Facile preparation of poly(N-isopropylacrylamide)-based hydrogels via aqueous Diels-Alder click reaction

Article abstract of DOI:10.1016/j.polymer.2010.02.008

The study reports a facile method of preparing poly(N-isopropylacrylamide)- based hydrogels by means of the Diels-Alder reaction. First, polymeric dienes were synthesized by free radical copolymerization between N-isopropylacrylamide (NIPA) and furfuryl methacrylate (FM), with 2, 2′-azobisisobutyronitrile (AIBN) as an initiator, and polymeric dienophile was obtained by a coupling reaction of poly(ethylene glycol) (PEG) and N-maleoyl-l-leucine (LMI) under N, N′-dicyclohexylcarbodiimide (DCC). Afterwards, the resultant dienes and dienophiles were dissolved in water and put in a refrigerator remaining a temperature of 9 °C, gelation via Diels-Alder reaction was observed after some time. The samples obtained at different steps were characterized by FTIR, NMR, GPC, SEM, CD, etc. It was found that LCST of copolymers decreases with the increase of FM content in copolymers. And the disassembly time of the hydrogels is closely related to the temperature and the solvents used. The swelling behavior study by gravimetric measurement indicates the hydrogels possess thermosensitivity and exhibit considerable swelling in water. Due to the simplicity of synthesis and no need for initiator or catalyzer and organic solvent, the strategy described here could find a promising application in the preparation of hydrogels.

Full text of DOI:10.1016/j.polymer.2010.02.008

Polymer 51 (2010) 1694e1702
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Facile preparation of poly(N-isopropylacrylamide)-based hydrogels via aqueous
DielseAlder click reaction
*
Hong-Liang Wei , Zhe Yang, Hui-Juan Chu, Jing Zhu, Zhi-Cheng Li, Jin-Shi Cui
School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The study reports a facile method of preparing poly(N-isopropylacrylamide)- based hydrogels by means
of the DielseAlder reaction. First, polymeric dienes were synthesized by free radical copolymerization
between N-isopropylacrylamide (NIPA) and furfuryl methacrylate (FM), with 2, 20-azobisisobutyronitrile
(AIBN) as an initiator, and polymeric dienophile was obtained by a coupling reaction of poly(ethylene
Received 13 October 2009
Received in revised form
20 December 2009
Accepted 4 February 2010
Available online 12 February 2010
glycol) (PEG) and N-maleoyl-L
-leucine (LMI) under N, N0-dicyclohexylcarbodiimide (DCC). Afterwards,
the resultant dienes and dienophiles were dissolved in water and put in a refrigerator remaining
a temperature of 9 ꢀC, gelation via DielseAlder reaction was observed after some time. The samples
obtained at different steps were characterized by FTIR, NMR, GPC, SEM, CD, etc. It was found that LCST of
copolymers decreases with the increase of FM content in copolymers. And the disassembly time of the
hydrogels is closely related to the temperature and the solvents used. The swelling behavior study by
gravimetric measurement indicates the hydrogels possess thermosensitivity and exhibit considerable
swelling in water. Due to the simplicity of synthesis and no need for initiator or catalyzer and organic
solvent, the strategy described here could find a promising application in the preparation of hydrogels.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Thermosensitivity
DielseAlder reaction
Hydrogels
1. Introduction
appropriately adjusted by copolymerizing N-isopropylacrylamide
with a more hydrophilic monomer or a more hydrophobic mono-
Hydrogels are a class of cross-linked polymers that can swell in
water without dissolving [1]. As is well-known, living organs
possess fantastic functions in soft and wet gel-like state. Therefore,
hydrogels have been the research focus of many material scientists
as excellent wet materials which are suitable for making artificial
organs for medical treatments [2]. Especially, stimuli responsive or
so-called “smart” polymer gels have been studied extensively
during the past decades, which are capable to undergo quite strong
and abrupt physical or chemical changes in response to small
external changes such as pH, temperature, ionic strength, electric
and magnetic field, etc. [3]. By far, the most studied temperature-
sensitive polymer gel is cross-linked poly(N-isopropylacrylamide)
[4,5]. Due to its special properties, poly(N-isopropylacrylamide) is
a good candidate material for many applications, such as in artificial
muscles, drug delivery systems, separation membranes, catalysis
substrates, actuators, and chemical valves [6]. Poly(N-iso-
propylacrylamide) gels exhibit a temperature-induced volume
phase transition in water upon heating to above its lower critical
solution temperature (LCST) at 32 ꢀC, and the LCST can be
mer [7e11]. Below LCST, the poly(N-isopropylacrylamide) hydrogel
takes in a lot of water in its network, and thus exhibits a swollen
state. But above its LCST, its swollen state collapses and displays
an abrupt reduction in volume. And this process of phase transition
is thermoreversible [12,13]. These characteristics make poly
(N-isopropylacrylamide) hydrogels a particularly interesting class
of “intelligent” materials. Generally, polymer hydrogels can be
divided into two main classes: chemically cross-linked hydrogels,
and physically cross-linked hydrogels [14]. The hydrogels formed
by physical crosslink can perform a reversible gelesol transition
without catalyzer or initiator, but their mechanical strength is
lower than that of the hydrogels formed by chemical crosslink and
the loss in vivo is unavoidable due to humoral scour. Therefore,
much attention has been paid to the hydrogels formed by chemical
crosslink. But there are still some drawbacks in the traditional
preparation methods of hydrogels. One is that the hydrogels
formed by chemical crosslink usually contain catalyzer or initiator,
which is difficult to be gotten rid of and debases the biocompati-
bility of materials. Moreover, traditional hydrogel synthesis relies
upon uncontrolled crosslinking methods, such as radical chemistry,
which leads to poorly defined materials and increases the difficulty
in correlating the network structure with the final physical prop-
erties of the gel [15]. Therefore, methods of preparing hydrogels
* Corresponding author. Tel.: þ86 371 67756849; fax: þ86 371 67756718.
