Preparation of block-brush PEG-b-P(NIPAM-g-DMAEMA) and its dual stimulus-response

Article abstract of DOI:10.1016/j.polymer.2010.05.012

Well-defined dually responsive block-brush copolymer of poly(ethylene glycol)-b-[poly(N-isopropylacrylamide)-g-poly(N,. N-dimethylamino-ethylmethacrylate)], [PEG-b-P(NIPAM-g-PDMAEMA)] was successfully prepared by the combination of atom transfer radical polymerization (ATRP) and click chemistry based on azide-capped PDMAEMA and alkyne-pending PEG-b-PNIPAM copolymer. Azide-capped PDMAEMA was synthesized through ATRP of DMAEMA monomer using an azide-functionalized initiator of β-azidoethyl-2-bromoisobutyrate. Alkyne-pending PEG-b-PNIPAM copolymer was obtained through ATRP copolymerization of NIPAM with propargyl acrylate. The final block-brush copolymer was synthesized by the click reaction between these two polymer precursors. Because of characteristics of three different blocks, the copolymer exhibited dually thermo- and pH-responsive behavior. The responsive behaviors of block-brush copolymer were studied by laser light scattering, temperature-dependent turbidity measurement and micro differential scanning calorimetry. The phase transition temperature of block-brush copolymer increased with the decrease of pH value. At pH = 5.0, the copolymer displayed weak thermo-responsive behavior and might form uni-molecular micelles upon heating. At higher pH values, the block-brush copolymer aggregated intermolecularly into the micelles during the phase transition.

Full text of DOI:10.1016/j.polymer.2010.05.012

Polymer 51 (2010) 3039e3046
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Preparation of block-brush PEG-b-P(NIPAM-g-DMAEMA) and its dual
stimulus-response
*
Bo-Yu Zhang, Wei-Dong He , Wen-Tao Li, Li-Ying Li, Ke-Ren Zhang, Hao Zhang
Department of Polymer Science and Engineering, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 March 2010
Received in revised form
Well-defined dually responsive block-brush copolymer of poly(ethylene glycol)-b-[poly(N-iso-
propylacrylamide)-g-poly(N,N-dimethylamino-ethylmethacrylate)], [PEG-b-P(NIPAM-g-PDMAEMA)] was
successfully prepared by the combination of atom transfer radical polymerization (ATRP) and click
chemistry based on azide-capped PDMAEMA and alkyne-pending PEG-b-PNIPAM copolymer. Azide-
capped PDMAEMA was synthesized through ATRP of DMAEMA monomer using an azide-functionalized
4
May 2010
Accepted 7 May 2010
Available online 15 May 2010
initiator of b-azidoethyl-2-bromoisobutyrate. Alkyne-pending PEG-b-PNIPAM copolymer was obtained
through ATRP copolymerization of NIPAM with propargyl acrylate. The final block-brush copolymer was
synthesized by the click reaction between these two polymer precursors. Because of characteristics of
three different blocks, the copolymer exhibited dually thermo- and pH-responsive behavior. The
responsive behaviors of block-brush copolymer were studied by laser light scattering, temperature-
dependent turbidity measurement and micro differential scanning calorimetry. The phase transition
temperature of block-brush copolymer increased with the decrease of pH value. At pH ¼ 5.0, the
copolymer displayed weak thermo-responsive behavior and might form uni-molecular micelles upon
heating. At higher pH values, the block-brush copolymer aggregated intermolecularly into the micelles
during the phase transition.
Keywords:
Stimulus response
Block -brush copolymers
ATRP
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
more extra properties and applications [10]. Thus, well-defined
block-brush copolymer would combine the advantages of both
The macromolecular architecture of copolymers determines the
macroscopic properties of polymeric materials. Well-defined
segmented copolymers such as block and graft polymers which
have different functionalized chains, could self-assemble into
various micellar structure in segment-selective solvents and have
attracted great interests in physical chemistry and polymer science
block and graft polymer, such as distinct physicochemical attribu-
tions and lower melting viscosity for a given molecular weight [11].
Secondly, the linear triblock copolymer usually has low steric
hindrance and electric repulsion; each block could exhibit the
individual nature of polymer chain. On the contrary, block-brush
copolymers could have different solution properties and assembly
behavior from linear block copolymer due to the stronger interac-
tion among the blocks.
Using living/controlled radical polymerizations, such as atom
transfer radical polymerization (ATRP) [12] and reversible addition-
fragmentation chain transfer polymerization [13], well-defined
polymers could be easily synthesized. One of conventional ideas for
the preparation of regular branched copolymer is the usage of
macromonomers [14,15]. Although ATRP method could be
employed for some macromonomers, it inherently possesses
a lower reactivity because of steric hindrance and needs to adopt
small molecular ATRP initiator [16]. It seems hard to prepare block-
brush copolymer from the living/controlled polymerizations of
macromonomer with macroinitiator. Combined with other chem-
ical reactions having high efficiency and quantitative yield, such as
click chemistry (i.e. 1,3-dpolar cycloaddition of [3 þ 2] system)
[1e6]. Some amphiphilic block copolymers such as polystyrene-b-
poly(ethylene oxide) [7] and poly(2-vinylpyridine)-b-poly(ethylene
oxide) [8] could form various morphologic core-shell micelles.
