Synthesis and characterization of azobenzene-functionalized hyperbranched polymers

Article abstract of DOI:10.1080/15421406.2010.495658

Azobenzene functionalized hyperbranched polymers (HBAzPs) have been synthesized by polycondensation of 4,4'-di((2-hydroxyethoxy)azobenzene and 1,3,5-benzenetricarboxylic acid chloride in presence of triethylamine at room temperature through A2+B3 polymerization approach. The structure and properties of HBAzPs were characterized by 1H NMR, IR, GPC, DSC and UV-Visible spectrometry. Molecular weights as well as properties of HBAzPs were found to be greatly influenced by molar ratio of monomers, amount of triethylamine and monomer concentration. HBAzP solutions in THF showed absorbances at 356nm and 450nm corresponding to -* and n-* electronic transitions of azobenzene chromophores, respectively. A reversible trans-cis photoisomerization of HBAzPs was observed upon irradiation of UV and visible light. Taylor & Francis Group, LLC.

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Synthesis and Characterization
of Azobenzene-Functionalized
Hyperbranched Polymers
MD. Zahangir Alam a , Tomonari Ogata a , Yutaka Kuwahara a & Seiji
Kurihara a
a Faculty of Engineering, Department of Applied Chemistry and
Biochemistry , Kumamoto University , Kumamoto, Japan
Published online: 11 Nov 2010.
To cite this article: MD. Zahangir Alam , Tomonari Ogata , Yutaka Kuwahara & Seiji Kurihara (2010)
Synthesis and Characterization of Azobenzene-Functionalized Hyperbranched Polymers, Molecular
Crystals and Liquid Crystals, 529:1, 25-31
To link to this article: http://dx.doi.org/10.1080/15421406.2010.495658
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Mol. Cryst. Liq. Cryst., Vol. 529: pp. 25–31, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.495658
Synthesis and Characterization of
Azobenzene-Functionalized Hyperbranched
Polymers
MD. ZAHANGIR ALAM, TOMONARI OGATA,
YUTAKA KUWAHARA, AND SEIJI KURIHARA
Faculty of Engineering, Department of Applied Chemistry and
Biochemistry, Kumamoto University, Kumamoto, Japan
Azobenzene functionalized hyperbranched polymers (HBAzPs) have been
synthesized by polycondensation of 4,40-di((2-hydroxyethoxy)azobenzene and
1,3,5-benzenetricarboxylic acid chloride in presence of triethylamine at room tem-
perature through A2 þ B3 polymerization approach. The structure and properties
of HBAzPs were characterized by 1H NMR, IR, GPC, DSC and UV-Visible spec-
trometry. Molecular weights as well as properties of HBAzPs were found to be
greatly influenced by molar ratio of monomers, amount of triethylamine and mono-
mer concentration. HBAzP solutions in THF showed absorbances at 356 nm and
450 nm corresponding to p-pꢀ and n-pꢀ electronic transitions of azobenzene chromo-
phores, respectively. A reversible trans-cis photoisomerization of HBAzPs was
observed upon irradiation of UV and visible light.
Keywords A2 þ B3 polymerization; hyperbranched azopolymer; trans-cis
photoisomerization
Introduction
Hyperbranched polymers (HBPs) are highly branched macromolecules with
three-dimensional architecture. Because of their structural characteristics HBPs
show many interesting properties of the dendritic polymers such as good solubility,
low melting viscosity, and large amount of terminal functional groups. Dendrimers
have a well-controlled size and shape, and are prepared by tedious multistep reac-
tions through a divergent or convergent scheme whereas HBPs can be synthesized
very easily by a simple one-pot synthetic procedure. Thus HBPs offer a good alter-
native to the more perfect dendrimers. That’s why recently a great deal of attention
was devoted to HBPs [1–5].
Address correspondence to Seiji Kurihara, Faculty of Engineering, Department of
Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kumamoto-si,
Kumamoto 860-8555, Japan. Tel.: þ81-96-342-3678; Fax: 81-96-342-2679; E-mail: kurihara@
gpo.kumamoto-u.ac.jp
25

