Water-soluble triply-responsive homopolymers of N,N-Dimethylaminoethyl methacrylate with a terminal azobenzene moiety

Article abstract of DOI:10.1002/pola.24034

Novel water-soluble triply-responsive homopolymers of N,N- dimethylaminoethyl methacrylate (DMAEMA) containing an azobenzene moiety as the terminal group were synthesized by atom transfer radical polymerization (ATRP) technique. The ATRP process of DMAEMA

Full text of DOI:10.1002/pola.24034

Water-Soluble Triply-Responsive Homopolymers of N,N-
Dimethylaminoethyl methacrylate with a Terminal Azobenzene Moiety
XINDE TANG,1,2 XIAOCHAO LIANG,1 LONGCHENG GAO,2 XINGHE FAN,2 QIFENG ZHOU2
1Institute of New Materials, Shandong Jiaotong University, Jinan 250023, China
2Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of
Education, Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, China
Received 7 February 2010; accepted 17 March 2010
DOI: 10.1002/pola.24034
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Novel water-soluble triply-responsive homopolymers
of N,N-dimethylaminoethyl methacrylate (DMAEMA) containing
an azobenzene moiety as the terminal group were synthesized
by atom transfer radical polymerization (ATRP) technique. The
ATRP process of DMAEMA was initiated by an azobenzene de-
rivative substituted with a 2-bromoisobutyryl group (Azo-Br) in
the presence of CuCl/Me6TREN in 1,4-dioxane as a catalyst sys-
tem. The molecular weights and their polydispersities of the
resulting homopolymers (Azo-PDMAEMA) were characterized by
gel permeation chromatography (GPC). The homopolymers are
soluble in aqueous solution and exhibit a lower critical solution
temperature (LCST) that alternated reversibly in response to Ph
and photoisomerization of the terminal azobenzene moiety. It
was found that the LCST increased as pH decreased in the range
of testing. Under UV light irradiation, the trans-to-cis photoiso-
merization of the azobenzene moiety resulted in a higher LCST,
whereas it recovered under visible light irradiation. This kind of
polymers should be particularly interesting for a variety of
potential applications in some promising areas, such as drug
controlled-releasing carriers and intelligent materials because of
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the multistimuli responsive property.
2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 48: 2564–2570, 2010
KEYWORDS: atom transfer radical polymerization (ATRP); azo
polymers; azobenzene; lower critical solution temperature;
photoresponsive; pH-responsive; single-electron transfer living
radical polymerization; stimuli-responsive polymer; stimuli-
sensitive polymers; thermoresponsive; water-soluble polymers
can for example be pH,10 light,11,12 or ionic strength.13 Tem-
perature- and pH-sensitive polymers have been the popular
dual functional systems.2,3,6 Such polymers have numerous
potential applications, such as controlled drug delivery and
release.12 In nature, however, the change in behavior of a
macromolecule (proteins and nucleic acids) is often a result
of its response not to a single factor, but to a combination of
environmental changes.14 To mimic this feature, formulation
of materials, which can sense specific changes and respond
to multiple stimuli in a predictable manner would be of
great interest.15 Recently, Thayumanavan and coworkers
reported a novel triple stimuli sensitive block copolymer
assembly that respond to changes in temperature, pH, and
redox potential. This block copolymer design constitutes an
acid-sensitive THP-protected HEMA and a temperature-sensi-
tive PNIPAM with an intervening disulfide bond.14
INTRODUCTION Stimuli-responsive polymers change their
solubility, volume, configuration, and conformation reversibly
by external triggers, such as pH, temperature, and ionic
strength.1 Such stimuli-responsive polymers have been
extensively investigated for the development of smart mate-
rials for various applications, such as controlled drug deliv-
ery system, personal care, industrial coatings, oil exploration,
biological and membrane science, viscosity modifier, colloid
stabilization, and water remediation.2,3
Water-soluble thermoresponsive polymers exhibit a lower
critical solution temperature (LCST) in aqueous media, at
which these polymers in aqueous solution experience a
phase change from a soluble state to an insoluble state. LCST
derives from coil-to-globule transitions of single chains, and
depends on some factors including the polymer composition,
the nature of functional groups, pH, ionic strength, solvent,
polymer concentration, and other external stimuli.4,5 Poly-
(N-isopropylacryamide) (PNIPAM) is the most popular ther-
moresponsive polymer targeting biological applications.6
Light-responsive polymers have attracted much interest for
their applications in optical and mechanical devices because
of their ready accessibility.7 Most photo-responsive polymers
containing azobenzene groups exhibit photoinduced alter-
nation of their cloud point in aqueous solution as azoben-
zene moieties undergo a reversible trans-cis isomerization
The area of polymers responsive to a single stimulus has
been extended to polymers, which show a responsive behav-
ior to multiple stimuli.7–9 Other stimuli besides temperature
Correspondence to: X. Tang (E-mail: xdtang8033@163.com)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2564–2570 (2010) 2010 Wiley Periodicals, Inc.
