Thermoresponsive copolymers with pendant d-galactosyl 1,2,3-triazole groups: Synthesis, characterization and thermal behavior

Article abstract of DOI:10.1039/c5nj01334h

A galactose containing glycomonomer has been synthesized by copper catalyzed azide-alkyne cyclo-addition reaction (CuAAC) of 6-azido-6-deoxy-1,2:3,4-di-O-isopropylidene-α-d-galactopyranose with propargyl acrylate. This monomer was subjected to homopolymerization and copolymerization with N-isopropylacrylamide (NIPAm) at different compositions by free radical polymerization using 2,2′-azobis-isobutyronitrile (AIBN) as an initiator. The composition of the copolymer was determined by ¹H-NMR spectroscopy. Upon acid hydrolysis of acetonide protected polymers, water-soluble deprotected polymers were obtained. The polymers were characterized and confirmed by NMR, IR, GPC and thermal analytical techniques. The protected and deprotected copolymers showed a sharp cloud-point temperature and a linear correlation was obtained between the lower critical solution temperatures (LCST) and the concentration of glycomonomer in the copolymers. This allows tuning the thermal response by simply altering the copolymer composition. Water contact angle experiments showed the changes in the hydrophilicity of copolymers with compositions that supported the LCST results. The glass transition temperature of protected copolymers followed a regular monotonic decreasing trend with increasing glycomonomer content, whereas T_g of deprotected copolymers increased due to H-bond interaction. The attempts to develop thermoresponsive polymers having LCSTs at physiological temperature were successful.

Full text of DOI:10.1039/c5nj01334h

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Thermoresponsive copolymers with pendant
D-galactosyl 1,2,3-triazole groups: synthesis,
Cite this:DOI: 10.1039/c5nj01334h
characterization and thermal behavior†
Archana B. Dhumure,a Ajay B. Patil,a Anuja S. Kulkarni,a Irina Voevodina,b
Mariastella Scandolab and Vaishali S. Shinde*a
A galactose containing glycomonomer has been synthesized by copper catalyzed azide–alkyne cyclo-
addition reaction (CuAAC) of 6-azido-6-deoxy-1,2:3,4-di-O-isopropylidene-a-D-galactopyranose with
propargyl acrylate. This monomer was subjected to homopolymerization and copolymerization with
N-isopropylacrylamide (NIPAm) at different compositions by free radical polymerization using 2,20-azobis-
isobutyronitrile (AIBN) as an initiator. The composition of the copolymer was determined by 1H-NMR
spectroscopy. Upon acid hydrolysis of acetonide protected polymers, water-soluble deprotected polymers
were obtained. The polymers were characterized and confirmed by NMR, IR, GPC and thermal analytical
techniques. The protected and deprotected copolymers showed a sharp cloud-point temperature and a
linear correlation was obtained between the lower critical solution temperatures (LCST) and the
concentration of glycomonomer in the copolymers. This allows tuning the thermal response by simply
altering the copolymer composition. Water contact angle experiments showed the changes in the
hydrophilicity of copolymers with compositions that supported the LCST results. The glass transition
temperature of protected copolymers followed a regular monotonic decreasing trend with increasing
glycomonomer content, whereas Tg of deprotected copolymers increased due to H-bond interaction. The
attempts to develop thermoresponsive polymers having LCSTs at physiological temperature were successful.
Received (in Montpellier, France)
26th May 2015,
Accepted 12th August 2015
DOI: 10.1039/c5nj01334h
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polymer chain plays a major role in determining the LCSTs of
these polymers. With a LCST approaching the normal body
1. Introduction
Thermoresponsive polymers are a class of smart polymers that temperature, PNIPAm is an attractive system for studies related
show a response to temperature.1 These polymers exhibit to pharmaceutical and biomedical applications.4 In order to
thermodynamic lower critical solution temperature (LCST) develop a thermoresponsive polymer for a specific end applica-
and show inverse solubility behaviour with an increase in tion, amongst various approaches,5 LCST can be optimized by a
temperature. The classical example is the solution of poly(N- tailor-made approach by changing the ratios of two different
isopropylacrylamide) [PNIPAm] in water which undergoes coil- monomer concentrations in copolymerization and also by
to-globule transition at LCST in the range of 31–33 1C.2 Below slightly varying the chemical structure of the monomers by
its LCST, the polymer is completely soluble in water, while introducing hydrophobic/hydrophilic groups in the chain.
it becomes insoluble and phase-separates above its LCST.