Copolymers consisting of two or more different polymeric blocks,
such as poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic
acid), exhibit stimulus-response behavior at different temperatures
and pHs, which have potential to be used as the carriers for gene
and drug delivery [9]. Besides block copolymers, graft or brush
copolymers exhibit the similar behavior, with the additional facts
that the branched structure makes the micellization more
complicated than block copolymers and provides the micelles with
*
Corresponding author. Tel.: þ86 551 3601699; fax: þ86 551 3606743.
E-mail address: wdhe@ustc.edu.cn (W.-D. He).
0
032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.05.012

3040
B.-Y. Zhang et al. / Polymer 51 (2010) 3039e3046
[
17,18], not only the controlled molecular weight and high mono-
After the removal of most solvent by rotary evaporation, the filtrate
was precipitated in cold diethyl ether. PEG-Cl was collected by
filtration and dried at room temperature under vacuum for 24 h
(7.9 g, yield: 79%). The end-group functionality was determined by
proton nuclear magnetic resonance ( H NMR) spectroscopy and
reached the unity.
dispersity have been achieved [19], but also polymers with more
complex architectures have been obtained. The macromolecules
with various architectures such as star block copolymers [20,21],
comb-like copolymers [22], H-shape copolymers [23] and theta-
shape copolymers [24] have been synthesized. So this methodology
could also be extended to the preparation of well-defined block-
brush copolymer.
Poly(ethylene glycol) (PEG) has been popularly used as the
water-soluble block in constructing the copolymers because of its
biocompatibility and physicochemical properties [25]. Poly(N-iso-
propylacrylamide) (PNIPAM) is a wildly-used thermal responsive
polymer [26]. There are a lot of research reports on the thermo-
dependent micellization of PEG/PNIPAM copolymers [27e29]. Poly
1
2.3. Preparation of PEG-b-P(NIPAM-co-ProA) copolymer by ATRP
method
A solution of propargyl alcohol (2.00 g, 0.49 mol), triethylamine
(5.50 g, 0.05 mol), hydroquinone (0.050 g) and THF (100 mL) was
cooled in ice-water bath. Acryloyl chloride (4.80 g, 0.05 mol) was
added dropwise in 20 min. The mixture was stirred in ice-water
bath for 1 h and at room temperature for 15 h. After the salt of
triethylammonium chloride was filtered off, the solvent was
removed by rotary evaporation. Then, propargyl acrylate (ProA)
(
N,N-dimethylaminoethyl methacrylate) (PDMAEMA) is a weak
base with pKa about 7.0 and is water-soluble within wide pH range.
It shows pH-responsive behavior due to the protonation of the
tertiary amine group [30]. The diblock copolymer of PEG-b-
PDMAEMA shows a potential for application in gene delivery
system [31] and encapsulation of antiparasitic compounds for the
treatment of leishmaniasis [32]. Recent research reports show that
the combination of different functions within a single polymer can
contribute to an overall improvement of carrier efficiency [33].
Combination of PEG, PNIPAM and PDMAEMA would offer thermal
and pH dual response, and good efficiency in the controlled drug
delivery system.
was obtained by further vacuum distillation (4.30 g, yield: 82%).
1
H NMR spectroscopy (CDCl
eC^CH), 5.85 (d, 1H, CH
CHe), 6.34 (d, 1H, CH
¼ CHe).
3
,
d
, ppm): 2.45 (s, 1H, eC^CH), 4.63
(s, 2H, eOCH
2
2
]CHe), 6.10 (q, 1H, CH
2
]
2
Copolymer of PEG-b-P(NIPAM-co-ProA) with low ProA content
was synthesized via ATRP copolymerization of NIPAM and ProA
using PEG-Cl as ATRP macro-initiator. NIPAM (1.13 g, 10.0 mmol),
PEG-Cl (0.25 g, 0.05 mmol), ProA (27 mg, 0.25 mmol) and DMF
(4.0 mL) were added into a polymerization vial equipped with
a magnetic stirring bar. After NIPAM and PEG-Cl were completely
dissolved, the mixture was frozen. Then, Me6TREN (0.028 g,
0.1 mmol) and CuCl (10 mg, 0.1 mmol) in water (2.0 mL) were
added. The mixture was degassed by three freezeepumpethaw
cycles. After the vial was sealed under vacuum, the polymerization
This study aims to the synthesis of well-defined block-brush
copolymer, poly(ethylene glycol)-b-poly[N-isopropylacrylamide-g-
N, N-dimethylaminoethyl methacrylate] [PEG-b-P(NIPAM-g-
DMAEMA)] and the fundamental investigation of its dual response.
ꢀ
2
. Experimental
reaction was allowed to proceed under stirring at 25 C. After 10 h,
the vial was opened and the reaction mixture was diluted with THF.
2.1. Materials
2 3
The thinned mixture was passed through an activated basic Al O
column to remove copper complex. After removing most THF by
rotary evaporation and precipitating the solution in diethyl ether,
the product of PEG-b-P(NIPAM-co-ProA) was collected by filtration
and dried under vacuum at room temperature for 24 h (1.10 g, yield
81%). The chemical composition was determined by H NMR
spectroscopy and relative molecular weight was characterized by
gel permeation chromatograph (GPC).