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transition under light irradiation.16 In such polymers the po-
lar cis- form of the azobenzene unit has increased aqueous
solubility in comparison with its relatively nonpolar trans-
form, the balance between polar and nonpolar moieties
has a great impact on the transition temperature. Tempera-
ture- and light-dual-responsive polymers are of special
importance, and there have been several reports on tempera-
ture-responsive PNIPAM copolymers, which contain light-
responsive azobenzene groups on the side chain of the poly-
mers.16–19 Molecular brushes, with a dual response to light
and temperature, were prepared by copolymerizing 4-meth-
acryloyloxyazobenzene and DMAEMA units into the side
chains by ATRP.20 Recently, Wang et al. synthesized triple-
stimuli (photo/pH/thermo) responsive copolymers of
poly(N-isopropylacrylamide-co-N-hydroxymethylacrylamide-
co-2-diazo-1,2-naphthoquinonemethylacrylamide) (P(NIPAM-
co-NHMA-co-DNQMA)).21
by stirring in glacial acetic acid, filtering, and washing with
acetone three times, followed by drying in vacuum at room
temperature overnight. Tris(2-(dimethylamino)ethyl)amine
(Me6TREN) was prepared following the procedure described
in the literature.27
Synthesis of 4-Hydroxy-40-ethoxyazobenzene
Following the procedure described in the literature,28
4-hydroxy-40-ethoxyazobenzene was prepared by a diazo-
coupling reaction between 4-ethoxyaniline and phenol in the
presence of sodium nitrite and hydrochloric acid. 1H NMR
(CDCl3, 400 MHz): d ¼ 7.9 (d, 4H), 7.0 (d, 4H), 4.1 (m, 2H),
1.5 (t, 3H).
Synthesis of 4-Hydroxyethoxy-40-ethoxyazobenzene
A mixture of 4-hydroxy-40-ethoxyazobenzene (12.1 g), anhy-
drous potassium carbonate (20.0 g), potassium iodide (1 g),
and ethanol (120 mL) was stirred and heated to reflux.
Excess of 2-chloroethanol (15.0 mL) was added dropwisely
and refluxed for 24 h. The mixture was poured into water,
and the precipitate was filtered off, then dissolved in THF
and filtered. The THF solution was concentrated and recrys-
tallized from ethanol to yield orange solid. 1H NMR (CDCl3,
400 MHz): d ¼ 7.9 (d, 4H), 7.0 (d, 4H), 4.2 (t, 2H), 4.1
(t, 2H), 4.0 (m, 2H), 1,9 (s, 1H), 1.5 (t, 3H).
Atom transfer radical polymerization (ATRP) as one of the
effective controlled radical polymerization methods, has been
widely used to synthesize predetermined molecular weight
polymers.22 ATRP can result in relatively lower polydisper-
sity to that synthesized by free radical polymerization. Gen-
erally, a simple alkyl halide can be used to initiate the poly-
merization. In addition, the synthesis of the initiator can be
exploited as a means to functionalize the terminus of the
resulting polymers. For instance, thermally and chemically
stable initiators with complicated structure, such as photo-
responsive moieties for ATRP can be used for the synthesis
of polymers with functionalized chain ends. Akiyama et al.
