Synthesis and characterization of azobenzene-functionalized poly(styrene)-b-poly(vinyl acetate) via the combination of RAFT and " click" chemistry

Article abstract of DOI:10.1016/j.polymer.2010.04.052

Here, we described a strategy for preparing well-defined block copolymers, poly(styrene)-b-poly(vinyl acetate) (PS-b-PVAc), containing middle azobenzene moiety via the combination of the reversible addition-fragmentation chain transfer (RAFT) polymerizati

Full text of DOI:10.1016/j.polymer.2010.04.052

Polymer 51 (2010) 3083e3090
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of azobenzene-functionalized poly
(styrene)-b-poly(vinyl acetate) via the combination of RAFT and “click” chemistry
*
Xiaoqiang Xue, Jian Zhu, Zhengbiao Zhang, Zhenping Cheng, Yingfeng Tu, Xiulin Zhu
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science of Soochow (Suzhou) University, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Here, we described a strategy for preparing well-defined block copolymers, poly(styrene)-b-poly(vinyl
acetate) (PS-b-PVAc), containing middle azobenzene moiety via the combination of the reversible
addition-fragmentation chain transfer (RAFT) polymerization and “click” chemistry. Firstly, a novel RAFT
Received 19 December 2009
Received in revised form
7 March 2010
Accepted 26 April 2010
Available online 20 May 2010
agent containing
carbonyl)prop-2-yl-9H-carbazole-9-carbodithioate (EACDT), was synthesized and used to mediate the
RAFT polymerization of styrene (St). Well-defined -alkyne end-functionalized poly(styrene) (PS) was
obtained. Secondly, the RAFT polymerization of vinyl acetate (VAc) was conducted using functionalized
RAFT reagent with -azide structure in Z group, O-(2-azidoethyl) S-benzyl dithiocarbonate (AEBDC).
Well-defined -azide end-functionalized poly(vinyl acetate) (PVAc) was obtained. Afterwards, the
resulting -alkyne terminated PS was coupled by “click” chemistry with the azide terminated PVAc. The
a-alkyne and azobenzene chromophore in R group, 2-(3-ethynylphenylazophenoxy
a
Keywords:
Click chemistry
Reversible addition-fragmentation chain
transfer (RAFT)
u
u
a
block copolymer, PS-b-PVAc, was obtained with tailored structures. The products from each step were
characterized and confirmed by GPC, 1H NMR, IR and differential scanning calorimetry (DSC) examina-
tion. Kinetics of the trans-cis-trans isomerization from azobenzene chromophore in PS-b-PVAc and PS
were investigated in CHCl3 solutions.
Azo polymer
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
system of i-Bu3Al/2,20-bipyridine/TEMPO [7], iodides [8], cobalt-
mediated [9], iron-catalyzed [10] radical polymerization, and photo-
The “living”/controlled free radical polymerization (LFRP) tech-
niques have been fast developed in the past decades. Many efficient
systems have been well developed especially for the atom transfer
radical polymerization (ATRP) [1], reversible addition-fragmenta-
tion chain transfer (RAFT) polymerization [2] and nitroxyl radical
mediated polymerization (NMP) [3]. The development of LFRP
makes it convenient to realize precise controlling over the poly-
merization process (controlled molecular weights and low poly-
dispersity index) under mild conditions. Many complex structure
polymers covered with a wide monomers range have been synthe-
sized via LFRP, such as block, star, and other complex architectures
for styrene (St), (meth)acrylate and so on. However, it is still a chal-
lenge to control free radical homo- and copolymerization of vinyl
acetate (VAc), a typical unconjugated vinyl monomer, due to the
high reactivity of its propagation radicals, which undergo radical-
eradical termination and chain transfer on the polymer and
monomer [4]. There are limited LFRP techniques to control the free
radical polymerization of VAc: RAFT polymerization using xanthates
[5] or dithiocarbamates as RAFT agents [6], NMP of VAc using the
living radical polymerization [11].
Till very recently reported by Rizarrdo et al. it was able to
prepare block copolymer of VAc with conjugated vinyl monomer,
such as styrene and (meth)acrylate, using a new class of “switch-
able” RAFT agents [12]. On the other hand, there were several
routes reported for the synthesis of well-controlled PS-b-PVAc
block copolymers by combination of different methods. For
example, Zi-Chen Li et al. obtained well-defined block copolymer
PS-b-PVAc by combination of ATRP and RAFT polymerization [13].
Combination of cobalt-mediated radical polymerization (CMRP)
and ATRP had also been performed to be a novel method [14].
Recently, Sharpless et al. successfully optimized 1,3-dipolar
cycloaddition of azide and terminal alkyne leading to 1, 2, 3-triazole
via copper (I) catalysis [15]. This concept of popular “click” chem-
istry gained most attention for its high efficiency, quantitative
yields and selectivity under mild reaction conditions [16]. This
novel method had been successfully applied to block copolymer PS-
b-PVAc synthesis. Stenzel and his coworkers synthesized two RAFT
agents with azide or alkyne group [17]. They used these RAFT
agents to prepare homopolymers of poly(vinyl acetate) and poly
(styrene) with terminal azide and alkyne group respectively. Then
well-defined block copolymer, PS-b-PVAc was prepared via “click”
chemistry between these two homopolymers.
* Corresponding author. Fax: þ86 512 65112796.