Methoxy poly(ethylene glycol) (PEG) (M
n
¼ 5000, BASF) was
dried over anhydrous toluene by azeotropic distillation. Acryloyl
chloride was purified by vacuum distillation. N-iso-
propylacrylamide (NIPAM, Kohjin Co., Japan) was purified by
recrystallization from a benzene/n-hexane mixture (65/35 v/v). 2-
Dimethylaminoethyl methacrylate (DMAEMA, Aldrich) was passed
1
through basic alumina column, then vacuum-distilled over CaH
2
0
00
000
000
prior to use. N, N, N , N , N , N -Hexamethyltriethylenetetramine
Me6TREN) was synthesized according to the method previously
2.4. Synthesis of azide end-functionalized PDMAEMA
(PDMAEMA-N )
3
(
0
0
described [34]. 2-Chloropropionyl chloride (CPC) and N, N, N , N ,
0
0
N -pentamethyldiethylenetriamine (PMDETA) were purchased
form Alfa Aesar and used without further purification. Triethyl-
amine was stirred with KOH for 12 h at room temperature, refluxed
with toluene-4-sulfonyl-chloride and distilled before use. Copper
The azide end-functionalized PDMAEMA was synthesized by
using AEBIB as ATRP initiator. DMAEMA (3.00 g, 19 mmol), AEBIB
(55.6 mg, 0.24 mmol), PMDETA (36.7 mg, 0.24 mmol), CuBr
(30.6 mg, 024 mmol) and isopropanol (4.0 mL) were added into
a 10-mL polymerization vial. After three freezeevacuumethaw
cycles, the vial was sealed under vacuum and immersed in a water
(
I) chloride and copper (I) bromide were washed with glacial acetic
acid, followed by washing with methanol and ethyl ether to remove
impurities, and then dried under vacuum and kept under N
ꢀ
2
bath thermostat at 50 C. After 8 h, the vial was opened. The
atmosphere. THF was dried over sodium/benzophenone and
distilled just before use. All other reagents were of analytical grade
reaction mixture was diluted with THF and passed through an
2 3
Al O column to remove Cu complex. After most THF was removed
and used as received. The azide-functionalized ATRP initiator of
azidoethyl 2-bromoisobutyrate (AEBIB) was synthesized according
to our previous report [35].
b
-
by rotary evaporation, the polymer of PDMAEMA-N
3
was obtained
ꢀ
by precipitation into hexane and dried in a vacuum oven at 25 C
for 24 h (2.1 g, yield: 70%). The chemical structure was determined
1
by H NMR spectroscopy and relative molecular weight was char-
2.2. Synthesis of 2-chloropropionylated PEG (PEG-Cl)
acterized by GPC.
A solution of PEG (10.00 g, 2 mmol), triethylamine (2.02 g,
0 mmol) and toluene (100 mL) were cooled in an ice-water bath.
2.5. Synthesis of PEG-b-P(NIPAM-g-DMAEMA) block-brush
copolymer
2
After 2-chloropropionyl chloride (2.52 g, 20 mmol) was added
dropwise in 20 min, the mixture was stirred at room temperature
for 12 h. The produced solid was removed by vacuum filtration.
Block-brush copolymer of PEG-b-P(NIPAM-g-DMAEMA) was
synthesized through the cycloaddition reaction between the alkyne

B.-Y. Zhang et al. / Polymer 51 (2010) 3039e3046
3041
Scheme 1. Schematic illustration of the synthesis of block-brush copolymer of PEG-b-P(NIPAM-g-DMAMEA).

3042
B.-Y. Zhang et al. / Polymer 51 (2010) 3039e3046
side-groups of PEG-b-P(NIPAM-co-ProA) and the azide end-group
of PDMAEMA-N Thus, PEG-b-P(NIPAM-g-DMAEMA) (0.50 g),
PDMAEMA (1.13 g), CuBr (13 mg), PMDETA (16 mg) and DMF
6.0 mL) were added into a 10-mL vial. Then the reaction mixture
methyl group originated from the 2-chloropropionyl group, and
that of d at 3.4 ppm to the methyl group from methoxy PEG. The
esterification conversion could be estimated from the relative
integral values of the signals of e and d. The ratio of e:d is close to 1,
indicating the complete conversion of the hydroxyl group of PEG.
In the second step, PEG-b-P(NIPAM-co-ProA) with low alkyne
content was synthesized through ATRP copolymerization of NIPAM
and propargyl acrylate with using 2-chloropropionylated PEG as
macro-initiator. The polymerization of NIPAM, using PEG-Cl/CuBr/
3
.
(
was degassed by three freezeepumpethaw cycles. The tube was
sealed under vacuum and the click reaction was allowed to proceed
ꢀ
under vigorous stirring at 30 C. After 48 h, the reaction was
stopped and the mixture was dialyzed against deionized water
using a dialysis membrane (cutoff molecular weight: 14,000) for
4
8 h, and subsequent lyophilized in a freeze drier (SIMENS FD5-3,
CuBr
2
/ME
6
TREN as the initiator/catalyst system in DMF/H
2
O (2/1, V/
ꢀ
1
Germany) for 48 h. Thus, PEG-b-P(NIPAM-g-DMAEMA) was
obtained, and its chemical composition and relative molecular
weigh was determined by
respectively.
V) at 25 C, is well controlled [37]. H NMR spectrum of PEG-b-P
(NIPAM-co-ProA) is shown as Fig. 1b. The copolymer composition is
determined by comparing the integration of PEG peak (a) at
3.7 ppm, NIPAM peak (b) at 4.0 ppm and ProA peak (c) at 4.7 ppm.