Recent technological developments focus a great deal on the
Thermally induced phase separation is attributed to the break- biocompatibility and biodegradability of the materials used for a
ing of polymer–water hydrogen bonding and the formation of variety of biological applications such as artificial drug delivery
polymer–polymer bonding at the critical temperature.3 The vehicles or molecules.6 Glycopolymers, sugar containing macro-
balance of hydrophilic and hydrophobic constituents of the molecules, are of great interest due to their improved bio-
compatibility.7 Also, being biomimetics with naturally occurring
a Department of Chemistry, Savitribai Phule Pune University (Formerly, University
of Pune), Pune 411007, India. E-mail: vsshinde@chem.unipune.ac.in;
Fax: +91 20 25691728; Tel: +91 20 25601395
glycoconjugates, synthetic glycopolymers were found to play a
significant role in many biomedical applications via multivalent
binding interactions, commonly known as the glyco-cluster effect.8
With these attractive features, the pendant sugar of these polymers
shows great affinity for some specific proteins, known as lectins,
and forms stable complexes. For example, mannose containing
polymers were strongly bound with Concanavalin A (ConA),9 while
b Department of Chemistry ‘G. Ciamician’, University of Bologna, Via Selmi 2,
40126 Bologna, Italy
† Electronic supplementary information (ESI) available: 1H and 13C-NMR, COSY
and HSQC spectra of compound 5, TGA curves of P-100 and D-100 under N2 and
air flow. See DOI: 10.1039/c5nj01334h
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N-acetylglucosamine containing polymeric spherical brushes and sodium ascorbate was purchased from S. D. Fine Chemicals
exhibited selective binding to wheat germ agglutinin (WGA).10
Ltd. Dialysis bags (1000 MWCO) were purchased from Spectra/Por.
Click chemistry strategy is not only used as a versatile tool for the 2,20-Azobis-isobutyronitrile (AIBN), copper sulfate pentahydrate,
design of complex polymer architectures, but it can also be used for zinc chloride, tosyl chloride and formic acid (85%) were purchased
the synthesis of bioactive and stable glycopolymers.11 One of the from Spectrochem. AIBN and NIPAm were recrystallized in metha-
most popular click reactions is Cu-catalyzed Huisgen 1,3-dipolar nol. Thin layer chromatography was performed on pre-coated
azide and alkyne cycloaddition (CuAAC) with the formation of plates (0.25 mm, silica gel 60 F254). Visualization was made by
1,4-disubstituted 1,2,3-triazole rings.12 The CuAAC is highly efficient absorption of UV light or by thermal development after spraying
and stereoselective, and it requires mild reaction conditions and is with 3.5% solution of 2, 4-dinitrophenylhydrazine in methanol/
compatible with different functional groups. The CuAAC strategy H2SO4 or with basic aqueous potassium permanganate solution or
has been used both to synthesize glycomonomers and to modify with 10% solution of phosphomolybdic acid (PMA) in ethanol. All
glycopolymer scaffolds by the post-functionalization approach. other chemicals were of analytical grade and were used as received.
