Dye-incorporated water-soluble polymer via click triazole formation

Article abstract of DOI:10.1016/j.dyepig.2012.01.012

A click triazole-polymerization is proposed for the synthesis of a water-soluble polymer dye. 4,4′-Diazidostilbene-2,2′-disulfonate (DASS) sodium salt reacted with a diacetylenic derivative of para-methyl red (PMR) in water, which was catalyzed by copper

Full text of DOI:10.1016/j.dyepig.2012.01.012

Dyes and Pigments 94 (2012) 217e223
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Dye-incorporated water-soluble polymer via click triazole formation
,
Byungkwon Jang a, Sung Yoon Kim b, Jung Yun Do a
*
a Department of Chemistry Education, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea
b Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
A click triazole-polymerization is proposed for the synthesis of a water-soluble polymer dye. 4,40-Dia-
zidostilbene-2,20-disulfonate (DASS) sodium salt reacted with a diacetylenic derivative of para-methyl
red (PMR) in water, which was catalyzed by copper (I) ion or accelerated under ultrasound irradiation.
DASS formed a soluble polymer with a carboxylate salt of PMR showing a water solubility of
120 g/100 mL but it generated an insoluble polymer with an alkyl ammonium salt. The prepared poly-
meric dye underwent color transition at pH 5e6, which is higher than that of PMR and similar to that of
ortho-methyl red (OMR). The quinonoid canonical structure of PMR generated a purple water solution
and was dominant in the polymer solution at pH below 5. Intra-molecular hydrogen bonding between
the triazoles and azo nitrogens was presumed to have induced the formation of the quinonoid structure.
The color change of the polymer was reproducible during successive up/down pH cycles.
Ó 2012 Elsevier Ltd. All rights reserved.
Article history:
Received 6 November 2011
Received in revised form
11 January 2012
Accepted 14 January 2012
Available online 24 January 2012
Keywords:
Water-soluble polymer dye
Para-Methyl red
Click polymerization
Acid-base indicator
Quinonoid structure
Hydrogen bonding
1. Introduction
in a fuel cell [16]. Two azido groups of DASS are known to undergo
a click reaction with triple bonds to generate water-swelling poly-
Extensive research has been conducted to investigate polymer
dyes and their applications such as fiber, hair dyes, solid-state
polymeric dye lasers, and chromatography [1e3]. Organic dyes
have been modified for electric and optical processes with polymer
backbones [4e6]. Physical and chemical polymer bonding some-
times improves the inherent optical properties of organic dyes [7,8].
Some inter- and intra-molecular interactions of dyes contribute to
specific applications such as sensor, switches and semiconductors
[9]. Most neutral organic dyes are soluble in organic solvents and
transformed for water-media uses. Hydrophilic polymer encapsu-
lation is a useful approach for water applications such as drug
delivery or tracers [10,11]. Another is the host-guest concept, where
a guest dye is surrounded by hosts such as cyclodextrin [12] or
dendrimer [13]. The direct synthesis of water-soluble polymer dyes
remains an issue because of functional group tolerance.
mers. The repeating unit with two sulfonate ions is chemically
stable, hydroscopic and transparent in visible wavelengths. The
triazole linker is able to hold protons in an acidic solution and to
enhance the ion mobility of the membrane. Furthermore, a dye-
attached polymer as a pH indicator enables visual pH sensing or
tracing in water applications [17e19]. Therefore, the synthesis of
click polymers of DASS and a dye is valuable in numerous applica-
tions. Dipolar azo dyes have vivid colors due to their p-conjugated
structures. Some azo compounds such as methyl orange or methyl
red (MR) are a popular acid-base indicator with the protonation-
deprotonation of the azo group and amino nitrogen as well as
carboxylic and sulfonic acid for specific color changes [20,21]. The
basic strength of various nitrogen atoms and thereby thermody-
namic equilibrium decides the transition pH of the indicator.
In the research, hydrophilic click polymers were developed
for water-soluble dyes via polymerization with a water-soluble,
difunctional monomer of 4,40-diazidostilbene-2,20-disulfonate
(DASS) sodium salt. DASS has been used as a photo-cross-linking
agent to interconnect polymer chains under UV radiation [14,15]
and has often been introduced to improve the polymer membrane
2. Experimental detail
2.1. Material and instrumentations
The starting materials and reagents were used as purchased
from Aldrich Chemical Co., except for 4,40-diazidostilbene-2,20-
disulfonic acid disodium salt tetrahydrate (DASS) purchased from
Fluka. 1H and 13C NMR spectra were recorded on 300 MHz and
75 MHz Varian NMR spectrometer system, respectively. IR spectra
were measured on a Thermo Nicolet FT-IR. The molecular weight of
* Corresponding author. Fax: þ82 51 581 2348.
E-mail address: jydo@pusan.ac.kr (J.Y. Do).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.01.012