E-mail address: xlzhu@suda.edu.cn (X. Zhu).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.04.052

3084
X. Xue et al. / Polymer 51 (2010) 3083e3090
Azobenzene-based materials have gained a great deal of atten-
vacuum at room temperature. Other reagents were purified using
tion due to potential application in fascinating photo-responsive
variations [18,19], such as optical data storage, liquid crystal
displays, optical switching, holographic surface relief gratings
(SRGs) and so on. Meanwhile, azobenzene-terminated polymers
have also been applied in photochromic probes [20] and photo-
responsive polymers [21]. Lydie Viau et al. reported azobenzene-
based ATRP initiator to synthesize photoisomerizable star-shaped
polymer by atom transfer radical polymerization (ATRP) of methyl
methacrylate (MMA) [22]. Photoisomerization properties of such
new metallochromophores have been studied. Our group also
prepared well-defined azobenzene-terminated poly(methyl
methacrylate) (PMMA) [23] via ATRP and poly(styrene) (PS) [24] via
RAFT polymerization, respectively. These polymers showed typical
trans-cis isomerization behavior under UV light irradiation.
standard procedure before use.
2.2. Synthesis of 2-(3-ethynylphenylazophenoxy-carbonyl)prop-2-
yl-9H-carbazole-9-carbodithioate (EACDT)
The
a-alkyne RAFT reagent EACDT containing azobenzene
chromophore was synthesized according to the route showed in
Scheme 2.
2.2.1. 30-Ethynylphenyl-(4-hydroxy)azobenzene (1)
3-Ethynylaniline (5.85 g, 50 mmol) was added dropwise to
a solution of concentrated HCl (37%, 15 mL) in deionized water
(30 mL). The mixture was stirred in an ice bath to keep the reaction
temperature at 0e5 ꢂC. Then a water solution (10 mL) of sodium
nitrite (3.50 g, 50.7 mmol) was added within 10 min. The mixture
was stirred at 0e5 ꢂC for further 60 min. A yellow transparent
diazonium salt solution was obtained. A coupling solution was
prepared as follows: phenol (8 g, 85 mmol), NaOH (4 g, 100 mmol)
and NaHCO3 (4.2 g, 50 mmol) was dissolved in 250 mL of water
under vigorous stirring at 0e5 ꢂC. Then the diazonium salt solution
was added dropwise to the coupling solution with the temperature
of 0e5 ꢂC. The final mixture was stirred at 5 ꢂC 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 recrystallization from ethanol. Compound 1 was
obtained as red-orange crystal (10.0 g, yield: 90.0%).
Herein, we synthesized a novel
a-alkyne RAFT reagent con-
taining azobenzene chromophore, 2-(3-ethynylphenylazophenoxy
carbonyl)prop-2-yl-9H-carbazole-9-carbodithioate (EACDT) and
a functionalized xanthate agent with
u-azido-Z group, O-(2-azi-
doethyl) S-benzyl dithiocarbonate (AEBDC) [25], as shown in
Scheme 1. Subsequently, the RAFT polymerization of styrene and
VAc was carried out using EACDT and AEBDC as RAFT reagent,
respectively. Afterwards, the resulting
a-alkyne terminated poly
(styrene) (PS) was coupled by “click” chemistry with the
u
-azido-
terminated poly(vinyl acetate) (PVAc) to obtain well-defined block
copolymer poly(styrene)-b-poly(vinyl acetate) containing middle
azobenzene moiety. The trans-cis-trans isomerization of PS-b-PVAc
and PS with azobenzene chromophore was investigated in CHCl3
solution. It showed adjustable isomerization rate before and after
“click” reaction. The current work opened the opportunity for
synthesizing conjugated and unconjugated vinyl block copolymers
with azobenzene structure incorporated.
The characteristic analytical data involved are as follows: 1H
NMR (400 MHz, CDCl3),
d (TMS, ppm): 8.04e7.97 (s, 1H, ArH),
7.94e7.83 (m, 3H, ArH), 7.62e7.52 (d, 1H, ArH), 7.50e7.42 (m, 1H,
ArH), 7.00e6.91 (d, 2H, ArH), 5.36e5.27 (s, 1H, ArOH), 3.17e3.10
(s,1H, ArC^CH); Elemental analysis: calculated (%): C 75.66, H 4.54,
N 12.60; found (%): C 75.31, H 4.34, N 13.11.
2.2.2. 2-Bromo-2-methylpropionic acid 4-(3-ethynyl-phenylazo)
phenyl ester (2)
2. Experimental section
The compound 1 (6.66 g, 30 mmol), dry THF (150 mL) and dry
triethylamine (6.06 g, 60 mmol) was added to a 250 mL three-
necked flask. The solution was stirred in an ice bath. A solution of
2-bromoisobutyryl bromide (10.35 g, 45 mmol) in dry THF (30 mL)
was added dropwise to the mixture with the temperature at
0e5 ꢂC. The reaction mixture was vigorously stirred for another 3 h
at 0e5 ꢂC, and then at room temperature overnight. After filtration,
the filtrate was evaporated under vacuum. The remaining yellow
mixture was dissolved in dichloromethane and washed with 5%
Na2CO3 aqueous solution followed with deionized water for three
times. The organic solution was dried with anhydrous MgSO4
overnight. Then, dichloromethane solvent was evaporated under
a reduced pressure. The final crude product was purified by column
chromatography (silica gel, ethyl acetate/petroleum ether ¼ 1:5) to
yield compound 2 as saffron solid (6.32 g, 62.1%).
