A straightforward protocol for the highly efficient preparation of main-chain azo polymers directly from bisnitroaromatic compounds by the photocatalytic process

Article abstract of DOI:10.1021/acs.macromol.5b00048

A novel strategy for the synthesis of main-chain Azo polymers directly from bisnitroaromatic compounds by the photocatalytic process has been achieved under mild conditions. This approach avoids the tedious synthesis of Azo monomers and proceeds with a hi

Full text of DOI:10.1021/acs.macromol.5b00048

Article
pubs.acs.org/Macromolecules
A Straightforward Protocol for the Highly Efficient Preparation of
Main-Chain Azo Polymers Directly from Bisnitroaromatic
Compounds by the Photocatalytic Process
Laibing Wang, Xiangqiang Pan, Yin Zhao, Yang Chen, Wei Zhang,* Yingfeng Tu, Zhengbiao Zhang,
Jian Zhu, Nianchen Zhou, and Xiulin Zhu
Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu Key Laboratory of Advanced Functional Polymer
Design and Application, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou Industrial
Park, Suzhou 215123, China
S
* Supporting Information
ABSTRACT: A novel strategy for the synthesis of main-chain Azo polymers
directly from bisnitroaromatic compounds by the photocatalytic process has
been achieved under mild conditions. This approach avoids the tedious
synthesis of Azo monomers and proceeds with a high monomer conversion
(∼100%) and excellent selectivity (∼100%) but without generating a
significant amount of inorganic wastes and impurities (Cu2+ or azoxy groups)
existing in main-chain Azo polymers compared to previous methods. The
polymerization was monitored by UV−vis, FT-IR, and MALDI-TOF-MS,
indicating that the reaction process proceeded with formation of the azoxy
repeating units from the corresponding bisnitroaromatic monomers and the
reduction of corresponding azoxy repeating units to the azo repeating units.
Furthermore, the recyclable heterogeneous photocatalysts represent an
attractive green process for production of main-chain Azo polymers.
Scheme 1. Different Strategies for the Synthesis of Azo
Polymers
INTRODUCTION
■
Over the past few decades, polymers containing aromatic Azo
and its derivatives (Azo polymers) have drawn considerable
attention owing to their outstanding mechanical, unique
physicochemical properties, and the well-known reversible
trans−cis photoisomerization of the Azo group.1,2 As compared
with side-chain Azo polymers, the main-chain Azo polymers
show much more attractive thermal stability and optoelectronic
properties, such as good conductivities (up to 10−4 Ω−1 cm−1),
liquid crystallinity, and nonlinear optical properties.3,4 How-
ever, relatively few studies have been done in this area, partly
due to the lack of a straightforward method to prepare main-
chain Azo polymers under mild conditions.5 Usually, the main-
chain Azo polymers can be prepared by several methods,
including polycondensation of Azo monomers, coupling
polymerization of primary aromatic diamines, and Azo coupling
reaction of monomers with diazonium salts and electron-rich
aromatic units as depicted in Scheme 1. The first method,
including amidation,6 esterification,7 Suzuki−Miyaura cou-
pling,8 click chemistry,9 and other coupling reactions,10 has
been reported to prepare main-chain Azo polymers, however,
needing sophisticated design and elaborate synthesis of Azo
monomers prior to polymerization. The second method, the
oxidative coupling polymerization of aromatic diamines, first
independently reported by Kotlyarevskii et al. and Bach,
commonly employs a copper−amine complex as catalyst.11,12
The obtained main-chain Azo polymers from this strategy
contain a substantial amount of copper (Cu2+) (about to one
copper atom per 3.5 repeating units), which results from the
Received: January 8, 2015
Revised: February 10, 2015
© XXXX American Chemical Society
A
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co-occurrence of a symmetric aromatic hydroxylation side
reaction. Furthermore, azoxy groups are also found as the other
impurity existed in the final polymers. The third method, Azo
coupling reaction, was first employed to functionalize vinyl
polymers in acetic acid medium by Katz and co-workers.13
Subsequently Wang et al. effectively expanded this method to
prepare linear, hyperbranched, and block Azo polymers in
organic solvent.14 Nevertheless, this method is confined to
electron-rich aromatic compounds and/or stoichiometric
amounts of bisdiazonium salts and polyphenols and generates
a significant amount of inorganic wastes. Moreover, most of
nitrogen sources in bisdiazonium salts, Azo monomers, and
aromatic diamines are from aromatic nitro compounds, which
are much stabler than bisdiazonium salts and aromatic
diamines. Therefore, developing an efficient method for the
preparation of main-chain Azo polymers directly from
bisnitroaromatic monomers will be of scientific and practical
interest.
and selectivity of recovered catalysts, combined with the
utilization of solar energy under mild conditions, will pave a
novel avenue for the preparation of main-chain Azo polymer
materials.
