Synthesis and phase structures of dendronized polymers with dendritic azobenzene side groups based on mesogen-jacketed liquid crystalline polymers

Article abstract of DOI:10.1080/15421406.2013.837996

The first- and second-generation dendronized polymers, poly (2, 5-bis {[3, 5-di (4′-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (PG1) and poly (2, 5-bis {3, 5-di [3, 5-di (4′-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styre

Full text of DOI:10.1080/15421406.2013.837996

Mol. Cryst. Liq. Cryst., Vol. 592: pp. 28–43, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.837996
Synthesis and Phase Structures of Dendronized
Polymers with Dendritic Azobenzene Side
Groups Based on Mesogen-Jacketed Liquid
Crystalline Polymers
CHANG-AN YANG,∗ YING LU, GANGYONG LI,
AND ZHOUBING HUANG
Department of Chemistry and Chemical Engineering, Hunan Institute of Science
and Technology, Yueyang, Hunan Province, P. R. China
The first- and second-generation dendronized polymers, poly (2, 5-bis {[3, 5-di (4ꢀ-
methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (PG1) and poly (2,
5-bis {3, 5-di [3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl}
styrene) (PG2), were successfully synthesized, and their phase structures were investi-
gated by differential scanning calorimetry (DSC), polarized light microscopy (PLM),
and one-dimensional wide-angle X-ray diffraction (1D WAXD). The results show that
the liquid crystalline (LC) phase structures of the dendronized polymers containing
dendritic azobenzene side groups depend strongly on the molecular weights (MWs)
and the generation of dendritic azobenzene side groups. Compared the low MWs PG2
with the low MWs PG1, the degree of order in LC phase increased with increasing the
generation of dendritic azobenzene side groups.
Keywords Dendritic azobenzene group; dendronized polymer; liquid crystals; phase
structure
1. Introduction
Dendronized polymers have attracted much attention in the last decades because of their
unique structure, properties, and potential applications in the field of catalyst support,
electroopic materials, and biological mimic, and so forth [1–3]. Dendronized polymers are
a special class of nanoscopic molecules with a linear main-chain and pendant dendrons
side groups at each repeating unit, which may adopt an extended and/or rod-like confor-
mation due to steric congestion. For the dendronized polymers, it can be synthesized via
different strategies including macromonomer route, graft-to route, and graft-from route
[4–8].
Over the last 20 years, various dendronized polymers were synthesized and reported
by Fre´chet [1,2,8], Tomalia [9], Schlu¨ter [4–6,10,11], Percec [3,12–20], Xi, and Chen
∗Address correspondence to Chang-An Yang, Department of Chemistry and Chemical Engi-
neering, Hunan Institute of Science and Technology, Yueyang 414006, Hunan Province, P. R. China.
Tel: +86-730-8640122; Fax: +86-730-8640122. E-mail: Chang anyang@163.com
28

Azobenzene Dendritic Polymers
29
[21–23]. The research showed that the phase behaviors and associated material proper-
ties of dendronized polymers were influenced by many various parameters, such as the
type of dendron, the length of the tail of dendron, the generation of dendron, and so
on. Dendronized polymers with Fre´chet-type benzyl ether dendrons could not form ex-
tended and stiff conformation and are amorphous [2,8]. However, dendronized polymers
containing Percec-type dendron side groups could form cylindrical conformation due to
the spatial requirement of bulky dendrons, which develop cub phase, hexagonal columnar
(ΦH) phase, columnar nematic (ΦN), tetragonal phase, and so on [12–20]. Schlu¨ter-type
ionic dendronized polymers also could self-assemble into hexagonal columnar (ΦH) phase,
columnar nematic (ΦN), tetragonal phase, and rectangular phase [4,5,11]. In addition,
the effects of the generation and length of the tail of dendron on the chain conforma-
tion and phase behaviors of dendronized polymer have been investigated by Mezzenga
[5]. For the first-generation dendronized polymers with short-tailed lipid, the polymer
showed an amorphous structure. By increasing the length of the lipid tail, lamellar phase
could be observed. When the length of the lipid tail of dendrons was kept at fixed length,
the first-generation dendronized polymer only formed a lamellar phase because of the
small steric hindrance, similarly as linear polymer lipid complexes. With increasing gen-
eration, however, the main-chain was forced to take an extended and stiff conformation
because of the steric hindrance between the bulky dendrons, leading to a columnar tetrag-
onal phase rather than a lamellar phase. Moreover, the chain conformation of polymers
also plays an important role in controlling their phase behaviors and associated material
properties. Percec reported a general strategy for the rational control of polymer confor-
mation through the self-assembly of quasi-equivalent monodendritic side groups attached
to flexible backbones. At low molecular weights (MWs), the conical monodendrons as-
sembled to produce a spherical polymer with random-coil backbone conformation. On
increasing the MWs, the self-assembly pattern of the monodendritic units changed to
give cylindrical polymers with extended backbones [14]. Recently, the dendronized poly-
mers with different end-functional groups have also been reported, and their properties
could be induced by changing the end-functional groups. Most of works concentrate on
the effect of the type of the end-functional groups (e.g., flexible alkyl, amine, hydroxyl,
and ionic mesogens) and on their material properties [4,5,24,25]. However, few works
have been done on the synthesis of dendronized polymer with dendritic azobenzene side
groups.
Mesogen-jacketed liquid crystalline polymers (MJLCPs) are a special class of side-
chain liquid crystalline polymers (SCLCPs) in which the mesogenic units are laterally
attached to the polymer backbone without or with very short spacers [26]. The steric
hindrance produced by the bulky and rigid side groups will force the main-chain to exhibit
an extended and stiff conformation similar to dendronized polymers [4,5,11–20]. In the
past two decades, systematic research on the structure–property relationships for MJLCPs
with different side groups has been done [27–33]. Recently, the functionalization of this
system has been researched. The first- and second-generation dendronized carbazoles were
introduced by the “graft through” method into MJLCPs for the first time [34]. The phase
structures and optoelectronic properties of the resulting polymers were investigated. The
results were shown that the first-generation polymer formed a hexatic columnar nematic
phase, while the second-generation one exhibited a columnar nematic phase. This stiff main-
chain conformation and the dendritic side group structure could reduce the intramolecular
interactions and aggregations of the carbazole side groups and prevent the formation of
excimers, which would be beneficial to the optoelectronic properties. Azobenzene mesogen
is one of the typical functional units, which can undergo photon-driven reversible cis–trans