2.1. Materials
3-Ethynylaniline (ꢀ98%; Aldrich), phenol (analytical reagent;
Shanghai Chemical Reagent Co. Ltd, Shanghai, China), carbazole
(98%; Aldrich), carbon disulfide (analytical reagent; Shanghai
Chemical Reagent Co. Ltd, Shanghai, China), 2-bromoisobutyryl
bromide (98%; Aldrich), and sodium azide (ꢀ99.5%; Aldrich) were
used as received. Vinyl acetate (99%; Shanghai Chemical Reagents
Co.) was purified by passing over a column of basic alumina and
subsequently distilled under reduced pressure and kept in
a refrigerator at ꢁ15 ꢂC. Styrene (analytical reagent; Shanghai
Chemical Reagent Co. Ltd, Shanghai, China) was washed with an
aqueous solution of sodium hydroxide (5 wt%) three times and then
with deionized water until neutralization. After being dried with
anhydrous magnesium sulfate, the monomers were distilled twice
under reduced pressure, and kept below ꢁ15 ꢂC. 2,20-Azobisiso-
butyronitrile (AIBN, 99%; Shanghai Chemical Reagent Co. Ltd,
China) was recrystallized three times from ethanol and dried in
The characteristic analytical data involved are as follows: 1H
NMR (400 MHz, CDCl3),
d (TMS, ppm): 8.07e8.03 (s, 1H, ArH),
8.02e7.97 (d, 2H, ArH), 7.93e7.88 (d, 1H, ArH), 7.63e7.57 (d, 1H,
ArH), 7.52e7.44 (m, 1H, ArH), 7.35e7.28 (d, 2H, ArH), 3.17e3.13
(s, 1H, ArC^CH), 2.12e2.08 (s, 6H, eCH3); Elemental analysis:
calculated (%): C 58.04, H 4.07, N 7.55; found (%): C 57.82, H 4.33, N
7.83.
2.2.3. 2-(3-Ethynylphenylazophenoxycarbonyl)prop-2-yl-9H-
carbazole-9-carbodithioate (EACDT)
RAFT agent EACDT was synthesized according to the following
procedure: carbazole (1.67 g, 10 mmol) was added to a suspension
of KOH (0.56 g, 10 mmol) in DMSO (25 mL) under vigorous stirring.
The reaction mixture was stirred overnight at room temperature.
Scheme 1. Chemical structures of 2-(3-ethynylphenylazophenoxycarbonyl)prop-2-yl-
9H-carbazole-9-carbodithioate (EACDT) and O-(2-azido-ethyl) S-benzyl dithiocar-
bonate (AEBDC).

X. Xue et al. / Polymer 51 (2010) 3083e3090
3085
NaNO2
0-5 oC
O
Et3N
NH2
N
N
OH
N
N
O
Br
THF
2-bromoisobutyryl bromide
phenol
2
1
S
O
2
KOH
DMSO
CS2
N
S
O
N N
NH
EACDT
Scheme 2. Synthetic route of 2-(3-ethynylphenylazophenoxycarbonyl)prop-2-yl-9H-carbazole-9-carbodithioate (EACDT).
Carbon disulfide (0.76 g, 10 mmol) was added dropwise into this
2.4. RAFT polymerization of styrene using EACDT as RAFT agent
mixture, and the resultant reddish solution was stirred for 10 h at
room temperature, followed by adding slowly compound 2 (3.71 g,
10 mmol) in DMSO (20 mL). The final mixture was stirred for 12 h at
room temperature. The resultant reaction mixture was poured into
large amount of deionized water, and orange red solid was obtained
by filtration. The crude product was purified by recrystallization
three times from ethanol. The pure EACDT was obtained as yellow
crystal (yield: 2.72 g, 51%).
The procedure of RAFT polymerization of styrene (St) was as
follows: A dry 5 mL ampoule tube was filled with St (3 mL,
26.1 mmol), EACDT (0.0463 g, 0.087 mmol) ([St]0: [EACDT]0 ¼ 300:
1) and anisole (2 mL). The content was purged with argon for
10 min to eliminate the oxygen. Then the ampoule was flame-
sealed. The polymerization reaction was performed in an oil bath
by a thermostat at 115 ꢂC and terminated after 12 h. The contents
were diluted with 10 mL tetrahydrofuran (THF) and precipitated
into 250 mL methanol. The precipitate was filtrated and dried to
a constant weight at room temperature in vacuum. The conversion
of polymer was determined by gravimetry.
The characteristic analytical data involved are as follows: 1H
NMR (CDCl3, d): 8.42e8.48 (d, 2H, carbazoleeH), 7.95e8.06 (m, 5H,
carbazoleeH and AreH), 7.83e7.94 (d, 1H, AreH), 7.57e7.62 (d, 1H,
ArH), 7.43e7.52 (m, 3H, carbazoleeH and AreH), 7.33e7.41 (m, 4H,
carbazoleeH and AreH), 3.13e3.17 (s, 1H, ArC^CH), 2.02e2.10
(s, 6H, eCH3). UVevis: lmax ¼ 315 nm (CH3Cl). Elemental analysis:
calculated (%): C 69.77, H 4.34, N 7.87, S 12.02; found (%): C 69.79, H
4.50, N 7.91, S 12.35. The 1H NMR spectrum of EACDT is shown in
Fig. 1.
2.5. RAFT polymerization of vinyl acetate (VAc) using AEBDC
as RAFT agent
The
u-azido-terminated poly(vinyl acetate) (PVAc) was
synthesized as described in a previous report [25]. A dry 10 mL
ampoule tube was filled with 10 mL (118 mmol) of VAc, 0.448 mg
(1.77 mmol) of AEBDC and 48.4.6 mg (0.295 mmol) of AIBN with
a predetermined molar ratio ([VAc]0: [AEBDC]0: [AIBN]0 ¼ 200: 3:
0.5). The content was purged with argon for 20 min to eliminate the
oxygen. Then the ampoule was flame-sealed and placed in an oil
bath held by a thermostat at 80 ꢂC to polymerize. After a preset
reaction time, the polymerization was terminated. The reaction
mixture was diluted with 10 mL THF and precipitated in 250 mL of
hexane. The polymer was obtained by filtration and dried at room
temperature in vacuum to a constant weight. The conversion of
polymerization was determined gravimetrically.
2.3. Synthesis of O-(2-azidoethyl) S-benzyl dithiocarbonate
(AEBDC)
AEBDC was synthesized as described in a previous report [25].
