude product and the crude product was purified
by silica gel column chromatography using petroleum ether as
eluent. Petroleum ether was removed under reduced pressure to
give white powder (11.8 g, 59.4% yield). Purity: 99.83% (HPLC).
2
oxide prior to use. Triethylamine was distilled from CaH prior to
use. Toluene was distilled from sodium and benzophenone prior to
use. Standard narrow polystyrene for GPC calibration was pur-
chased from Agilent Technologies (Part No.: PL2010-0501).
Found: C, 50.63%; H, 5.77%. Anal. Calcd for C14
H
19
B
10Cl: C, 50.82%;
ꢂ1
H, 5.79%. FT-IR (KBr, cm ): 3059.7 (Ar-H), 2577.7 (B-H), 1590.1
1
2.2. Measurements
(C¼C), 1492.2 (C¼C), 1450.1, 1075.4, 836.2. H NMR (500 MHz,
CDCl
3
):
d
7.42 (d, J ¼ 8.2 Hz, 1H), 7.35 (d, J ¼ 8.8 Hz, 1H), 7.26 (d,
13
All NMR spectra were measured on a Bruker AVANCE Ⅲ 500
J ¼ 5.4 Hz,1H), 7.18 (d, J ¼ 7.7 Hz,1H), 7.11 (t, J ¼ 5.6 Hz,1H). C NMR
(126 MHz, CDCl ): 136.67 (s), 131.86 (s), 130.61 (s), 130.43 (s),
129.31 (s), 128.52 (s), 128.48 (s), 85.29 (s), 84.01 (s). MS (MALDI-
TOF) m/z: Mþ calculated for C14 10Cl 330.22; found 330.22.
spectrometer. High performance liquid chromatography (HPLC)
analysis was performed on an Alliance2695-2696 instrument.
Elemental analysis was performed on an elemental analysis Vario
EL series. Inductively coupled plasma-atomic emission spectrom-
etry (ICP) was performed on an Optima2000DV instrument. The FT-
IR spectra were obtained using a Thermo Nicolet Nexus 470 Fourier
transform infrared (FT-IR) spectrometer. MALDI-TOF-MS measure-
ment was performed on a Waters MALDI micro MX. The wide-angle
X-ray diffraction (WAXD) measurements were undertaken on a D/
Max 2400. The gel permeation chromatography (GPC) analysis was
3
d
19
H B
2.5. Synthesis of 1,2-bis(4-chlorophenyl)ethyne
To 500 mL flask, equipped with a magnetic stirrer and a rubber
septum, was charged with 1.26 g (1.80 mmol) of PdCl (PPh ) , 1.14 g
2 3 2
(5.85 mmol) of CuI, 42.9 g (180 mmol) of 1-chloro-4-iodobenzene.
The flask was purged with dry nitrogen. DBU (72.0 mL), thrime-
thylsilyethynylene (13.8 mL), distilled water (1.30 mL), and aceto-
nitrile (250 mL) were then added by syringe. The reaction flask was
carried on the Agilent PL-GPC 50 with two PL 7.5
mm MIXED-D
columns and one 5 m guard column. The system was calibrated
m
ꢁ
against standard narrow polystyrene and conducted by using THF
as eluent. Scanning electron microscopy (SEM) images were ob-
tained with a QUANTA 450. The samples were gold coated. A
working distance of approximately 5e5.5 mm and an accelerating
voltage of 30 kV were used. Transmission electron microscopic
covered in aluminum foil and was stirred at 60 C for 48 h. The
reaction mixture was poured into 500 mL of distilled water. The
precipitate was filtered and washed with distilled water. The pure
product was obtained by recrystallization from chloroform in 85%
yield (18.9 g). Purity: 99.63% (HPLC). Found: C, 67.9%; H, 3.09%. Anal.
ꢂ1
(
TEM) image was obtained with a Tecnai F30. Thermogravimetric
Calcd for C14
H
8
Cl
2
: C, 68.04%; H, 3.26%. FT-IR (KBr, cm ): 1908.5,
1
analysis (TGA) was performed on a Mettler TGA/SDTA851 ther-
mogravimetric analysis instrument in nitrogen or air atmosphere at
1492.5 (C¼C), 1396.3, 1088.9, 1011.2, 825.6, 656.9.