30
C. -A. Yang et al.
isomerization [35]. Liquid crystalline (LC) polymers containing azobenzene mesogens have
been widely studied for potential applications in specific areas, such as reversible optical
data storage, optical switching, polarization holography, and so forth [36]. Moreover, they
only form low-ordered LC phase, i.e., nematic (N), smectic A (SmA), and smectic C
(SmC) phases [37]. When the dendritic azobenzene side groups were introduced by the
“graft through” method into MJLCPs for the first time, the polymers shall exhibit versatile
LC properties and be expected to be novel optoelectronic materials using for a special
purpose because of the unique structure.
In the present work, we report the syntheses and phase behaviors of poly(2, 5-bis
{[3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (de-
noted as PG1 in Scheme 1) with the first-generation dendritic azobenzene side groups
and poly(2, 5-bis {3, 5-di [3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxy-
carbonyl} styrene) (denoted as PG2 in Scheme 1) with the second-generation dendritic
azobenzene side groups, and investigate the effect of the MWs and the generation of den-
dritic azobenzene side groups on the LC behaviors of dendronized polymers. We expect
that the LC behaviors of the novel polymers can be rationally tailored by changing the
MWs and/or the generation of dendritic azobenzene side groups of dendronized polymers.
Moreover, the chemical structure of the monomers was confirmed by elemental analysis
and 1H NMR, and the corresponding polymers were studied by gel permeation chromatog-
raphy (GPC), differential scanning calorimetry (DSC), polarized light microscopy (PLM),
and one-dimensional wide-angle X-ray diffraction (1D WAXD).
2. Experimental
2.1. Materials
The compound of vinylterephthalic acid was synthesized using the method previously
reported [38]. Azobisisobutyronitrile (AIBN) was purified by recrystallization from ethanol.
Scheme 1. Synthetic route of the monomers (G1, G2) and the corresponding polymers (PG1, PG2).