The unit ratio of EG:NIPAM:ProA and number-averaged molecular
weight by NMR (MnNMR) are listed in Table 1. GPC trace of PEG-b-P
1
H NMR spectroscopy and GPC,
2.6. Characterizations
(NIPAM-co-ProA) is shown in Fig. 2, indicating a monomodal and
The relative molecular weight and its distribution were deter-
symmetrical elution peak with MnGPC of 29,700 and M /M of 1.09.
w
n
mined on a Waters 150C Gel Permeation Chromatography (GPC)
equipped with Waters 1515 HPLC pump, Waters 2414 differentia
3.2. Preparation of PDMAEMA-N
1H NMR spectrum of PDMAEMA-N
is shown in Fig. 1c. The
3
3
3
refractive index detector and three Ultrastyragel columns (500, 10
4
ꢀ
ꢀ
and 10 A in series) at 30 C, using monodispersed polystyrene as
calibration standard. DMF was used as eluent at a flow rate of
peaks at 4.03 (b), 2.59 (c), 2.27 (d), 1.67e2.12 (e) and 0.71e1.26 (f)
ppm are the characteristic signals of PDMAEMA. The peak (a) at
3.46 ppm corresponds to the signal of the methylene protons
originating from the used initiator. By comparing the integration of
the peaks at 3.46 and 4.03 ppm, the polymerization degree of
1.0 mL/min.
1H NMR spectroscopy (300 MHz) measurement for chemical
structure and MnNMR was performed on Bruker DMX300 spec-
trometer in CDCl
NMR, neutral D
3
. For the investigation of thermal response by
O was used as the solvent. Fourier transform
2
3
PDMAEMA-N is 80.
infrared (FTIR) spectra were recorded on a Bruker VECTOR-22 IR
spectrometer in KBr pellets.
IR analysis of PDMAEMA-N
3
shows an absorption peak at
ꢁ1
2100 cm , which corresponds to the asymmetric stretching
vibration of the azide group (Fig. 3). This confirms the successful
synthesis of the azide end-functionalized PDMAEMA. The GPC trace
The optical transmittance of PEG-b-P(NIPAM-g-DMAEMA)
aqueous solution at a wavelength of 500 nm was acquired on
a Unico UV/vis 2802PCS spectrophotometer. A thermostatically
controlled cuvette was employed. The lower critical solution
temperature (LCST) was defined as the temperature corresponding
to the initial abrupt decrease in transmittance. The polymer
concentration was 2.0 mg/mL.
of PDMAEMA-N
molecular distribution (M
3
(Fig. 2) also shows narrow and symmetrical
/M
¼ 1.15).
w
n
3.3. Click reaction between PEG-b-P(NIPAM-co-ProA) and
PDMAEMA-N
3
Micro-DSC measurements were carried out on a VP DSC from
MicroCal. The volume of the sample cell is 0.509 mL. The reference
cell was filled with deionized water. The sample solution at 2.0 mg/
Block-brush copolymer of PEG-b-P(NIPAM-g-DMAEMA) was
obtained through the click reaction between the alkyne side-
groups of PEG-b-P(NIPAM-co-ProA) and the azide end-group of
ꢀ
ꢀ
mL was degassed at 25 C for 0.5 h and equilibrated at 10 C for 2 h
ꢀ
before the heating process at the heating rate of 1.0 C/min.
3
PDMAEMA-N . Usually, we used alkyne-functionalized solid
Dynamic laser light scattering (DLS) study was performed on
a modified commercial laser light scattering spectrometer (ALV/SP-
25) equipped with an ALV-5000 multi-s digital time correlator and
support to remove the remaining azide-functionalized polymer as
our previous report [35]. However, because the molar ratio of azide
group to alkyne group was unity at the present work, we acquired
pure PEG-b-P(NIPAM-g-DMAEMA) without any precursor
1
a HeeNe laser (output power ¼ 10 mW,
l
¼ 632 nm). Before DLS
measurement, the solutions were dust-freed with a Millipore filter
3
PDMAEMA-N after the dialysis.
ꢀ
(
0.45
mm). Scattered light was collected at a fixed angle of 30 for
Fig. 3 shows FTIR spectrum of the block-brush PEG-b-P(NIPAM-
g-DMAEMA) after click reaction. The characteristic absorbance of
duration of 10 min. All data were averaged over three
measurements.
ꢁ
1
azide group (2106 cm ) absolutely disappears, indicating the full
conversion of PDMAEMA-N . GPC trace of PEG-b-P(NIPAM-g-
3
3
. Results and discussion
DMAEMA) is shown in Fig. 2 and shifts to higher molecular weight
compared with those of PEG-b-P(NIPAM-co-ProA) and PDMAEMA-
The block-brush copolymer of PEG-b-P(NIPAM-g-DMAEMA)
3
N , and its polydispersity becomes a little broader. However, it still
was synthesized as shown in Scheme 1. Herein, we synthesized
PEG-b-P(NIPAM-co-ProA) having alkyne side-groups and
keeps unimodal and is free from the tailing in both lower and
higher molecular weight region. The result reveals that no residue
PDMAEMA-N
3
having azide end-group separately by ATRP poly-
3
of PEG-b-P(NIPAM-co-ProA) or PDMAEMA-N remains after the
merization, and then obtained PEG-b-P(NIPAM-g-DMAEMA)
click reaction and dialysis. The number-averaged molecular weight
through click chemistry.
measured by GPC (MnGPC, 65,400) using polystyrene standards is
lower than that by H NMR (MnNMR, 91,400), reflecting that the
1
3
.1. Synthesis of PEG-b-P(NIPAM-co-ProA)
block-brush copolymers possess smaller hydrodynamic volume
than the linear analogue [38,39].