H
NMR
(500 MHz, CDCl ):
3
d
7.44 (d, J ¼ 8.6 Hz, 1H), 7.32 (d, J ¼ 8.6 Hz, 1H).
ꢁ
ꢂ1
ꢁ
13
a heating rate of 20 C min from 30 to 1000 C. The differential
scanning calorimetry (DSC) was measured on Mettler DSC822 DSC
C NMR (126 MHz, CDCl
3
): d 134.54 (s), 132.80 (s), 128.77 (s),
121.45 (s), 89.18 (s).
ꢂ1
ꢁ
C
under a nitrogen flow (50 mL min ) at a heating rate of 10
min from 25 C to 500 C.
ꢂ1
ꢁ
ꢁ
2.6. Synthesis of 1,2-bis(4-chlorophenyl)-carborane
2
.3. Synthesis of 1-chloro-4-(2-phenylethynyl)benzene
1,2-Bis(4-chlorophenyl)-carborane was prepared using the
similar procedure for preparation of 1-(4-chlorophenyl)-2-phenyl-
carborane except for using 1,2-bis(4-chlorophenyl)ethyne as re-
A 500 mL flask, equipped with a magnetic stirrer and a rubber
septum, was charged with 1.26 g (1.80 mmol) of PdCl (PPh , 1.14 g
5.85 mmol) of CuI, 21.5 g (90.0 mmol) of 1-chloro-4-iodobenzene.
2
3
)
2
agent (55% yield). Found: C, 45.69%; H, 4.99%. Anal. Calcd for
ꢂ1
(
C
14
H
18
B
10Cl
2
: C, 46.03%; H, 4.96%. FT-IR (KBr, cm ): 3080.6 (Ar-H),
1
The flask was purged with dry nitrogen. DBU (72.0 mL), 1-
ethynylbenzene (10.0 mL), distilled water (1.30 mL), and
2569.7 (B-H), 1597.3 (C¼C), 1493.2 (C¼C), 1402.6, 829.2. H NMR
(500 MHz, CDCl ):
3
d
7.35 (d, J ¼ 8.8 Hz, 4H), 7.15 (d, J ¼ 8.8 Hz, 4H).

226
S. Cheng et al. / Polymer 115 (2017) 224e231
13C NMR (126 MHz, CDCl
28.75 (s), 84.16 (s). MS (MALDI-TOF) m/z: Mþ calculated for
365.17; found 365.17.
): d 136.97 (s), 131.85 (s), 129.06 (s),
was subsequently placed into a boron nitride crucible and trans-
ferred into a tube furnace. The sample was then heated under argon
3
1
C
ꢁ
ꢁ
ꢁ
H
14 18
B
10Cl
2
to 1000 C at a heating rate of 5 C/min and held at 1000 C for 2 h.
The obtained sample was named as PPB-1000. PPB-1200, PPB-
1400, and PPB-1500 were prepared using similar procedure and
2.7. Synthesis of model compound MCPB
ꢁ
pyrolyzed at 1200, 1400, and 1500 C for 2 h, respectively. The
A 25 mL Schlenk flask equipped with a magnetic stirrer was
element composition and ceramic yields of the resulting ceramics
are summarized in Table 5. Ni content by ICP analysis: PPB-1000,
charged with 0.065 g (0.10 mmol, 0.1 equiv) of NiCl
2
(Ph
3 2
) , 0.052 g
ꢂ2
ꢂ2
ꢂ2
(
0
0
0.20 mmol, 0.2 equiv) of triphenylphosphine, 0.016 g (0.10 mmol,
.1 equiv) of bipyridine, 0.20 g (3.1 mmol, 3.1 equiv) of zinc, and
.030 g (0.20 mmol, 0.2 equiv) of sodium iodide. The flask was
1.93 ꢀ 10 %; PPB-1200, 1.87 ꢀ 10 %; PPB-1400, 1.90 ꢀ 10 %;
ꢂ2
PPB-1500, 1.78 ꢀ 10 %.