There are very few reports which follow the first protocol, while
the second synthetic route was used by many researchers being
2.2. Synthesis of the glycomonomer
more convenient to obtain glycopolymers by attaching different
A triazole containing glycomonomer 5 was synthesized from
sugar moieties to preformed polymer scaffolds. For example, Okoth
D-galactose as shown in Scheme 1. 6-Azido-6-deoxy-1,2:3,4-di-O-
et al. described CuAAC between propargyl galactoside with azide
isopropylidene-a-D-galactopyranose (4) was synthesized by a
polymers using a microwave assisted protocol.13 Otman et al.
reported protocol.20
obtained amphiphilic polymers by click chemistry of mannose
A mixture of 6-azido-6-deoxy-1,2:3,4-di-O-isopropylidene-a-D-
azide and poly(propargyl acrylate-co-N-vinyl pyrrolidone).14 Stenzel
galactopyranose (2.0 g, 5.405 mmol) and propargyl acrylate
and coworkers synthesized thermoresponsive glycopolymers via
(0.596 ml, 5.405 mmol) in 10 ml of THF : water (1 : 1) was taken.
CuAAC of galactose azide and the alkyne functionalized block
After stirring for 10 min, sodium ascorbate (20 mg, 0.101 mmol)
polymer of HEMA (2-hydroxy ethyl methacrylate) and DEGMA
and copper sulfate (20 mg, 0.0801 mmol) were added. The
(diethylene glycol methyl ether methacrylate).15 The resulting glyco-
reaction was stirred at room temperature. After completion of
polymers were then tested for inhibition of ricin. Haddleton and
the reaction (as monitored by TLC), the reaction mixture was
coworkers reported libraries of glycopolymers using CuAAC chem-
extracted with ethyl acetate. The organic layer was dried over
istry. Their approach involved the grafting of different sugar azides
anhydrous sodium sulfate. Column purification of the product
onto clickable polymer backbones with alkyne functionalities.16 The
afforded a white crystalline solid (2.5 g, yield 95%).
same group also prepared a mannose containing glycomonomer by
Mp: 122–123 1C; Rf = 0.4 (ethyl acetate/hexane, 6 : 4); IR
CuAAC of the mannose azide and propargyl methacrylate and
(KBr, cmꢀ1): 1728 (CQO), 1634 (CQC) cmꢀ1
.
subsequently polymerized target glycoprotein mimics.17
1H NMR (500 MHz, CDCl3), d (ppm): 7.81 (s, 1H, CH of triazole
The current work utilizes CuAAC chemistry for the synthesis of
a monomer containing a triazole ring by Cu-catalyzed 1,3-dipolar
cycloaddition of the 6-azido-6-deoxy-1,2:3,4-di-O-isopropylidene-
a-D-galactopyranose with propargyl acrylate. Subsequently, the
glycomonomer was copolymerized with NIPAm by free radical
polymerization. Upon acid hydrolysis, water-soluble deprotected
polymer analogues were obtained. The composition of the copo-
lymer was determined by 1H-NMR spectroscopy. The polymer
properties were characterized thoroughly for their thermosensi-
tivity by using temperature dependent UV and NMR techniques.
Furthermore, these newly synthesized thermoresponsive glyco-
polymers were studied by thermal property and contact angle
measurements. One of the motivations for developing these poly-
mers was to achieve thermoresponsive polymers with tunable
LCSTs and also to take advantage of the triazole ring for its rigidity
and bioactivity.18 Such thermosensitive glycopolymers are known
for controlling lectin interaction or bacterial aggregation.19
0
0
ring, H5 ), 6.44 (dd, J = 1.5 and 17.1 Hz, 2H, H9 a), 6.14 (dd, J = 10.5
0
0
and 17.1 Hz, 1H, H8 ), 5.85 (dd, J = 1.5 and 10.5 Hz, 2H, H9 b), 5.53
(d, J = 5.3 Hz, 1H, H1), 5.31 (ABq, J = 12.8 and 3.9 Hz, 2H, H6 a,b),
4.60–4.66 (m, 2H, H6a, H3), 4.5 (dd, J = 6.2 and 14.3 Hz, 1H, H6b),
4.34 (dd, J = 2.4 and 4.8 Hz, 1H, H2), 4.18–4.20 (m, J = 7.6 Hz, 2H,
0
2. Experimental section
2.1. Materials
Scheme 1 Synthesis of glycomonomer (a) acetone, ZnCl2, cat. H2SO4, rt,
6 h, 81%. (b) TsCl, pyridine, cat. DMAP, 0 1C – rt, 8 h, 75%. (c) NaN3, TBAI,
DMF, 110 1C, 72 h, 51%. (d) Propargyl acrylate, CuSO4ꢁ5H2O, Na ascorbate,
THF : H2O (1 : 1), rt, 1 h, 90%.