26
MD. Z. Alam et al.
Introducing azobenzene-chromophore can significantly modify the properties of
HBPs. Azobenzene molecules exist in two isomeric states: trans-form which are
rod-shaped and cis-form which possess a bent shape. It is well known that a
reversible trans-cis photoisomerization can be observed by irradiation of UV and
visible light respectively. The bent structure causes a large effect on molecular
arrangements of azobenezene molecules. The two states of azobenzene molecules
have distinct absorption spectra which allow reversible storage of data.
Azobenzene-functionalized linear polymers have been studied and optimized for a
wide range of applications such as liquid crystal displays and devices, reversible
optical storage systems, nonlinear optical waveguides, photorefractive switches,
and holographic gratings [6–9]. Recently hyperbranched polymers functionalized
with azobenzene chromophores have been designed and prepared from AB2
monomers [10–12]. But, HBPs prepared by one-step polycondensation of AB2 type
monomers possess highly irregular structures and large molecular weight distribu-
tions [13,14]. In this study we were envisioned to synthesize HBAzPs through
A2 þ B3 polymerization approach as a lot of monomers are commercially available.
Experimental
An A2 type monomer 4,40-di(2-hydroxyethoxy)azobenzene (DH2Az) was
synthesized in good yield and purity. Hyperbranched azopolymers were synthesized
by condensation polymerization of DH2Az and trimesoyl chloride (TMC) in pres-
ence of triethylamine (TEA) as basic catalyst in dioxane (DO) at room temperature
through A2 þ B3 polymerization approach. Gel permeation chromatography (GPC)
analyses using tetrahydrofuran (THF) as eluent revealed molecular weights and
1
polydispersity indexes of HBAzPs. H NMR and IR spectroscopy were also used
for characterization of HBAzPs. Differential Scanning Calorimetry (DSC) experi-
ments were performed on a Seiko SSC-5020 instrument at a temperature rate of
10ꢁC per minute. Polarizing optical microscopy (POM) observations were made with
Olympus BHSP polarizing microscope equipped with a Mettler FP 80 hot stage
controlled by Mettler unit. The heating and cooling cycles were performed at a rate
of 10ꢁC minꢂ1. Reversible tran-cis photoisomerization behaviors of HBAzPs were
studied upon irradiation of UV (366 nm) and visible light (436 nm), respectively.
Synthesis of 4,40-di(2-hydroxyethoxy)azobenzene, DH2Az
Synthesis of 4,40-dihydroxyazobenzene (DHAz). 20.0 g (181 mmol) of p-
aminophenol was dissolved in 200 ml of 1 M HCl solution and was cooled to 0ꢁC
in an ice bath. 12.6 g (181 mmol) of NaNO2 dissolved in 300 ml of water was
added dropwise to solution of p-aminophenol at 0ꢁC. 400 ml of pre-cooled
methanol was added to this diazotized solution and the solution was added
drop-wise to 17.1 g (181 mmol) of phenol dissolved in 120 ml 3 M aqueous
potassium hydroxide solution. This solution was stirred at room temperature for
2 h. Then methanol was removed from the reaction mass by evaporation and was
acidified with aqueous HCl up to a pH value of 4.0. The resulting precipitate was
collected by filtration and washed with water until neutralization. The crude
DHAz was purified by recrystallization from ethanol=water mixture and dried
under vacuum. Yield: 56.1%, melting point: 218ꢁC.