sealed with a septum, evacuated and refilled with nitrogen three
3
. Results and discussion
times, and charged with 1 mL of THF via syringe. The catalyst
ꢁ
mixture was stirred and heated in an oil bath at 60 C for several
3.1. Preparation and characterization of monomers and polymers
minutes. Once the color of the catalyst mixture changed from deep
green to dark red, 0.33 g (1.0 mmol) of 1-(4-chlorophenyl)-2-
phenyl-carborane was added under nitrogen flow. The reaction
The synthetic route of model monomer and bischloride mono-
ꢁ
mer is illustrated in Scheme 1. 1-Chloro-4-(2-phenylethynyl)ben-
zene and 1,2-bis(4-chlorophenyl)ethyne were synthesized via
Sonogashira coupling reaction. The reaction proceeded smoothly
and gave the target monomers in high yields. 1-(4-Chlorophenyl)-
continued at 60 C for 12 h. The reaction mixture was then
precipitated into 10% hydrochloric acid to remove excess zinc and
extracted with chloroform. Then the organic layer was washed with
brine and dried over anhydrous sodium sulfate overnight, followed
by vacuum evaporation. The crude product was finally passed
through silica gel column chromatography using petroleum ether
as an eluent. Petroleum ether was removed under reduced pressure
2-phenyl-carborane and 1,2-bis(4-chlorophenyl)-carborane were
synthesized using a similar procedure according to a previous
literature [6]. The chemical structures of 1-chloro-4-(2-
phenylethynyl)benzene, 1-(4-chlorophenyl)-2-phenyl-carborane,
1,2-bis(4-chlorophenyl)ethyne, and 1,2-bis(4-chlorophenyl)-car-
borane were confirmed by FT-IR, H-NMR, C-NMR, and elemental
analysis (The FT-IR, H-NMR, and
chlorophenyl)-2-phenyl-carborane were listed in the supporting
information). The H-NMR and C-NMR spectra of 1,2-bis(4-
chlorophenyl)-carborane were showed in Fig. 1. The broad peak at
to give needle-crystal (92% yield). Found: C, 56.97%; H, 6.49%. Anal.
ꢂ1
Calcd for C28
(
NMR (500 MHz, THF):
H
38
B
20: C, 56.92%; H, 6.48%. FT-IR (KBr, cm ): 3062.6
1
13
1
Ar-H), 2571.9 (B-H), 1605.3 (C¼C), 1495.2 (C¼C), 1444.6, 688.6. H
1
13
C-NMR spectra of 1-(4-
d
7.53 (dd, J ¼ 13.4, 5.0 Hz, 4H), 7.39 (d,
13
J ¼ 8.6 Hz, 2H), 7.23 (t, J ¼ 7.3 Hz,1H), 7.16 (t, J ¼ 7.7 Hz, 2H). C NMR
1
13
(
(
126 MHz, THF):
s), 128.37 (s), 126.66 (s), 85.32 (s), 84.65 (s). MS (MALDI-TOF) m/z:
20 590.49; found 590.49.
d 140.77 (s), 131.11 (s), 130.72e130.39 (m), 130.26
d
1.5e3.5 ppm was assigned to the protons of o-carborane cage [39].
Mþ calculated for C28
38
H B
All of the protons and carbons in 1-(4-chlorophenyl)-2-phenyl-
carborane and 1,2-bis(4-chlorophenyl)-carborane were detected as
expected, confirming that 1-(4-chlorophenyl)-2-phenyl-carborane
and 1,2-bis(4-chlorophenyl)-carborane were successfully synthe-
sized. In the FT-IR spectra of 1-(4-chlorophenyl)-2-phenyl-carbor-
ane and 1,2-bis(4-chlorophenyl)-carborane (see supporting
2.8. Synthesis of PPB
A 25 mL Schlenk flask equipped with a magnetic stirrer was
charged with 0.26 g (0.40 mmol, 0.1 equiv) of NiCl (Ph , 0.21 g
0.80 mmol, 0.2 equiv) of triphenylphosphine, 0.062 g (0.40 mmol,
2
3 2
)
(
ꢂ1
information), the absorption peak at 2596 cm was attributed to
the stretching vibration of B-H in carborane cage.