1
Firstly, 2-chloropropionylated PEG as ATRP macro-initiator was
H NMR spectrum of PEG-b-P(NIPAM-g-DMAEMA) displays the
synthesized according to the literature [36] and its NMR spectrum
characteristic signals of PEG, PNIPAM and PDMAMEA, as shown in
is shown in Fig. 1a. The signal of e at 1.7 ppm is described to the
Fig.1d. Furthermore, the integration ratio of PDMAEMA signal (d) at

B.-Y. Zhang et al. / Polymer 51 (2010) 3039e3046
3043
Fig. 1. 1H NMR spectra of PEG-Cl (a), PEG-b-P(NIPAM-co-ProA) (b), PDMAEMA-N
(c) and PEG-b-P(NIPAM-g-PDMAEMA) (d) in CDCl .
3 3
2
4
.59 ppm to that of the signal (b and c) of PNIPAM and PDMAEMA at
.06 ppm is determined to be 0.8, which is the same as the
not observed [42e44]. In our current study, the thermal-sensitive
behavior to PNIPAM block and the influence of cationic PDMAEMA
block at different pHs are focused. Thermal and pH dual responsive
expected value based on the charged amount. By the combination
of GPC result, we could conclude that each PDMAEMA-N chain has
been successfully stuck to the backbone. MnNMR of PEG-b-P(NIPAM-
g-DMAEMA) was calculated according to the integration values of
PEG signal (a), PNIPAM signal (d) and PDMAEMA signal (b þ c) and
the result is listed in Table 1.
3
behavior of PEG-b-P(NIPAM-g-DMAEMA) is investigated by the
1
combination of
H NMR analysis at different temperatures,
dynamic light scattering, temperature-dependent turbidimetry
study and micro-DSC.
1H NMR spectra of block-brush PEG-b-P(NIPAM-g-DMAEMA)
with D
2
O as the solvent at two temperatures are shown in Fig. 4.
ꢀ
PEG-b-P(NIPAM-g-DMAEMA) is well dissolved at 25 C and all
proton signals are normally observed. Compared with H NMR
1
3.4. Stimulus-response behavior of PEG-b-P(NIPAM-g-DMAEMA)
It is well known that PNIPAM homopolymer is temperature-
ꢀ
ꢀ
responsive and its LCST in aqueous solution is 32 C [40].
PDMAEMA homopolymer has LCST varied from 40 to 50 C with
decreasing molecular weight and its LCST value strongly depends
on the ionization of the dimethylamino group [41]. However, for
the block copolymers composed of PNIPAM and PDMAEMA, the
thermal responsive behavior concerning with PDMAEMA block was
Table 1
Molecular characterizations of different polymers in this study.
Polymer
NMR result
GPC result
Molar ratio
of units
M
nNMR
M
nGPC
w n
M /M
PEG-Cl
PEG-b-
P(NIPAM-co-ProA)
PDMAEMA-N
PEG-b-(PNIPAM-
g-PDMAEMA)
EG:2-CPC ¼ 113:1
EG:NIPAM:ProA ¼
113:200:5
DMAEMA:AEBIB ¼ 80:1
EG:NIPAM:DMAEMA ¼
113:199:393
5100
28,200
5100
29,700
1.02
1.09
3
12,500
91,400
12,900
65,400
1.15
1.37
3
Fig. 2. GPC traces of PDMAEMA-N , PEG-b-P(NIPAM-co-ProA) and PEG-b-P(NIPAM-g-
DMAEMA).

3044
B.-Y. Zhang et al. / Polymer 51 (2010) 3039e3046
Table 2
Main variations of chemical shift and integrate ratio in 1H NMR spectra of PEG-b-P
(NIPAM-g-DMAEMA) with temperature and solvent.
ꢀ
ꢀ
ꢀ
O/40 C
Proton signal
CH
in EGa
CH(CH in NIPAM
OCH in DMAEMA
CH N(CH
DMAEMA
CH N(CH
DMAEMA
CDCl
3
/25
C
D
2
O/25
C
D
2
3.66 ppm (1.00)b
4.06 ppm (2.18)
3.75 ppm (1.00)
3.97 ppm (0.44)
4.25 ppm (1.75)
2.81 ppm (1.73)
3.75 ppm (1.00)
4.09 ppm (0.24)
4.36 ppm (1.73)
2.82 ppm (1.71)
2
3 2
)
2
2
3
)
2
in
in
2.59 ppm (1.74)
2.27 ppm (5.21)
2
3
)
2
2.41 ppm (5.22)
2.40 ppm (5.20)
a
The integrate of methylene protons of PEG is taken as unity.
The value in the brackets stands for the relative integration of the signal.
b
DMAEMA) aqueous solution at pH ¼ 5.0, 7.0 and 9.0. In acidic
aqueous solution with pH ¼ 5.0, PEG-b-P(NIPAM-g-DMAEMA)
shows weak thermo-response. The solution remains transparent
Fig. 3. FTIR spectra of the PDMEAME-N
3
and PEG-b-P(NIPAM-g-DMAEMA).