D-Galactose, sodium azide, tetrabutyl ammonium iodide (TBAI)
were purchased from Merck. Propargyl acrylate and N-
isopropylacrylamide (NIPAm) were procured from Sigma Aldrich
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H4, H5),1.5 (s, 3H, CH3), 1.4 (s, 6H, 2 ꢂ CH3), 1.3 (s, 3H, CH3); 13
C
MWCO: 1000) against double distilled water for two days and
NMR: (125 MHz, CDCl3): d 165.7 (C-70), 142.3 (C-40), 131.3 (C-90), freeze-dried. Finally, the deprotected polymer (D-series) was
128.1 (C-80), 125.0 (C-50), 109.7 (C-7/C-8), 108.9 (C-8/C-7), 96.2 (C-1), obtained as solid white powder (120 mg, yield 70%).
71.1 (C-4/ C-5), 70.74 (C-3), 70.3 (C-2), 67.13 (C-5/C-4), 57.8 (C-60),
50.5 (C-6), 24.3, 24.7, 25.7, 25.8 (4 CH3).
2.5. Characterization techniques
1H and 13C-NMR spectra were recorded on Varian Mercury 300
2.3. Synthesis of homopolymers and copolymers
and Bruker Avance AV-500 spectrometers in CDCl3 and D2O as
solvents. The solvent signals were used as chemical shift
markers. Temperature-dependent 1H-NMR spectra of the copo-
lymers in D2O were recorded using a Bruker AV-500 spectro-
meter. FT-IR spectra were recorded on a Shimadzu FT-IR 8400
spectrophotometer. Molecular weights and molecular weight
distribution of polymers were determined by gel-permeation
chromatography (GPC) (Waters) by using THF as an eluent at a
flow rate of 1 ml minꢀ1 at 25 1C (Thermo Separation Products)
equipped with spectra series UV 100 and spectra system RI 150
detectors. Sample concentration was 2 mg mlꢀ1 and narrow
molecular weight distribution polystyrenes were used as cali-
bration standards. Polymeric solutions of 0.2% in double
distilled water were used for determining cloud point tempera-
tures using a JASCO V-630 spectrophotometer with peltier
assembly. Absorbance at 500 nm was measured at a tempera-
ture scanning rate of 1 1C minꢀ1. Thermogravimetric measure-
ments were carried out using a TA-TGA 2950 instrument. The
analyses were performed at 10 1C minꢀ1 from room tempera-
ture to 600 1C both under nitrogen and air flow. Differential
scanning calorimetry (DSC) was performed using a 2010 TA
thermal analyzer at a heating rate of 20 1C minꢀ1 under
nitrogen flow. The temperature range explored was from room
temperature to 180 1C. Quench cooling was applied between
scans. The glass transition temperature (Tg) was taken as
midpoint of the stepwise increase of the specific heat asso-
ciated with the transition. Static water contact angle measure-
ments were performed on samples coated on glass slides by
means of a Laurell (WS-650-23NPP) spin coater under ambient
conditions. The contact angle experiments were performed
using an optical contact angle and surface tension meter KSV’s
CAM 100 (KSV, Espoo, Finland) using Milli-Q water. The water
drop profile images were collected every second in a time range
of 0–60 s. A minimum of five drops per sample was analyzed
Homopolymers (P-100 and NIPAm) and copolymers (P-5, P-10,
P-20, and P-25) were prepared by free-radical polymerization
using AIBN at 70 1C in 1,4-dioxane for 24 h according to the
feed ratio given in Table 1. In a typical procedure for synthesis
of P-5, glycomonomer 5 (0.074 g, 0.2 mmol), NIPAm (0.426 g,
3.76 mmol) and AIBN (6.6 mg, 0.0397 mmol) were taken in
1,4-dioxane (3.0 ml) and nitrogen gas was purged for 20 min
(Scheme 2). The reaction mixture was stirred at 70 1C for 24 h
under nitrogen atmosphere. After completion of the reaction,
the mixture was sticky and semisolid. The content was pre-
cipitated in distilled diethyl ether and again dissolved in
chloroform and re-precipitated in diethyl ether, thrice. The
resultant product was dried under reduced pressure at 50 1C
for two days. All protected polymers (P-series) were obtained as
white solids in quantitative yield.