0
0
.1 equiv) of bipyridine, 0.81 g (12 mmol, 3.1 equiv) of zinc, and
.12 g (0.80 mmol, 0.2 equiv) of sodium iodide. The flask was sealed
Ni(0)-catalyzed coupling reaction of aryl chlorides is an effective
approach for the formation of aromatic carbon-carbon bonds. Many
studies on the mechanism of this reaction have resulted in a better
understanding of the role of reactant structure, ligands, tempera-
ture, and reducing metal on the reaction. The presence of electron-
withdrawing groups ortho or para to the reactive site accelerates
the reaction rate by activating that site to oxidative addition by the
with a septum, evacuated and refilled with nitrogen three times,
and charged with 4 mL of THF via syringe. The catalyst mixture was
ꢁ
stirred and heated in an oil bath at 60 C for several minutes. Once
the color of the catalyst mixture changed from deep green to dark
red, 1.5 g (4.0 mmol) of 1,2-bis(4-chlorophenyl)-carborane was
ꢁ
added under nitrogen flow. The reaction continued at 60 C for 12 h.
The reaction mixture was then precipitated into 200 mL of 40% HCl/
methanol to remove excess zinc and stirred overnight. The pre-
cipitate was collected by suction filtration, washed with saturated
sodium bicarbonate and methanol, redissolved in THF, and repre-
cipitated into methanol. The polymer was extracted with methanol
for 24 h and dried in a vacuum oven to give 1.0 g of white powder
(
85% yield). Found: C, 57.12%; H, 5.85%. Anal. Calcd for (C14
H
18
B
10
)
n
:
ꢂ1
C, 57.12%; H, 6.16%. FT-IR (KBr, cm ): 3043.6 (Ar-H), 2593.9 (B-H),
1
(
1
607.9 (C¼C), 1496.2 (C¼C), 1396.6, 1005.9, 831.6.
H NMR
500 MHz, THF): 5.90e5.74 (d, 37H), 5.73e5.57 (d, 37H), 5.47 (t,
d
13
J ¼ 9.3 Hz, 1H), 5.41 (t, J ¼ 7.6 Hz, 2H). C NMR (126 MHz, THF):
d
8
138.80 (s), 129.32 (s), 128.64 (d, J ¼ 42.1 Hz), 126.42 (s), 124.72 (s),
ꢂ2
3.34 (s). Ni content by ICP analysis: 2.32 ꢀ 10 %.
2.9. Preparation of ceramic
In a typical pyrolysis of PPB, a sample specimen disks (0.6e0.8 g)
of PPB with a diameter of 2.54 cm was prepared by press-molding
PPB powder at room temperature. The compacted polymer disk
Scheme 1. Synthetic route of model monomer and bischloride monomer.

S. Cheng et al. / Polymer 115 (2017) 224e231
227
Table 1
a
The reaction condition of model study.
ꢁ
Entry Temperature ( C) Time (h) Solvent Halide (equiv) HPLC yield (%)
1
2
3
60
60
60
12
12
12
THF
THF
THF
e
94.5
97.1
100.0
NaBr (0.2)
NaI (0.2)
a
2 3 2
Reaction conditions: NiCl (Ph ) (0.1 equiv), triphenylphosphine (0.2 equiv),
bipyridine (0.1 equiv), zinc (3.1 equiv). Equivalents were given with respect to the
model monomer, 1-(4-chlorophenyl)-2-phenyl-carborane.