EXPERIMENTAL SECTION
■
Materials. Bromohexane (98%, J&K Chemical), 1-bromododecane
(98%, J&K Chemical), 1,12-dibromododecane (98%, J&K Chemical),
ethyl formate (98%, J&K Chemical), dodecylmagnesium bromide (1.0
M solution in diethyl ether, J&K Chemical), chloroauric acid (≥98%,
Aldrich), carbazole (90%, Aldrich), zirconium dioxide (99.9%, Beijing
Dk Nano Technology Co., Ltd.), 9H-fluorene (98%, Acros), and 2,7-
dinitro-9,9-dioctyl-9H-fluorene (monomer A) (Longrepharm Labo-
ratories (China), Ltd.) were used as received without further
purification. All solvents were dried with 4 Å molecular sieves before
use. Other chemicals were purchased from Shanghai Chemical
Reagent Co. Ltd., Shanghai, China, and used without further
purification except mentioned specifically. The synthesis and
characterization of nitroaromatic compounds and bisnitroaromatic
monomers are described in the Supporting Information.
Basically, developing an efficient approach for synthesis of
functional small Azo compounds, prior to preparing the
corresponding Azo polymers, is of prime importance.
Compared to other methods, such as Azo coupling reaction,15
Mills reaction,16 oxidation of anilines, and so on,17,18 reductive
coupling of aromatic nitro derivatives showed many advantages
for synthesizing Azo compounds, including wider availability,
enhanced stability of substrates, higher tolerance of various
functionalities, one-pot procedure, and so on.1f,i As matters
stand, a myriad of reducing agents are innovated and developed
for producing Azo compounds directly from nitroarenes, which
included LiAlH4,19 NaBH4,20 Mg/HCO2HNEt3,21 In(OTf)3/
Et3SiH,22 Pd/KOH,23 etc. The major limitations of these
procedures, however, are excessive use of reducing agent, lack
of selectivity, and forming environmentally unfriendly by-
products.1f Moreover, some methods suffer from difficulty
avoiding the existence of the azoxybenzene, which becomes the
major obstacle in preparing main-chain Azo polymers with high
purity.24 Thus, developing the efficient approach along with
high selectivity and effective conversion for preparing Azo
monomers/polymers is urgently needed. Recently, supported
gold nanoparticles (AuNPs) were reported by facilitating target
specific synthesis of Azo compounds with high yields and good
selectivity directly from nitroarenes.25 Up to now, AuNPs with
different supports, such as TiO2, CeO2, ZrO2, and Mg4Al-HT,
have been successfully reported as heterogeneous catalysis to
prepare Azo compounds from nitroarenes.26 More recently,
Zhu and co-workers prepared ZrO2-supported AuNPs by a
photocatalytic process to synthesize Azo compounds directly
from nitroarenes in one step.27 This unique methodology
prevents the formation of harmful byproducts from reducing
agents, meanwhile avoiding unstable intermediates and use of
higher temperature and pressure. If direct reduction of
bisnitroaromatic compounds can be successfully conducted by
above strategy, the first preparation of main-chain Azo
polymers would be achieved in a much more controlled,
simplified, and greener way (Scheme 1).
Characterization. 1H and 13C nuclear magnetic resonance (NMR)
spectra were recorded on a Bruker nuclear magnetic resonance
instrument (300 MHz) using deuterated chloroform (CDCl3) as the
solvent and tetramethylsilane (TMS) as the internal standard at room
temperature. The number-average molecular weight (Mn) and
polydispersity (PDI = Mw/Mn) of polymers were determined by a
HLC-8320 size exclusion chromatograph (SEC, TOSOH) equipped
with a refractive index detector, using two TSKgel Super Mutipore
HZ-N columns (4.6 × 150 mm, 3 μm particle size) in series with
molecular weights ranging from 7 × 102 to 2 × 105 g/mol. THF was
used as the eluent at a flow rate of 0.35 mL/min and 40 °C. These
samples were calibrated with PS standards purchased from TOSOH.