E-mail address: wtroy68@yahoo.com.cn (H.-L. Wei).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.02.008

H.-L. Wei et al. / Polymer 51 (2010) 1694e1702
1695
have been constantly explored so as to improve the properties of
hydrogels [16,17]. The DielseAlder reaction is a chemoselective
reaction and water has an extraordinary rate-accelerating effect on
the reaction process [18e21]. On the other hand, DielseAlder
reaction is thermally reversible, whose reaction degree can be
controlled by temperature [22e28]. It is also one representation of
“click”-typed reactions [29]. Furthermore, DielseAlder reaction can
be applied to the preparation of chiral polymer [30]. Based on these
properties of DielseAlder reaction, it can be speculated that Diel-
seAlder reaction is a promising reaction for the preparation of
hydrogels. Chujo et al. reported the first example of a thermally
reversible hydrogel through the covalent bond, and the hydrogel
was prepared by means of intermolecular DielseAlder reaction
between furan-modified poly(N-acetylethylenimine) (PAEI) and
maleimide-modified PAEI [31]. Later, they prepared polymer
hybrids utilizing the DielseAlder reaction [26,32]. Liu et al. repor-
ted gel formation through DielseAlder in N, N-dimethylacetamide
from polymers based on maleimide-containing polyamides and
a tri-functional furan compound [33]. More recently, we demon-
strated a novel gelation process based on the aqueous DielseAlder
reaction of poly(N, N-dimethylacrylamide-co-furfuryl methacry-
late) and N-[4-(formyl polyethylene glycol ester) bismaleimide. It
was found that water can accelerate DielseAlder reaction while
DMF can accelerate retro-DielseAlder reaction. Swelling/shrinking
kinetics indicates that the as-prepared hydrogels have high
swelling ratio and can respond to temperature [34]. But its ther-
mosensitivity is lower, so we replaced N, N-dimethylacrylamide by
N-isopropylacrylamide, a most used monomer to prepare ther-
mosensitive hydrogel, to improve its thermosensitivity. In order to
Co., Ltd, China. Poly(ethylene glycol) 2000 (PEG2k) was imported
from Japan and distributed domestically. 2, 20-Azobisisobutyroni-
trile (AIBN) was purified by crystallization from methanol 1,
4-Dioxane was distilled then dried over molecular sieve for 2 d.
Triethylamine was distilled then dried over KOH. All other reagents
used were of analytical grade.
2.2. Synthesis of poly(N-isopropylacrylamide-co-furfuryl
methacrylate) (PIPAFM)
PIPAFM was synthesized by copolymerization of FM and NIPA. In
brief, FM and NIPA were charged into a round-bottom flask with
a magnetic stirrer under high pure nitrogen, with1, 4-dioxane as
a solvent. Monomer and initiator concentrations were 0.3 and 5 ꢁ
l0ꢂ3 mol/L, respectively. The flask was immersed in an oil bath held
at 70 ꢀC for 24 h. Afterwards, the flask was removed from the bath
and the contents were immediately dropped into an excess of
diethyl ether to afford the precipitates. The precipitated samples
were washed with the precipitant and dried under vacuum until
constant weight was attained. For the sake of expression, the
copolymers prepared here are denoted PIPAFM-n, where n stands
for a feed molar ratio of NIPA to FM. To afford a direct comparison,
the homopolymers of NIPA and FM, denoted PNIPA and PFM,
respectively, were also prepared.
2.3. Synthesis of N-maleoyl-L-leucine polyethylene glycol ester
bismaleimide (PEGeLMI)
A water-soluble dienophile of PEGeLMI was prepared by the
coupling reaction of PEG and N-maleoyl- -leucine (LMI). LMI was
enhance the biocompatibility, we use N-maleoyl-
L-leucine (LMI) to
L
synthesize polymeric dienophiles, which possess chirality and can
be used to prepare chiral hydrogels. As a result, thermosensitive
chiral hydrogels were obtained in our laboratory.
synthesized by following the literature method [35]. Typically,
PEGeLMI was synthesized by the below three steps, as shown in
Scheme 1.
