Soluble main-chain azobenzene polymers via thermal 1,3-dipolar cycloaddition: Preparation and photoresponsive behavior

Article abstract of DOI:10.1021/ma902671m

Two soluble polymers containing azobenzene chromophore in main chain were successfully synthesized from α-azide, ω-alkyne A-B type azobenzene monomers, 3′-ethynylphenyl[4-hexoxyl(2-azido2-methylpropionate)phenyl] azobenzene (EHPA) and 3′-ethynylphenyl[4-(

Full text of DOI:10.1021/ma902671m

2704 Macromolecules 2010, 43, 2704–2712
DOI: 10.1021/ma902671m
Soluble Main-Chain Azobenzene Polymers via Thermal 1,3-Dipolar
Cycloaddition: Preparation and Photoresponsive Behavior
Xiaoqiang Xue, Jian Zhu, Zhengbiao Zhang, Nianchen Zhou, Yingfeng Tu, and Xiulin Zhu*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and
Materials Science of Soochow University, Suzhou 215123, China
Received December 2, 2009; Revised Manuscript Received February 9, 2010
ABSTRACT: Two soluble polymers containing azobenzene chromophore in main chain were successfully
synthesized from R-azide, ω-alkyne A-B type azobenzene monomers, 30-ethynylphenyl[4-hexoxyl(2-azido-
2-methylpropionate)phenyl]azobenzene (EHPA) and 30-ethynylphenyl[4-(4-azidobutoxy)phenyl]azobenzene
(EAPA), via thermal 1,3-dipolar cycloaddition in bulk. Compared to the polymers obtained from Cu(I)-
catalyzed 1,3-dipolar cycloaddition (“click” chemistry), the polymers obtained from thermal 1,3-dipolar
cycloaddition showed good solubility in common solvents like CHCl3 and THF and good film-forming
ability. The polymers were thermally stable up to 330 ꢀC. The structures of the main-chain azobenzene
1
polymers were characterized by gel permeation chromatography (GPC), H NMR, UV-vis, and FT-IR
spectra. The photoinduced trans-cis isomerization of the polymers in chloroform (CHCl3) solution was
examined. With illumination of linearly polarized Krþ laser beam at 413.1 nm, surface relief gratings formed
on PEHPA2 spin-coating films were investigated.
Introduction
“click” chemistry is shown as a powerful tool in material synthe-
sis.13 Recently, there have been a few papers of azobenzene
Over the past two decades, polymers containing azobenzene
moieties (azobenzene polymers) have been the subject of intensive
research due to their potential applications in photoresponsive
and optoelectronic fields,1-4 such as optical data storage, liquid
crystal displays, optical switching, and holographic surface relief
gratings (SRGs). The key attractive features are unique reversible
trans-cis-trans isomerization cycles of the azobenzene chromo-
phore upon exposure to UV or visible light, which allows orien-
tational distribution perpendicular to the direction of the polari-
zation. When azobenzene polymers in thin film are irradiated
with linear polarized light, the photoisomerization possibly
induced large-scale shape and dipole movement of the rigid
azobenzene chromophore to produce surface relief gratings
(SRGs).5 This property had attracted much attention since it
was first reported by Natansohn and Tripathy in 1995.6 For
decades, many kinds of such azobenzene polymers have been
developed, including azobenzene chromophore in the side chain6,7
or in the polymer main chain.8 Compared to the side-chain
polymers, main-chain azobenzene polymers showed good ther-
mal stability, which plays an important role in photoinduced
dichroism, birefringence, nonlinear optical properties, and sur-
face profile gratings.9 However, synthesis of the main-chain
azobenzene polymers is not easy,8-10 as it is normally synthesized
by condensation polymerization, and the resulting polymer is
hard to be dissolved. The synthesis of main-chain azobenzene
polymers is a challenge to polymer chemists, and that calls for the
introduction of a new method.
polymers prepared by step-growth polymerization using “click”
chemistry.14,15 Xiaoqin Shen et al.14 synthesized 1,2,3-triazole-
linked azobenzene dendrons of four generations by click reac-
tion and investigated trans-cis isomerization of the polymers.
Li and co-workers15 reported new azobenzene-containing hyper-
branched and dendronized polymers through “click” chemistry
reactions with copper(I) catalysis and demonstrated good second-
order nonlinear optical properties. However, those main-chain
polymers synthesized via “click” chemistry were found to have
solubility problem, unless long alkyl chains were attached to the
benzene rings.16,17 Furthermore, these materials showed poor
film-forming abilities and mechanical properties, which made it
hard to prepare clarity film device.18 Thus, it was hard to further
investigate these polymer properties for photoresponsive appli-
cations.
The poor solubility of polymers may be caused by the regular
structure of polymer backbone as well as the formation of
polymer complexes due to the coordination of the copper ion
with the nitrogen-rich heteroatom rings.16,18,19 Fortunately, in
the absence of copper catalysis and organic solvents, the thermal
1,3-dipolar cycloaddition reaction of monomers with azide and
alkyne groups yield polymers containing irregular structures of
1,4- and 1,5-substituted 1,2,3-triazole mixtures with high conver-
sion.20 The obtained main-chain polymers showed excellent
solubility and film-forming ability.18,20
Following this route, in this paper we selected the A-B type
monomer 30-ethynylphenyl-[4-(4-azidobutoxy)phenyl]azobenzene
(EAPA), which formed polymers insoluble in common organic
In the 1980s, 1,3-dipolar cycloaddition was systematically
studied by Huisgen.11 Since then, the copper(I)-catalyzed Huis-
gen 1,3-dipolar cycloaddition of azide and terminal alkyne
(typical model of “click” chemistry)12 has drawn widespread
attention due to its high efficiency, quantitative yields, and
selectivity under mild reaction conditions. For these reasons,
solvents polymerized via CuSO4 5H2O/sodium ascorbate/H2O
3
“click” way,21 to polymerize via the alternate way, e.g., through
the thermal 1,3-dipolar cycloaddition reaction. The following
property investigations of the polymers showed that the obtai-
ned polymers were soluble in tetrahydrofuran (THF) and N,N-
dimethylformamide (DMF). However, the attempt to prepare a
uniform film for further photoresponsive characterization was
*Corresponding author: e-mail xlzhu@suda.edu.cn, Tel þ86-512-
65111278, Fax þ86-512-65112796.