reported the ATRP-based synthesis of a PNIPAM derivative
with a single photoresponsive azobenzene unit at one of its
termini and the cloud point shift in aqueous solution
response upon exposure to light.23
Synthesis of 4-Ethoxy-40-(2-(2-bromoisobutyryloxy)-
ethoxy)azobenzene (Azo-Br)
Excess of 2-bromoisobutyryl bromide (2 mL) was dropwisely
added to the solution of 4-hydroxyethoxy-40-ethoxyazoben-
zene (0.87 g) and triethylamine (2 mL) in THF. After stirring
for 4 h, the solution was diluted with CH2Cl2 and the insolu-
ble solid was removed by filtration. The solution was con-
centrated and the residue was purified through silica gel
column chromatography to yield 4-ethoxy-40-(2-(2-bromoiso-
butyryloxy)-ethoxy)azobenzene. 1H NMR (CDCl3, 400 MHz):
d ¼ 7.8 (d, 4H), 7.0 (d, 4H), 4.5 (t, 2H), 4.3 (t, 2H), 4.1
(m, 2H), 1,9 (s, 6H), 1.5 (t, 3H).
Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) has
antibacterial, hemostatic, and anticancer activity.24 In addi-
tion, PDMAEMA is responsive to changes in pH as well as
temperature, and the thermal behavior changes significantly
with pH.25,26 Polymers that combine PDEMEMA with azoben-
zene can expected to be triply-responsive to pH, tempera-
ture, and light. In this work, triply-responsive PDMAEMA
homopolymers with a terminal azobenzene moiety were syn-
thesized by ATRP and their multi-stimuli responsive proper-
ties were investigated. To the best of our knowledge, this is
the first triple-stimuli responsive homopolymer of which the
LCST in aqueous solution can be modulated by pH and light,
respectively.
Synthesis of Poly(N-isopropylacrylamide) Homopolymer
(Azo-PNIPAM)
In a Schlenk flask, Az-Br (40 mg, 0.1 mmol), NIPAM (1.36 g,
12 mmol), deoxygenated DMF/H2O mixture (2.25 mL, v/v ¼
8:1), CuBr (15 mg), and Me6TREN (28 lL) were charged.
Then the mixture was frozen in liquid nitrogen, and a vac-
uum was applied. After three freeze-pump-thaw cycles, the
flask was sealed in vacuum and the reaction lasted 4.5 h at
ambient temperature. The mixture was diluted with THF,
passed through a neutral aluminum column, and precipitated
into an excess of hexane. The precipitation process was car-
ried out three times. The product was dried in a vacuum
oven at ambient temperature.
EXPERIMENTAL
Materials
All chemicals and solvents were commercially available and
used as received unless otherwise stated. 2-Bromoisobutyryl
bromide was purchased from Creator Chemical Company of
China. N-Isopropylacrylamide (NIPAM) was purchased from
Shanghai Wujing Chemicals of China and used without fur-
ther purification. N,N-Dimethylaminoethyl methacrylate
(DMAEMA), purchased from Feixiang Chemicals (Jiangsu,
China) was used as received. CuCl and CuBr were purified
Synthesis of Poly(N,N-dimethylaminoethyl methacrylate)
Homopolymer (Azo-PDMAEMA)
To optimize the reaction conditions, Azo-PDMAEMA products
with varying feed ratios, reaction temperatures, reaction
time, and solvents were synthesized. A typical preparation
procedure was as follows: In a Schlenk flask, Az-Br (40 mg,
0.1 mmol), deoxygenated DMAEMA (1.5 g, 10 mmol) and
HOMOPOLYMERS OF N,N-DIMETHYLAMINOETHYL METHACRYLATE, TANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1,4-dioxane (1 mL), CuCl (10 mg), and Me6TREN (28 lL)
were charged, immediately the mixture was frozen in liquid
nitrogen, and a vacuum was applied. After three freeze-
pump-thaw cycles, the flask was sealed in vacuum and
ꢀ
placed in an oil bath preheated at 60 C. The reaction lasted
10 h and the reaction was stopped by dilution with THF.
Then the mixture was passed through a neutral aluminum
column. The filtrate was concentrated, and the polymer was
recovered by precipitation into an excess of hexane. The pre-
cipitation process was carried outꢀthree times. The product
was dried in a vacuum oven at 30 C.