bipyridine as ligands. THF was chosen as the solvent because THF
has been shown to minimize reduction and other side reactions
[
(
33]. Additionally, polar aprotic solvent such as dimethylacetamide
DMAC) and dimethylformamide (DMF) can lead to cage degrada-
tion reaction of o-carborane [43e45]. Without addition of sodium
halide (Table 1 entry 1), the MCPB was obtained in a high yield of
94.5% according to high performance liquid chromatography
analysis (HPLC), suggesting that 1,2-bis(4-chlorophenyl)-carborane
could be used for Ni(0)-catalyzed polycondensation. The polymer-
ization of 1,2-bis(4-chlorophenyl)-carborane was performed under
this model reaction condition. However, only low molecular weight
oligomer was obtained, and number-average molecular weight of
3
the oligomer was determined to be 1.0 ꢀ 10 according to gel
permeation chromatography analysis (GPC). According to the pre-
vious reports, halide ions, especially iodide, enhance the reaction
rate of Ni(0)-catalyzed homocoupling reactions [40,46]. In our
model study, the addition of sodium halide indeed accelerated the
reaction rate of Ni(0)-catalyzed homocoupling reaction, and the
model reaction with addition of 0.2 equiv sodium iodide gave an
excellent yield of 100%. High molecular weight polymer was pre-
pared under the optimized reaction conditions, and the number-
average molecular weight of the obtained polymer was deter-
3
mined to be 12.4 ꢀ 10 by GPC (Table 2).
MCPB and PPB were characterized by 1H-NMR, 13C-NMR, 11B-
NMR, and FT-IR spectroscopies as well as elemental analysis. As
shown in Fig. 2, FT-IR spectra of MCPB and PPB were very similar.
Fig. 1. 1H-NMR and 13C-NMR spectra of 1,2-bis(4-chlorophenyl)-carborane in CDCl
.
3
ꢂ1
Absorption band at 2596 cm in FT-IR spectra of MCPB and PPB
Ni(0) complex, and thus aryl chlorides with electron-withdrawing
substituents show higher reactivity and give higher yields of
coupling products [33,40]. The electron-withdrawing property of o-
carborane cage [41,42] should increase the Ni(0)-catalyzed reac-
tivity of 1,2-bis(4-chlorophenyl)-carborane, and the polymerization
of 1,2-bis(4-chlorophenyl)-carborane via Ni(0)-catalyzed coupling
reaction may lead to high molecular weight polymers. In order to
verify the reactivity of 1,2-bis(4-chlorophenyl)-carborane, a model
reaction of 1-(4-chlorophenyl)-2-phenyl-carborane was conducted
was ascribed to the stretching vibration of B-H in carborane cage
1
[
47e49]. H-NMR spectra of MCPB and PPB in THF-d and their as-
1
signments were shown in Fig. 3. As shown in Figure 3, H-NMR
spectrum of PPB showed two doublets and two smaller triplets.
These two doublets at
hydrogen atoms in benzene ring of the polymer chain. While the
two smaller triplets at 7.1 and 7.2 ppm were attributed to the
d 7.54 and 7.36 ppm corresponded to the two
d
hydrogen atoms in benzene of the polymer end groups and were
used to calculate the degree of polymerization. It's worth
mentioning that all the end groups of the polymer chain were
(Scheme 2), and the reaction condition was listed in Table 1. The
model reactions were performed in THF using NiCl (Ph as a
2
3 2
)
1
phenyl groups rather than p-chlorophenyl groups based on H-
catalyst, Zn as a reducing agent, and triphenylphosphine and
NMR study of PPB, which was attributed to the reduction reaction
of arylnickel intermediates with protic sources during the reaction
post-processing [40]. The degree of polymerization (DP) calculated
1
from end group analysis by H-NMR was 18.3, and thus the
1
number-average molecular weight of PPB calculated from H-NMR
3
was 5.4 ꢀ 10 g/mol. While number-average molecular weight
3
determined by GPC was 12.4 ꢀ 10 , this difference was attributed to
13
the different hydrodynamic volumes of PPB and polystyrene. C-
NMR spectra of MCPB and PPB were shown in Fig. 4. All carbon
atoms were well assigned as expected, confirming that polymeri-
zation of 1,2-bis(4-chlorophenyl)-carborane was successfully pro-
ceed. Aromatic carbon atoms of the polymer end groups were well
13
detected in the C-NMR spectrum, indicating that number-average
molecular weight of PPB could not be too high, which was consis-
1
11
tent with H-NMR analysis. B-NMR spectra of PPB and MCPB
11
exhibited two absorption peaks, as shown in Fig S4. The B signals
of the carborane in PPB were significantly broadened as compared
Scheme 2. Synthesis of MCPB and polymerization of 1,2-bis(4-chlorophenyl)-
carborane.