pubs.acs.org/Macromolecules
Published on Web 02/17/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 6, 2010 2705
Scheme 1. Synthetic Route of 30-Ethynylphenyl[4-hexoxyl(2-azido-2-methylpropionate)phenyl]azobenzene (EHPA)
unsuccessful due to its poor film-forming ability. To further
improve solubility and film-forming ability of the obtained main-
chain azobenzene polymer, an EAPA analogue A-B type azoben-
zene monomer, 30-ethynylphenyl[4-hexoxyl(2-azido-2-methyl-
propionate)phenyl]azobenzene (EHPA), which contained long
hexyl chain and ester, was synthesized. The corresponding poly-
mer PEHPA via the thermal 1,3-dipolar cycloaddition reaction in
bulk showed good thermal stability and trans-cis-trans isomeri-
zation ability. Meanwhile, SRGs formed on the polymer PEH-
PA’s spin-coating films were investigated for the first time with
illumination of linearly polarized Krþ laser beam at 413.1 nm.
ArCtCH) (spectrum was provided in the Supporting Informa-
tion, Figure S1). Elemental analysis: Calculated (%): C 75.66,
H 4.54, N 12.60. Found (%): C 75.31, H 4.34, N 13.11.
30-Ethynylphenyl[4-(6-hydroxyhexoxy)phenyl]azobenzene (2).
A solution of compound 1 (8.88 g, 40.0 mmol), 6-chlorohexanol
(5.44 g, 40.0 mmol), potassium carbonate (5.52 g, 40.0 mmol), a
catalytic amount of potassium iodide, and 150 mL of N,N-
dimethylformamide (DMF) was prepared in a 250 mL round-
bottom flask under vigorous stirring. The solution was stirred
under reflux at 110 ꢀC for 5 h. After cooling to room tempera-
ture, the mixture was poured into 500 mL of water under
vigorous stirring. The precipitate was collected by filtration,
washed with deionized water three times, and dried under
vacuum. The final crude product was purified by recrystalliza-
tion from ethanol to yield compound 2 as yellow solid (11.9 g,
92.1%). 1H NMR (CDCl3), δ (TMS, ppm): 7.99 (s, 1H, ArH),
7.95-7.87 (d, 2H, ArH), 7.87-7.82 (d, 1H, ArH), 7.56-7.51 (d,
1H, ArH), 7.48-7.41 (m, 1H, ArH), 7.05-6.96 (d, 2H, ArH),
4.10-4.00 (m, 2H, ArOCH2), 3.73-3.62 (m, 2H, -CH2OH),
3.12 (s, 1H, ArCtCH), 1.89-1.78 (m, 2H, -CH2-), 1.67-1.35
(m, 4H, -CH2-) (spectrum was provided in Supporting Infor-
mation, Figure S2). Elemental analysis: Calculated (%): C 74.51,
H 6.88, N 8.69. Found (%): C 74.71, H 6.62, N 8.32.
Experimental Section
Materials. 3-Ethynylaniline (g98%; Aldrich), phenol (analytical
reagent; Shanghai Chemical Reagent Co. Ltd., Shanghai, China),
6-chlorohexanol (98%; Aldrich), 2-bromoisobutyryl bromide
(98%; Aldrich), 1,4-dibromobutane (analytical reagent; Shanghai
Chemical Reagent Co. Ltd., Shanghai, China), and sodium azide
(g99.5%; Aldrich) were used as received. N,N-Dimethylforma-
mide (Analytical reagent; Shanghai Chemical Reagent Co. Ltd.,
Shanghai, China) was purified by vacuum distillation over anhy-
drous calcium hydride (CaH2) before use. Other reagents were
purified using standard procedure before use. EAPA (1H NMR
and 13C NMR spectrum of EAPA is shown in the Supporting
Information, Figures S6 and S7) was synthesized according to the
method described in the literature.21
6-[4-(3-Ethynylphenylazo)phenoxy]hexyl-2-bromo-2-methylpro-
panoate (3). Compound 2 (9.66 g, 30.0 mmol), freshly distilled
THF (100 mL), and dry triethylamine (4.04 g, 40.0 mmol) were
added to a 250 mL three-necked flask. The solution was stirred
in an ice bath. A solution of 2-bromoisobutyryl bromide (8.05 g,
35.0 mmol) in dry THF (30 mL) was added dropwise to the
mixture at 0-5 ꢀC. The reaction mixture was vigorously stirred
for another 5 h at 0-5 ꢀC and then at room temperature
overnight. After filtration, the filtrate was dried under vacuum.
The remaining yellow mixture was dissolved in dichloromethane
and washed with 5% Na2CO3 aqueous solution and deionized
water for three times. After being dried with anhydrous MgSO4
overnight, dichloromethane was evaporated under reduced
pressure. The final crude product was purified by column chro-
matography (silica gel, ethyl acetate/petroleum ether = 1:5) to
Synthesis of 30-Ethynylphenyl[4-hexoxyl(2-azido-2-methyl-
propionate)phenyl]azobenzene (EHPA). The synthetic route of
EHPA is shown in Scheme 1, and detailed synthetic procedures
and characterization data follow.
30-Ethynylphenyl(4-hydroxy)azobenzene (1). 3-Ethynylaniline
(5.85 g, 50.0 mmol) was added dropwise to a solution of
concentrated HCl (37.0%, 15 mL) in deionized water (30 mL).
The mixture was stirred in an ice bath to keep the reaction
temperature at 0-5 ꢀC. Then a water solution (10 mL) of sodium
nitrite (3.50 g, 50.7 mmol) was added slowly within 10 min.
A yellow transparent diazonium salt solution was obtained,
after reacted for 60 min at 0-5 ꢀC. Meanwhile, a coupling
solution was prepared as follows: phenol (8.00 g, 85.0 mmol),
NaOH (4.00 g, 100 mmol), and NaHCO3 (4.20 g, 50.0 mmol)
were dissolved in 250 mL of water under vigorous stirring at 0-5ꢀC.