Firstly, maleic anhydride (9. 9 g, 0.1 mol), acetic acid (50 mL) and
2. Experimental
L-leucine (13.1 g, 0.1 mol) were charged into one-neck flask with
a magnetic stirrer. After being stirred for 24 h at room temperature,
the reaction mixture was distilled under reduced pressure to
remove acetic acid and then dried under vacuum, (S)-N-maleamic
2.1. Materials
Furfuryl methacrylate (>95.0%) was purchased from TCI, Japan.
N, N0-Dicyclohexylcarbodiimide (DCC) and N-isopropylacrylamide
(NIPA) were purchased from SigmaeAldrich (Shanghai) Trading
acid-L-leucine (LMA) was obtained as a white powder. mp: 133e137
ꢀC. (lit. mp 134e136.5 ꢀC) [35].
Secondly, LMA (5.0 g, 22 mmol) was suspended into dry
toluene (200 mL) in a there-neck flask with a mechanical agitator,
then H3PO4 (85%, 0.7 mL) and triethylamine (6.5 mL, 47 mmol)
were added in sequence. The resultant mixture was refluxed for
Co., Ltd. Maleic anhydride (99%) and L-leucine (98%) were obtained
from Sinopharm Chemical Reagent Co., Ltd, China. 2, 20-Azobisi-
sobutyronitrile (AR) was produced by Shanghai Shanpu Chemical
Scheme 1. Synthesis route of water-soluble dienophile (PEGeLMI).

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H.-L. Wei et al. / Polymer 51 (2010) 1694e1702
Table 1
1.5 h with vigorous stirring. Afterwards, the mixture was cooled to
room temperature and organic solution was concentrated. The
obtained residue was neutralized with 0.5 N HCl until pH ¼ 3, and
extracted with ethyl acetate. The organic phase was fully washed
with water and saturated NaCl aqueous solution, then dried over
Na2SO4, and concentrated under reduced pressure, crystallized in
Characterization of some copolymers.
Copolymer NIPA/FM NIPA/FM molar
Yield (%) Mn
Mw
PDI
molar feed ratio in copolymer
(by 1H NMR)
PIPAFM-5
5
4.3
5.8
82
90
86
80
89
9071 21 045 2.32
8625 19 149 2.22
10 695 21 926 2.05
18 791 34 200 1.82
10 450 20 482 1.96
PIPAFM-10 10
PIPAFM-15 15
PIPAFM-20 20
PIPAFM-25 25
n-hexane to give (S)-N-maleoyl-L-leucine (LMI) as a white powder
7.4
10.4
14.5
2.0 g (Yield: 43%). mp: 89e91 ꢀC. (lit. mp 88e89.5 ꢀC) [35]. FTIR
(KBr, thin film, cmꢂ1): 1736 and 1709 (C]O); 1607 (C]C); 1402
and 1387 (]CeH); 1273 (CeN); 1176 (CeO); 698 (cis-CH]CH).
1H NMR (400 MHz, CDCl3,
d, ppm): 6.75 (s, 2H, eCH]CHe),
test-tubes, respectively, then 0.2 g of dry gel was added to each
tube. After heated to different temperatures, disassembly time
based on retro-DA reaction was determined when the gel
disappeared.
4.82e4.78 (dd, H, eCH), 2.31e1.83 (m, 2H, eCH2),1.58 (m,1H, eCH),
0.94e0.91 (dd, 6H, eCH3).
Finally, a novel water-soluble dienophile was prepared by the
coupling of LMI with PEG2k under DCC. PEG was selected as the
main structural component of dienophile due to its hydrophilicity
and biocompatibility [36,37]. As a typical example, synthesis
process of PEGeLMI was conducted as follows. PEG2k (4.00 g, 4
mmol) and LMI (1.86 g, 8.8 mmol) were dissolved in dry
dichloromethane. After being cooled to 0 ꢀC, a solution of DCC in
dichloromethane was added dropwise, and the mixture was stirred
at room temperature for 24 h under nitrogen atmosphere. The
resulting mixture was filtered, washed with dichloromethane. The
filtrate was precipitated in a large amount of diethyl ether. The
precipitate was filtered out and dried under vacuum to constant
weight (Yield: 90%).
2.6. Characterization
FTIR spectra were measured using Shimadzu IR Prestige-21 FTIR
spectrometer at room temperature, in the range from 4000 to 500
cmꢂ1, with a resolution of 2 cmꢂ1 and 20 scans. Samples were
prepared by well dispersing the complexes in KBr and compressing
the mixtures to form disks. The 1H NMR spectra were recorded at
room temperature on a Bruker DPX-400 NMR instrument with
tetramethylsilane (TMS) as internal standard. Gel permeation
chromatography (GPC) analysis was carried out with an SHIMADZU
LC-10AVP chromatographic system equipped with a (SHIMADZU)
shim-pack GPC-803 chromatographic column. THF was used as
eluent at a flow rate of 1.0 mL/min. Monodispersed polystyrene
standards were used to obtain a calibration curve. Scanning elec-
tron microscopy experiments were performed by a model S-4800
field emission scanning electron microscope (Hitach: HIGH-TECH-
NOGIES CORPORATION, Japan). After freeze-drying, samples were
placed in a sputter coater and covered by a thin gold film. After-
wards, they were fixed in the microscope support by a carbon tape.