2.4. Deprotection of polymers
Deprotection of the 1,2:3,4-di-O-isopropylidene group of glycopo-
lymers was done under mild acidic conditions by using a formic
acid solution (85%). The protected polymer (200 mg) was dissolved
in 20 ml of formic acid solution and stirred for 48 h at room
temperature. The resulting solution was dialyzed (Spectra/Por,
Table 1 Summary of copolymers of NIPAm with the glycomonomer
Sample
code
Feed ratio
NIPAm: 5
Compositiona
NIPAm: 5
Mn
(g moleꢀ1
)
PDIb
P-100
P-25
P-20
P-10
P-5
0 : 100
75 : 25
80 : 20
90 : 10
95 : 05
100 : 00
0 : 100
3.06 : 1
3.86 : 1
8.13 : 1
17.6 : 1
100 : 00
5800
7286
9320
18 600
—
1.68
1.85
1.77
1.22
—
PNIPAm
—
—
a
b
Determined from 1H NMR integrations. Determined from GPC
(PS calibration).
Scheme 2 Synthesis of copolymers of NIPAm and glycomonomer 5.
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and the average value is reported together with the standard weight (Mn) of the polymers was found to be in the range of
deviation.
5.8 ꢂ 103 ꢀ18.6 ꢂ 103 g molꢀ1 with polydispersity index (PDI) of
1.22–1.85.
Upon acid hydrolysis, the glycohomopolymer and its copo-
lymers were converted into hydrophilic polymers. These poly-
mers were dialyzed against double distilled water to remove
byproducts of hydrolysis and impurities. Resultant deprotected
3. Results and discussion
3.1. Synthesis and characterization
As shown in Scheme 1, D-galactose was trapped in pyranose form
to get 1,2:3,4-di-O-isopropylidene-a-D-galactopyranose 2, which
upon tosylation gave compound 3. Tosylated compound 3 upon
treatment with sodium azide in DMF at 110 1C afforded azido
compound 4. In the final step, glycomonomer 5 was prepared by
Cu-catalyzed 1,3-dipolar cycloaddition of galactose azide 4 and
propargyl acrylate. The formation of galactose containing mono-
mer 5 was confirmed by 1H-NMR, 13C-NMR, COSY and HSQC
spectroscopic techniques (Fig. S1–S4 in the ESI†).
1
hydrophilic polymers (D-series) were characterized by H-NMR
spectroscopy. The absence of isopropylidene proton peaks at
1.3–1.5 ppm confirmed the deprotection as seen in Fig. 2. FTIR
spectroscopy also offered the evidence of deprotection. Fig. 3
shows the FTIR spectra of both protected and deprotected
glycohomopolymer. The deprotected polymers showed a broad
absorption peak at around 3400 cmꢀ1 due to free hydroxyl
groups of deprotected sugar.