ꢀ
h
even at 40 C while hydrodynamic radius (R ) is not obtained due to
the sharp fluctuation in scattering intensity during DLS
measurement.
spectra with CDCl
of the signals of CH(CH
DMAEMA (c, 4.25 ppm). Other assignments of the concerning
protons and the relative integration of the proton signals based on
methylene protons of EG unit are summarized in Table 2.
When the solution is heated to 40 C, PNIPAM chain is dehy-
drated and collapsed, leading to the variation in H NMR spectrum
of PEG-b-P(NIPAM-g-DMAEMA) as shown in Fig. 4 and Table 2. The
chemical shift of CH(CH ) in NIPAM (b) decreases obviously while
3 2
3
as the solvent, the main change is the separation
in NIPAM (b, 3.97 ppm) and OCH in
As reported previously, PEG-b-PNIPAM aqueous solution
3
)
2
2
ꢀ
exhibited obvious thermo-response at the temperature of 40 C
[45]. The reason for our observation may be explained as followed.
Firstly, the protonization degree of PDMAEMA units is high and
PDMAEMA chains have better solubility in water at low pH value
since pKa of PDMAEMA is 7.0. Thus, the steric hindrance of
stretching PDMAEMA chains would prevent PEG-b-P(NIPAM-g-
DMAEMA) molecules from inter-molecular aggregation to large
micelles. At the same time, PDMAEMA chains are positively
charged and their electric repulsion between PDMAEMA chains
would have the same effect. In this case, PEG-b-P(NIPAM-g-
DMAEMA) molecules tend to form unimer micelles by the intra-
molecular collapse of PNIPAM chains. Moreover, better hydrophi-
licity of PDMAEMA chains with higher protonization leads to the
increase of PNIPAM LCST.
ꢀ
1
others in NIPAM hardly change when the temperature rises to
ꢀ
4
0 C, resulting from the rupture of hydrogen-bonding of NIPAM
amide groups with water, which decreases the hydrophilicity of
PNIPAM chains. Furthermore, the integration ratio of CH(CH in
NIPAM (b) to PEG signal (a) at 3.75 ppm reduces about 46% while
the integration ratio of OCH in DMAEMA (c) is almost unchanged.
This evidence means that PEG-b-P(NIPAM-g-DMAEMA) undergoes
the formation of micellar aggregates consisting of the condensed
PNIPAM core and the stretched corona of PDMAEMA and PEG
chains.
Through dynamic light scattering, we studied the stimulus-
response behavior of block-brush PEG-b-P(NIPAM-g-DMAEMA) at
different temperatures and pHs. Fig. 5 shows the temperature
dependence of the scattering intensity of PEG-b-P(NIPAM-g-
3 2
)
2
When pH increases to 7.0, PDMAEMA becomes partially
deprotonated and less hydrophilic. The decrease of electric repul-
sion between PDMAEMA chains offers PNIPAM chains the possi-
bility to aggregate inter-molecularly above LCST [46]. Thus, block-
brush copolymer shows obvious thermo-response and its LCST gets
ꢀ
down to 34 C. When pH increases to 9.0, PDMAEMA chains turn
highly deprotonated, so LCST of block-brush copolymer further
ꢀ
decreases to 28 C, even lower than LCST of homo-PNIPAM. The
variation tendency of LCST is consistent with the decrease of
PDMAEMA hydrophilicity as the aqueous solution turns less and
less acidic. The result clearly demonstrates that PEG-b-P(NIPAM-g-
Fig. 4. 1H NMR spectra of PEG-b-P(NIPAM-g-DMAEMA) in D
up).
O at 25 C (low) and 40 C
ꢀ
ꢀ
Fig. 5. Variation of DLS scattering intensity of PEG-b-P(NIPAM-g-DMAEMA) aqueous
solution (0.1 mg/mL) at a fixed scattering angle of 30 at different pHs.
2
ꢀ
(

B.-Y. Zhang et al. / Polymer 51 (2010) 3039e3046
3045
Fig. 8. Temperature dependence of the specific heat capacity (C
p
) of PEG-b-P(NIPAM-
h
Fig. 6. Temperature dependence of R of PEG-b-P(NIPAM-g-DMAEMA) micelles in
g-DMAEMA) aqueous solution (2.0 mg/mL) at pH ¼ 7.0 and 9.0 during the heating
aqueous solution (0.1 mg/mL) during the heating process at pH ¼ 7.0 and 9.0. The
ꢀ
process. The heating rate was 1.0 C/min.
inseted is the hydrodynamic diameter distribution of PEG-b-P(NIPAM-g-DMAEMA)
ꢀ
micelles at 28.7 and 32.0 C (pH ¼ 9.0).
ꢀ
ꢀ
molecular micelles. LCST shifts from 33.5 C at pH ¼ 7.0 to 28.1 C at
pH ¼ 9.0 while the phase transition becomes sharp at pH ¼ 9.0.
Micro-DSC analysis is further employed to investigate the
thermal phase transition of PEG-b-P(NIPAM-g-DMAEMA) block-
brush copolymer in the aqueous solutions with a concentration of
2.0 mg/mL at pH ¼ 7.0 and 9.0, and the results are shown in Fig. 8. It
is obviously seen that the endothermic peak of block-brush
copolymer in the aqueous solution with pH ¼ 7.0 is much broader
DMAEMA) has both pH and thermal responsive behavior, and LCST
of PEG-b-P(NIPAM-g-DMAEMA) can be modulated by pH value of
aqueous solution.