Ultraviolet−visible (UV−vis) absorption spectra of the samples were
recorded on a Shimadzu UV-2600 spectrophotometer at room
temperature. FT-IR spectra were recorded on a Bruker TENSOR-27
FT-IR spectrometer using KBr pellets. Atomic absorption spectrom-
etry was detected by a Varian Spectra 220FS atomic absorption
spectrometer. Elemental analysis of C, H, and N was measured with an
EA1110 CHNO-S instrument. Matrix-assisted laser desorption/
ionization time-of-flight mass spectrometry (MALDI-TOF-MS)
measurement was performed using a Bruker UltrafleXtreme
(MALDI-TOF/TOF) mass spectrometer equipped with a 355 nm
Nd:YAG laser. Both matrix (2-[(2E)-3-(4-tert-butylphenyl)-2-methyl-
prop-2-enylidene]malononitrile (DCTB)) (at a concentration of ca.
20 mg/mL) and all samples were dissolved in chloroform (at a
concentration of ca. 10 mg/mL). The cationizing agent sodium
trifluoroacetate was dissolved in ethanol (at a concentration of ca. 10
mg/mL). The matrix and cationizing salt solutions were mixed in a
ratio of 10/1 (v/v). This mixture solution (0.5 μL) was placed on a
metal sample plate. The sample was air-dried at ambient temperature.
Transmission electron microscopy (TEM) was recorded on HITACHI
HT7700 employing an accelerating voltage of 120 kV. The purity of
the products was determined on a high-performance liquid chromato-
graph (HPLC, mode 515, Waters) using acetonitrile as the eluent at
30 °C. The light intensity was measured by UV radiometer fabricated
by a photoelectric instrument factory of Beijing Normal University.
Preparation of Main-Chain Azo Polymers. To a mixture of
monomer A (80 mg), potassium hydroxide (5.6 mg), 3 mL of toluene,
and 3 mL of isopropyl alcohol was added catalyst (33 mg) under room
temperature. The solution was bubbled with argon for 10 min, and
then the ampule was flame-sealed and magnetically stirred under the
illumination of a Xe lamp (3.1 mW/cm2) at 45 °C for 12 h. The
ampule was opened, and the resulting mixture was precipitated into
250 mL of methanol. The polymer (PA) was obtained by filtration and
dried under vacuum at 40 °C to constant weight with a yield of 96%.
The Mn and Mw/Mn values were determined to be 28 000 g/mol and
2.63, respectively. PBs and PCs were prepared with the similar
procedures.
In this work, we developed a novel approach for preparing
the main-chain Azo polymers directly from bisnitroaromatic
compounds photocatalyzed by ZrO2-supported AuNPs under
mild conditions. With high monomer conversion, this novel
strategy produces relatively high molecular weight (Mn up to
29 800 g/mol) Azo polymers with good selectivity. Moreover,
the recovered photocatalysts still exhibited highly catalytic
efficiency after twice reutilization. The retention of the activity
B
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RESULTS AND DISCUSSION
Table 1. Photocatalyzed Reduction of Nitroaromatic
Compounds in Different Experimental Conditions
■
a
The photocatalysts, ZrO2-supported AuNPs developed by Zhu
et al., presented a high conversion and high selectivity for the
reduction of nitrobenzene to azobenzene under irradiation with
b
entry
IPA/toluene (mL/mL)
t (h)
conv (%)
yield (%)
1
2
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
2
8
8
2
4
6
8
8
100
100
100
48
10
>98
>98
1
above 400 nm light.21 The above excellent performance of
b
ZrO2-supported AuNPs motivated us to test the possibility of
preparing main-chain Azo polymers, directly from the reduction
of bisnitroaromatic compounds. The ZrO2-supported AuNPs,
with 3.0 wt % gold loading on the ZrO2 powder (particle size
less than 100 nm), were prepared as described by Zhu’s
group,27b which was confirmed by atomic absorption spectra
and transmission electron microscopy (TEM) images (Figure
S1). Similarly, the ZrO2-supported AuNPs show obvious
absorption around 450−650 nm, and the size of gold
nanoparticles is about 6−8 nm. As reported, the gold content
existing in photocatalysts determined the catalytic activity of
the catalyst; too low or high gold content (1.5 or 5 wt %)
presented poor performance (lower monomer conversion and
selectivity).27b Therefore, the ZrO2-supported AuNPs with
suitable 3 wt % gold content were designed and employed to
conduct the preparation of main-chain Azo polymers via the
photocatalyzed reduction of bisnitroaromatic compounds
(xenon-lamp light irradiation with about 3.1 mW/cm2
measured by UV radiometer, UV-420).