Working distance, spot size, pressure and accelerating voltage were
adjusted for each sample in order to obtain the best resolution.
The thermal behavior or LCST of polymer aqueous solutions was
observed by transmittance measurements using a TU-1900 dual-
beam UVevisible spectrophotometer (Beijing purkinje general
2.4. Gelation by DielseAlder reaction
A certain quantity of PEGeLMI was added into a prescribed
amount of PIPAFM-n aqueous solution, and the total concentration
of the admixture was 15 wt%. The resulting mixture was stirred to
make the PEGeLMI dissolved, and then put into refrigerator at 9 ꢀC
and viewed the change of solegel (Scheme 2).
2.5. Disassembly by retro-DA reaction
5
mL of Dimethylformamide (DMF), dimethylsulphoxide
(DMSO), xylenes, 1, 4-dioxane, and water were put into five 10 mL
Scheme 2. Chemical structure of hydrogels formed by DA reaction.

H.-L. Wei et al. / Polymer 51 (2010) 1694e1702
1697
maintaining at a temperature of 9 ꢀC until the gelation occurred.
The gelation time was visually determined when the mixture did
not flow by inverting the vials.
The swelling behavior of dried hydrogels was studied by
a general gravimetric method. A certain amount of dry gel was
incubated in distilled water at 37 ꢀC, and the swollen weight for
each sample was recorded at regular time intervals after excess
surface water was blotted carefully with filter paper. The procedure
was repeated until there was no further weight increase. While the
temperature increased gradually the swollen hydrogels began to
shrink. The temperature was maintained constant for 12 h after
increased by each 3 ꢀC, and then the weight of shrunk hydrogel was
measured. The swelling ratio (SR) was calculated by the following
equation:
SR ¼ ðm1 ꢂ m0Þ ꢁ 100%=m0
where m0 stands for the initial weight of dried gel, and m1 stands
for the weight of the swelling gel at a particular temperature and
a prescribed time interval.
Fig. 1. FTIR spectra of polymers: (a) PFM; (b) PIPAFM-5; (c) PIPAFM-10; (d) PIPAFM-15;
(e) PIPAFM-20; (f) PIPAFM-25; (g) PNIPA.
instrument Co., Ltd., China) equipped with
a temperature
3. Results and discussions
controller. The samples for the UVevis experiments were prepared
in quartz colorimetric cell (10 mm in thickness) with a concentra-
tion of 1 g/L. As a reference we used a colorimetric cell filled with
doubly distilled water. All extinction measurements were per-
formed at a wavelength of 500 nm. The transmittance of the
copolymer solutions was recorded as a function of temperature.
The temperature was varied at a step width of 0.5 K/10 min. LCSTs
of the polymer solutions at different concentrations were defined
as the temperature at which the optical transmittance of the
solution reduced to 10% of its original value.
Chirality of the hydrogels was characterized by circular
dichroism (CD). CD spectra were recorded on a JASCO J-810 spec-
trophotometer at room temperature. Before measurement, LMI and
PEGeLMI were dissolved in water respectively and the xerogels
were prepared to be thin films.
3.1. Synthesis of PIPAFM
When using toluene as a solvent of copolymerization, the
solution become cloudy after reacting for some time, so we use
1,4-dioxane as a solvent in order to keep the solution transparent
during polymerization. In order to suppress the gelation which may
be attributed to lability of the carbon-5 of the furan ring during
polymerization of furan derivatives, we reduce the initial total
concentration of monomers to 0.3 mol/L, which is obtained from
the reference and our experiment results [34,38]. The GPC data
show that all the obtained macromers have an unimodal peak with
a polydispersity index (PDI) around 2 (Table 1). Molar feed is
different from molar ratio in copolymer, which is relevant to
reactivity ratios of NIPA and FM and detailed research on this aspect
is ongoing.
The gelation time of the mixture was measured by a vial
inversion method. Aqueous solutions containing a prescribed
amount of PIPAFM and PEGeLMI were put into a refrigerator
FTIR spectra of PFM (a), PIPAFM-5 (b), PIPAFM-10 (c), PIPAFM-15
(d), PIPAFM-20 (e), PIPAFM-25 (f), PNIPA (g) are presented in Fig. 1.
Fig. 2. 1H NMR spectra of PIPAFM-25 (a), PIPAFM-15 (b), PIPAFM-5 (c) in DMSO-d6.