Characterization
1H NMR spectra were measured using a Bruker 400 MHz
spectrometer at 25 ꢀC. Chemical shifts were referenced to
tetramethyl silane. The average molecular weights and mo-
lecular weight distributions of the obtained polymers were
determined by a gel permeation chromatography (GPC) sys-
tem, which was equipped with a Waters 2414 refractive
index detector, a Waters 1525 binary HPLC pump, and three
Waters Styragel columns (HT2, HT3, and HT4). The columns
ꢀ
were thermostated at 35 C. THF at a flow rate of 1 mL/min
was used as the eluent. The molecular weight was calibrated
by using the polystyrene standards.
Phase transitions of the Azo-PDMAEMA homopolymer in
deionized water were observed by optical transmittance
changes, which were monitored at 525 nm with a spectro-
photometer (662, Metrohm, Swizerland). The sample cell
was thermostated with a heating circulator (C25P, Thermo
Haar, Germany). The measurement was conducted with a
SCHEME 1 The synthetic routes of the initiator Azo-Br.
synthesized by the esterification reaction of the terminal
hydroxyl group with 2-bromoisobutyryl bromide following
the procedure described in the literature.29
ꢀ
programmed temperature increasing from 25 C to a certain
value at a rate of 0.2 ꢀC/min. The LCST was defined as the
temperature at 50% transmittance of solution. All measure-
ments were repeated three times.
Synthesis of Azo-PDMAEMA
Solutions of the Azo-PDMAEMA homopolymer (2 mg/mL)
for phase transition observation were prepared using 0.05 M
potassium hydrogen phthalate buffer at pH 4.00 (25 ꢀC),
0.025 M mixed phosphꢀate buffer at pH 6.86 (25 ꢀC), deion-
ized water (pH 8.5, 25 C), 0.01 M sodium tetraborate buffer
at pH 9.18 (25 C), 0.05 M sodium carbonate/sodium dicar-
bonate buffer at pH 10.3, and 0.05 M sodium carbonate/
sodium dicarbonate buffer at pH 11.3, respectively.
Generally, DMAEMA was used as a cationic monomer as the
pKa of PDMAEMA is 7.5.30 In the synthesis of copolymers
containing PDMAEMA by ATRP mentioned previously, differ-
ent amine ligands, such as N,N,N0,N0,N00-pentamethyldiethy-
lentriamine (PMDETA), 1,1,4,7,10,10-hexamethyltriethylene-
tetramine (HMTETA), or 2,20-bipyridine (bpy) were used to
control polymerization.31–33 Matyjaszewski et al. synthesized
amphiphilic block copolymers of DMAEMA and n-butyl meth-
acrylate (BMA) by ATRP in 2-propanol/water mixtures using
a methoxy-poly(ethylene glycol) macroinitiator. The resulting
products showed controlled molecular weights and low poly-
dispersities (Mw/Mn ¼ 1.11–1.47).33 Recently, CuCl/Me6T-
REN catalyst system was used for the ATRP copolymeriza-
tion of DMAEMA and NIPAM, and the resultant copolymers,
P(NIPAM-co-DMAEMA), exhibited narrower polydispersity
(1.36–1.55).
ꢀ
To investigate the photoresponsive property of the polymers,
UV irradiation was carried out with a 100 W high-pressure
mercury lamp coupled with UV filters (<360 nm), and the
actual intensity was estimated to be 150 mW/cm2.
RESULTS AND DISCUSSION
Synthesis of Azo-Br
In this work, the homopolymerization of DMAEMA was initi-
ated by an azobenzene derivative, Azo-Br, the synthetic route
of which was shown in Scheme 1. 4-Hydroxy-40-ethoxyazo-
benzene was prepared by a diazo-coupling reaction between
4-ethylaniline and phenol in the presence of sodium
nitrite and hydrochloric acid, and a subsequent reaction of
4-hydroxy-40-ethoxyazobenzene with 2-chloroethanol gives
rise to 4-hydroxyethoxy-40-ethoxyazobenzene. Azo-Br was
However, for the PDMAEMA homopolymers, the molecular
weight distributions are much higher than those of the
corresponding copolymers. The reaction-time-dependent
changes in the molecular weights of PNIPAM homopolymer
and PDMAEMA homopolymer were verified by Okano and
coworkers.34 It was found that under the same conditions, at
the onset of the ATRP procedure, the number averaged
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TABLE 1 Characterization of Azo-PNIPAM and Azo-PDMAEMA
the conditions here occur within a short time, in accordance
with the mechanism of SET-LRP.36 In addition, at ambient
temperature, the conversions of monomer and the molecular
weights are comparatively low. In this work, we attempted
to synthesize such homopolymers using 1,4-dioxane as sol-
vent under higher temperature. The resulting products (P4
and P5) possess higher molecular weights (>20,000 Da) and
lower polydispersities.