The characteristic analytical data involved are as follows: 1H NMR
(CDCl3, d): 7.25e7.40 (m, 5H, AreH), 4.76 (t, 2H, CH2eOe), 4.42
(s, 2H, CH2eSe), 3.65 (t, 2H, CH2eN3). Elemental analysis: calcu-
lated (%): C 47.41, H 4.38, N 16.59; found: C 47.24, H 4.55, N 16.45.
e
f
2.6. “Click” reactions of the
a-alkyne-terminated poly(styrene) (PS)
b
with the -azido-terminated poly(vinyl acetate) (PVAc)
u
h
h
f
c
b
b
b
e
a
S
O
d
S
O
N N
N
The synthetic route was shown in Scheme 3. A dry 10 mL
ampoule tube was filled with a solution of PS (0.1 mmol), PMDETA
(0.5 equiv), and PVAc (1 equiv) in THF (10 mL). The content was
purged with argon for 10 min to remove oxygen. Then CuBr
(0.5 equiv) was added under argon atmosphere. The content was
purged with argon for another 10 min. After that, the ampule was
flame-sealed. “Click” reaction was performed at 80 ꢂC for 24 h. The
reaction mixture was passed through a column of neutral alumina.
The polymer was obtained by precipitating reaction mixture into
250 mL petroleum ether followed by filtration and vacuum drying.
a
f
b
f
e
g
f
h
g
b
a
e
d
c
2.7. Analysis and characterizations
9
8
7
6
5
4
3
2
1
0
The number-average molecular weights (Mns) and molecular
weight distributions (Mw/Mns) of the polymers were determined
with a Waters 1515 gel permeation chromatographer (GPC)
Chemical shift/ppm
Fig. 1. 1H NMR spectrum of the RAFT agent EACDT in CDCl3.

3086
X. Xue et al. / Polymer 51 (2010) 3083e3090
S
S
O
N
3
N
S
O
N N
O
S
EACDT
St
AEBDC
VAc
S
O
H
C
2
N
S
O
N N
S
m
N
3
O
S
n
O
O
Click chemistry
S
O
H
2
N
S
C
O
N N
N
m
N
N
S
O
S
n
O
O
Scheme 3. Synthetic route of block copolymers poly(styrene)-b-poly(vinyl acetate) containing azobenzene moiety.
equipped with a refractive index detector, using HR1, HR3, and HR4
column with a molecular weight range of 100e500,000 calibrated
with PS and PMMA standard samples. THF was used as the eluent at
a flow rate of 1.0 mL minꢁ1 operated at 30 ꢂC. 1H NMR spectra were
recorded on an INOVA 400 MHz nuclear magnetic resonance (NMR)
instrument, using CDCl3 as solvent, tetramethylsilane (TMS) as the
internal standard. Elemental analysis of C, H, N and S were con-
ducted with an EA1110 CHNO-S instrument. The UVevis spectra
were determined on a Hitachi U-3900 spectrophotometer at room
temperature. Differential scanning calorimetry (DSC) was per-
formed using a TA instruments DSC2010 with a heating/cooling
rate of 10 ꢂC minꢁ1 under a continuous nitrogen flow. FT-IR spectra
were recorded on a Nicolette-6700 FT-IR spectrometer.
end-functionalized polymers (PS). The RAFT polymerization of St
was carried out in anisole using EACDT as RAFT agent and thermal
initiation at 115 ꢂC with molar ratio of [St]0: [EACDT]0 ¼ 300: 1.
Detailed experimental conditions and results were listed in Table 1.
In order to increase the degree of end-functionality, the monomer
conversion was controlled to less than 50%. After the polymeriza-
tion period of 12 h and 15 h, two poly(styrene) (PS), e.g. PS1 and PS2
were obtained with monomer conversions of 11.7%, and 16.5%.
Narrow molecular weight distributions (Mw/Mns ꢃ 1.15) and good
agreement between the number-average molecular weights
measured by GPC (Mn(GPC)) and theoretical values (Mn(th)) indicated
that the EACDT was an effective RAFT agent for St polymerization.
The theoretical values Mn(th) were calculated via Equation (1):
MnðthÞ ¼ ½Stꢄ0=½EACDTꢄ0 ꢅ Mst ꢅ Conversion þ MEACDT
(1)
3. Results and discussions
3.1. RAFT polymerization of styrene (St) and vinyl acetate (VAc)
Table 1
Characteristics of the end-functionalized homopolymers PS, PVAc and block
copolymers PS1-b-PVAc, PS2-b-PVAc.
According to reference [26], dithiocarbamates can be used as
efficient RAFT agents to control the free radical polymerization of
conjugated vinyl monomers due to significantly enhancement of
the activity by introducing electron-withdrawing groups on
nitrogen atom or the delocalization of the nitrogen electron pair.
Moreover, the structure of the N-group within dithiocarbamate has
significant effects on the activity of thiocarbonylthio compounds
for the RAFT polymerization of St. Our group reported that dithio-
carbamate derived from carbazole group in N-group was effective
RAFT agent for the RAFT polymerization of St [24,27]. The poly-
merizations showed good living characteristics. Thus, a novel
f
g
Sample
T (ꢂC) Time (h) Con./Yield(%) Mn(GPC)
Mn(th)
Mw/Mn
PS1a
115
115
80
80
80
12
15
12
24
24
11.7d
16.5d
27.9d
95.2e
94.7e
3800
5000
1800
5100
6700
3600
5100
1600
e
1.10
1.12
1.19
1.34
1.21
PS2a
PVAcb
PS1-b-PVAcc
PS2-b-PVAcc
e
a
[St]0: [EACDT]0
¼
300: 1, at 115 ꢂC in anisole with thermal initiation.
St/anisole ¼ 3:2 (v/v).
b
[VAc]0: [AEBDC]0: [AIBN]0 ¼ 200: 3: 0.5, at 80 ꢂC in bulk.
c
Polymers obtained via “click” chemistry using CuBr and PMDETA as catalyst
system at 80 ꢂC for 24 h.
a
-alkyne RAFT agent containing azobenzene chromophore, 2-(3-
d
Conversion determined by gravimetry.