Scheme 2 depicts the synthesis of copolymers of the glycomo-
nomer with NIPAm by conventional free radical polymerization
as per feed ratio given in Table 1. The 1H-NMR spectra of
protected copolymers: P-5, P-10, P-20, P-25 and their comparison
with the spectra of two homopolymers: PNIPAm and glycohomo-
polymer (P-100) are shown in Fig. 1. Polymer formation is
confirmed by the disappearance of olefinic proton signals and
appearance of –CH and –CH2 (1.5–2.5 ppm) peaks for a polymer
backbone and a peak due to the triazole proton at 7.81 ppm.
The 1H-NMR spectra of P-100 shows a characteristic peak of
the H-1 proton (anomeric) at 5.53 ppm of the galactopyranose
ring, which is absent in the 1H NMR spectrum of PNIPAm and
shows the appearance of a new peak at 7.81 ppm, assigned to
the triazole proton. As the percentage of the glycomonomer
content increases in copolymers from P-5 to P-25, the intensity
of these characteristic peaks (at 5.53 and 7.81 ppm) increases
distinctly. The composition of copolymers was quantitatively
determined from 1H-NMR spectra by referring to the ratio of
integrations of H-1 proton of the galactopyranose ring and
tertiary –CH proton of the isopropyl group of NIPAm. These
values were compared with the feed ratios. There is a fairly
good agreement with the values of the feed ratios, showing
that copolymer composition can be easily controlled by the
feed ratio. The molecular weights of the polymers were deter-
mined by GPC. As shown in Table 1, number average molecular
Fig. 2 1H-NMR spectra of (a) P-100 (in CDCl3) (b) D-100 (in D2O).
Fig. 1 Comparison of 1H NMR spectra homopolymers and copolymers.
Fig. 3 FTIR spectra of P-100 and D-100.
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3.2. Thermosensitivity studies
Fig. 4 shows temperature dependent absorbance at 500 nm of
synthesized polymers. The PNIPAm homopolymer clearly
showed an LCST (cloud point temperature) at 34.3 1C. The
LCSTs of protected copolymers were found to be less than that
of the PNIPAm homopolymer due to an increase in the overall
hydrophobic content of copolymers. It was observed that as the
amount of glycomonomer increases, the LCST of the copolymer
decreases and also the solubility of the protected polymer in
water decreases. The LCSTs of two protected polymers (P-20,
P-25) were not recorded as they were insoluble in water. The
LCSTs of deprotected polymers were higher than that of PNI-
PAm and of protected copolymers. This interesting observation
was attributed to an increase in the hydrophilicity of polymers
after deprotection. All hydroxyl groups of the sugar moiety
become free after deprotection and can easily form hydrogen
bonding with water even at temperatures higher than LCST of
PNIPAm. These observations also support the fact that copoly-
merizing NIPAm with a hydrophobic comonomer resulted in
lowering of the LCST, whereas copolymerizing NIPAm with a
hydrophilic comonomer resulted in a higher LCST.5b It is worth
mentioning that we were successful to tailor the LCST of
polymer D-5 at B37 1C, which is the physiological temperature
of the human body. This interesting result provides a strong
impetus towards the biological utility of the developed glyco-
polymer. The linear correlation between the LCST and concen-
tration of the glycomonomer was obtained for deprotected
copolymers (Fig. 5), which could be helpful for designing
tailor-made polymers with desired LCST.
Fig. 5 Linear correlation between LCST and % mole glycomonomer.
We have successfully demonstrated the mechanism of phase
transition by a temperature dependent 1H-NMR experiment.
The deprotected copolymer (D-5 with a cloud point temperature
of 37 1C) was dissolved at a concentration of 1.0 mg mlꢀ1 in
D2O solvent. The sample was heated to the measuring tem-
perature and left for 30 min to reach thermal equilibrium. The
1H-NMR spectrum was confirmed by 100 s relaxation delay
and 64 scans. As shown in Fig. 6, when the temperature was
Fig. 6 1H NMR spectra of the D-5 polymer in D2O at different
temperatures.
less than the LCST, all proton signals were well resolved
and distinct. However, upon increasing the temperature, the
peaks were shifted downfield. At 45 1C, peaks a, b, and c
(due to methyl protons of PNIPAm, backbone protons and the
tertiary proton of the isopropyl group of PNIPAm respectively)
broadened and disappeared subsequently. This could be
attributed to phase separation of polymer chains above
the LCST due to the breakdown of hydrogen bonding at
elevated temperature.