Homopolymers
Homopolymera
Mn,GPC
Mw/Mn
b
b
Azo-PNIPAM
4,800
8,900
1.28
2.56
Azo-PDMAEMA
a
Feed molar ratio: [monomer]/[Azo-Br]/[CuBr]/[ Me6TREN] ¼ 120:1:1:1;
reaction temperature: ambient temperature (22 ꢀC); reaction time: 4.5 h;
solvent: DMF/H2O mixture (2.25 mL, v/v ¼ 8:1).
b
Determined by GPC.
Phase Transition of Azo-PDMAEMA in Aqueous Solution
PDMAEMA is both temperature- and pH-sensitive,43 and azo-
benzene moiety is light-responsive. Therefore, Azo-
PDMAEMA could be triply-responsive to temperature, pH,
and light. The homopolymers were soluble in deionized
water and formed pale yellow transparent solution at ambi-
ent temperature. To investigate the phase transition of the
Azo-PDMAEMA homopolymers, in this section, we take the
homopolymer P4 for example.
molecular weight of PDMAEMA was larger than that of PNI-
PAM, and polydispersity for PDMAEMA was significantly
higher than that for PNIPAM,35 which indicates that the
monomer reactivity of DMAEMA is higher than that of
NIPAM. These results suggest that the catalyst (CuCl/Me6T-
REN) in 2-propanol was inadequate for controlled polymer-
ization of DMAEMA.34 In our contrastive experiments, the
homopolymerizations of NIPAM and DMAEMA were, respec-
tively, initiated by Azo-Br under the identical conditions. The
number-averaged molecular weights and the polydispersities
of the obtained polymers are summarized in Table 1. These
results demonstrate that the catalyst (CuBr/Me6TREN) in
DMF/H2O mixture can not realize the controlled homopoly-
merization of DMAEMA under similar condition.
Effect of Temperature
The temperature-sensitive character of homopolymer P4 was
examined by a spectrophotometer. The temperature depend-
ence of the light transmittance at 525 nm for an aqueous
PDMAEMA solution at a concentration of 0.6 g/L was shown
Figure 1. With increasing temperature to a critical value, the
solution was transformed from transparent to opaque
sharply at 58 ꢀC, which is a typical LCST of the Azo-
PDMAEMA homopolymer. At a concentration of 2.0 g/L, the
LCST changed to 53 ꢀC, which demonstrates that the homo-
polymer concentration has an impact on the LCST.
To further optimize the polymerization conditions, we syn-
thesized a series of Azo-PDMAEMA homopolymers using the
CuCl/Me6TREN catalyst system in DMF/water mixture or
1,4-dioxane. Characteristics of the resulting homopolymers
under different reaction conditions including temperature,
reaction time, and solvent are summarized in Table 2. Under
the same reaction conditions, the molecular weight of homo-
polymer P1 synthesized in bulk state without solvent was
higher than that of P2 in the solvent of DMF/H2O mixture.