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H.-L. Wei et al. / Polymer 51 (2010) 1694e1702
Fig. 3. 13C NMR spectrum of PIPAFM-5 in DMSO-d6.
The PFM spectrum shows the band at 1733 cmꢂ1 assigned to the
C]O stretching mode and the bands at 1266 cmꢂ1,1223 cmꢂ1 and
1157 cmꢂ1 attributed to the symmetric and asymmetric CeOeC
stretching mode. In the PNIPA spectrum, the amide I band (C]O
stretch) emerges at 1653 cmꢂ1, the amide II band (NeH vibration)
at 1543 cmꢂ1, and the methyl groups (in isopropyl group) at 1362
and 1386 cmꢂ1. The broad peak at the range from 3200 to 3600
cmꢂ1 belongs to the NeH or OeH vibration. Moreover, the char-
acteristic absorptions of NIPA and FM emerge in the spectra of
copolymers (curve b, c, d, e, f) with a little shift. It can be seen that
the intensity of the peak at about 1733 cmꢂ1 becomes weaker and
the intensity of the peak at about 1653 cmꢂ1 becomes stronger
from curve b to curve f as the NIPA to FM molar ratio augments.
1H NMR spectra of PIPAFM-25 (a), PIPAFM-15 (b), PIPAFM-5 (c)
were recorded in DMSO at room temperature (Fig. 2). The peak at
4.96 ppm is assigned to the eOeCH2 protons of FM, and the signals
at 7.65 ppm and 6.44e6.50 ppm correspond to the protons of the
furan ring, whereas the broad peak at 7.17e7.32 ppm belongs to
the eNH protons. The copolymer compositions were estimated by
comparison of the integrated intensities of resonance signals at 7.65
ppm and 3.84 ppm, which varies in accordance with the molar feed
ratio (Table 1). 13C NMR spectrum of PIPAFM-5 was also recorded in
DMSO at room temperature. As shown in Fig. 3, the signals at 173,
149, 144, 111, 58, 42, 36, 30, 22 ppm correspond to different carbons
of the copolymer.
Fig. 5. LCST for aqueous solutions of polymer as a function of FM content.
3.2. Temperature-responsive property of PIPAFM-n and PNIPA
In this study, we selected the most popular thermosensitive poly
(N-isopropylacrylamide) as main components of polymeric dienes
for the purpose of in situ hydrogel formation via DielseAlder
chemistry. In a pure poly(N-isopropylacrylamide) system, there
generally exists a hydrophilic/hydrophobic balance in the NIPA unit
resulting from the hydrophilic (amide group) and hydrophobic
(isopropyl group) regions of the PNIPA. The temperature sensitivity
of the PNIPA hydrogel is attributed to its uniquely rapid alteration
in hydrophilicity and hydrophobicity of the matrix [39]. LCST is the
typical feature of temperature-sensitive PNIPA-based materials and
it means the critical temperature where hydrophobic interactions
of isopropyl groups of PNIPA outweigh the hydrophilic nature of the
amide groups on the pendant groups [40].
Below its LCST, the hydrophilic groups in the side chains bond to
water molecules through hydrogen bonds, and PNIPA hydrogel
subsequently exhibit swelling state. However, as the external
Fig. 4. Transmittance changes for aqueous solutions of polymers as a function of
temperature: PIPAFM-10 (a), PIPAFM-15 (b), PIPAFM-20 (c), PIPAFM-25 (d), PIPAM-30
(e), PIPAFM-40 (f), PIPAFM-50 (g), PNIPA (h).
Fig. 6. FTIR spectra of PEGeLMI (a) and PEG (b).

H.-L. Wei et al. / Polymer 51 (2010) 1694e1702
1699
Fig. 7. 1H NMR spectra of LMI (A) and PEGeLMI (B).
temperature increase, the hydrogen bonds are broken and the
hydrophobic interactions among the hydrophobic groups become
dominant. As a result, entrapped water molecules via hydrogen
bonds in hydrogel are released from the network, leading to
collapse of polymer chains and phase separation of hydrogel [41].
The LCST of PNIPA can be adjusted by copolymerizing NIPA with
other comonomers. It is found that the incorporation of ionic
hydrophilic moieties into the PNIPA hydrogel networks would
enhance its LCST and the gels become sensitive towards pH,
whereas hydrophobic moieties decrease the LCST [3,42]. Herein, it
can be seen from the following experiment result that the LCST of
PIPAFM-n is lower than the LCST of PNIPA due to hydrophobicity of
PFM.
Fig. 9. FTIR spectra of copolymer PIPAFM-10 (a) and the corresponding hydrogel (b).
can be seen that all the polymer solutions show a transmittance
change when the temperature increases, which indicates all the
polymer solutions are temperature-responsive. In addition, the
thermosensitivity increases with the decrease of the FM content in
polymer, which shows more NIPA content is of advantage to retain
the thermosensitivity of polymer. Fig. 5 shows the relationship
between the LCST of the copolymers and the FM content. The LCST
of the copolymer solutions shifts to a lower temperature with the
increase of FM content because the hydrophobcity of the copoly-
mers can be improved by increasing the FM content in copolymer.