Fig. 6 shows the temperature-dependent of hydrodynamic
radius (R
h
) of block-brush PEG-b-P(NIPAM-g-DMAEMA) in aqueous
of copol-
solution (0.1 mg/mL) at two pHs. At pH ¼ 7.0 and 9.0, R
h
ymer solution increases from about 5 nm for individual block-brush
molecule to 90e100 nm for the micelles of inter-molecular aggre-
gation above LCST. The observed value of LCST decreases with the
increase of pH. The inset in Fig. 6 shows typical hydrodynamic
radius distributions of block-brush copolymer at two temperatures
for pH ¼ 9.0. The averaged hydrodynamic radius of block-brush
than that with pH ¼ 9.0. Moreover, the onset temperature of C
p
ꢀ
increase is 28.0 and 34.0 C for pH ¼ 7.0 and 9.0, respectively,
indicating the decrease of phase transition temperature of PEG-b-P
(NIPAM-g-DMAEMA) with the increase of pH value. These results
are in good agreement with DLS analysis and turbidimetry
measurement. It has been reported that the enthalpy change for
linear PNIPAM chains is independent of its chain length, and
ꢀ
copolymer micelles is 25 and 82 nm at 28.7 and 32.0 C, respec-
tively. The result should be ascribed to the collapse and aggregation
ꢀ
of PINIPAM chains during phase transition. However, even at 55 C
enthalpy change (
values of
H for PNIPAM phase transition at pH ¼ 9.0 and 7.0 is
5.61 kJ/mol and 3.67 kJ/mol in our case, respectively. It should be
noted that
H value for PNIPAM phase transition at pH ¼ 7.0 is
DH) is in the range of 5.5e7.5 kJ/mol [47,48]. The
and pH ¼ 9.0, the phase transition concerning with PDMAEMA is
D
not observed.
Transmittance of PEG-b-P(NIPAM-g-DMAEMA) aqueous solu-
tion (2 mg/mL) at three pHs was measured in the temperature
D
smaller than linear PNIPAM. This is because hydrophilic PEG and
PDMAEMA segments tend to solubilize PNIPAM segment and
increase LCST of block-brush copolymer. In other words, not all of
the PNIPAM units take part in the rupture of hydrogen-bonding
with water during thermal transition [49]. When pH is 9.0,
PDMAEMA chains are highly deprotonated and show little influ-
ence on enthalpy change for PNIPAM chains. At pH ¼ 5.0,
PDMAEMA is highly protonated, leading to the hardly observation
ꢀ
range of 20e50 C. As shown in Fig. 7, PEG-b-P(NIPAM-g-DMAEMA)
aqueous solution (pH ¼ 5.0) keeps transparent almost without the
decrease in transmittance, suggesting no formation of large-sized
aggregates of PEG-b-P(NIPAM-g-DMAEMA) at this pH value. With
the increase of pH to 7.0 and 9.0, obvious decrease in transmittance
is observed, which is contributed to the formation of large inter-
p
of C change.
4. Conclusions
Block-brush copolymer, poly(ethylene glycol)-b-poly[N-iso-
propylacrylamide-g-2-(dimethylamino)ethyl methacrylate], was
synthesized successfully by the combination of ATRP and click
chemistry. Because of the controlled features of ATRP and high
efficiency of click chemistry, the obtained block-brush copolymer
1
has well-defined chemical structure, which is confirmed by GPC, H
NMR and FTIR. PEG-b-P(NIPAM-g-DMAEMA) exhibits pH and
thermal dual responsive behavior. The combination of tempera-
ture-varied NMR, DLS, turbidimetry and micro-DSC reveals that the
thermal responsive behavior of PEG-b-P(NIPAM-g-DMAEMA)
depends on pH value. At acidic condition, PEG-b-P(NIPAM-g-
DMAEMA) shows weak thermal response and the phase transition
of PNIPAM segment may lead to the formation of uni-molecular
micelles. At neutral or basic condition, PEG-b-P(NIPAM-g-
Fig. 7. Transmittance change of PEG-b-P(NIPAM-g-DMAEMA) aqueous solution (2 mg/
mL) with temperature.

3046
B.-Y. Zhang et al. / Polymer 51 (2010) 3039e3046
DMAEMA) exhibits strong thermal response and may form the
micelles of inter-molecular aggregates. LCST of PEG-b-P(NIPAM-g-
DMAEMA) increases with pH decrease.
[19] Vora A, Singh K, Webster DC. Polymer 2009;50:2768e74.
[
20] Petrova S, Riva R, Jerome C, Lecomte P, Mateva R. Eur Polym
009;45:3442e50.
21] Liu JQ, Tao L, Xu JT, Jia ZF, Boyer C, Davis TP. Polymer 2009;50:4455e63.
22] Bernard J, Favier A, Davis TP, Barner-Kowollik C, Stenzel MH. Polymer
006;47:1073e80.
23] Gungor E, Durmaz H, Hizal G, Tunca U.
008;46:4459e68.
[24] Shi GY, Pan CY. J Polym Sci A Polym Chem 2009;47:2620e30.
J
2
[
[
2
Acknowledgments
[
J Polym Sci A Polym Chem
2
The financial supports of China Science and Technology Ministry
[25] Chen CP, Kim JS, Liu DJ, Rettig GR, McAnuff MA, Martin ME, et al. Bioconjugate
Chem 2007;18:371e8.