On the other hand, the homopolymerization of DMAEMA
using CuCl/Me6TREN in DMF/H2O prefers to follow the
mechanism of single-electron transfer living radical polymer-
ization (SET-LRP).36–42 Because the rapid disproportionation
of CuCl/Me6TREN in DMF/H2O generates too much highly
reactive nascent Cu(0) too quickly, and therefore, activation
is too rapid. Combined the molecular weights of the result-
ant homopolymers in Table 2 (P2 and P3) with that of Azo-
PDMAEMA in Table 1, it seems that to prolong reaction time
can not affect dramatically the molecular weight. This dem-
onstrates that the homopolymerization of DMAEMA under
As known, at low temperature, the PDMAEMA homopolymer
chains exist in random coil conformation because of the H-
bonding interactions between the homopolymer and the
water molecules. However, as the temperature raises to a
critical value, polymer chains could shrink into a globular
structure because of the hydrophobic interactions between
N,N-dimethylaminoethyl groups.44 It is known that tempera-
ture-induced phase separation of solutions is mainly driven
by increased interactions between hydrophobic moieties on
the polymers because of a reduction of structural water
around polymer side groups with increasing temperature.21
In this work, incorporation of hydrophobic azobenzene moi-
ety relatively increases the hydrophobicity of the Azo-
PDMAEMA homopolymers
TABLE 2 Reaction Conditions and Characteristics of the Azo-PDMAEMA Homopolymers
Polymera
Temperature (ꢀC)
Reaction Time (h)
Solvent
Mn,GPC
Mw/Mn
b
b
P1
P2
P3
P4
P5
a
22
22
22
60
60
8
8
no solvent
DMF/H2O
10,500
6,700
2.60
2.45
2.64
2.28
2.23
18
10
10
DMF/H2O
7,900
1,4-dioxane
1,4-dioxane
23,600
26,900
b
Feed molar ratio: [Monomer]/[Azo-Br]/[CuCl]/[Me6TREN] ¼ 120:1:1:1.
Determined by GPC.
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about 31 ꢀC. Even at pH 11.3, the LCST still keeps stable.
However, in the pH range of 5.0–10.0, the polyelectrolytes
are partially charged, and the hydrophilic/hydrophobic prop-
erties change significantly and may cause many intriguing
self-assembly variations.46
FIGURE 1 Transmittance of the homopolymer aqueous solu-
tion at different temperature (a-0.6 g/L; b-2.0 g/L)
Effect of pH
As demonstrated in previous literature, PDMAEMA solutions
exhibit pH-responsive behavior because of protonation/
deprotonation of N,N-dimethylaminoethyl groups in
PDMAEMA component. PDMAEMA chains gradually shrink
and precipitate form solution because of the deprotonation
of amine groups.45
The influence of pH on the LCST of the Azo-PDMAEMA
homopolymers was investigated in buffer solutions. As
shown in Figures 2 and 3, the LCST of the homopolymer
shifted toward higher temperatures with pH varying from
4.0 to 11.3. At pH 4.0, tertiary amine groups are fully proto-
nated, and the PDMAEMA chains are so hydrophilic that they
completely enter into an aqueous phase. Therefore, the
homopolymer aqueous solution shows no LCST. Above pH
10.0, tertiary amine groups become uncharged, and thus, the
PDMAEMA chains preferentially stay in the hydrophobic
phase. The LCST of the homopolymer aqueous solution is
FIGURE 3 Triply-responsive Azo-PDMAEMA homopolymer P4
in different pH aqueous solution (glass tubes from left to right:
1-pH 4.0 potassium hydrogen phthalate buffer solution; 2-pH
6.8 mixed phosphate buffer solution; 3-deionized water solu-
tion (pH 8.5); 4-pH 9.2 sodium tetraborate buffer solution) at
different temperature (A: 26 ꢀC, B: 42 ꢀC, C: 53 ꢀC, D: 68 ꢀC).
FIGURE 2 Effect of pH on the LCST of the homopolymer aque-
ous solution (2.0 g/L).
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no. 20134020), the Visiting Scholar Project of Shandong
Province of China (grant no. 2008001), and the Science
Research Fund of Shandong Jiaotong University of China
(grant no. Z200802).
REFERENCES AND NOTES
1 Fujii, S.; Suzaki, M.; Yusa, S.; Muraoka, M.; Nakamura, Y. J
Polym Sci Part A: Polym Chem 2009, 47, 3431–3443.
2 Gil, E. S.; Hudson, S. M. Prog Polym Sci 2004, 29, 1173–1222.
3 Dobrynin, A. V.; Rubinstein, M. Prog Polym Sci 2005, 30,
1049–1118.
4 Gao, J.; Wu, C. Macromolecules 1997, 30, 6873–6876.
5 Lu, X.; Hu, Z.; Schwartz, J. Macromolecules 2002, 35,
9164–9168.
FIGURE 4 LCST variations of the homopolymer aqueous solu-
tions (2.0 g/L) with different pH before (a) and after (b) UV
irradiation.