Moreover, on the basis of the results in Fig. 5, it is revealed that the
LCST of the copolymers could be easily controlled by varying the
feed ratio of the comonomers. As is well-known, phase transition
We estimated the LCSTs of the polymers in water by means of
transmittance measurements of the aqueous solutions of polymer.
Fig. 4 shows the transmittance change in the polymer solutions. It
Fig. 8. SEM images of the hydrogel surfaces: (a) PIPAFM-10; (b) PIPAFM-15; (c) PIPAFM-20; (d) PIPAFM-25.

1700
H.-L. Wei et al. / Polymer 51 (2010) 1694e1702
Table 2
found to be porous, as is well-known, porous structure is important
to tissue engineering scaffold.
The FTIR spectra of the gel show important changes compared
with those of the initial polymers, which clearly confirms that the
DA reaction has taken place.
Disassembly time of the hydrogels (from PIPAFM-15) at different temperatures and
solvents.
Solvents
Water DMSO
DMF
Xylenes
Dioxane
Dielectric constant [43]
Disassembly time (80 ꢀC)
79.7
Noa
46.6
Noa
36.7
Noa
2.3
2.21
Noa
Noa
e
Noa
FTIR spectra of copolymer PIPAFM-10 (a) and the corresponding
hydrogel (b) are shown in Fig. 9. In the spectrum of copolymer
PIPAFM-10, the band at about 3071 cmꢂ1 is attributed to the ]CeH
stretching mode, and the band at 1730 cmꢂ1 is due to ester carbonyl
in FM segments, and the band at 1652 cmꢂ1 is from carbonyl
stretching vibration in NIPA while the band at 1541 cmꢂ1 belongs to
bending vibration of NeH. The peaks at 1234 cmꢂ1, 1173 cmꢂ1, 1017
cmꢂ1are due to CeH deformation vibration in furan ring, whereas
the peaks at 978 cmꢂ1, 922 cmꢂ1 are produced by out-of-plane
deformation vibration, at 746 cmꢂ1 CeH out-of-plane bending
vibration, at 600 cmꢂ1 cyclic-deformation vibration. After forming
gel, the differences are mostly in the carbonyl region and in the low
frequency area. The bands at 1652 cmꢂ1 and 1730 cmꢂ1 shift to
1648 cmꢂ1 and 1711 cmꢂ1, respectively, possibly due to peaks of
different carbonyl overlapping each other, and the typical bands
arising from the presence of the furan rings disappeared. In addi-
tion, ]CeH stretching also shifts from 3071 cmꢂ1 to 3064 cmꢂ1
after gelation, which may indicate the disappearance of conjugative
double-bond. It was also confirmed that DA reaction took place
during gelation via a blank experiment, in which a solution of
copolymer or PEGeLMI alone was left in the same experimental
conditions, and crosslinking did not occur. Hence it can be inferred
with confidence that gelation was due to DA reaction.
Disassembly time (100 ꢀC) Noa
315 min 600 min Noa
Disassembly time (135 ꢀC)
e
25 min
35 min
180 minb
a
The gel didn't disappear in 600 min.
The gel is most disassembled.
b
temperature control is very important in some biomedical
applications.
3.3. Synthesis of PEGeLMI
The FTIR spectra of PEGeLMI (a) and PEG (b) are shown in Fig. 6.
In PEGeLMI sample, the two peaks around 1714 and 1745 cmꢂ1
(C]O) are attributed to the carbonyl group stretch in PEGeLMI. The
ꢂ1
peaks at 1400 and 1352 cm
are produced by isopropyl. The
stretch of CeN (1253 cm ꢂ1) and C]C (1634 cm ꢂ1) is also observed
in the spectrum of PEGeLMI.
1H NMR spectra of LMI (A) and PEGeLMI (B) are presented in
Fig. 7. It can be seen that the characteristic peaks of LMI emerge in
the spectrum of PEGeLMI. The protons in PEG give peaks around
4.43e3.44 ppm. The degree of functionality of PEGeLMI can be
estimated by comparison of the integrated intensities of resonance
signals at 6.74 ppm and 4.43e3.44 ppm, which is about 63%.
3.4. Gelation by DA reaction
3.5. Disassembly by retro-DA reaction
After a certain amount of PIPAFM-n was dissolved in 5 mL of
water in a vial (20 mm in diameter and 45 mm in length), then to
this added a prescribed amount of PEGeLMI, making the total
concentration of PIPAFM-n and PEGeLMI 15 wt%, gelation was
observed after a period of time. It was found that gelation time
increased with the decrease of FM content in copolymer and the
gelation times of PIPAFM-10, PIPAFM-15, PIPAFM-20, PIPAFM-25
were about 20 h, 35 h, 55 h, 76 h, respectively. The rate of gelation
was faster when the content of furanic monomer in the copolymer
is higher. SEM images of freeze-dried hydrogels via DA reaction of
PIPAFM-n and PEGeLMI are shown in Fig. 8. The structure was
Disassembly times based on retro-DA reaction in different
solvents are presented in Table 2. When the temperature is below
80 ꢀC, the gels were not disassembled in all five solvents after 10 h.