Azobenzene Dendritic Polymers
31
Chlorobenzene was washed with H2SO4, NaHCO3, and distilled water separately and was
distilled from calcium hydride. Tetrahydrofuran (THF, AR; Beijing Chemical Co., China)
and triethylamine (Acros, 99%; J&K Scientific Ltd., China) were heated under reflux over
calcium hydride for at least 8 hr and distilled before use. All other reagents were used as
received from commercial sources.
2.2. Measurements
Elemental analysis was carried out with an Elementar Vario EL instrument. 1H NMR spectra
were recorded on a Bruker ARX400 spectrometer at room temperature, using deuterated
chloroform (CDCl3) as the solvent and tetramethylsilane (TMS) as the internal standard.
The apparent number-average MW (Mn) and MW distribution (Mw/Mn) were measured
on a GPC (Waters 1515) instrument with a set of HT3, HT4, and HT5. The μ-Styragel
columns used THF as an eluent, and its flow rate was 1.0 mL min−1 at 35◦C. Calibration
was made with polystyrene standards (PS).
The thermogravimetric analysis (TGA) was performed on a TA SDT 2960 instrument
at a heating rate of 20◦C min−1 in nitrogen atmosphere.
DSC examination was carried out on a TA DSC Q100 calorimeter with a programed
heating procedure in nitrogen. The sample size was about 5 mg and encapsulated in
hermetically sealed aluminum pans, whose weights were kept constant. The temperature
and heat flow scale at different cooling and heating rates were calibrated using standard
materials such as indium and benzoic acid.
LC texture of the polymers was examined under PLM (Leica DM-LM-P) coupled with
a Mettler-Toledo hot stage (FP82HT). The films with thickness of ∼10 μm were casted
from THF solution and slowly dried at room temperature.
1D WAXD experiments were performed on a Philips X’ Pert Pro diffractometer with
a 3 kW ceramic tube as the X-ray source (Cu Kα) and an X’ celerator detector. The
reflection peak positions were calibrated with silicon powder (2θ > 15◦) and silver behenate
(2θ < 10◦). The sample stage is set horizontally, and a temperature control unit (Paar
Physica TCU 100) in conjunction with the diffractometer was utilized to study the structure
evolutions as a function of temperature. The heating and cooling rates in the WAXD
experiments were 10◦C min−1
.
2.3. Synthesis of Monomers
The synthetic route of the monomers, 2, 5-bis {[3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy
azobenzene) benzyl] oxycarbonyl} styrene) (denoted as G1) and 2, 5-bis {3, 5-di [3, 5-di
(4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl} styrene) (denoted as G2),
and the corresponding polymers was illustrated in Scheme 1. The experimental details of
the monomers’ synthesis and characterization are described as follows.
2.3.1. Synthesis of 4-(6-bromohexyloxy) -4ꢀ-methoxy azobenzene. 4-(6-Bromohexyloxy)-
4ꢀ-methoxy azobenzene was prepared according to literature [39].
1H NMR (400 MHz, CDCl3, δ, ppm): 7.89–7.86 (m, 4H, Ar-H), 7.01–6.97 (m, 4H, Ar-
H), 4.06–4.02 (m, 2H, -CH2-), 3.89 (s, 3H, -OCH3), 3.45–3.42 (m, 2H, -CH2Br), 1.93–1.82
(m, 4H, -CH2-), 1.55–1.51 (m, 4H, -CH2-).
2.3.2. Synthesis of Vinylterepthaloyl Chloride. An amount of 2-vinylterephthalic acid
(1.92 g, 10 mmol) was mixed with thionyl chloride (20 mL) in a round-bottom flask

32
C. -A. Yang et al.
(100 mL). The mixture was refluxed for about 2 hr to get a clear solution. The excess
thionyl chloride was removed by evaporation under reduced pressure. The residue was
washed twice by petroleum ether. A pale yellow liquid of vinylterepthaloyl chloride was
obtained.
2.3.3. Synthesis of Methyl 3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzoate (1).
Compound 1 was obtained by the etherification of methyl 3, 5-dihydroxybenzoate with 4-
(6-bromohexyloxy)-4ꢀ-methoxy azobenzene. In a three-neck round-bottom flask (1000 mL)
equipped with a condenser, N2 inlet–outlet, and mechanical stirrer, a mixture of K2CO3
(16.56 g, 120 mmol), acetone (200 mL) was thoroughly degassed with N2 for 1 hr. Methyl
3, 5-dihydroxybenzoate (3.36 g, 20 mmol) and 4-(6-bromohexyloxy)-4ꢀ-methoxy azoben-
zene (15.64 g, 40 mmol) were added, and the mixture was heated to 70◦C. After 48 hr,
the reaction was found to be complete by thin-layer chromatography (TLC). No side prod-
ucts were observed. The reaction mixture was filtrated. Acetone was evaporated under
reduced pressure to obtain a salmon pink solid. Afterward, the crude product dissolved in
1
dichloromethane was purified by column chromatography (silica gel, CH2Cl2). H NMR
(400 MHz, CDCl3, δ, ppm): 7.89–7.85 (m, 8H, Ar-H), 7.18–7.16 (m, 2H, Ar-H), 7.01–6.97
(m, 8H, Ar-H), 6.64–6.63 (m, 1H, Ar-H), 4.06–3.93 (m, 8H, -CH2-), 3.90 (s, 3H, -OCH3),
3.88 (s, 6H, -OCH3), 1.83–1.79 (m, 8H, -CH2-), 1.56–1.54 (m, 8H, -CH2-).
2.3.4. Synthesis of 3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl alcohol (2).
Compound 2 was prepared by the reduction of methyl 3, 5-di (4ꢀ-methoxy-4-oxy hexyloxy
azobenzene) benzoate (1) with LiAlH4. Compound 1 (7.88 g, 10 mmol) in dry THF
(200 mL) was added slowly to a suspension of LiAlH4 (0.38 g, 10 mmol) in dry THF
(100 mL) at 0◦C under a flow of N2. After the addition was complete, the mixture was reacted
for further 2 hr at room temperature. The reaction was found to be complete by TLC. Water
was then added slowly with vigorous stirring to terminate the reaction, and diluted HCl
was used to adjust the pH of the mixture to 7. The product was extracted with CH2Cl2. The
extracts were dried over anhydrous magnesium sulfate (anhydrous MgSO4) and condensed
to give a yellow solid. Afterward, the crude product dissolved in dichloromethane was
purified by column chromatography (silica gel, CH2Cl2/acetone, 50/1). 1H NMR (400 MHz,
CDCl3, δ, ppm): 7.90–7.86 (m, 8H, Ar-H), 7.01–6.97 (m, 8H, Ar-H), 6.52–6.51 (m, 2H,
Ar-H), 6.38–6.37 (m, 1H, Ar-H), 4.62 (s, 2H, -CH2-), 4.06–3.96 (m, 8H, -CH2-), 3.88 (s,
6H, -OCH3), 1.85–1.80 (m, 8H, -CH2-), 1.56–1.55 (m, 8H, -CH2-).
2.3.5. Synthesis of 2, 5-bis {[3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxy-
carbonyl} styrene (Monomer G1, 3). To a solution of 2 (15.20 g, 20 mmol), NEt3 (8 mL),
and DMAP (7.34 g, 60 mmol) in dried THF (100 mL) was added vinylterephthal chloride
(10 mmol) at 0◦C–5◦C. After stirring overnight, a few drops of H2O were added to quench
the reaction. The mixture was then partitioned between CH2Cl2 and water. The organic layer
was washed with dilute hydrochloric acid, aqueous solution of sodium bicarbonate, and
brine, and dried over anhydrous MgSO4 and further evaporated under reduced pressure to
obtain a yellow solid. The crude product dissolved in dichloromethane was purified by col-
umn chromatography (silica gel, CH2Cl2), followed by recrystallization from THF/diethyl
ether to obtain the monomer G1, 3. 1 H NMR (400 MHz, CDCl3, δ, ppm): 8.29–7.95 (m, 3H,
Ar-H); 7.91–7.87 (m, 16H, Ar-H), 7.49–7.42 (q, 1H, = CH-); 7.02–6.98 (m, 16H, Ar-H),
6.57 (s, 4H, Ar-H), 6.43 (s, 2H, Ar-H); 5.79–5.74 (dd, 1H, = CH2); 5.44–5.42 (dd, 1H,
= CH2); 5.30, 5.28 (s, 4H, -OCH2-), 4.03–3.96 (m, 16H, -OCH2-), 3.88 (s, 12H, -OCH3),