Yield determined by gravimetry.
Mn(GPC): Molecular weight determined by GPC.
Mn(th): Theoretical molecular weight calculated via Equation (1).
ethynylphenylazophenoxycarbonyl)prop-2-yl-9H-carbazole-9-car-
bodithioate (EACDT), was synthesized and used to mediate the
RAFT polymerization of styrene (St) for preparing well-defined
e
f
g

X. Xue et al. / Polymer 51 (2010) 3083e3090
3087
where, [St]0 and [EACDT]0 were the initial concentration of St and
3.3. End group analysis and thermal characterization
EACDT, respectively, MSt and MEACDT were the molecular weights of
St and EACDT.
VAc can be polymerized under controlled manner via RAFT or
MADIX polymerization using xanthates as chain transfer agents.
To further confirm the molecular structures, all the polymers
were characterized via 1H NMR and FT-IR spectroscopy. Fig. 3
showed the 1H NMR spectroscopy of the homopolymers PS2,
PVAc and block copolymer PS2-b-PVAc. In the spectrum of PS2, the
characteristic signals corresponding to the protons of the alkyne
group (a) were observed at around 3.02e3.16 ppm, which indicated
that the alkyne group did not participate in RAFT polymerizations.
The chemical shifts at around 7.70e8.30 ppm were due to phenyl
protons of the azobenzene group (b) and carbazole (b). These
results indicated that the moiety of the RAFT agent was attached to
the end of PS2. Furthermore, the molecular weight (Mn
Our group successfully synthesized
u-azido-functionalized
xanthate, O-(2-azidoethyl) S-benzyl dithiocarbonate (AEBDC) [25],
and used as chain transfer agent in the polymerization of VAc. The
polymerization results showed that AEBDC was an effective chain
transfer agent for controlling the polymerization of VAc. Thus, well-
controlled poly(vinyl acetate) (PVAc) was obtained with molecular
weight of 1800 g/mol and molecular weight distributions of 1.19.
¼ 5300 g/mol), calculated from the 1H NMR spectrum
(NMR)
3.2. “Click” chemistry between PS and PVAc
(Equation (2)) was close to the value obtained by GPC (Mn
¼ 5000 g/mol). From these data, it can also be estimated the
(GPC)
The obtained
involved in “click” chemistry with
u
-azido-functionalized PVAc was subsequently
-alkyne terminated PS to
degree of end-functionality by Mn(GPC)/Mn(NMR). The result indi-
cated that 86% (5000/5800) chains of the polymers were end-
functionalized with the azobenzene and alkyne group. The Mn(NMR)
was given by Equation (2):
a
prepare block copolymers, poly(styrene)-b-poly(vinyl acetate) (PS-
b-PVAc), containing middle azobenzene chromophore. According
to reference [28], the copper (I) with PMDETA as ligand was effi-
cient catalyst system for the 1,3-dipolar cycloaddition of organic
azides with terminal alkynes. Herein, “click” chemistry of PS and
PVAc was conducted using CuBr and PMDETA as catalyst system,
THF as solvent at 80 ꢂC. Detailed experimental conditions and
results were given in Table 1. PS with molecular weight of 3800 g/
mol and 5000 g/mol was combined with PVAc with molecular
weight of 1800 g/mol. The obtained block copolymers showed
corresponding molecular weight of 5100 g/mol and 6700 g/mol,
which were close to the sum value of its original polymers. GPC
curves of the homopolymers and block copolymers were shown in
Fig. 2. There was an obvious peak shift from the homopolymers to
the copolymer product after the “click” coupling. The molecular
weight increased from 3800 g/mol to 5100 g/mol. However, the
molecular weight distributions of the polymers showed a slight
increase, e.g. from 1.12 of PS and 1.19 of PVAc to 1.21 of block
copolymer. The reason was due to the presence of small portion of
remaining homopolymers. These homopolymers may derive from
the nonend-functionalized polymers, PS and PVAc, in the systems.
Furthermore, it was difficult to operate at the perfect stoichiometry
1: 1 with homopolymers [17,29], which should be resulted in the
remaining of homopolymer after couple reaction.
ꢀ
ꢁ
ꢂ
I1:06e2:30 ꢁ 6 I7:69e8:33
Mn;NMR
¼
ꢅ MSt þ MEACDT
(2)
3
8
I1.06e2.30 is the integral of the signals (d) at 1.06e2.30 ppm, and
I7.69e8.33 is the integral of the signals (a) at 7.69e8.33 ppm in Fig. 3
(PS2).
The spectrum of PVAc in Fig. 3 also confirmed that the RAFT
agent was successfully attached to the end of polymer chain.
Similarly, the molecular weight of PVAc can be calculated from the
1H NMR spectrum (Mn(NMR) ¼ 1900 g/mol). Thus, the degree of end-
functionality (Mn(GPC)/Mn(NMR)) was 95% (1800/1900). These results
were similar to our previous report [25].
Afterwards, the resulting
a-alkyne terminated PS2 was coupled
via “click” chemistry with the
u
-azido-terminated PVAc. The
PS2
PVAc
PS2-
b
-PVAc
14
16
18
20
22
24
26
28
30
Retention time / minutes
Fig. 2. The GPC curves of the homopolymers PS2 (Mn(GPC)
¼
5000 g/mol, Mw
/
Fig. 3. 1H NMR spectra of the homopolymers PS2 (Mn(GPC)
¼
5000 g/mol, Mw/
Mn ¼ 1.12), PVAc (Mn(GPC) ¼ 1800 g/mol, Mw/Mn ¼ 1.19) and copolymer PS2-b-PVAc (Mn
Mn ¼ 1.12), PVAc (Mn(GPC) ¼ 1800 g/mol, Mw/Mn ¼ 1.19) and copolymer PS2-b-PVAc (Mn
¼ 6700 g/mol, Mw/Mn ¼ 1.21).