When the temperature is at 100 ꢀC, the gels were not disassembled
in xylenes, dioxane and water, but the gel disappeared in DMSO
within 315 min and disappeared in DMF within 600 min. When the
temperature is at 135 ꢀC, the gel in DMSO disappeared within 25
min, and the gel in DMF disappeared within 35 min, at the same
time the gel in xylenes most disappeared after 180 min. All these
facts indicate the gel is stable in water and retro-DA reaction is
interrelated with the solvents used. It seems that bigger polarity of
Fig. 10. 1H NMR spectra of the mixture from the disassembly of hydrogel in DMSO-d6.

H.-L. Wei et al. / Polymer 51 (2010) 1694e1702
1701
Fig. 11. CD spectra of LMI (a), PEGeLMI (b) and representative hydrogel (c).
organic solvents is of advantage to disassembly of the gel. Addi-
tionally, the disassembly time shortens with the increase of
temperature. When DMSO-d6 was used as a solvent of disassembly
of the gel, 1H NMR spectrum of the resultant mixture was recorded
directly after the gel disappeared. As shown in Fig. 10, the special
bands of furans reappear after disassembly, which confirms the
occurrence of the retro-DA reaction.
Deswelling rate was faster at the beginning and became slower
when temperature was higher than 34 ꢀC, reaching a deswelling
limit gradually. It also could be seen from Fig. 13 that the rate of
thermosensitivity increased with the decrease in the cross-link
density. The reason may be that the less cross-link density has less
bondage to PNINA segments, which benefit to bring thermosensi-
tivity of PNIPA into play.
4. Conclusions
3.6. CD analysis
In conclusion, thermosensitive hydrogels based on PNIPA were
prepared by means of clean and efficient DA reaction in water. The
preparation process is facile and the reaction conditions are mild
with neither initiator nor organic solvent. The gelation time can be
controlled by changing the component of copolymer, whereas
swelling behavior can be adjusted by varying temperature and
As is shown in Fig. 11, the CD spectra of LMI and PEGeLMI have
similarity in shape and position. The broad peaks around 320 nm
are produced from conjugative system in LMI and PEGeLMI. After
forming gel by DA reaction, the broad peak disappears because of
disappearance of conjugative system. However, from180 nm to 380
nm, the gel produces obvious CD signals (varying between 3.56 and
ꢂ1.73 mdeg), which indicates chiral structures exist.
3.7. Swelling behavior
The swelling ratios of the as-prepared hydrogels were measured
in distilled water at 17 ꢀC. After reaching the swelling equilibrium,
these hydrogels were left at 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40,
43, 46, 49, 52, 55, 58, 61 ꢀC in turn in the thermostatic water bath
for 12 h before measuring the swelling ratio changes. As depicted in
Fig. 12, the hydrogels have high swelling ratios in water at 17 ꢀC,
which reach swelling equilibrium in about 12 h, and the equilib-
rium swelling ratio increases with the decrease in cross-link
density. In order to study the thermosensitivity of the hydrogels,
the swelling behavior of the hydrogels was measured between
a broad temperature range from 10 ꢀC to 61 ꢀC. It was found that the
swelling ratios of the hydrogels have temperature sensitivity, just
as expected. As shown in Fig. 13, when temperature increased from
10 ꢀC to 61 ꢀC, the swelling ratio of the hydrogels decreased.
Fig. 12. The swelling ratio of the hydrogels as a function of time at 17 ꢀC from PIPAFM-20
(a), PIPAFM-15 (b), PIPAFM-10 (c).

1702
H.-L. Wei et al. / Polymer 51 (2010) 1694e1702
[6] Liang L, Liu J, Gong X. Langmuir 2000;16:9895e9.
[7] Zhang GR, Bowyer A, Eisenthal R, Hubble J. Adv Polym Tech 2008;27
(1):27e34.
[8] Topp MDC, Dijkstra PJ, Talsma H, Feijen J. Macromolecules 1997;30:8518e20.
[9] Maeda T, Kanda T, Yonekura Y, Yamamoto K, Aoyagi T. Biomacromolecules
2006;7:545e9.
[10] Zhao Q, Sun J, Ling Q, Zhou Q. J Polym Sci Part A Polym Chem 2008;46:
6594e603.
[11] Chang C, Wei H, Feng J, Wang ZC, Wu XJ, Wu DQ, et al. Macromolecules
2009;42:4838e44.
[12] Zhang XZ, Zhang JT, Zhuo RX, Chu CC. Polymer 2002;43:4823e7.
[13] Lang YY, Jiang TY, Li SM, Zheng LY. J Appl Polym Sci 2008;108:3473e9.