Azobenzene Dendritic Polymers
33
1.87–1.83 (m, 16H, -CH2-), 1.56–1.55 (m, 16H, -CH2-). Anal. Calcd. for C100H108N8O16:
C, 71.58; N, 6.68; H, 6.49; found: C, 71.41; N, 6.61; H, 6.48.
2.3.6. Synthesis of 3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl chloride (4).
Compound 4 was obtained by the chlorination of 2 with SOCl2. Into a three-neck round-
bottom flask (500 mL), equipped with magnetic stirrer and addition funnel, 2 (7.60 g,
10 mmol) was dissolved in dry CH2Cl2 (200 mL), and a mixture of SOCl2 (1.79 g,
15 mmol) and CH2Cl2 (10 mL) was added dropwise slowly at room temperature. After
the addition was complete, the mixture was reacted for further 2 hr at room temperature.
The reaction was found to be complete by TLC. Water was then added slowly to terminate
the reaction. The organic layer was washed with water, and dried over anhydrous MgSO4
and further evaporated under reduced pressure to obtain a yellow solid. The crude product
dissolved in dichloromethane was purified by column chromatography (silica gel, CH2Cl2).
1H NMR (400 MHz, CDCl3, δ, ppm): 7.92–7.89 (m, 8H, Ar-H), 7.04–7.00 (m, 8H, Ar-H),
6.55–6.54 (m, 2H, Ar-H), 6.43–6.42 (m, 1H, Ar-H), 4.53 (s, 2H, -CH2-), 4.09–3.97 (m, 8H,
-CH2-), 3.91 (s, 6H, -OCH3), 1.89–1.85 (m, 8H, -CH2-), 1.59–1.58 (m, 8H, -CH2-).
2.3.7. Synthesis of Methyl 3, 5-di [3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene)
benzyloxy] benzoate (5). Compound 5 was obtained by the etherification of 3, 5-
dihydroxybenzoate with methyl 3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl
chloride. In a three-neck round-bottom flask (1000 mL) equipped with a condenser, N2
inlet–outlet, and mechanical stirrer, a mixture of K2CO3 (16.56 g, 120 mmol), acetone
(200 mL) was thoroughly degassed with N2 for 1 hr. Methyl 3, 5-dihydroxybenzoate
(3.36 g, 20 mmol) and 4 (31.14 g, 40 mmol) were added, and the mixture was heated to
70◦C. After 48 hr, the reaction was found to be complete by TLC. No side products were ob-
served. The reaction mixture was filtrated. Acetone was evaporated under reduced pressure
to obtain a salmon pink solid. Afterward, the crude product dissolved in dichloromethane
was purified by column chromatography (silica gel, CH2Cl2). 1H NMR (400 MHz, CDCl3,
δ, ppm): 7.90–7.87 (m, 16H, Ar-H), 7.31–7.30 (m, 2H, Ar-H), 7.03–6.99 (m, 16H, Ar-H),
6.82–6.81 (m, 1H, Ar-H), 6.59–6.57 (m, 4H, Ar-H), 6.44–6.43 (m, 2H, Ar-H), 5.01 (s, 2H,
-CH2-), 4.07–3.97 (m, 16H, -CH2-), 3.92 (s, 3H, -OCH3), 3.90 (s, 12H, -OCH3), 1.87–1.84
(m, 16H, -CH2-), 1.56–1.58 (m, 16H, -CH2-).
2.3.8. Synthesis of 3, 5-di [3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyloxy]
benzyl alcohol (6). Compound 6 was prepared by the reduction of methyl 3, 5-di [3, 5-di
(4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyloxy] benzoate with LiAlH4. Into a three-
neck round-bottom flask, equipped with a condenser, ice bath, N2 inlet–outlet, and magnetic
stirrer, LiAlH4 (0.38 g, 10 mmol) was suspended in dry THF (20 mL), and the solution
of 5 (16.52 g, 10 mmol) in dry THF (200 mL) was added dropwise very slowly. After
the addition was complete, the mixture was reacted for further 2 hr at room temperature.
The reaction was found to be complete by TLC. Water was then added slowly to terminate
the reaction, and then diluted HCl was added to dissolve the precipitate. The product was
extracted with CH2Cl2. The extracts were dried over anhydrous MgSO4, and condensed to
give a yellow solid. Afterward, the crude product dissolved in dichloromethane was purified
by column chromatography (silica gel, CH2Cl2/acetone, 50/1). 1H NMR (400 MHz, CDCl3,
δ, ppm): 7.91–7.87 (m, 16H, Ar-H), 7.03–6.99 (m, 16H, Ar-H), 6.63–6.62 (m, 2H, Ar-H),
6.58–6.57 (m, 4H, Ar-H), 6.56–6.55 (m, 1H, Ar-H), 6.43–6.42 (m, 2H, Ar-H), 4.98 (s, 2H,
-CH2-), 4.64 (s, 2H, -CH2-), 4.07–3.97 (m, 16H, -CH2-), 3.81 (s, 12H, -OCH3), 1.86–1.84
(m, 16H, -CH2-), 1.56–1.58 (m, 16H, -CH2-).