Then the diazonium salt solution was added dropwise to the
coupling solution within 20 min at 0-5 ꢀC. The final mixture
was reacted at 5 ꢀC for 3 h. The precipitate was collected by
filtration, washed with deionized water three times, and dried
under vacuum. The crude product was purified by recrystalliza-
tion from ethanol. Compound 1 was then obtained as red-
orange crystal (10.0 g, yield 90.0%). 1H NMR (CDCl3), δ
(TMS, ppm): 8.04-7.97 (s, 1H, ArH), 7.94-7.83 (m, 3H, ArH),
7.62-7.52 (d, 1H, ArH), 7.50-7.42 (m, 1H, ArH), 7.00-6.91
(d, 2H, ArH), 5.36-5.27 (s, 1H, ArOH), 3.17-3.10 (s, 1H,
1
yield compound 3 as saffron solid (10.6 g, 75.1%). H NMR
(CDCl3), δ (TMS, ppm): 8.00 (s, 1H, ArH), 7.95-7.89 (d, 2H,
ArH), 7.89-7.84 (d, 1H, ArH), 7.58-7.53 (d, 1H, ArH),
7.49-7.43 (m, 1H, ArH), 7.03-6.98 (d, 2H, ArH), 4.24-4.17
(m, 2H, ArOCH2), 4.09-4.02 (m, 2H, -CH2O-), 3.13 (s, 1H,
ArCtCH), 1.94 (s, 6H, -CH3), 1.90-1.80 (m, 2H, -CH2-),
1.80-1.70 (m, 2H, -CH2-), 1.62-1.42 (m, 4H, -CH2-)
(spectrum was provided in Supporting Information, Figure
S3); Elemental analysis: Calculated (%): C 61.15, H 5.77,
N 5.94. Found (%): C 60.92, H 5.53, N 5.83.
30-Ethynylphenyl[4-hexoxyl(2-azido-2-methylpropionate)phenyl]-
azobenzene (EHPA). Compound 3 (9.40 g, 20.0 mmol), DMF
(200 mL), sodium azide (1.92 g, 30.0 mmol), and deionized water
(10 mL) were added to a 500 mL round-bottomed flask

2706 Macromolecules, Vol. 43, No. 6, 2010
Xue et al.
Scheme 2. Synthetic Route of Polymers PEAPA1 and PEHPA1 by Cu(I)-Catalyzed 1,3-Dipolar Cycloaddition (“Click” Chemistry) of
30-Ethynylphenyl[4-(4-azidobutoxy)phenyl]azobenzene (EAPA) and 30-Ethynylphenyl[4-hexoxyl(2-azido-2-methylpropionate)phenyl]azobenzene
(EHPA), Respectively, Using 5% CuSO4 5H2O and 10% Sodium Ascorbate as Catalyst in DMF at Room Temperature, and Polymers
3
PEAPA2 and PEHPA2 by Thermal 1,3-Dipolar Cycloaddition
equipped with a stir bar and a condenser. The mixture was
vigorously stirred under reflux at 80 ꢀC for 24 h and then cooled
to room temperature. Then water (300 mL) was added. The mix-
ture was extracted with ethyl acetate (3ꢀ100 mL). The organic
layer obtained was dried with anhydrous MgSO4 overnight,
filtered, and evaporated in a reduced pressure. The final crude
product was purified by column chromatography (silica gel,
ethyl acetate/petroleum ether = 1:8) to yield EHPA as yellow
of polymerization was determined gravimetrically (0.396 g yield:
91.4%). The obtained polymer PEHPA1 was soluble in chloro-
form, THF, and DMF. 1H NMR and 13C NMR spectra of poly-
mer PEHPA1 were obtained with CDCl3 as solvent. The number-
average molecular weight (Mn) and molecular weight distribution
(Mw/Mn) values were determined by GPC calibrated with PS
standards (Mn,GPC =18 000 g mol-1, Mw/Mn =2.00).
Preparation of PEHPA2 and PEAPA2 by Thermal 1,3-Dipo-
lar Cycloaddition. The synthetic route is shown in Scheme 2.
A dry 10 mL ampule tube was filled with EAPA (0.638 g, 20.0
mmol) and then placed in an oil bath at 80 ꢀC. A transparent and
viscous liquid was obtained. The reaction mixture polymerized
continuously at 80 ꢀC for 12 h. Thereafter, the polymerization
was transferred into another oil bath at 120 ꢀC for 6 h. The
sample was postcured at 150 ꢀC for 6 h, and the polymerization
was terminated. The contents were diluted with 20 mL of
tetrahydrofuran (THF) and precipitated into 250 mL of metha-
nol. The precipitate (PEAPA2) was filtrated and dried to a
constant weight at room temperature in vacuum. The conver-
sion of polymerization was determined gravimetrically (0.520 g,
yield: 81.5%). The 1H NMR spectrum of polymer PEAPA2 was
obtained with CDCl3 as solvent. The Mn and Mw/Mn values were
determined by GPC (Mn,GPC=29 400 g mol-1, Mw/Mn=2.44).
Polymer PEHPA2 was prepared using similar procedures
(0.749 g, yield 86.5%). 1H NMR and 13C NMR spectra of poly-
mer PEHPA2 were obtained with CDCl3 as solvent. The Mn
1
solid (7.10 g, 82.0%). H NMR (CDCl3), δ (TMS, ppm): 8.02
(s, 1H, ArH), 7.99-7.80 (m, 3H, ArH), 7.66-7.52 (d, 1H, ArH),
7.52-7.41 (m, 1H, ArH), 7.07-6.91 (d, 2H, ArH), 4.29-4.12
(m, 2H, ArOCH2), 4.12-3.95 (m, 2H, -CH2N3), 3.15 (s, 1H,
ArCtCH), 1.92-1.62 (m, 4H, -CH2CH2-), 1.62-1.40 (m, 10H,
-CH2CH2- and -CH3) (spectrum was provided in Supporting
Information, Figure S4). 13C NMR (CDCl3), δ (TMS, ppm):
173.1, 162.1, 152.7, 146.9, 133.8, 129.3, 126.2, 125.2, 123.5,
123.2, 114.9, 83.3, 78.0, 68.3, 66.1, 63.4, 29.3, 28.7, 25.89, 24.7
(spectrum was provided in Supporting Information, Figure S5).