[14] Haraguchi K, Farnworth R, Ohbayashi A, Takehisa T. Macromolecules 2003;
36:5732e41.
[15] Malkoch M, Vestberg R, Gupta N, Mespouille L, Dubois PA, Mason F, et al.
Chem Commun 2006;26:2774e6.
[16] Wang Z-C, Xu X-D, Chen C-S, Wang G-R, Cheng S-X, Zhang X-Z, et al. React
Funct Polym 2009;69:14e9.
[17] Xu X-D, Chen C-S, Lu B, Wang Z-C, Cheng S-X, Zhang X-Z, et al. Macromol
Rapid Commun 2009;30:157e64.
[18] Graziano G. J Phys Org Chem 2004;17:100e1.
[19] Mubofu EB, Engberts JBFN. J Phys Org Chem 2004;17:180e6.
[20] Sun XL, Yang LC, Chaikof EL. Tetrahedron Lett 2008;49:2510e3.
[21] Hill KW, Taunton-Rigby J, Carter JD, Kropp E, Vagle K, Pieken W, et al. J Org
Chem 2001;66:5352e8.
[22] Gheneim R, Perez-Berumen C, Gandini A. Macromolecules 2002;35:7246e53.
[23] Chen X, Dam MA, Ono K, Mal A, Shen H, Nutt SR, et al. Science 2002;295:
1698e701.
[24] Costanzo PJ, Demaree JD, Beyer FL. Langmuir 2006;22:10251e7.
[25] Adzima BJ, Aguirre HA, Kloxin CJ, Scott T, Bowman CN. Macromolecules
2008;41:9112e7.
Fig. 13. The swelling ratio of the hydrogels as a function of temperature from PIPAFM-20
(a), PIPAFM-15 (b), PIPAFM-10 (c).
crosslinking density. And the disassembly time of the hydrogels is
affected by temperature and solvents used. The method presented
here has potential application in biomaterials. The study on the
application of the as-prepared hydrogels is currently in progress.
[26] Imai Y, Itoh H, Naka K, Chujo Y. Macromolecules 2000;33:4343e6.
[27] Watanabe M, Yoshie N. Polymer 2006;47:4946e52.
[28] Teramoto N, Arai Y, Shibata M. Carbohydr Polymer 2006;64:78e84.
[29] Kim TD, Luo J, Tian Y, Ka JW, Tucker NM, Haller M, et al. Macromolecules 2006;
39:1676e80.
Acknowledgments
The authors would like to acknowledge the National Natural
Science Foundation of China (50773018), the Education Department
of Henan Province (2008A430003), and Henan University of Tech-
nology for financial support. The authors also gratefully acknowl-
edge a grant from Zhengzhou Science and Technology bureau.
[30] Itsuno S. Prog Polym Sci 2005;30:540e58.
[31] Chujo Y, Sada K, Saegusa T. Macromolecules 1990;23:2636e41.
[32] Adachi K, Achimuthu AK, Chujo Y. Macromolecules 2004;37:9793e7.
[33] Liu YL, Hsieh CY, Chen YW. Polymer 2006;47:2581e6.
[34] Wei HL, Yang Z, Zheng LM, Shen YM. Polymer 2009;50:2836e40.
[35] Gao H, Isobe Y, Onimura K, Oishi T.
J Polym Sci Part A Polym Chem
References
2007;45:3722e38.
[36] Bhattarai N, Ramay HR, Gunn J, Matsen FA, Zhang M. J Control Release 2005;
103:609e24.
[1] Yi F, Zheng S. Polymer 2009;50:670e8.
[37] Thompson MS, Vadala TP, Vadala ML, Lin Y, Riffle JS. Polymer 2008;49:345e73.
[38] Goiti E, Huglin MB, Rego JM. Polymer 2001;42:10187e93.
[39] Zhang X, Wu D, Chu C-C. Biomaterials 2004;25:4719e30.
[40] Zhang X-Z, Lewis PJ, Chu C-C. Biomaterials 2005;26:3299e309.
[41] Han J, Wang K, Yang D, Nie J. Int J Biol Macromol 2009;44:229e35.
[42] Fundueanu G, Constantin M, Bortolotti F, Cortesi R, Ascenzi P, Menegatti E. Eur
Polym J 2007;43:3500e9.
[2] Nakajima T, Furukawa H, Tanaka Y, Kurokawa T, Osada Y, Gong JP. Macro-
molecules 2009;42:2184e9.
[3] Mohan YM, Premkumar T, Joseph DK, Geckeler KE. React Funct Polym 2007;
67:844e58.
[4] Musch J, Schneider S, Lindner P, Richtering W. J Phys Chem B 2008;112:
6309e14.
[5] Zhang J, Xie R, Zhang S-B, Cheng C-J, Ju X-J, Chu L-Y. Polymer 2009;50:
[43] Smallwood IM. Handbook of organic solvent properties. London: Amold; 1996.
2516e25.