34
C. -A. Yang et al.
2.3.9. Synthesis of 2, 5-bis {3, 5-di [3, 5-di (4ꢀ-methoxy-4-oxy hexyloxy azobenzene) benzyl]
oxycarbonyl} styrene (Monomer G2, 7). To a solution of 6 (15.20 g, 20 mmol), NEt3
(8 mL), and DMAP (7.34 g, 60 mmol) in dried THF (100 mL) was added vinylterephthal
chloride (10 mmol) at 0◦C–5◦C. After stirring overnight, a few drops of H2O were added
to quench the reaction. The mixture was then partitioned between CH2Cl2 and water.
The organic layer was washed with dilute hydrochloric acid, aqueous solution of sodium
bicarbonate, and brine, and dried over anhydrous MgSO4 and further evaporated under
reduced pressure to obtain a yellow solid. The crude product dissolved in dichloromethane
was purified by column chromatography (silica gel, CH2Cl2), followed by recrystallization
from THF/diethyl ether to obtain the monomer G2, 7. 1H NMR (400 MHz, CDCl3, δ, ppm):
8.26–8.01 (m, 3H, Ar-H); 7.97–7.94 (m, 32H, Ar-H), 7.46–7.38 (q, 1H, =CH-); 7.00–6.96
(m, 32H, Ar-H), 6.66 (s, 4H, Ar-H), 6.58 (s, 2H, Ar-H), 6.54 (s, 8H, Ar-H), 6.39 (s, 4H,
Ar-H); 5.77–5.72 (dd, 1H, =CH2); 5.42–5.39 (dd, 1H, =CH2); 5.29, 5.27 (s, 4H, -OCH2-),
4.95 (s, 8H, -OCH2-), 4.01–3.93 (m, 32H, -OCH2-), 3.87 (s, 24H, -OCH3), 1.81–1.78 (m,
32H, -CH2-), 1.53–1.51 (m, 32H, -CH2-). Anal. Calcd. for C204H220N16O32: C, 71.89; N,
6.58; H, 6.51; found: C, 71.76; N, 6.53; H, 6.49.
2.4. Polymerization
The polymers were each obtained by conventional solution radical polymerization (see
Scheme 1), typically carried out as described in the following example.
Monomer G1 (0.5 g, 0.3 mmol), 0.01 M of AIBN (150 μL) in chlorobenzene solution,
and dried chlorobenzene (1.00 g) were placed in a 25 mL reaction tube with a magnetic stir
bar. After three freeze-pump-thaw cycles, the tube was sealed under vacuum. Polymeriza-
tion was carried out at 60◦C for 24 hr. The sample was diluted with THF and precipitated
into a large volume of hot acetone. The sample was purified by similarly re-precipitating
three times from THF into hot acetone, and dried overnight at room temperature in
vacuo.
3. Results and Discussion
3.1. Synthesis of the Monomers and the Polymers
As shown in Scheme 1, the monomers G1 and G2 were successfully synthesized
through multistep reactions. The preparation and characterization of the materials, 4-(6-
bromohexyloxy)-4ꢀ-methoxy azobenzene, has been described elsewhere [39]. First, methyl
3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzoate was prepared by the etherifica-
tion of methyl 3, 5-dihydroxybenzoate with 4-(6-bromohexyloxy)-4ꢀ-methoxy azobenzene
in a hot potassium salt solution of acetone. Then, 3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy
azobenzene) benzyl alcohol was synthesized through the reduction between methyl 3, 5-di
(4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzoate and LiAlH4. Finally, the monomer G1
was obtained by the esterification of vinylterepthaloyl chloride and 3, 5-di (4ꢀ-methoxy-4-
oxyhexyloxy azobenzene) benzyl alcohol. The crude product dissolved in dichloromethane
was purified by column chromatography (silica gel, CH2Cl2), followed by recrystalliza-
tion from THF/diethyl ether to obtain the monomer G1. The synthetic protocol to prepare
monomer G2 was as follows: 3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl chlo-
ride was obtained by the chlorination of 3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene)
benzyl alcohol with SOCl2. Methyl 3, 5-di [3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene)
benzyloxy] benzoate was prepared by the etherification of methyl 3, 5-dihydroxybenzoate