Elemental analysis: Calculated (%): C 66.49, H 6.28, N 16.16.
Found (%): C 66.12, H 6.35, N 16.50. FTIR (KBr, Figure 4
(EHPA)): γmax/cm-1 3260, 2950, 2100, 1720, 1600, 1500, 1470,
1260, 1150, 1000, and 900.
Preparation of PEHPA1 and PEAPA1 by Cu(I)-Catalyzed
1,3-Dipolar Cycloaddition (“Click” Chemistry). The main-chain
azobenzene polymer PEAPA1 (Scheme 2) was synthesized as
described in a previous report.21 According to the last work, the
obtained polymers PEAPA1 was insoluble in common solvents
such as chloroform, THF, and DMF. The monomer EHPA
contained long hexyl chains and ester group should improve the
solubility of corresponding azobenzene polymer PEHPA1. The
typical experiment process of PEHPA1 was followed. EHPA
and Mw/Mn values were determined by GPC (Mn,GPC
=
31 900 g mol-1, Mw/Mn =1.91).
Preparation of the Polymer Film and Optical Characterization.
Thin films of the polymer PEHPA2 were obtained by the
following procedure. A CHCl3 solution of the polymer PEAPA2
(0.05 g/mL) was filtered through 0.22 μm pore size filter. Thin
films were obtained via spin-coating the solution onto clean
glass slide at 3000 rpm. The thicknesses of homogeneous thin
films were measured to be about 225 and 456 nm by Ambios
Technology XP-2 Stylus Profiler and subsequently dried under
vacuum for 24 h at room temperature. The films were allowed to
store in a desiccator for further study.
(0.433 g, 10.0 mmol) and CuSO4 5H2O (0.002 50 g, 0.500 mmol)
3
were dissolved in 10 mL of DMF in a single-necked flask of
50 mL, and then a solution of sodium ascorbate (0.196 g, 1.00
mmol) in deionized water (0.5 mL) was added dropwise to the
mixture under vigorous stir at room temperature. The reac-
tion mixture was vigorously stirred in sealed system for 24 h.
After that, the content was poured into 250 mL of methanol.
The polymer was obtained by filtration and dried at room
temperature in vacuum to a constant weight. The conversion
The procedure for SRGs fabrication can be found in a related
reference (illustrated in Figure 1).22 The SRGs measurement

Article
Macromolecules, Vol. 43, No. 6, 2010 2707
was performed at room temperature with linearly polarized Krþ
laser beam (413.1 nm, 200 mW/cm2) as the light source and
optically inscribed on the film with p-polarized interfering laser
beam. The surface images of the SRGs were detected using
atomic force microscopy (AFM, NT-MDT SOLVER P47-PRO).
The dynamic diffraction efficiency of the positive first order was
measured using an unpolarized low-power diode laser beam
(650 nm) in transmission mode.
UV-vis spectra were determined on a Hitachi U-3900 spectro-
photometer at room temperature.
Results and Discussion
Synthesis of Polymers via 1,3-Dipolar Cycloaddition. The
step-growth click chemistry of EAPA as monomer was
carried out efficiently using 5% CuSO4 5H2O and 10%
3
Analysis and Characterizations. The number-average mole-
cular weight (Mn) and molecular weight distribution (Mw/Mn) of
the polymers were determined from a Waters 1515 gel permea-
tion chromatograph (GPC) equipped with a refractive index
sodium ascorbate as the catalyst in DMF. As our previously
report,21 the formed polymer, PEAPA1, was insoluble in
common organic solvents. The reason may be caused by
regioregular 1,2,3-triazole and azobenzene group in the main
chain21 and the formation of complexes of copper ion with
polymer triazole rings in final product.19 In order to improve
the solubility of this kind main-chain azobenzene polymer,
monomer EHPA (analogue of EAPA) containing long alkyl
chain and ester structures was synthesized following
Scheme 1. This monomer was polymerized under the same
˚
detector, using HR1 (pore size: 100 A, 100-5000 Da), HR2
˚
˚
(pore size: 500 A, 500-20 000 Da), and HR4 (pore size 10 000 A,
50-100 000 Da) columns (7.8ꢀ300 mm, 5 μm beads size) with a
molecular weight range of 100-500 000 Da, calibrated with PS
standard samples. Tetrahydrofuran (THF) was used as the
eluent at a flow rate of 1.0 mL min-1 operated at 30 ꢀC. GPC
samples were injected using a Waters 717 plus autosampler. 1H
NMR and 13C NMR spectra were recorded on an INOVA
400 MHz nuclear magnetic resonance (NMR) instrument, using
CDCl3 as the solvent and tetramethylsilane (TMS) as the inter-
nal standard. Differential scanning calorimetry (DSC) was
performed using a TA Instruments DSC2010 with a heating/
cooling rate of 10 ꢀC min-1 from 25 to 600 ꢀC under a conti-
nuous nitrogen flow. The DSC2010 instrument was calibrated
by pure indium for temperature and enthalpy changes. Samples
(3-5 mg) were crimped in standard aluminum pans. First-order
transitions were taken at the maximum point of endothermic or
exothermic peaks and the glass transition temperature (Tg) at
the midpoint of the heat capacity jump. Thermogravimetric
analysis (TGA) was performed at a heating rate of 10 ꢀC/min
from room temperature to 600 ꢀC under a continuous nitrogen
flow of 50 mL min-1 with a TA Instruments SDT-2960TG/
DTA. The temperature of thermal degradation (Td) was mea-
sured at the point of 5% weight loss relative to the weight at
room temperature. FT-IR spectra were recorded on a Nicolette-
6700 FT-IR spectrometer. Elemental analysis of C, H, and N
was conducted with an EA1110 CHNO-S instrument. The
condition of EAPA, e.g., using 5% CuSO4 5H2O and 10%
3
sodium ascorbate as the catalyst. The obtained polymer,
PEHPA1, was soluble in common organic solvents, such as
CHCl3, THF, and DMF. The number-average molecular
weight (Mn) of PEHPA1 was 18 000 g/mol by gel permeation
chromatography (GPC) using THF as the eluent and poly-
styrene as the calibration standard (shown in Table 1). This
result indicates that the introducing of long alkyl chain and
ester structures successfully improved the solubility of main-
chain azobenzene polymers.