c
3
d
4
d
5
65
38
d
6
d
96
>96
>99
>97
7
100
100
e
8
a
Reaction conditions: isopropyl alcohol (3 mL), NOFL (80 mg),
catalyst (33 mg), KOH (5.6 mg), 8 h. The photocatalyzed reactions
b
were conducted in an argon atmosphere at 45 °C. Yields measured by
c
d
e
HPLC analysis. NOFL (150 mg). HONB (80 mg). HONB (110
mg).
solvent would result in the precipitation of target polymers due
to their poor solubilities in IPA.
The investigation highlighted herein demonstrated an
enormous potential to prepare main-chain Azo polymers by
the directly photocatalyzed reduction of bisnitroaromatic
monomers. The conjugated monomer A (2,7-dinitro-9,9-
dioctyl-9H-fluorene), B (3,6-dinitro-9-(pentacosan-13-yl)-9H-
carbazole) (Scheme S2, Figures S6a and S7a), and uncon-
jugated monomers C (1,12-bis(2-(dodecyloxy)-4-
nitrophenoxy)dodecane) (Scheme S3, Figures S6b and S7b)
were designed and employed to synthesize main-chain Azo
polymers (Figure 1). Further efforts to explore the photo-
Two nitroaromatic compounds, 2-nitro-9,9-dioctyl-9H-fluo-
rene (NOFL) and 1-(hexyloxy)-4-nitrobenzene (HONB), were
designed (Schemes S2 and S1, Figures S2−S4) and employed
as the model compounds to confirm the catalytic activity of
AuNPs (Scheme 2). Mixed solvents (toluene/isopropyl alcohol
Scheme 2. Photocatalytic Reduction of Model Nitroaromatic
Compounds
(IPA)) were used to ensure the complete dissolution of
polymers in the following procedure. Multiple conditions and
diverse solvents mixed with IPA were investigated, and the
results are shown in Table 1 and Table S1. As expected, the
AuNPs photocatalysts efficiently photocatalyzed the reduction
of NOFL and HONB with ≥96% conversion and ≥96%
selectivity (Table 1, entries 2, 3, and 6−8) in optimized
conditions. In addition, the conversion and selectivity of NOFL
and HONB increased along with the reaction time as shown in
Table 1. The photocatalyzed reduction of NOFL and HONB
was almost completed within 8 h. Meanwhile the AuNPs
exhibited excellent catalytic capability in IPA/toluene (3/3, v/
v) mixed solvents, compared to that in IPA/(tetrahydrofuran,
dichloromethane, chlorobenzene, anisole, acetone, or chloro-
form) (3/3, v/v) (entries 2−8 in Table S1). The color change
before and after reaction also partially supported the results
above (Figure S5). High volume ratio of toluene in the mixed
solvent would decrease the catalytic capability and, thus, led to
the incomplete transformation of azoxybenzene to azobenzene
unit. Furthermore, a high volume ratio of IPA in the mixed
Figure 1. Plausible mechanism for the photocatalyzed polymerization
of bisnitroaromatic monomers.
catalyzed polymerization behavior of bisnitroaromatic mono-
mers were implemented with compound A as the model
monomer as presented in Table 2. The conjugated Azo
polymers were successfully obtained with high monomer
conversions and molecular weight of PA increased with
reaction time and up to 29 800 g/mol.