¼ 6700 g/mol, Mw/Mn ¼ 1.21).
(GPC)
(GPC)

3088
X. Xue et al. / Polymer 51 (2010) 3083e3090
< 365 nm
N N
N N
T or >400 nm
trans- form
cis- form
PVAc
PS
Scheme 4. Illustration of the trans-cis-trans isomerization process of block polymer
PS2-b-PVAc.
and PVAc are immiscible [30], there only one glass transition point
showed in block copolymer at 75 ꢂC, which was between that of
precursor homopolymer PS2 and PVAc. The reason may be due to
the low molecular weight of PS and PVAc in the block copolymer
[31]. No obvious phase separation occurs in current block
copolymer.
Fig. 4. FT-IR spectra of the homopolymers PS2 (Mn(GPC) ¼ 5000 g/mol, Mw/Mn ¼ 1.12),
PVAc (Mn(GPC)
¼
1800 g/mol, Mw/Mn
¼
1.19) and copolymer PS2-b-PVAc (Mn
¼ 6700 g/mol, Mw/Mn ¼ 1.21).
(GPC)
3.4. Photo and thermal isomerization behaviors
Azobenzene polymers exhibited photo or thermal isomeriztion
behavior, which undergoes conversion from trans- to cis-forms
under the irradiation of 365 nm ultraviolet and reverse cis-to-trans
forms under thermal energy in dark [32] (see Scheme 4). Thus,
photoisomeriztion behavior of azobenzene-functionalized homo-
polymer PS2 and block polymer PS2-b-PVAc were studied in
chloroform solution with the irradiation of 365 nm UV light. The
thermal recovery behavior was investigated in the dark at 60 ꢂC.
Firstly, the UVevis absorption changes of block polymer PS2-b-
PVAc after different period of UV irradiation were shown in Fig. 6
(A). The maximum absorption at 323.5 nm was the characteristic
obtained block copolymer PS2-b-PVAc containing azobenzene
moiety was characterized by 1H NMR spectra, as shown in Fig. 3
(PS2-b-PVAc). Compared with the spectra of PS and PVAc, the
spectrum of block copolymer showed both typical signals of PVAc
and PS. The signals at around 3.02e3.16 ppm due to the alkyne
group in the terminated PS were disappeared after “click” reaction.
The characteristic signals corresponding to the phenyl protons of
the azobenzene group and carbazole were still observed at around
7.70e8.30 ppm in block copolymer spectrum. All of these signals
confirmed the structure of azobenzene-functionalized block
copolymer PS2-b-PVAc. Moreover, the success of the “click” reac-
tion can also be confirmed from FT-IR spectroscopy in Fig. 4. The
signals at 3295 cmꢁ1 (Fig. 4, PS2) and 2106 cmꢁ1 (Fig. 4, PVAc)
assigned to the alkyne and azide groups in PS2 and PVAc dis-
appeared after the reaction. The FT-IR spectra indicated the high
efficiency of the click reaction of PS2 and PVAc.
Thermal property of the polymers was evaluated by differential
scanning calorimetry (DSC). All the polymerꢁs1were heated or cooled
with a heating/cooling rate of 10 ꢂC min under a continuous
nitrogen flow. The second DSC heating curves of the homopolymers
PS2, PVAc and copolymer PS2-b-PVAc were given in Fig. 5. The
results showed typical glass transition temperature (Tg) of homo-
polymers PS2 and PVAc at 85 ꢂC and 3 ꢂC, respectively. Although PS
intense
pe
p* transition of azobenzene (trans-form) before UV
exposure. Upon irradiation with 365 nm light, the trans-form of
azobenzene changed to the cis-form (weak nep* transition). The
absorption maximum at 323.5 nm was quickly decreased, whereas
the intensity of the weak nep* transition band at about 450 nm
increased slightly. However, the trans- form of the azobenzene did
not completely disappear, which indicated the incomplete photo-
isomerization [33]. Similar characteristic behavior of homopolymer
PS2 was also observed.
The photoisomerization kinetics of PS2-b-PVAc was plotted in
Fig. 6 (b). The rate of trans-cis photoisomerization was analyzed
from the absorption of 323.5 nm with different 365 nm light
PS2-b-PVAc
75 o
C
PS2
85 o
C
3 o
C
PVAc
-20
0
20
40
60
80
100 120 140 160
o
Temperature
C
Fig. 6. (A): UVevis absorption changes of block polymer PS2-b-PVAc with different
irradiation time of 365 nm UV light; (B): First-order for trans-cis photoisomerization of
block polymer PS2-b-PVAc. The concentration of the solution is 5.0 ꢅ 10ꢁ4 M in
chloroform solution under different time interval at room temperature.
Fig. 5. Differential scanning calorimetry (DSC) of the homopolymers PS2 (Mn
¼ 5000 g/mol, Mw/Mn ¼ 1.12), PVAc (Mn(GPC) ¼ 1800 g/mol, Mw/Mn ¼ 1.19) and
(GPC)
copolymer PS2-b-PVAc (Mn(GPC) ¼ 6700 g/mol, Mw/Mn ¼ 1.21).

X. Xue et al. / Polymer 51 (2010) 3083e3090
3089
Furthermore, the trans-cis-trans isomerization of PS-b-PVAc was
also observed in chloroform solution. The ke and kH of PS-b-PVAc
was 0.0027 and 2.2 ꢅ 10ꢁ4 sꢁ1, respectively.
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PS2-b-PVAc
Acknowledgments
The financial supports of this work by the National Natural
Science Foundation of China (Nos. 20874069, 50803044, 20904036
and 20974071), the Specialized Research Fund for the Doctoral
Program of Higher Education contract grant (No. 200802850005),
the Program of Innovative Research Team of Soochow University
and the Qing Lan Project are gratefully acknowledged.