3.3. Contact angle measurements
To demonstrate the changes in hydrophilicity, water contact
angle (WCA, y) measurements were performed on the homo-
polymers and on selected copolymers. The contact angle values
Fig. 4 Cloud points of PNIPAm, protected and deprotected copolymers.
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Table 2 Water contact angle of protected and deprotected samples
Sample
Water contact anglea (1)
Standard deviation
PNIPAm
P-5
P-20
P-100
D-20
D-100
43.0
60.4
68.2
69.6
37.2
27.6
1.4
0.9
0.5
1.6
1.5
1.9
a
Values taken 30 s after drop deposition.
Fig. 7 Water contact angle as a function of time. Exemplary drops taken
after 30 s from drop deposition on samples: (a) P-100, (b) PNIPAm and
(c) D-100.
taken 30 s after drop deposition are listed in Table 2. Compared
with PNIPAm (y = 431), the glycohomopolymer (P-100) in its
protected form is much more hydrophobic (y = 69.61), whereas
after deprotection (D-100), it becomes more hydrophilic than
PNIPAm (y = 27.61). Hydrophilicity changes are also highlighted
by the different trend of the y vs. t curves (see Fig. 7), where
Fig. 8 TGA weight loss curves: (a) derivative curves and (b) under N2 flow
of PNIPAm, P-100 and protected copolymers.
Table 3 Thermal properties of protected polymers and of PNIPAm
strong water/polymer interaction causes
a faster contact
TGA
DSC
angle decrease with time. The curves of protected copolymers
lie between those of PNIPAm and P-100, according to the
expected composition-dependent hydrophilicity changes. Analo-
gously, deprotected copolymer D-20 has a behavior intermediate
between those of PNIPAm and D-100. The WCA results confirm
that differences of hydrophobicity underpin the observed
changes in the LCST.
Residue at
600 1C (%)
Sample
Tmax (1C)
Tg (1C)
Dcp ( J gꢀ1 deg)
PNIPAm
P-5
P-10
P-20
P-25
409
4.04
5.30
4.55
6.73
7.70
19.31
140
134
126
121
118
94
0.61
0.55
0.59
0.50
0.54
0.46
351, 402
365, 402
363, 399
360, 392
351
P-100
3.4. Thermal analysis
Fig. 8a shows the thermogravimetric (TGA) curves of protected about 60 1C, with PNIPAm being more thermally stable. Com-
polymer samples together with that of PNIPAm, while Fig. 8b parison of Tmax values of copolymers with those of PNIPAm and
shows the derivative TGA curves in the T-range of the main P-100 (Table 3) and analysis of the relative intensities of the
degradation. After an initial weight loss possibly associated weight drops (Fig. 8) that change according to the change in
with some humidity absorbed by the environment, a main copolymer composition suggest the correlation of the first and
weight loss occurs on a rather broad temperature range second weight loss steps in copolymers with the content of the
between 300 1C and 450 1C (Fig. 8a). The derivative curves glycomonomer and NIPAm respectively.
clearly reveal that the degradation behavior of the copolymers
The large solid residue at 600 1C of homopolymer P-100 in
includes two weight loss processes, whose sharpness and the TGA experiments run under nitrogen flow (Fig. 8b) is worth
intensity gradually change with composition. The temperature noticing. In analogous TGA analyses run under air flow, the
location of such phenomena (Tmax), taken at the peak of the residue at 600 1C is found to approach zero in all samples,
derivative curves, is reported in Table 3 together with the Tmax including P-100 (curve shown as Fig. S5 in the ESI†). This result
values of the two homopolymers. The latter show a difference of indicates that in the presence of oxygen, the substances
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Fig. 10 DSC curves of PNIPAm and protected samples: P-100 and
copolymers. The glass transition temperature (Tg) is marked on the curves.