This strategy was also expanded to the other bisnitroaromatic
monomers (B, C). Owing to the excellent solubility of
monomer A in the mixed solvents, the molecular weight of
PA is much higher than that of PB and PC (Table 2, Table S2,
Figure S8) in the same polymerization conditions. The other
C
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Table 2. Photocatalyzed Polymerization of Bisnitroaromatic
a
Monomers at Different Experimental Conditions
PB-
24h
PC-
24h
entry
PA-2h PA-4h
PA-8h PA-12h
t (h)
2
4
8
12
24
24
conv (%)
77
96
95
96
95
92
Mn,GPC (g/mol)
2300
1.13
21600
2.49
29800
4.11
28000
2.63
4100
1.16
4600
1.50
Mw/Mn
a
The reaction was carried out with 80 mg of bisnitroaromatic
monomers (A, B, and C) in argon atmosphere at 45 °C, and the
polymers were precipitated in methanol. Isopropyl alcohol (3 mL),
toluene (3 mL), catalyst (33 mg), KOH (5.6 mg).
polymerization results of monomers B and C at various
conditions are given in Table S2. In addition, the dosage of
monomer C has no obvious effect on the monomer conversion,
however, which slightly affected the molecular weights of the
obtained polymers (Table S2, entries 5−7). The reason may be
attributed to the low catalytic efficiency resulted from
aggregation of catalysts in high content of catalyst and less
catalytic points in low content of monomer.
1
Figure 3. H NMR spectra of monomer A and PA-12h in CDCl3.
were clearly observed. The characteristic peaks of methyne
group close to N atoms of the carbazole unit in PB at 4.62 ppm
and the methoxy group in PC at 4.06 ppm were also confirmed
1
by H NMR spectra.
The polymerization processes of monomers A, B, and C
were monitored by UV−vis (Figure 2a−c) and FT-IR (Figure
S9). As presented in Figure 2a−c, the UV−vis spectra of the
monomers (A, B, and C) and the corresponding polymers
(PAs, PBs, and PCs) with different reaction time showed
significantly stepwise red-shifts (125 nm from 340 to 465 nm
for monomer A and PA, 38 nm from 368 to 406 nm for
monomer B and PB, and 35 nm from 340 to 375 nm for
monomer C and PC) with the elongation of conjugated
Furthermore, a thorough study of the obtained polymer
structure was carried out by matrix-assisted laser desorption
ionization time-of-flight mass spectrometry (MALDI-TOF-
MS).30 The MALDI-TOF-MS spectra of PAs with different
reaction time (PA-2h, PA-12h) are presented in Figure 4. As
expected, a representative peak of m/z at 2665.0 corresponding
to 5-mer of PA-2h is in good agreement with the calculated
mass ([M′ + Na]+, calcd: 2664.9), and a regular interval 432.3
Da was observed in PA-2h, agreeing well with the molar mass
of the corresponding azoxy repeating unit. A representative
peak of m/z at 2563.4 corresponding to 5-mer of PA-12h with
a proton is in good agreement with the calculated mass ([M +
H]+, calcd: 2562.9), and a regular interval 416.4 Da was
observed in PA-12h, which is attributed to the molar mass of
the corresponding Azo repeating unit in PA chain. Meanwhile,
similar experimental data were obtained in MALDI-TOF-MS
spectra of PBs and PCs with different reaction time as given in
Figure S11. These data agree well with UV−vis, FT-IR, and 1H
NMR studies as discussed above. In order to study the
polymnerization mechanism, the reaction time was reduced to
0.5 h for the photocatalyzed reduction reaction of NOFL and
monomer A. Indeed only the azoxy products with single N−O
systems and suggested the polymerization processes.28
A
further evidence for successful formation of PA, PB, and PC
was provided by FT-IR spectra. As presented in Figure S9, the
absorption peaks around 1602, 1594, and 1585 cm−1 attributed
to the stretching vibration of Azo group in PA, PB, and PC
chains12a became obvious stronger along with the reaction time,
respectively. Simultaneously, the nitro absorption around 1342,
1334, and 1350 cm−1 became weaker along with the reaction
time,29 which leveled off after 8 h. The weaker nitro absorption
peaks are ascribed to the stretching vibration of nitro groups in
PA, PB, and PC chain ends.
The chemical structures of PA, PB, and PC were
1
characterized by H NMR (Figure 3 and Figure S10). The
1
characteristic chemical shifts from 7.75 to 8.73 ppm, 7.30 to
9.50 ppm, and 6.90 to 7.55 ppm were attributed to the protons
of the aromatic rings in the main-chain Azo polymers PA, PB,
and PC, respectively. The characteristic peaks of methylene
group close to the 9-position of fluorene in PA at 2.20 ppm
bond are generated evidenced by H NMR, MALDI-TOF-MS,
and FT-IR, rather than two N−O bonds. These data are very
similar to those of productes obtained in 2 h and not shown
here. It turns out that the overall polymerization reaction
process involved two main steps as depicted in Figure 1, which
Figure 2. UV−vis absorption spectra of (a) bisnitroaromatic monomers A and corresponding polymers PAs, (b) B and PBs, and (c) C and PCs with
different reaction time in THF (Cn,A, Cn,C, Cn,PAs, Cn,PCs = 0.02 g/L; Cn,B, Cn,PBs = 0.05 g/L).