0
1000
2000
3000
Time (s)
4000
5000
6000
References
Fig. 7. First-order for thermal cis-trans isomerization of block polymer PS2-b-PVAc.
The concentration of the solution is 5.0 ꢅ 10ꢁ4 M in chloroform solution under
different time interval at 60 ꢂC in the dark room.
[1] (a) Wang JS, Matyjaszewski K. Macromolecules 1995;28:7901e10;
(b) Kato M, Kamigaito M, Sawamoto M, Higashimura T. Macromolecules
1995;28:1721e3;
(c) Matyjaszewski K, Xia J. Chem Rev 2001;101:2921e90;
(d) Kamigaito M, Ando T, Sawamoto T. Chem Rev 2001;101:3689e745.
[2] (a) Chiefari J, Chong YK, Ercole F, Krstina J, Jeffery J, Le PT. Macromolecules
1998;31:5559e60;
irradiation time in chloroform solution. The first-order rate
constants kexp of photoisomerization was determined by Equation
(3) [32]:
(b) Chong YK, Le PT, Moad G, Rizzardo E, Thang SH. Macromolecules
1999;32:2071e4;
ꢀ
ꢂ
AN ꢁ A0
AN ꢁ At
(c) Lowe AB, McCormick CL. Prog Polym Sci 2007;32:283e351;
(d) Moad G, Rizzardo E, Thang SH. Aust J Chem 2006;59:669e92;
(e) Moad G, Rizzardo E, Thang SH. Polymer 2008;49:1079e131.
[3] (a) Sciannamea V, Jerome R, Detrembleur C. Chem Rev 2008;108:1104e26;
(b) Solomon DH, Rizzardo E, Cacioli P. U.S. Patent 4581429; 1985. Ć(c) Moad G,
Rizzardo E, Solomon DH. Macromolecules 1982;15:909e14;
(d) Hawker CJ, Bosman AW, Harth E. Chem Rev 2001;101:3661e88.
[4] Orienti I, Di PA, Luppi B, Zecchi V, Arch P. Arch Pharm Pharm Med Chem
2000;333:421e4.
In
¼ ꢁkexpt
(3)
where AN, A0 and At are absorbance at 323.5 nm after 365 nm light
irradiation at infinite time, time zero and time t, respectively. The
isomerization rate constants kexp of PS2-b-PVAc was 0.0027 sꢁ1
.
According to this method, the kexp of PS2 was 0.0047 sꢁ1, which was
about nearly 2 times faster than that of PS2-b-PVAc, which was
consistent with the result obtained in the literature [34]. The reason
was considered due to sterically hindering effect of the polymer
chain configuration.
[5] (a) Simms RW, Davis TP, Cunningham MF. Macromol Rapid Commun
2005;26:592e6;
(b) Stenzel MH, Davis TP, Barner-Kowollik C. Chem Commun 2004;13:
1546e7;
(c) Coote ML, Radom L. Macromolecules 2004;37:590e6;
(d) Stenzel MH, Cummins L, Roberts GE, Davis TP, Vana P, Barner-Kowollik C.
Macromol Chem Phys 2003;204:1160e8.
Secondly, thermal cis-trans isomerization of the irradiated
sample PS2-b-PVAc was investigated at 60 ꢂC in dark room. The
absorption at 323.5 nm was slowly restored to the initial state of
PS2-b-PVAc at about 2 h, which means the equilibrium between cis
and trans is established. The kinetics of the cis-trans thermal
isomerizations of PS2-b-PVAc in dark were fitted satisfactorily to
Equation (4):
[6] Destarac M, Charmot D, Franck X, Zard SZ. Macromol Rapid Commun
2000;21:1035e9.
[7] Mardare D, Matyjaszewski K. Macromolecules 1994;27:645e9.
[8] (a) Iovu MC, Matyjaszewski K. Macromolecules 2003;36:9346e54;
(b) Koumura K, Satoh K, Kamigaito M, Okamoto Y. Macromolecules
2006;39:4054e61.
[9] (a) Bryaskova R, Detrembleur C, Debuigne A, Jérôme R. Macromolecules
2006;39:8263e8;
(b) Debuigne A, Caille JR, Jérôme R. Macromolecules 2005;38:5452e8;
(c) Debuigne A, Caille JR, Jérôme R. Angew Chem Int Ed 2005;44:1101e4.
[10] (a) Wakioka M, Baek KY, Ando T, Kamigaito M, Sawamoto M. Macromolecules
2002;35:330e3;
(b) Xia JH, Paik HJ, Matyjaszewski K. Macromolecules 1999;32:8310e4.
[11] (a) Yoshida E. Colloid Polym Sci 2010;288:73e8;
(b) Otsu T, Matsunaga T, Doi T, Matsumoto A. Eur Polym J 1995;31:67e78;
(c) Ueda N, Kamigaito M, Sawamoto M. Polym Prepr Jpn 1997;46:149e52.
[12] (a) Benaglia M, Chiefari J, Chong YK, Moad G, Rizzardo E, Thang SH. J Am Chem
Soc 2009;131:6914e5;
ꢀ
ꢂ
AN ꢁ A0
AN ꢁ At
Ln
¼ kHt
(4)
where AN, A0 and At are absorbance at 323.5 nm at time infinite,
time zero and time t, respectively. The first-order cis-trans thermal
isomerization was shown in Fig. 7. The cis-trans rate constant (kH) of
PS2-b-PVAc was 2.2 ꢅ 10ꢁ4
s
ꢁ1. These results confirmed that the
isomerization of azobenzene in the polymer is reversible by the
photo- and thermo procedures, which had a potential application
in photochromic probes.
(b) Benaglia M, Chen M, Chong YK, Moad G, Rizzardo E, Thang SH. Macro-
molecules 2009;42:9384e6.