However, attempts on film photoproperty investigations
of PEHPA1 were unsuccessful due to its poor film-forming
ability via spin-coating. Thus, an alternative procedure was
used to improve the film-forming ability of polymer. As
mentioned in the Introduction, thermal 1,3-dipolar cyclo-
addition of alkynes and azides produced polymer with regio-
random structure in good yields, and the introduction of
copper ion into polymer system was also hindered. There-
fore, monomer EAPA and EHPA were attempted to poly-
merize via thermal 1,3-dipolar cycloaddition to obtain
corresponding polymer PEAPA2 and PEHPA2, respec-
tively. The reactions were carried out in bulk with a tem-
perature procedure of 80 ꢀC/12 h, 120 ꢀC/6 h, and 150 ꢀC/6 h.
Figure 2 shows the appearance of reaction mixture
(monomer EHPA) before and after successive step-growth
polymerization. The obtained polymer (PEHPA2) showed
as a clear solid. The number-average molecular weights of
the obtained polymers PEAPA2 and PEHPA2 were esti-
mated by GPC as 29 400 and 31 900 g/mol. The number-
average molecular weight of PEHPA2 (Mn = 31 900) was
higher than that of PEHPA1 (Mn = 18 000 g/mol). As pre-
dicted, this thermal 1,3-dipolar cycloaddition polymeriza-
tion of monomer EAPA afforded polymer PEAPA2 with
good solubility in common solvents, such as THF and DMF.
Figure 1. Experimental setup for recording and detecting surface relief
gratings (SRGs): PBS, a polarization beam splitter; M, a mirror; HWP,
a half-wave plate; LD, a laser diode that emits 650 nm beam; D, a first-
order diffracted beam intensity detector.
Table 1. Characteristics Data of Polymers PEAPA1, PEAPA2, PEHPA1, and PEHPA2
conversiona (%)
Mnb (g/mol)
Mwb (g/mol)
Mw/Mn
Tgc (ꢀC)
Tdd (ꢀC)
solubility
b
sample
catalyst
PEAPA1e
PEAPA2f
PEHPA1e
PEHPA2f
CuSO4/SA
thermal
CuSO4/SA
thermal
89.3
81.5
91.4
86.5
134
78
88
357
352
337
340
29 400
18 000
31 900
71 900
36 000
61 000
2.44
2.00
1.91
THF,g DMFg
CHCl3,g THF, DMF
CHCl3, THF, DMF
83
a Conversion determined by gravimetry. b Determined by gel permeation chromatography (GPC) using tetrahydrofuran (THF) as the eluent. Mn: the
number-average molecular weight. Mw: the weight-average molecular weight. Mw/Mn: molecular weight distribution. c The glass transition temperature
(Tg) was measured using a TA Instruments DSC2010 with a heating/cooling rate of 10 ꢀC min-1 under a continuous nitrogen flow. d The temperature of
thermal degradation (Td) was measured at the point of 5% weight loss relative to the weight at room temperature with a TA Instruments SDT-2960TG/
e
DTA. Polymers PEAPA1 and PEHPA1 were obtained by Cu(I)-catalyzed 1,3-dipolar cycloaddition of 30-ethynylphenyl[4-(4-azidobutoxy)-
phenyl]azobenzene (EAPA) and 30-ethynylphenyl[4-hexoxyl(2-azido-2-methylpropionate)phenyl]azobenzene (EHPA), respectively, using 5% CuSO4
3
5H2O and 10% sodium ascorbate as catalyst in N,N-dimethylformamide (DMF) at room temperature. f Polymers PEAPA2 and PEHPA2 were obtained
by thermal 1,3-dipolar cycloaddition in bulk of EAPA and EHPA, respectively, with a temperature procedure of 80 ꢀC/12 h, 120 ꢀC/6 h, and 150 ꢀC/6 h.
g Soluble in THF, DMF, CHCl3 (chloroform).

2708 Macromolecules, Vol. 43, No. 6, 2010
Xue et al.
Figure 2. Images of monomer 30-ethynylphenyl[4-hexoxyl(2-azido-2-
methylpropionate)phenyl]azobenzene (EHPA) in a dry 10 mL ampule
tube via thermal 1,3-dipolar cycloaddition: before polymerization (A)
at 80 ꢀC and after polymerization (B) at 150 ꢀC. The polymerization
reactions were carried out in bulk with a temperature procedure of
80 ꢀC/12 h, 120 ꢀC/6 h, and 150 ꢀC/6 h.
Figure 4. FT-IR spectra of 30-ethynylphenyl[4-hexoxyl-(2-azido-2-methyl-
propionate)phenyl]azobenzene (EHPA): (A); PEHPA2 after polymeri-
zation thermal 1,3-dipolar cycloaddition polymerization of the mono-
mer EHPA in bulk at 80 ꢀC/12 h (Mn,GPC=4300 g/mol) (B); 80 ꢀC/12 h
þ 120 ꢀC/6 h þ 150 ꢀC/6 h (Mn,GPC=31 900 g/mol) (C); and PEHPA1
(Mn,GPC = 18 000 g/mol) catalyzed 1,3-dipolar cycloaddition (“click”
chemistry) of the monomer EHPA using 5% CuSO4 5H2O and 10%
3
sodium ascorbate as catalyst in DMF at room temperature (D).
reacted to form triazole ring after thermal activated step-
growth polymerization. However, all of these final polymers
showed a weak absorption at about 2100 cm-1, which were
caused by the remaining of terminal unreacted azide group.
The polymerization course of PEHPA2 was also followed
by GPC. When the reaction mixture of monomer EHPA was
polymerized continuously at 80 ꢀC for 12 h, the number-
average molecular weight of PEHPA was 4300 g/mol. After
the polymerization was further carried out at 120 ꢀC for 6 h,
the number-average molecular weight of PEHPA increased
to 18 600 g/mol. The final polymer showed the number-
average molecular weight of 31 900 g/mol after further reac-
tion at 150 ꢀC for 6 h. It can be seen that the number-average
molecular weights measured by GPC increased with the
reaction time and temperature.