228
S. Cheng et al. / Polymer 115 (2017) 224e231
Table 2
a
Molecular weight and Thermal data for PPB.
( ꢀ 10ꢂ3
)
M
( ꢀ 10ꢂ3
)
PDI
2.5
T
( C)
ꢁ
T
( C)
ꢁ
b
N
T
10 ( C)c
ꢁ
N
/air
Polymer
M
n
w
g
5
2
/air
2
PPB
12.4
30.7
>450
706/>1000
>1000/>1000
a
ꢁ
3 2
(Ph ) (0.1 equiv), triphenylphosphine (0.2 equiv), bipyridine (0.1 equiv), zinc (3.1 equiv), NaI (0.2 equiv), 60 C, 12 h. Equivalents were
Polymerization conditions: NiCl
2
given with respect to the bischloride monomer, 1,2-bis(4-chlorophenyl)-carborane.
b
5
1
% weight loss temperatures determined in nitrogen and air, respectively.
0% weight loss temperatures determined in nitrogen and air, respectively.
c
Fig. 4. 13C-NMR spectra of MCPB and PPB in THF-d.
Fig. 2. FT-IR spectra of MCPB and PPB.
Fig. 3. 1H-NMR spectra of MCPB and PPB in THF-d.
Fig. 5. Wide angle X-ray diffraction pattern for PPB.
to the signals of the MCPB, suggesting that the mobility of the
carborane groups in PPB was greatly restricted by polymer chain
compared with MCPB.
Table 3
The solubility of PPB.
a
Solvent
CHCl
3
THF
þ
DMAC
e
DMSO
e
DMF
e
NMP
þ
During the polymerization of 1,2-bis(4-chlorophenyl)-carbor-
ane, PPB precipitated from the polymerization system, which pre-
vented the further increase of molecular weight of PPB. The
precipitated polymer could redissolve in NMP and excess THF,
indicating that precipitation of the polymer was not caused by
cross-linking reaction. This phenomenon was attributed to the
crystalline character of PPB, which was verified by XRD analysis. As
Solubility
e
a
Key: þ, soluble at room temperature (0.05 g of PPB in 1 mL of solvent), -,
insoluble.
3.2. Thermal analysis of PPB
ꢁ
ꢁ
shown in Fig. 5, four sharp peaks were observed at 2
2
q
¼ 14.8 , 16.7 ,
The differential scanning calorimetry (DSC) curve of PPB (Fig S5)
was featureless line without any sign of glass transition in the
temperature range of 25e450 C, while the exothermic peak of
ꢁ
ꢁ
1.6 , and 26.5 , revealing its crystalline character. Solubility of PPB
is summarized in Table 3 and PPB can be well dissolved in NMP at
room temperature.
ꢁ

S. Cheng et al. / Polymer 115 (2017) 224e231
229
decomposition reaction of PPB began to appear when the tem-
perature was above 450 C. In consideration of the high rigidity of
3.3. Ceramic conversion reactions of PPB
ꢁ
the polymer chain, it was reasonable that the glass transition
Pyrolysis was performed in a tube furnace under an argon at-
mosphere at 1000, 1200, 1400, and 1500 C for 2 h to afford PPB-
ꢁ
ꢁ
temperature (T
g
) of PPB was above 450 C. DSC curve of PPB showed
ꢁ
no sign of endothermic melting peak up to 500 C. Since the
melting point of the MCPB was 281.8 C (measured by DSC), a
crystalline melting point for PPB of >500 C seemed reasonable.