Synthesis of Hyperbranched Azopolymers
27
Synthesis of 4,40-di(2-hydroxyethoxy)azobenzene, DH2Az
4,40-Dihydroxyazobenzene 20.0 g (90.5 mmol), potassium carbonate 25.0 g
(181 mmol), 2-chloroethanol 15.3 g (181 mmol) and a small amount of potassium
iodide was dissolved in 500 ml DMF in a flask and the reaction mass was stirred
at 100ꢁC for 48 h. After the reaction time is over the reaction mass was poured into
a large amount of water and precipitate was collected by filtration. The crude
DH2Az was purified by recrystallization from ethyl acetate and dried under vacuum.
Yield: 24.0%, Melting point: 198ꢁC. Anal.: Calcd. (%); H; 6.00, C; 63.56, N; 9.27.
Found: (%); H; 5.95, C; 63.75, N; 8.89.
Synthesis of HBAzPs
Required amounts of DH2Az and TMC dissolved in dioxane were taken in a flask
equipped with a stirrer, gas-inlet and gas-outlet tubes. Nitrogen gas was passed
through the polymerization solution for about one hour. TEA dissolved in dioxane
was added slowly to A2 þ B3 polymerization solution with a dropping funnel. After
addition of TEA the polymerization solution was stirred at room temperature under
nitrogen for the desired period. Then the polymerization solution was poured into
methanol to obtain the precipitate of HBAzPs. The precipitated polymer was filtered
off and was purified by CHCl3=methanol reprecipitation technique, and dried under
vacuum at room temperature.
Results and Discussions
HBAzPs were synthesized by condensation polymerization of DH2Az and TMC in
presence of TEA at room temperature through A2 þ B3 polymerization approach.
The synthetic route, structure of monomers and HBAzPs are shown in Scheme 1.
All polycondensation reactions were carried out at room temperature due to very
high reactivity of TMC. Molar ratio and concentration of monomers, amount of
TEA and polymerization time were varied to explore their effects on A2 þ B3 poly-
merization. In this study polymerizations were carried out with monomer ratios of
A2:B3 ¼ 3:2 and 1:1 corresponding to stoichiometric amount and excess of B func-
tionalities, respectively. Table 1 shows the polymerization conditions and character-
ization of the resultant HBAzPs. It is observed that monomer ratio has a crucial
influence on the properties of HBAzPs. With stoichiometric monomer ratio and little
excess of TMC monomer higher molecular weight HBAzPs were obtained. But,
possibility of gelation was also higher. Gelation can be avoided only by optimization
of polymerization conditions. Apparent molecular weights of HBAzPs determined
by GPC using polystyrene as standard were found to be relatively low. But the actual
molecular weights are supposed to be three to five times higher than the obtained
value since highly branched polymers are known to have smaller hydrodynamic vol-
ume than their linear analogues [15–17].
The data in Table 1 also reveal the effect of monomer concentration and time on
molecular weights of HBAzPs. Molecular weights of HBAzPs were found to be
increased with an increase in monomer concentration. But the possibility of gelation
is higher in case of concentrated polymerization solution. Molecular weights of
HBAZPs increased a little with an increase in polymerization time. In the polycon-
densation of DH2Az and TMC, a relatively fast reaction occurred at the beginning

28
MD. Z. Alam et al.
Scheme 1. Synthesis of HBAzPs through A2 þ B3 polymerization approach.
compared to the subsequent propagation. So, polymerization time exhibited a little
effect on molecular weight of HBAzPs.
TEA which is a basic catalyst in nature was used as condensing agent in this
A2 þ B3 polymerization. A significant effect of TEA on molecular weight of HBAzPs
was observed. Molecular weights of HBAzPs were found to increase with an increase
in the amount of TEA. It is assumed that TEA enhanced polycondensation reac-
tions. But, gelation was found to be occurred in case of using an excess amont of
TEA. In the synthesis of HBAzPs gelation can be avoided by optimizing the reaction
conditions.
Figure 1 shows the IR spectra of monomers (i) DH2Az, (ii) TMC and (iii)
HBAzP-I. Monomer DH2Az exhibited –OH group characteristic absorption peak
at 3,290 cmꢂ1. In the IR spectra of TMC the absorption peak corresponding to
Table 1. Polymerization conditions and characterization of HBAzPs
BAz20H
mmol
TMC
mmol
TEA
mmol
DO
ml
Time
h
HBAzPs
Mw
Mw=Mn
HBAzP I
1.00
1.00
1.00
0.50
0.50
0.50
0.50
0.75
0.67
0.67
0.37
0.37
0.50
0.50
2.0
2.0
2.0
1.25
2.0
1.0
1.0
10
20
50
20
20
20
30
24
6
3900
2500
2200
2900
1.53
1.30
1.50
1.23
HBAzP II
HBAzP III
HBAzP IV
HBAzP V
HBAzP VI
HBAzP VII
14
24
Gel
24
24
3000
2400
1.30
1.32