Figure 3. FT-IR spectra of monomer 30-ethynylphenyl[4-(4-azidobu-
toxy)phenyl]azobenzene (EAPA) (A); PEAPA2 after thermal 1,3-dipo-
lar cycloaddition polymerization of the monomer EAPA in bulk at
80 ꢀC/12 h (B); 80 ꢀC/12 h þ 120 ꢀC/6 h þ 150 ꢀC/6 h (C); and PEAPA1
catalyzed 1,3-dipolar cycloaddition (“click” chemistry) of the monomer
EAPA using 5% CuSO4 5H2O and 10% sodium ascorbate as catalyst
To further confirm the polymerization reaction, 1H NMR
spectra of PEHPA1 and PEHPA2 were investigated and are
shown in Figure 5. Compared to the 1H NMR spectrum of
EHPA (monomer, Figure 1), the chemical shift of the alkyne
group at around 3.15 ppm in EHPA (h in Figure 1) rapidly
decreased in the spectra of PEHPA1 and PEHPA2, which
indicated the consuming of alkyne group during the step-
growth polymerization. A weak chemical shift at 3.15 ppm
3
in DMF at room temperature (D).
Most importantly, the obtained PEHPA2 showed good film-
forming property, which would be significantly favored in
the application of optics device, such as SRG.
Structural and Thermal Characterization of the Polymers.
The course of polymerization reaction was first followed by
Fourier transform infrared (FT-IR) spectra. The FT-IR
spectra of PEAPA2 and PEHPA2 at different polymeriza-
tion stages are shown in Figure 3, respectively. The signals at
3280 cm-1 (ꢁC-H), 2148 cm-1 (CꢁC) and 2109 cm-1 (-N3)
(Figure 3A) were the characteristic absorption peaks of the
alkyne and azide groups in EAPA. As shown in Figure 3B,C,
the characteristic absorption peaks at 3280, 2148, and 2109 cm-1
decrease as the reaction proceeds, which indicated the gra-
dual consumption of the reactive groups during the thermal
reaction. In addition, the FT-IR spectra of PEAPA2 were
similar to that of PEAPA1 in Figure 3D. Similar changes in
FT-IR during the polymerization of EHPA are shown in
Figure 4. The characteristic absorption peaks of the alkyne at
3270 cm-1 and azide groups at 2100 cm-1 of EHPA
(Figure 4A) rapidly decreased under thermal (Figure 4B,C)
or Cu(I)-mediated (Figure 4D) 1,3-dipolar cycloaddition reac-
1
showed in the H NMR of polymers should be assigned to
the remaining alkyne group at the terminal of polymer main
chain. After polymerization, the signals of methyl groups at
1.31-1.62 ppm (j in Figure 1) adjacent to azide functionality
shifted to 2.86-2.10 ppm (g in Figure 5). A new resonance
signal at 8.27 ppm (a in Figure 5) appeared and can be
assigned to the proton of the 1,2,3-triazole ring (1,4-regio-
isomer) (PEHPA1 and PEHPA2). Furthermore, the mole-
cular weight can be calculated from the 1H NMR spectrum
according to the ratio of the polymer terminal groups to the
polymer main chain, using the following equation: Mn,NMR =
433*I3.58-4.31/4I3.15, where Mn,NMR is the number-averaged
1
molecular weight calculated from H NMR, I3.58-4.31 is the
integral of the signals (e and f) at 3.58-4.31 ppm, and I3.15 is
the integral of the signals at 3.15 ppm in Figure 5. It can be
estimated that the molecular weights of PEHPA1 and PEH-
PA2 were 8400 and 14 800 g/mol, respectively. The number-
averaged molecular weights obtained by NMR were lower
than the GPC values (Mn,GPC), which could be due to dis-
crepancies in PS standards used for calibrate GPC curves.
tions. On the other hand, the absorption peak at 1050 cm-1
,
corresponding to triazole group, emerged and increased after
thermal treatments, indicating the formation of triazole
group during polymerization process. These results indicated
that the alkyne and azide groups in these two monomers were

Article
Macromolecules, Vol. 43, No. 6, 2010 2709
Figure 6. 13C NMR spectrum of the polymer PEHPA2 prepared by
thermal 1,3-dipolar cycloaddition of 30-ethynylphenyl[4-hexoxyl(2-azido-
2-methylpropionate)phenyl]azobenzene (EHPA) in bulk with a tempe-
rature procedure of 80 ꢀC/12 h, 120 ꢀC/6 h, and 150 ꢀC/6 h (Mn,GPC
=
31 900 g mol-1, Mw/Mn = 1.91) in CDCl3.
Figure 5. 1H NMR spectra of the polymer PEHPA1 prepared by
catalyzed 1,3-dipolar cycloaddition (“click” chemistry) of the monomer,
30-ethynylphenyl[4-hexoxyl(2-azido-2-methylpropionate)phenyl]azoben-
172.0 ppm (n0), 65.7 ppm (e0), and 27.1 ppm (a0) correspond-
ing to the 1,5-regioisomer. Therefore, the 1,4-regioisomeric
ratios (F1,4) of PEHPA2 can be calculated by the 13C NMR
signals as 77% from the similar eqs 1 or 2.
zene (EHPA), using 5% CuSO4 5H2O and 10% sodium ascorbate
3
as catalyst in DMF at room temperature (Mn,GPC = 18000 g mol-1
,
Thus, the NMR results confirmed that the alkyne and
azide groups in EAPA and EHPA successfully formed the
triazole ring in PEAPA2 (see Supporting Information:
Figure S8, PEAPA2, Mn,GPC = 4300 g mol-1, Mw/Mn =
1.86) and PEHPA2 via thermal 1,3-dipolar cycloaddition
reaction. It also proved that the thermal 1,3-dipolar cycload-
dition reactions of terminal alkyne and azide groups pro-
ceeded in a usual regiorandom way including 1,4-regio-
isomer and 1,5-regioisomer.