Azobenzene Dendritic Polymers
35
with 3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl chloride in a hot potassium
salt solution of acetone. Then, 3, 5-di [3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene)
benzyloxy] benzyl alcohol was synthesized through the reduction between methyl 3, 5-di
[3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyloxy] benzoate and LiAlH4. The
last step was completed by the esterification of vinylterepthaloyl chloride and 3, 5-di
[3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyloxy] benzyl alcohol. The crude
product dissolved in dichloromethane was purified by column chromatography (silica gel,
CH2Cl2), followed by recrystallization from THF/diethyl ether to obtain the monomer G2.
1
The chemical structure of the monomers G1 and G2 was confirmed by H NMR (see
Fig. 1).
The polymers PG1 were each obtained by conventional solution radical polymerization.
The polymers synthesized are fusible and soluble in common organic solvents including
THF, dimethylformamide (DMF), chlorobenzene, and so forth. However, it is difficult for
the polymer PG2 to obtain high MW materials because the bulky dendritic fragments
inhibit the formation of polymers completely. One may speculate that the polymer PG2
molecular resembled a capsule or a cage-like environment in which the polymerizable
groups were jacketed by their own dendritic coat. As we know, due to the high mass of
the dendritic monomers (MW = 3408 for monomer G2), their molar concentration for
the same weight percent concentration as of vinylterephthalic acid was 18 times lower.
The increasing of concentration at the polymerization place increased dramatically the rate
of polymerization and decreased initiator efficiency since the polymerizable groups were
located in a very small volume fraction while the radical initiator was distributed uniformly
in the reaction mixture. The high mass of monodendrons and their self-assembly forced
slow their diffusion and the monodendrons were not able to migrate from their assembly
to the reactive growing chain. However, the high concentration of monomer G2 was found
to be difficultly obtained in common organic solvents and be difficultly polymerized to a
sufficiently high MW [13,14]. The polymers synthesized are fusible and soluble in common
organic solvents including THF, DMF, CH2Cl2, CHCl3, and so forth. GPC analysis was
carried out to determine the apparent MW and MW distributions of the polymers, and the
molecular characterization of the polymers is summarized in Table 1.
Table 1. GPC, DSC, and TGA results and thermotropic properties of the polymers
Transition (◦C) and
corresponding
enthalpy change
Polymera Mnb (×10−4
)
Mw/Mn
Tg (◦C)c
(J/g)c
Td (N2) (◦C)d
b
PG1-1
PG1-2
PG1-3
PG2
2.28
4.27
5.38
2.72
1.32
1.57
1.49
1.11
48
49
48
45
161 (2.21)
162 (0.23) 184 (0.15)
164 (0.07) 185 (0.37)
138 (2.59)
353
356
364
349
aPG1 and PG2 represented the first-generation dendronized polymer and the second-generation
dendronized polymer.
bDetermined by GPC in THF using polystyrene standards.
cEvaluated by DSC during the second heating cycle at a rate of 10◦C min−1
.
d5% weight loss temperature evaluated by TGA under a nitrogen atmosphere at a heating rate of
20◦C min−1
.

36
C. -A. Yang et al.
Figure 1. 1H NMR spectra of the monomers G1 (a) and G2 (b) in CDCl3-d.
3.2. Phase Transitions and Phase Structures
TGA was employed to characterize the thermal stabilities of the polymers. As could be
seen from Table 1, the polymers were quite stable, 5% weight loss occurring only above
340◦C in nitrogen atmosphere. The phase transitions of the polymers were studied by DSC,
and the transition temperatures and corresponding enthalpy changes were also summarized