6 Rzaev, Z. M. O.; Dincer, S.; Piskin, E. Prog Polym Sci 2007,
32, 534–595.
7 Jochum, F. D.; Borg, L.; Roth, P. J.; Theato, P. Macromole-
cules 2009, 42, 7854–7862.
Effect of Light
To investigate the influence of light on the LCST of homo-
polymer aqueous solution, UV light was used to irradiate the
solution. LCST variations of the polymer aqueous solutions
(2.0 g/L) with different pH value before and after UV irradia-
tion were shown in Figure 4. It could be found that the LCST
after UV irradiation was higher than that before irradiation.
8 Jochum, F. D.; Theato, P. Macromolecules 2009, 42, 5941–5945.
9 Roy, D.; Cambre, J. N.; Sumerlin, B. S. Chem Commun 2009,
2106–2108.
10 Li, D.; He, Q.; Yang, Y.; Moehwald, H.; Li, J. Macromolecules
2008, 41, 7254–7256.
11 Wang, G.; Tong, X.; Zhao, Y. Macromolecules 2004, 37,
The phase transition of the thermoresponsive polymer is a
result of the change in its hydrophilic-hydrophobic balance.
Therefore, change in the molecular structure that increase its
overall hydrophilicity results in an increase of the LCST of
the polymer.21 The Azo-PDMAEMA homopolymer has an azo-
benzene terminal unit. Under the UV irradiation, the photo-
isomerization of azobenzene resulted in the transition from
the trans form to the cis form. The cis form of azobenzene
exhibits more hydrophilic than the trans form in solution,
which results in a dramatically increase of its LCST. In addi-
tion, under the visible light, the LCST will be recovered to its
original value, which demonstrates that the procedure could
be reversible.
8911–8917.
12 Lee, H.; Wu, W.; Oh, J. K.; Mueller, L.; Sherwood, G.;
Peteanu, L.; Kowalewski, T.; Matyjaszewski, K. Angew Chem Int
Ed 2007, 46, 2453–2457.
13 Magnusson, J. P.; Khan, A.; Pasparakis, G.; Saeed, A.;
Wang, W.; Alexander, C.
10852–10853.
J Am Chem Soc 2008, 130,
14 Klaikherd, A.; Nagamani, C.; Thayumanavan, S. J Am Chem
Soc 2009, 131, 4830–4838.
15 Ganta, S.; Devalapally, H.; Shahiwala, A.; Amiji, M. J Control
Release 2008, 126, 187–204.
16 Kungwachakun, D.; Irie, M. Macromol Chem Rapid Commun
1988, 9, 243–246.
CONCLUSIONS
17 Sumaru, K.; Kameda, M.; Kanamori, T.; Shinbo, T. Macro-
In this work, we reported water-soluble triply-responsive
Azo-PDMAEMA homopolymers via ATRP. By the optimization
of polymerization conditions, the polymers with higher
molecular weights and lower polydispersities were obtained.
The LCST of the polymers can be modulated not only by the
pH of aqueous solution but also by light. The terminal azo-
benzene moiety, which can undergo reversible photoisomeri-
zation under alternative UV/visible light irradiation, results
in a changeable hydrophilicity in aqueous solution. Such
homopolymers combine pH-, photo-, and thermoresponsive
properties in a single macromolecule. These novel triply-
responsive homopolymers can be applied in some promising
areas including life science and material science.
molecules 2004, 37, 4949–4955.
18 Desponds, A.; Freitag, R. Langmuir 2003, 19, 6261–6270.
19 Akiyama, H.; Tamaoki, N. J Polym Sci Part A: Polym Chem
2004, 42, 5200–5214.
20 Lee, H.; Pietrasik, J.; Matyjaszewski, K. Macromolecules
2006, 39, 3914–3920.
21 Yu, Y.-Y.; Tian, F.; Wei, C.; Wang, C.-C. J Polym Sci Part A:
Polym Chem 2009, 47, 2763–2773.
22 Matyjaszewski, K.; Xia, J. Chem Rev 2001, 101, 2921–2990.
23 Akiyama, H.; Tamaoki, N. Macromolecules 2007, 40,
5129–5132.