1000, PPB-1200, PPB-1400, and PPB-1500, respectively. The ce-
ramics after pyrolysis were compact black disks, and the black color
indicated that the ceramics contained excess carbon. SEM images
(Fig S7) of the ceramic samples showed that the matrix of the ce-
ramics was compact and with some microscale cracks. Elemental
analysis data and ceramic yields were summarized in Table 5.
Ceramic yields of PPB were in the range of 91.1e92.6%, which were
higher than that reported in the previous studies [29,36,58]. The
elemental analysis showed that all ceramics had B:C ratios lower
ꢁ
ꢁ
The thermal stability of PPB was evaluated by TGA under ni-
trogen and air atmosphere, and the thermograms were shown in
Fig. 6. The weight loss temperatures of 5% and 10% (T
5
and T10) and
ꢁ
char yields at 1000 C for PPB were summarized in Table 4. As
shown in Fig. 6, PPB showed a two-step weight loss pattern
ꢁ
ꢁ
beginning at about 400 C and 600 C, and char yield of PPB
ꢁ
remained constant over the temperature range of 850e1000 C. A
4
than the 4:1 ration of B C, suggesting the presence of excess carbon.
Additionally, the presence of the free carbon was also confirmed by
ꢁ
high char yield of 93.2% at 1000 C in nitrogen was finally obtained.
DTG curve analysis was used to provide more information on the
degradation process. As shown in Fig S6, the thermal degradation
process of PPB in nitrogen atmosphere could be divided into two
degradation stages. Assuming that hydrogen atoms in benzene
were lost at first degradation stage and hydrogen atoms in car-
borane were lost at second degradation stage, the theoretical
weight loss at first degradation stage and second degradation stage
was 2.8% and 3.4%, respectively. In fact, weight loss of the two
stages was 3.3% and 3.5% according to TGA analysis, which was
agreed with the theoretical weight loss. So the peak of weight loss
Raman analysis. As shown in Fig S8, the absorption bands at 1344
and 1594 cm were attributed to graphite in ceramics [19]. In
ꢂ1
consideration of the low content of hydrogen (<0.3%) in the ce-
ramics, assuming that all of the hydrogen was lost and all the car-
bon was retained during the ceramic conversion, and then PPB
would convert to boron-carbide ceramics according to the
following equation to give a theoretical ceramic yield of 93.8%.
ꢁ
Ceramic yield (Fig. 6) after TGA study up to 1000 C in nitrogen was
93.2% and was very close to theoretical ceramic yield as shown in
the following equation. In addition, the elemental composition of
PPB-1000 was very close to the theoretical elemental composition,
ꢁ
at 536 C in the first thermal degradation stage could be attributed
to the loss of hydrogen in benzene, while the peak of weight loss at
which were B
As shown in Fig. 7, the TEM micrograph of PPB-1500 reveals
lattice-plane structures of B C and graphite nanocrystals in matrix
4 4
C$C4.73 and B C$C4.6, respectively.