The calorimetric behavior of all samples, both protected and
deprotected, was studied by means of differential scanning
calorimetry (DSC). Fig. 10 shows the DSC curves of protected
polymers and of PNIPAm. The only thermal event observed in the
DSC curves is an endothermal baseline shift associated with the
glass transition temperature, whose temperature and specific
heat increment is listed in Table 3. The absence of any crystal-
lization/melting phenomena indicates that all analyzed samples
are totally amorphous. This result reflects the absence of regular
chirality along the chain, the presence of bulky and complex side
chains and randomness of comonomer distribution. In addition,
as commonly expected in random copolymers, the glass transi-
tion temperature changes with composition and is intermediate
between those of the two homopolymers.
Fig. 9 TGA curves under N2 flow of protected (solid lines) and depro-
tected polymers (dotted lines): (a) P-5 and D-5; (b) P-25 and D-25;
(c) P-100 and D-100.
The Tg of both protected and deprotected polymers are
s
plotted in Fig. 11 as a function of the weight fraction (w) content
of the glycomonomer in copolymers. The dependence of Tg on
forming the solid residue under N2 at 600 1C oxidize and yield
volatile products.
After deprotection, all copolymers as well as the glycohomo-
polymer were subjected to TGA analysis. Fig. 9 compares the
TGA curves before and after deprotection (under N2 flow) of the
glycohomopolymer and two copolymers containing 5 mol%
and 25 mol% of the glycomonomer. Upon deprotection, the
TGA curve of the glycohomopolymer (Fig. 9c) remarkably
changes, denoting a decrease of thermal stability and a very
consistent increase of the solid residue at 600 1C (from ca. 20%
in P-100 to 35% in D-100). This behaviour of lower thermal
stability and larger solid residue is also apparent in the TGA
curves of the copolymers in Fig. 9, being more marked, as
expected, in the copolymer with the higher glycomonomer
content (25 mol%, Fig. 9b).
As already described for the protected polymers, also in the
case of deprotected samples when the purge gas in TGA
experiments is switched from N2 to air, the curves show an
additional high temperature weight loss that brings the solid
residue close to zero (Fig. S6 in the ESI†).
Fig. 11 Tg as
a function of the glycomonomer weight fraction of
protected (K), deprotected (J) polymers and PNIPAm (E). The line
represents Fox equation (eqn (1)) applied to protected copolymers.
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the composition of protected copolymers is compared with that References
predicted for random copolymers by the Fox equation:21
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H-bonds, are formed between monomer units in copolymers.22
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In conclusion, we have explored the copper catalyzed azide–
alkyne click reaction (CuAAC) for the synthesis of a triazole
containing glycomonomer from cheaply available D-galactose
as a starting material. The synthetic protocol is simple, highly
efficient, functional group tolerant and in aqueous medium.
The homopolymerization as well as copolymerization of the
glycomonomer with N-isopropylacrylamide (NIPAm) at differ-
ent compositions afforded well-defined thermoresponsive gly-
copolymers. Protected polymers upon acid hydrolysis yielded
hydrophilic polymers. Thermosensitivity of synthesized glyco-
polymers has been evaluated by the cloud point method and
strongly supported by a temperature dependent NMR study.
The protected and deprotected copolymers exhibited the lower
and higher cloud point temperatures respectively, when com-
pared with that of PNIPAm. This observation was attributed to a
change in hydrophilicity as well predicted by water contact
angle results. A peculiar Tg-composition behavior associated
with H-bond interaction in deprotected copolymers was evi-
denced by DSC.
Acknowledgements
We gratefully acknowledge the Department of Science and
Technology (DST, New Delhi) for financial support. VSS thanks
the Erasmus Mundus Action 2 INDIA4EU program for scholar-
ship. ABD is thankful to CSIR (New Delhi, INDIA) for junior
research fellowship.
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