Synthesis of Hyperbranched Azopolymers
29
Figure 1. IR-spectra of DH2Az, TMC and HBAzP-I.
ꢂ1
=
carbonyl (C O) group was observed at 1747 cm while this peak in the HBAzP-I
spectra is shifted to shorter wavelength (1725 cmꢂ1) due to the formation of ester.
The absorption peaks corresponding to the azobenzene chromophore of monomer
DH2Az and HBAzP-I were observed at 1601 and 1598 cmꢂ1, respectively. The
absorption peak at 3290 cmꢂ1 corresponding to –OH was fully disappeared in the
IR spectra HBAzP-I. These data reveal the successful formation of HBAzPs through
A2 þ B3 polymerization approach. 1H NMR spectra of DH2Az, TMC and HBAzPs
also confirmed the formation of HBAzPs. But, it was not possible to determine
degree of branching of HBAzPs with 1H NMR spectra due to the difficulty of assign-
ing dendritic, linear and terminal units accurately.

30
MD. Z. Alam et al.
Figure 2. DSC thermograms of HBAzP-I, HBAzP-II and HBAzP-IV.
Thermal properties of HBAzPs were determined by DSC and POM. Only
HBAzP-I showed a glass transition at 115ꢁC and isotropization temperature at
210ꢁC. Other HBAzPs mentioned in Table 1 did not show any glass transition tem-
perature. Other HBAzPs exhibited their thermal stability up to about 300ꢁC and
decomposed above this temperature. It is assumed that because of the structural
rigidity HBAzPs showed excellent morphological stability, with no glass transition
temperature detectable in the measured temperature region of the DSC analysis.
HBAzPs synthesized in this study exhibited a typical reversible trans-cis photo-
isomerization. Figure 3 showed the changes in UV-visible absorption spectra of
HBAzP-I in THF upon irradiation of (A) UV and (B) visible light. The UV-visible
absorption spectra of HBAZP-I in THF showed a strong absorption at 356 nm and a
weak absorption at 450 nm corresponding to p-pꢀ and n- pꢀ transition of trans-
azobenzene moiety, respectively. Upon UV light irradiation the intensity of p-pꢀ
Figure 3. Changes in absorption spectra of HBAzP-I solution in THF by (A) UV and (B) vis-
ible light irradiation.

Synthesis of Hyperbranched Azopolymers
31
transition band at 356 nm decreased and intensity of n-pꢀ transition band at 450 nm
gradually increased due to trans–cis photoisomerization. The photostationary state
was observed after an irradiation of 20s. The reversibility of cis-trans photoisomeri-
zation was confirmed by irradiation of visible light. The UV spectra recorded at dif-
ferent irradiation time were characterized by the presence of two isosbestic points at
about 283 and 392 nm, respectively. This reveals that only two absorbing species,
namely cis and trans isomers of azobenzene group are present in the solution.
Conclusion
HBAzPs were successfully synthesized by polycondensation of 4,40-di(2-
hydroxyethoxy)azobenzene and 1,3,5-benzenetricarboxylic acid chloride in presence
of triethylamine at room temperature through A2 þ B3 polymerization approach.
Molar ratio of monomers, monomer concentration and amount of TEA showed a
significant influence on properties of HBAzPs while polymerization time exhibited
a very little effect. HBAzPs showed excellent morphological stability due to their
structural rigidity. They showed reversible trans-cis photoisomerization by
irradiation of UV and visible light, respectively. Thus the resulting HBAzPs are
expected to find their applications in various photonic devices.
Acknowledgments
The financial support from the Japan Society for the Promotion of Science (No.
08045) to carry out this research is gratefully acknowledged.
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