Azobenzene Dendritic Polymers
37
Figure 2. DSC curves of dendronized polymers PG1 and PG2 during the second heating at a rate of
10◦C min−1
.
in Table 1. Figure 2 depicts a set of DSC heating trances of the three PG1 samples with
different MWs and one PG2 sample recorded at a rate of 10◦C min−1 during the second
heating process. From Fig. 2 and Table 1, it will be seen that all the glass transition
temperatures (Tg) of the three PG1 samples with different MWs were ∼48◦C, and the
glass transition temperatures (Tg) of the PG2 sample was ∼45◦C. The Tg of the polymers
decreased slightly with the generation number of the dendritic azobenzene side groups
increased. The polymer PG1-1 had two phase transitions, attributed to the glass transition
and the LC-isotropic phase transition, and the LC phase transition temperature (TLC-I) was
161◦C. For the polymer PG1-2, careful examination revealed that two LC phase transitions
were clearly observed after Tg upon heating and their LC phase transition temperatures
were 162◦C and 184◦C, respectively. In addition, the polymer PG1-3 also had two LC phase
transitions and the LC phase transition temperatures were 164◦C and 185◦C, respectively.
However, for the polymer PG2 had only two phase transitions, attributed to the glass
transition and the LC-isotropic phase transition, and the LC phase transition temperature
(TLC-I) was 138◦C.
The optical textures shown by the three PG1 samples with different MWs and one
PG2 sample were observed using PLM. At temperature below the highest phase transition
temperature (TLC-I = 161◦C), the polymer PG1-1 exhibited a representative schlieren
nematic texture (see Fig. 3a), which implied that the nematic phase formed. When the
sample was heated above their highest phase transition temperature, the schlieren nematic
texture disappeared and isotropic phase could be observed. Both the other two PG1 samples
with different MWs gave similar PLM textures. When the two samples were heated to a
temperature much higher than the highest phase transition temperature, the LC birefringence
was still observed and remained before decomposition. These phenomena were all similar
to other MJLCPs [32,33]. Using PG1-3 as an example, Figs. 3(b)–(d) show three typical

38
C. -A. Yang et al.
Figure 3. PLM images of the sample PG1-1 at (a) 150◦C; the sample PG1-3 at (b) 100◦C, (c) 175◦C,
and (d) 200◦C; the sample PG2 at (e) 120◦C. (Magnification: ×200).

Azobenzene Dendritic Polymers
39
Figure 4. 1D WAXD patterns of the sample PG1-1 obtained during the second heating at different
temperatures.
PLM images recorded at 100◦C, 175◦C, and 200◦C, respectively, corresponding to the three
LC phases. When the PG2 was heated above Tg, an obvious fan-shaped texture appeared
(see Fig. 3e), indicating that the smectic phase formed. When the sample was heated above
their highest phase transition temperature, however, the fan-shaped texture disappeared and
isotropic phase could also be observed. Combining the DSC and PLM experimental results
together, we consider that the PG1s with low MWs have one LC phase, and the PG1s with
higher MWs have three LC phases and share a same phase transition sequence. For the low
MW PG2, only one LC phase was observed.
To further elucidate the phase structures and transitions more clearly, 1D WAXD
experiments were performed at different temperatures. Herein, the samples PG1-1, PG1-3,
and PG2 will be used as the examples. Figure 4 depicts a set of 1D WAXD patterns of
the polymer PG1-1 recorded during heating from 40◦C to 170◦C. As shown in Fig. 4, no
diffraction peak can be identified in the low-angle region during the heating from 40◦C
to 170◦C, and an intense broad peak in the high-angle region was seen below the highest
phase transition temperature (T = 161◦C), which indicated that only a short range order
exists in the molecular lateral packing. When the sample is heated above 161◦C, a typical
amorphous pattern in the high-angle region is observed, implying the formation of the
isotropic phase (see Fig. 4). On the basis of the results of DSC and 1D WAXD, we confirm
that the LC transition of PG1-1 follows a sequence of nematic phase (N) ↔ isotropic
state.
Figure 5 represents a set of 1D WAXD patterns of the polymer PG1-3 recorded during
heating process. As shown in Fig. 5, five diffraction peaks at 2.82◦ (d spacing of 3.13 nm),
3.04◦ (d spacing of 2.93 nm), 4.76◦ (d spacing of 1.86 nm), 5.06◦ (d spacing of 1.75 nm),
and 5.64◦ (d spacing of 1.57 nm) can be indentified at 100◦C, and can be assigned as
(110), (200), (020), (120), and (220) diffractions of a 2D ΦR phase with a = 5.86 nm,
b = 3.70 nm, and γ = 90◦. When the sample was heated above the second phase tran-
sition temperature (164◦C), five diffraction peaks at 2.70◦ (d spacing of 3.27 nm), 2.97◦
(d spacing of 2.97 nm), 4.48◦ (d spacing of 1.97 nm), 4.97◦ (d spacing of 1.78 nm),