The obtained main-chain azobenzene polymers were char-
acterized by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) (Figure S9 and S10 in the
Supporting Information). The thermal properties of PEA-
PAs and PEHPAs are summarized in Table 1. The glass
transition temperatures (Tgs) of PEAPA1 and PEHPA1 were
134 and 88 ꢀC, which were higher than that of PEAPA2
(78 ꢀC) and PEHPA2 (83 ꢀC), respectively. This should be
attributed to the introducing of regioisomer via thermal 1,3-
dipolar cycloaddition, which reducing the regular packing of
polymer chain. TGA results of polymers showed that all
polymers were thermally stable up to 330 ꢀC under a nitrogen
atmosphere. The introduction of triazole ring into the poly-
mer backbone plays an additional favorable role in improv-
ing the thermal stability.21,23
Photoisomerization Behaviors. Under 365 nm ultraviolet
irradiation, azobenzene compounds undergo isomerization
from trans- to cis-forms and show photoisomerization beha-
vior. The UV-vis absorption spectra of PEHPA2 in CHCl3
solution were recorded at different time intervals until
photostationary state was reached (Figure 7). The absorp-
tion peak at around 350 nm was the characteristic intense
π-π* transition of trans-form azobenzene in polymer. After
irradiation with 365 nm UV light, the trans-form of azoben-
zene changed to the cis-form, which showed the absorption
peak at 440 nm in the UV-vis spectrum. Meanwhile, the
absorption peak of trans-form azobenzene at 350 nm de-
creased rapidly, while the intensity of the cis-form azoben-
zene (440 nm) slightly increased with the irradiation time.
Photoisomerization behavior of monomer EHPA and poly-
mer PEHPA1 was similar to that of PEHPA2.
Mw/Mn = 2.00) and PEHPA2 prepared by thermal 1,3-dipolar
cycloaddition of EHPA in bulk with a temperature procedure of 80 ꢀC/
12 h, 120 ꢀC/6 h, and 150 ꢀC/6 h (Mn,GPC = 31 900 g mol-1, Mw/Mn =
1.91) in CDCl3.
It should be noted that in the 1H NMR spectrum of
PEHPA2, besides of the strong resonance peaks at
2.06-2.10 ppm (g) and 8.27 ppm (a) for triazole ring of
1,4-regioisomer, there were two weak resonance peaks at
1.86 ppm (g0) and 7.70 ppm (a0). These two weak resonance
peaks can be assigned to the protons in methyl group
adjacent to 1,2,3-triazole ring of 1,5-regioisomer in polymer
PEHPA2. Thus, the regioisomer ratio in polymer PEHPA2
can be calculated from the ratio of the characteristic signals
in Figure 5 (PEHPA2). The 1,4-regioisomeric ratio (F1,4) was
given by eqs 1 and 2:
0
F1;4 ¼ Ig=ðIg þ Ig Þ
ð1Þ
ð2Þ
0
0
F1;4 ¼ Ia=ðIa þ Ia Þ
where Ig is the integral of the signals at 2.06-2.10 ppm (g),
0
Ig is the integral of the signals at 1.86 ppm (g0), Ia is the
0
integral of the signals at 8.27 ppm (a), and Ia is the integral of
the signals at 7.70 ppm (a0) in Figure 5 (PEHPA2). From the
eqs 1 and 2, the 1,4-regioisomeric ratios (F1,4, F1,40) of
PEHPA2 were calculated to be 78% and 76%, respectively.
The small difference should be caused by the error in integral
of 1H NMR spectrum. Moreover, the 13C NMR spectrum of
polymer PEHPA2 also indicated that polymer PEHPA was
successfully obtained by the thermal 1,3-dipolar cycloaddi-
tion reaction in bulk (see the Figure 6). The signals at 83.3
and 78.0 ppm (i and h in Figure S5) due to the alkyne group in
the monomer (EHPA) disappeared after 1,3-dipolar cyclo-
addition reaction. Two new resonance signals at 119.5 and
119.1 ppm (h in Figure 6) were assigned to the carbons in the
1,2,3-triazole ring. The signals at 173.1, 63.4, and 24.7 ppm
(m, e, and a in Figure S5) due to carbons adjacent to azide
group shifted to 171.5, 64.8, and 25.9 ppm (n, e, and a in
Figure 6) for the formation of 1,2,3-triazole ring after
polymerization. Besides of the strong resonance peaks n, e,
and a in Figure 6 for triazole ring of 1,4-regioisomer in
polymer PEHPA2, there were also weak resonance peaks at
The rate of trans-cis photoisomerization of EHPA, PEH-
PA1, and PEHPA2 in CHCl3 solution was analyzed. The

2710 Macromolecules, Vol. 43, No. 6, 2010
Xue et al.
Figure 9. AFM images of the SRGs formed on PEHPA2 (thermal 1,3-
dipolar cycloaddition of the monomer 30-ethynylphenyl[4-hexoxyl-
(2-azido-2-methylpropionate)phenyl]azobenzene (EHPA) in bulk with
a temperature procedure of 80 ꢀC/12 h, 120 ꢀC/6 h, and 150 ꢀC/6 h;
Mn,GPC = 18 000 g mol-1; Mw/Mn = 2.00) film via spin-coating on the
clean glass with thickness of 456 nm. 3D view of the SRG (A) and plane
view of the SRG (B).
Figure 7. UV-vis absorption changes of polymer PEHPA2 prepared
by thermal 1,3-dipolar cycloaddition of the monomer 30-ethynylphenyl-
[4-hexoxyl(2-azido-2-methylpropionate)phenyl]azobenzene (EHPA)
in bulk with a temperature procedure of 80 ꢀC/12 h, 120 ꢀC/6 h, and
150 ꢀC/6 h (Mn,GPC =31 900 g mol-1, Mw/Mn =1.91) under different
time interval during the irradiation time with 365 nm UV light in CHCl3
at room temperature. The concentration of solution is 5.0 ꢀ 10-6 M.