ꢁ
6
78 C in the second thermal degradation stage could be attributed
to the loss of hydrogen in carborane. Additionally, o-carborane
4
undergoes an irreversible thermal reaction resulting in the for-
of the ceramic sample. The measured d-spacings of about 0.24, 0.38,
and 0.35 nm can be assigned to the (021) plane and (012) plane in
B C and (002) plane in graphite, respectively [59]. As shown in
4
ꢁ
mation of the m-carborane on heating at 470e600 C in an inert
ꢁ
atmosphere, and further heating to about 650e700 C in inert at-
mosphere results in forming p-carborane [50e53]. So it is reason-
able to attribute the second degradation stage to the loss of
hydrogen in carborane. In contrast with TGA curve in nitrogen, TGA
curve of PPB in air showed a slight weight gain from 500 C, which
was attributed to oxidation of boron atoms in the carborane upon
exposure to oxidizing atmospheres at elevated temperatures
Fig. 8, XRD study of the bulk-ceramic residues indicated that boron
carbide crystallization for PPB began between 1000 and 1200 C,
ꢁ
which was consistent with the previous study [58]. The boron
carbide diffraction peaks began to appear in PPB-1200 and the
diffraction intensity increased as increased pyrolysis temperature,
indicating that high temperature promoted crystallization of boron
carbide. The size of the boron carbide nanocrystallite was calcu-
lated to be 20.33 nm for PPB-1500 by the Scherrer equation using
the full-width at half-maximum (fwhm) value of diffraction peak at
ꢁ
[54e57]. The decomposition reaction was predominant at elevated
temperature, so PPB began to lose weight when the temperature
ꢁ
was above 610 C under air atmosphere. PPB showed excellent
ꢁ
ꢁ
ꢁ
ꢁ
thermal and thermo-oxidative stability with T
5
at 707 C and
2q
¼ 37.5 . The diffraction peaks located at 2
q
¼ 26 and 42 in XRD
ꢁ
>
1000 C in nitrogen and air, respectively.
pattern of the ceramics were attributed to graphite [19,58].
In order to investigate the thermo-oxidative stability of the ce-
ramics, TGA study of the ceramics was performed on pieces of
ceramic sample in air. As shown in Fig. 9, the ceramics derived from
PPB showed excellent thermo-oxidative stability with char yields
ꢁ
range from 100.8% to 101.6% at 1000 C in air, and the weight gain of
the ceramic samples at 600 C in the TGA curve was attributed to
ꢁ
oxidation of boron in ceramics [54,56].
4. Conclusions
Under the
optimized
polymerization
condition,
a
poly(phenylene-carborane) (PPB) with moderate number-average
molecular weight was successfully synthesized via Ni(0)-
catalyzed polymerization of 1,2-bis(4-chloro-phenyl)-carborane
1
13
11
and well characterized by H-NMR, C-NMR, B-NMR, FT-IR, and
GPC. The number-average molecular weight of the obtained poly-
3
1
mer was determined to be 5.4 ꢀ 10 g/mol by H-NMR spectros-
copy analysis. The crystalline character of PPB was verified by XRD,
which prevented the further increase of molecular weight of PPB
during the polymerization. The glass transition temperature and
melting point of PPB were all above its decomposition temperature
ꢁ
Fig. 6. TGA curves of PPB in nitrogen and air.
(450 C) due to the high rigidity of the polymer chain. Based on TGA

230
S. Cheng et al. / Polymer 115 (2017) 224e231
Table 4
a
TGA data of PPB in air and nitrogen.
ꢁ
ꢁ
ꢁ
ꢁ
Polymer
T
5
/ C (N
2
)
T
10/ C (N
2
)
Char yield/% (N
93.2
2
)
T
5
/ C (air)
T
10/ C (air)
Char yield/% (air)
97.9
PPB
707
>1000
>1000
>1000
a
5
T : temperature for 5% weight loss; T10: temperature for 10% weight loss.
Table 5
a
Ceramic data of PPB derived ceramics.
Sample
Ceramic yield (%)
B (%)
C (%)
H (%)
composition
PPB-1000
PPB-1200
PPB-1400
PPB-1500
92.6
92.3
92.2
91.1
40.97
43.33
43.15
41.55
58.74
56.46
56.68
58.31
0.29
0.21
0.17
0.14
4
B C$C4.73
4
B C$C4.21
4
B C$C4.25
4
B C$C4.61
a
Boron content of the ceramics was calculated from contents of C and H (100%-C
%
-H%).
Fig. 9. TGA curves of the ceramics derived from PPB in air.
proven to be excellent sing-source precursors to boron-carbide/
carbon ceramic materials with high ceramic yield. Boron carbide
ꢁ
crystallization for PPB began between 1000 and 1200 C according
to XRD analysis. Boron carbide as well as graphite crystallization in
the ceramics was confirmed by TEM, XRD, and Raman analysis.
Acknowledgements
This work was supported by the National High Technology
Research; and Development Program (**863** Program) of China
Fig. 7. TEM image of PPB-1500.
(
(
No. 2015AA033802); and the National Science Foundation of China
21074017 and 51273029).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2017.03.036.
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