The authors gratefully acknowledge financial support from
the National Natural Science Foundation of China (grant
24 Huang, J.; Murata, H.; Koepsel, R. R.; Russel, A. J.; Matyjas-
zewski, K. Biomacromolecules 2007, 8, 1396–1399.
HOMOPOLYMERS OF N,N-DIMETHYLAMINOETHYL METHACRYLATE, TANG ET AL.
2569

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
25 Amalvy, J. I.; Unali, G. F.; Li, Y.; Granger-Bevan, S.; Armes,
S. P.; Binks, B. P.; Rodrigues, J. A.; Whitby, C. P. Langmuir
2004, 20, 4345–4354.
37 Nguyen, N. H.; Rosen, B. M.; Lligadas, G.; Percec, V. Macro-
molecules 2009, 42, 2379–2386.
38 Rosen, B. M.; Percec, V. J Polym Sci Part A: Polym Chem
26 Yamamoto, S.; Pietrasik, J.; Matyjaszewski, K. Macromole-
2008, 46, 5663–5697.
cules 2008, 41, 7013–7020.
39 Lligadas, G.; Percec, V. J Polym Sci Part A: Polym Chem
27 Ciampolini, M.; Nardii, N. Inorg Chem 1966, 5, 41–44.
2008, 46, 2745–2754.
28 Jankova, K.; Chen, X.; Kops, J.; Batsberg, W. Macromole-
40 Percec, V.; Guliashvili, T.; Ladislaw, J. S.; Wistrand, A.;
Stjerndahl, A.; Sienkowska, M. J.; Monteiro, M. J.; Sahoo, S. J
Am Chem Soc 2006, 128, 14156–14165.
cules 1998, 31, 538–541.
29 Stewart, D.; Imrie, C. T. Polymer 1996, 37, 3419–3425.
41 Feng, C.; Shen, Z.; Li, Y.; Gu, L.; Zhang, Y.; Lu, G.;
30 van de Wetering, P.; Moret, E. E.; Schuurmans-Nieuwen-
broek, N. M. E.; van Steenbergen, M. J.; Hennink, W. E. Biocon-
jugate Chem 1999, 10, 589–597.
Huang, X.
1811–1824.
J Polym Sci Part A: Polym Chem 2009, 47,
42 Tang, X. D.; Liang, X. C.; Yang, Q.; Fan, X. H.; Shen, Z. H.;
31 Topham, P. D.; Howse, J. R.; Crook, C. J.; Pamell, A. J.; Geo-
ghegan, M.; Jones, R. A. L. Polym Int 2006, 55, 808–815.
Zhou, Q. F.
4420–4427.
J Polym Sci Part A: Polym Chem 2009, 47,
32 Zhang, X.; Xia, J.; Matyjaszewski, K. Macromolecules 1998,
31, 5167–5169.
43 Alarcon, C. D. L.; Pennadam, S.; Alexander, C. Chem Soc
Rev 2005, 34, 276–285.
33 Lee, S. B.; Russell, A. J.; Matyjaszewski, K. Biomacromole-
cules 2003, 4, 1386–1393.
44 Sui, X.; Yuan, J.; Zhou, M.; Zhang, J.; Yang, H.; Yuan,
W.; Wei, Y.; Pan, C. Biomacromolecules 2008, 9,
2615–2620.
34 Nagase, K.; Kobayashi, J.; Kikuchi, A.; Akiyama, Y.; Kana-
zawa, H.; Okano, T. Biomacromolecules 2008, 9, 1340–1347.
45 Pantoustier, N.; Moins, S.; Wautier, M.; Degee, P.; Dubois,
35 Kurisawa, M.; Yokoyama, M.; Okano, T. J. Control Release
P.; Chem Commun 2003, 340–341.
2000, 69, 127–137.
46 Yan, Q.; Yuan, J.; Zhang, F.; Sui, X.; Xie, X.; Yin, Y.;
Wang, S.; Wei, Y. Biomacromolecules 2009, 10,
2033–2042.
36 Percec, V.; Popov, A. V.; Ramirez-Castillo, E.; Monteiro, M.;
Barboriu, B.; Weichold, O.; Asandei, A. D.; Mitchell, C. M. J Am
Chem Soc 2002, 124, 4940–4941.
2570
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