D
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Figure 4. MALDI-TOF-MS spectra of PA-2h (top) and PA-12h (bottom).
Figure 5. Absorption spectra of PA-4 solution during continuous 435 nm (a) and 546 nm (b) light irradiation in THF. The concentration of PA-4
(Mn,GPC = 28 000 g/mol, Mw/Mn = 2.63) is 0.02 g/L.
is slightly different as proposed by Zhu’s group.27b In the first
step, the azoxy repeating units were formed from the
corresponding bisnitroaromatic monomers. In the second
step, the Azo repeating units were formed via the reduction
of corresponding azoxy repeating units.
Considering the intrinsic ability of trans−cis photoisomeriza-
tion of Azo chromophore, the trans−cis photoisomerizations of
PA, PB, and PC were investigated by UV−vis spectra. As
evidenced in Figure 5 and Figures S12 and S13, the absorption
bands centered at 465/410/375 nm contributing to the π−π*
electronic transition of the tran-Azo in PA/PB/PC backbone
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were observed. Unusually, the solution of PA/PB showed a
slight decrease in the π−π* electronic transition of the tran-Azo
absorption around 465/410 nm upon the 435/404 nm light
irradiation (Figure 5 and Figure S12). It is likely due to the
rigid conjugated backbone structures of polymers.3b Expectedly,
the obvious trans−cis phoisomerization of Azo chromophore in
PC chain was observed upon time-dependent irradiation with
365 nm UV light (2.6 mW/cm2), which is evidenced by a sharp
decrease of the absorbance at 375 nm along with a concomitant
slight increase at 475 nm (Figure S13a). Subsequently, the cis−
trans photoisomerization was completed upon the 180 s visible-
light (435 nm, 0.9 mW/cm2) irradiation (Figure S13b).
To further confirm the effectiveness of the current
photocatalyst system, the catalyst (ZrO2-supported AuNPs)
was recycled and reused to catalyze the reduction of the
nitroaromatic (NOFL) and bisnitroaromatic (monomer A)
compounds. As shown in Figure 6, the recycled photocatalyst
Azo polymers with other topological structures and investigate
the application of conjugated Azo polymers synthesized via the
current approach.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Synthesis, H NMR spectra, 13C NMR spectra, TEM images,
GPC traces, FT-IR spectra, UV−vis spectra, and MALDI-TOF-
MS spectra of compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail weizhang@suda.edu.cn (W.Z.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support from the National Nature Science
Foundation of China (21374072), the Project of International
Cooperation of the Ministry of Science and Technology of
China (2011DFA50530), the college students’ innovation and
entrepreneurship training plan of Soochow University
(2013xj022), the Priority Academic Program Development
(PAPD) of Jiangsu Higher Education Institutions, and the
Program of Innovative Research Team of Soochow University
is gratefully acknowledged.
REFERENCES
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of nitroaromatic compound NOFL and monomer A.
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CONCLUSIONS
■
In summary, we have developed a novel approach to prepare
the main-chain Azo polymers directly from bisnitroaromatic
compounds photocatalyzed by ZrO2-supported AuNPs under
mild conditions. The most outstanding feature of this
photocatalyst is its excellent activity and selectivity in the
reduction of bisnitroaromatic compounds, which prevents the
formation of the azoxy units in the final polymer chains. The
other advantage of current system is the preparation of Azo
polymers from the direct reduction of bisnitroaromatic
compounds, without additional procedures for tedious design
and synthesis of aromatic dianilines or Azo monomers as
reported. The retention of activity and selectivity of recovered
catalysts and relatively lower reaction temperature and pressure,
combined with the potential to utilize solar energy, reveal an
attractive green process for preparation of main-chain Azo
polymers and will open a novel window for preparing Azo
polymer materials. Studies are ongoing in our group to prepare
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