(
2007CB936401), HK Earmarked Research Grant (403706) and Key
Project of NSFC (20934005) are greatly acknowledged. W-T Li, K-R
Zhang and H Zhang thank the support of National Undergraduate
Innovative Experiment Project of China Education Ministry and the
conveniences of measurements offered by Chemistry Experiment
Center of USTC.
[
26] Nuopponen M, Ojala J, Tenhu H. Polymer 2004;45:3643e50.
[27] Cai T, Marquez M, Hu ZB. Langmuir 2007;23:8663e6.
[28] Zhang WQ, Shi LQ, Wu K, An YG. Macromolecules 2005;38:5743e7.
[29] You YZ, Oupicky D. Biomacromolecules 2007;8:98e105.
[30] Xu YY, Bolisetty S, Drechsler M, Fang B, Yuan JY, Ballauff M, et al. Polymer
2008;49:3957e64.
[
[
31] Jiang X, Lok MC, Hennink WE. Bioconjugate Chem 2007;18:2077e84.
32] Auguste DT, Armes SP, Brzezinska KR, Deming TJ, Kohn J, Prud’homme RK.
Biomaterials 2006;27:2599e608.
References
[33] Manickam DS, Oupicky D. Bioconjugate Chem 2006;17:1395e403.
[
[
[
1] Discher DE, Eisenberg A. Science 2002;297:967e73.
2] Zhang LF, Eisenberg A. Science 1995;56:1728e31.
3] Talingting MR, Munk P, Webber SE, Tuzar Z. Macromolecules 1999;32:
[34] Ciampoli M, Nardi N. Inorg Chem 1966;5:41.
[35] Li J, He WD, Han SC, Sun XL, Li LY, Zhang BY. J Polym Sci A Polym Chem
2009;47:786e96.
1
593e601.
4] Yamato M, Akiyama Y, Kobayashi J, Yang J, Kikuchi A, Okano T. Prog Polym Sci
007;32:1123e33.
5] Kim TH, Huh J, Hwang J, Kim HC, Kim SH, Sohn BH, et al. Macromolecules
009;42:6688e97.
[36] Xu ZS, Hu XX, Li XQ, Yi CF. J Polym Sci A Polym Chem 2008;46:481e8.
[37] Masci G, Giacomelli L, Crescenzi V. Macromol Rapid Commun
2004;25:559e64.
[38] Beers KL, Gaynor SG, Matyjaszewski K, Sheiko SS, Moller M. Macromolecules
1998;31:9413e5.
[
[
2
2
[
[
[
[
6] Cui HG, Chen ZY, Zhong S, Wooley KL, Pochan DJ. Science 2007;317:647e50.
7] Yu K, Eisenberg A. Macromolecules 1996;29:6359e61.
8] Martin TJ, Prochazka K, Munk P, Webber SE. Macromolecules 1996;29:6071e3.
9] Reddy TT, Takahara A. Polymer 2009;50:3537e46.
[39] Neugebauer D, Zhang Y, Pakula T.
2006;44:1347e56.
[40] Riess G. Prog Polym Sci 2003;28:1107e70.
[41] Lowe AB, Billingham NC, Armes SP. Chem Commun 1997;11:1035e6.
[42] Butun V, Armes SP, Billingham NC. Polymer 2001;42:5993e6008.
[43] Triftaridou AI, Vamvakaki M, Patrickios CS. Polymer 2002;43:2921e6.
[44] You YZ, Zhou QH, Manickam DS, Wan L, Mao GZ, Oupicky D. Macromolecules
2007;40:8617e24.
J Polym Sci A Polym Chem
[
10] Weber C, Becer CR, Hoogenboom R, Schubertt US. Macromolecules
2009;42:2965e71.
[
[
[
11] Teertstra SJ, Gauthier M. Macromolecules 2007;40:1657e66.
12] Wang JS, Matyjaszewski K. Macromolecules 1995;28:7901e10.
13] Chiefari J, Chong YK, Ercole F, Krstina J, Jeffery J, Le TPT, et al. Macromolecules
[45] Tang XD, Liang XC, Yang Q, Fan XH, Shen ZH, Zhou QF. J Polym Sci A Polym
Chem 2009;47:4420e7.
1998;31:5559e62.
[14] Roos SG, Muller AHE, Matyjaszewski K. Macromolecules 1999;32:8331e5.
[15] Shinoda H, Matyjaszewski K. Macromolecules 2001;34:6243e8.
[16] Zhang ZB, Shi ZQ, Han X, Holdcroft S. Macromolecules 2007;40:2295e8.
[17] Tornoe CW, Christensen C, Meldal M. J Org Chem 2002;67:3057e64.
[18] Diaz DD, Punna S, Holzer P, McPherson AK, Sharpless KB, Fokin VV, et al. J
Polym Sci A Polym Chem 2004;42:4392e403.
[46] Xu XW, Liu C, Huang JL. J Appl Polym Sci 2008;108:2180e8.
[47] Shibayama M, Mizutani S, Nomura S. Macromolecules 1996;29:
2019e24.
[48] Shan J, Chen J, Nuopponen M, Tenhu H. Langmuir 2004;20:4671e6.
[49] Li LY, He WD, Li J, Han SC, Sun XL, Zhang BY. J Polym Sci A Polym Chem
2009;47:7066e77.