Figure 10. Diffraction efficiency as a function of the laser irradiation
time for PEHPA2 (thermal 1,3-dipolar cycloaddition of the monomer
30-ethynylphenyl[4-hexoxyl(2-azido-2-methylpropionate)phenyl]azoben-
zene (EHPA) in bulk with a temperature procedure of 80 ꢀC/12 h,
120 ꢀC/6 h, and 150 ꢀC/6 h; Mn,GPC =18000 g mol-1; Mw/Mn =2.00)
film via spin-coating on the clean glass with thickness of 225 and
456 nm.
Figure 8. First-order trans-cis isomerization kinetic of monomer 30-ethy-
nylphenyl[4-hexoxyl(2-azido-2-methylpropionate)phenyl]azobenzene
(EHPA), PEHPA1 (prepared by catalyzed 1,3-dipolar cycloaddition of
constants. The results confirmed there were the accordant
rates of trans-cis photoisomerization for EHPA, PEHPA1,
and PEHPA2, which showed first-order photoisomerization
behavior of azobenzene units.
EHPA using 5% CuSO4 5H2O and 10% sodium ascorbate as catalyst
3
in DMF at room temperature, Mn,GPC = 18000 g mol-1, Mw/Mn =
2.00), and PEHPA2 (prepared by thermal 1,3-dipolar cycloaddition of
the monomer EHPA in bulk with a temperature procedure of 80 ꢀC/
12 h, 120 ꢀC/6 h, and 150 ꢀC/6 h, Mn,GPC =31 900 g mol-1, Mw/Mn =
1.91) in chloroform solution under different time interval at room
temperature. The concentration of the solution is 5.0 ꢀ 10-6 M.
Photoinduced Surface Relief Gratings. The obtained poly-
mer PEHPA2 has good film-forming property, which pro-
vides the opportunity for the investigation of its surface relief
grating (SRG) ability. Thereby, thin films of polymer PEH-
PA2 were prepared via spin-coating on the clean glass. The
homogeneous thin film thicknesses were measured to be 225
and 456 nm. Then, the SRG measurement5-7 was performed
at room temperature with linearly polarized Krþ laser beam
and characterized by the inscription rates and the saturation
levels of SRGs formation. Atomic force microscopy (AFM)
observation indicated that surface relief structures with
regular spaces were fabricated on the film surfaces of PEH-
PA2. For the film with thickness of 456 nm, Figure 9 shows
AFM three-dimensional (A) and plane (B) images of the
alternate surface relief structures with regular spaces fabri-
cated on the polymer film surfaces after Krþ laser irradiated
for 1000 s at room temperature. From AFM images, the
surface modulation depth was about 74 nm, and the grating
spacing was about 1400 nm. Moreover, the rates of gratings
formation can be probed by measuring the first-order dif-
fraction efficiency of the SRG. Figure 10 shows the increase
photoisomerization kinetics of these compounds are plotted
in Figure 8. The results confirmed the first-order kinetic plot
for the trans-cis photoisomerization of EHPA, PEHPA1,
and PEHPA2. The first-order rate constant was determined
by formula 3
ꢀ
ꢁ
A¥ - At
A¥ - A0
ln
¼ -ket
ð3Þ
where A¥, A0, and At are absorbance at 350 nm at time
infinite, time zero, and time t, respectively. Deduced from
Figure 8, the photoisomerization rate constant, ke, of EHPA,
PEHPA1, and PEHPA2 was 0.014, 0.011, and 0.010 s-1
,
respectively. The rate constant of EHPA was the highest
among these three compounds due to the low trans-to-cis
transition resistance existed in small molecular. Two poly-
mers, e.g., PEHPA1 and PEHPA2, showed similar rate

Article
Macromolecules, Vol. 43, No. 6, 2010 2711
of the diffraction efficiency of SRG with the irradiation time.
The diffraction efficiency of the polymer film rapidly in-
creased to about 0.82% until saturation level at 1000 s. In the
same way, the diffraction efficiency of SRG with a thickness
of 225 nm film was up to about 0.28% with saturation level at
1300 s, which was much lower than that of sample with a
thickness of 456 nm.24
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Conclusion
Four polymer samples labeled as PEAPA1, PEAPA2, PEH-
PA1, and PEHPA2 were prepared from the monomer 30-ethy-
nylphenyl[4-(4-azidobutoxy)phenyl]azobenzene (EAPA) and
30-ethynylphenyl[4- hexoxyl(2-azido-2-methylpropionate)phenyl]-
azobenzene (EHPA). The polymers PEAPA1 and PEHPA1 were
obtained through copper(I) catalysis “click” chemistry technique,
while PEAPA2 and PEHPA2 were obtained through thermal 1,3-
dipolar cycloaddition technique. All of these polymers showed
high thermally resistance (Td above 330 ꢀC). The polymers
PEHPA2 and PEAPA2 showed better solubility than PEHPA1
and PEAPA1 in common organic solvents due to the formation
of regiorandom structures as well as no complex of copper ion
with polymer triazole rings. Most of all, PEHPA2 showed good
film-forming ability, which offered the opportunity to explore
these polymers for optical devices for the first time. The film
prepared from PEHPA2 showed efficient surface relief gratings
(SRGs) formation ability. The diffraction efficiency from SRG
with film thickness of 465 nm was measured to be about 0.82%.
The reported result provided a convenient alternative route to
preparation main-chain azobenzene polymers with good solu-
bility and film formation property. This should be favored
in the investigation of optical devices based on azobenzene poly-
mers.
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Acknowledgment. The financial support from the National
Nature Science Foundation of China (Nos. 20874069, 50803044,
20974071, and 20904036), the Specialized Research Fund for the
Doctoral Program of Higher Education contract grant (No.
200802850005), and the Qing Lan Project and the Program of
Innovative Research Team of Soochow University is gratefully
acknowledged.
Supporting Information Available: 1H NMR spectra of the
compounds, monomer EAPA, EHPA, polymer PEAPA2
(Mn,GPC = 4300 g mol-1, Mw/Mn = 1.86) in CDCl3; 13C NMR
spectra of the monomer EAPA, EHPA, polymer PEHPA2
in CDCl3; DSC and TGA spectra of polymers. This mate-
rial is available free of charge via the Internet at http://pubs.
acs.org.
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