40
C. -A. Yang et al.
Figure 5. 1D WAXD patterns of the sample PG1-3 obtained during the second heating at different
temperatures.
and 5.39◦ (d spacing of 1.64 nm) can be indentified at 175◦C, and can be assigned as
(110), (200), (300), (310), and (220) diffractions of a 2D ΦR phase with a = 5.94 nm,
b = 3.92 nm, and γ = 90◦. Notably, when the temperature overpass the third phase transi-
tion temperature (185◦C), it led to a suddenly enhancement of the diffraction peak intensity
at 2.74◦ (d spacing of 3.22 nm) and the peak position slightly shifted to higher 2θ angles,
in addition, peak at 2θ = 2.97◦ at 175◦C disappears and two distinct diffraction peaks at
4.76◦ (d spacing of 1.86 nm) and 5.49◦ (d spacing of 1.61 nm) could be observed at 200◦C.
The scattering vectors of the three peaks are found to be in the ratio 1:31/2:2. The three
peaks can be assigned as (100), (110), and (200) diffractions, demonstrating a long-range-
ordered hexagonal lattice with a = b = 3.22 nm and γ = 120◦. Further heating the sample,
the 2D long-range-ordered ΦH phase still remained before the sample decomposed. This
diffraction behavior is similar to that observed from MJLCPs containing two triphenylene
units in the side-chains [40]. The sample renders a diffuse scattering halo at 2θ of ∼20◦ (d
spacing of ∼0.44 nm) in the high-angle region at lower temperatures, which contributing
from the packing of azobenzene-containing side-chains. When the temperature exceeds
164◦C (the second phase transition temperature), the halo turns into a typical amorphous
halo, indicating that the first LC phase transition is the melting of SmA-like structure and
entering the isotropic phase.
Figure 6 depicts a set of 1D WAXD patterns of the sample PG2 recorded during heating
from 40◦C to 150◦C. As indicated by the arrows in Fig. 6, the two diffraction peaks at 2.94◦
(d spacing of 3.00 nm) and 4.44◦ (d spacing of 1.99 nm) can be indentified at 120◦C. The
scattering vectors of the two peaks are found to be in the ratio 2:3 and can be assigned
as (200) and (300) diffractions, indicating a typical smectic phase structure. When the
sample is heated above the LC phase transition temperature (TLC-I, 138◦C), the two peaks
disappear, indicating that the sample enters an isotropic state. Combining DSC and WAXD
results, it is proposed that for PG2, the LC phase transition seen is from the smectic phase
to isotropic.
Combined with DSC, PLM, WAXD experiments results, we propose that the LC phase
structures of the dendronized polymer containing dendritic azobenzene side groups depend

Azobenzene Dendritic Polymers
41
Figure 6. 1D WAXD patterns of the sample PG2 obtained during the second heating at different
temperatures.
strongly on the MWs of polymers. For the PG1-1 with low MW, it only forms nematic
phase at low temperature. Heating the polymer above the LC phase transition temperature,
the isotropic state can be observed. With increasing the MWs, however, the PG1-3 can
form a hierarchically ordered structure with double orderings on both the nanometer and
subnanometer length scales, most likely, the thick main-chains obtained by “jacketing” the
central rigid portion of side-chain to the polyethylene backbone construct a 2D centered
rectangular (ΦR) scaffold at lower temperature and a 2D hexagonal columnar (ΦH) at
higher temperature. The azobenzene-containing side-chains pack inside the main-chain
scaffold form smecitc A (SmA)-like structure and are perpendicular to the main-chains at
lower temperature. With increasing temperature, the side-chains lose gradually the order
and become isotropic state. The sequence of phase transitions of the polymer PG1-3 is
followed: 2D centered rectangular ΦR scaffold of main-chain and SmA-like structure of
side-chain ↔ 2D centered rectangular ΦR scaffold of main-chain and isotropic side-chain
↔ 2D long-range-ordered hexagonal columnar ΦH phase of main-chain and isotropic
side-chain. Further heating the polymer, the 2D long-range-ordered ΦH phase of main-
chain still remained before decomposed. Moreover, the generation of dendritic azobenzene
side groups does also play an important role in controlling their phase behaviors and
phase transitions. For the second-generation polymer PG2, it is difficult to obtain high
MW materials. However, we successfully synthesized lower MW PG2, which can form
an SmA-like packing. Compared the low MW PG2 with the low MW PG1-1, we found
that the degree of order in LC phase increased with increasing the generation of dendritic
azobenzene side groups.
4. Conclusions
The first- and second-generation novel dendronized polymers containing azobenzene meso-
gen were designed and successfully synthesized via free radical polymerization. These
are poly(2, 5-bis {[3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy azobenzene) benzyl] oxycarbonyl}
styrene) (denoted as PG1) and poly(2, 5-bis {3, 5-di [3, 5-di (4ꢀ-methoxy-4-oxyhexyloxy

42
C. -A. Yang et al.
azobenzene) benzyl] oxycarbonyl} styrene) (denoted as PG2). The chemical structure of
the monomers was confirmed by elemental analysis and 1H NMR, and the corresponding
polymers were studied by GPC, DSC, PLM, and 1D WAXD. The results show that the MWs
and the generation of dendritic azobenzene side groups of the polymers play important roles
in controlling their phase behaviors and phase transitions.
Acknowledgment
This work is supported by the National Natural Science Foundation of China (NNSFC
Grants: 21106037), the Scientific Research Fund of the Hunan Provincial Education De-
partment (No: 11A040), and Hunan Provincial Natural Science Foundation of China (No:
12JJ4015).
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