[13] Tong YY, Dong YQ, Du FS, Li ZC. Macromolecules 2008;41:7339e46.
[14] Debuigne A, Caille JR, Willet N, Jérôme R. Macromolecules 2005;38:9488e96.
[15] Kolb HC, Finn MG, Sharpless KB. Angew Chem Int. Ed 2001;40:2004e21.
[16] (a) Wang Q, Chan TR, Hilgraf R, Fokin VV, Sharpless KB, Finn MG. J Am Chem
Soc 2003;125:3192e3;
4. Conclusions
Well-defined block copolymers, poly(styrene)-b-poly(vinyl
acetate), PS-b-PVAc containing middle azobenzene moiety were
successfully synthesized by a combination of the reversible addi-
tion-fragmentation chain transfer (RAFT) and “click” chemistry.
This novel method provided an efficient way to prepare terminal
(b) Wu P, Feldman AK, Nugent AK, Hawker CJ, Scheel A, Voit B, et al. Angew
Chem Int Ed 2004;43:3928e32;
(c) Malkoch M, Schleicher K, Drockenmuller E, Hawker CJ, Russell TP, Wu P,
et al. Macromolecules 2005;38:3663e78.
[17] Quémener D, Davis TP, Barner-Kowollik C, Stenzel MH. Chem Commun
2006;48:5051e3.
[18] (a) Brown D, Natansohn A, Rochon P. Macromolecules 1995;28:6116e23;
(b) Kumar G, Neckers DC. Chem Rev 1989;89:1915e37;
(c) Delaire JA, Nakatani K. Chem Rev 2000;100:1817e45;
(d) Natansohn A, Rochon P. Chem Rev 2002;102:4139e75.
[19] (a) Gibons WM, Shannon PJ, Sun ST, Swetlin BJ. Nature 1991;351:49e50;
(b) Hafiz HR, Nakanishi F. Nanotechnology 2003;14:649e54;
(c) Rasmussen pH, Ramanujam PS, Hvilsted S, Berg RH. J Am Chem Soc
1999;121:4738e43;
functionalized block copolymer: firstly,
chromophore terminated poly(styrene) (PS), and
a
-alkyne and azobenzene
-azido-termi-
u
nated poly(vinyl acetate) (PVAc) were designed via RAFT tech-
nology. Secondly, “click” reactions were performed with the
combination of CuBr and PMDETA as catalyst system. PS-b-PVAc
block copolymers were demonstrated by GPC, 1H NMR, FT-IR
spectra and differential scanning calorimetry (DSC) analysis.
(d) Rochon P, Batalla E, Natansohn A. Appl Phys Lett 1995;66:136e8.

3090
X. Xue et al. / Polymer 51 (2010) 3083e3090
[20] (a) Kronek J, Luston J, Bohme F, Komber H. Macromol Symp 2001;164:105e15;
(b) Kronek J, Luston J, Bohme F, Komber H. Macromol Symp 2001;170:301e10;
(c) Peptu C, Bogdan VH, Simionescu BC, Adamus G, Kowalczuk M, Nunzi JM. J
Polym Sci Part A Polym Chem 2009;47:534e47.
[21] (a) Akiyama H, Tamaoki N. Macromolecules 2007;40:5129e32;
(b) Jochum FD, Borg LZ, Roth PJ, Theato P. Macromolecules 2009;42:7854e62;
(c) You X, Chen X, Zou G, Su W, Zhang QJ, He PS. Chem Phys Lett
2009;482:129e33;
[25] Chen F, Cheng ZP, Zhu J, Zhang W, Zhu XL. Eur Polym J 2008;44:1789e95.
[26] (a) Mayadunne RTA, Rizzardo E, Chiefari J, Chong YK, Moad G, Thang SH.
Macromolecules 1999;32:6977e80;
(b) Chiefari J, Mayadunne RTA, Moad CL, Moad G, Rizzardo E, Postma A, et al.
Macromolecules 2003;36:2273e83.
[27] Zhou D, Zhu XL, Zhu J, Yin HS.
J Polym Sci Part A Polym Chem
2008;43:4849e56.
[28] (a) Golas PL, Tsarevsky NV, Sumerlin BS, Matyjaszewski K. Macromolecules
2006;39:6451e7;
(d) Barrio J, Oriol L, Alcala R, Sanchez C. Macromolecules 2009;42:7954e62.
[22] Viau L, Bidault S, Maury O, Brasselet S, Ledoux I, Zyss J, et al. J Am Chem Soc
2004;126:8386e7.
(b) Chan TR, Hilgraf R, Sharpless KB, Fokin VV. Org Lett 2004;6:2853e5.
[29] Gao H, Matyjaszewski K. Macromolecules 2006;39:4960e5.
[30] Huang X, Huang Z, Huang J. J Polym Sci Part A Polym Chem 2000;38:914e20.
[31] Widmarier JM, Meyer GC. J Therm Anal 1982;23:193e9.
[32] Sin SL, Gan LH, Hu X, Tam KC, Gan YY. Macromolecules 2005;38:3943e8.
[33] Monteiro MJ, De Brouwer H. Macromolecules 2001;34:349e52.
[34] Erdogan T, Gungor E, Durmaz H, Hizal G, Tunca U. J Polym Sci Part A Polym
Chem 2006;44:1396e403.
[23] Wang G, Zhu XL, Cheng ZP, Zhu J.
J Polym Sci Part A Polym Chem
2005;43:2358e67.
[24] (a) Xue XQ, Zhang W, Cheng ZP, Zhu J, Zhu XL. J Polym Sci Part A Polym Chem
2008;46:5626e37;
(b) Wan XM, Zhu XL, Zhu J, Zhang ZB, Cheng ZP. J Polym Sci Part A Polym
Chem 2007;45:2886e96.