218
B. Jang et al. / Dyes and Pigments 94 (2012) 217e223
small molecules was recorded on an Agilent 1200LC/1100 MSD SL
mass spectrometer. Elemental analyses were carried out using
a LECO 932 CHNS elemental analyzer. UVeVis spectra were
measured on a Varian Cary 5000 spectrometer. Aqueous GPC
measurement was carried out using sulfonated-DVB column (Jordi-
Gel Aqueous GPC Column, using 0.2 M aqueous sodium acetate and
THF) to give molecular weights calibrated by poly(acrylic acid)
standards. Thermal analyses of polymers were conducted on
a Mettler Toledo TGA/SDTA 851e analyzer. The ultrasound (20 kHz)
was generated using a VCX-500P Ultrasonic Processor (SONICS).
The pH was kept with buffer using HCl/Na2HPO4 and NaOH/
NaHCO3.
After being stirred for 12 h, the solution was neutralized with
aqueous HCl (35%, 3.4 mL) and precipitated with additional water
(40 mL) to afford 3a (overall 74% yield, white powder).
2a: 1H NMR (CDCl3):
d
¼ 7.31 (s, 2 H), 6.83 (s, 1 H), 4.75 (s, 4 H),
3.91 (s, 3 H), 2.58 (s, 2 H). 13C NMR (CDCl3):
d
¼ 166.7, 158.7, 132.3,
109.0, 107.7, 78.2, 76.3, 56.3, 52.7. LC-MS, m/z: 245.02 [M þ 1]þ.
Anal. Calcd for C14H12O4: C, 68.85; H, 4.95. Found: C, 68.84; H, 4.98.
3a: 1H NMR (CDCl3):
d
¼ 7.38 (s, 2 H), 6.89 (s, 1 H), 4.76 (s, 4 H),
2.59 (s, 2 H). 13C NMR (CD3OD):
d
¼ 168.0, 158.9, 132.7, 108.8, 106.8,
78.2, 76.1, 55.7. IR (KBr): 3350e2700 (br, CO2H), 3294, 2115, 1688
(C]O), 1609, 1246 cmꢁ1. LCeMS, m/z: 231.01 [M þ 1]þ. Anal. Calcd
for C13H10O4: C, 67.82; H, 4.38. Found: C, 67.77; H, 4.19.
2.2. Preparation of 1a
2.5. Preparation of water-soluble polymer 3b
A 50 mL-round bottom flask was charged with 0.76 g (5.0 mmol)
of methyl 4-hydroxybenzoate, propargyl bromide (6 mmol), and
N,N-dimethyl formamide (DMF, 10 mL). The mixture was stirred
with potassium carbonate (0.83 g) over 12 h at room temperature.
The resulting mixture was poured into water (100 mL) and
extracted twice with diethyl ether (200 mL ꢀ 2). The combined
organic layer was washed with 150 mL of water and dried over
MgSO4. The solution was concentrated and purified by a chro-
matographic separation using ethyl acetate (EA)/hexane (1/7, v/v)
to afford 1a (73% yield, white powder).
In a 25 mL-round bottom flask were placed 3a (0.46 g,
2.00 mmol) and NaOH (80 mg) in water (8 mL). After adding DASS
(1.08 g, 2.00 mmol), CuSO4 (52 mg), and sodium ascorbate (80 mg),
the mixture was warmed to 60 ꢂC and agitated for 48 h to yield
a ball-shaped, phase-separated mass. The solid was dissolved in
water (30 mL) and precipitated by the addition of ethanol (150 mL)
and keeping at low temperature. The solid was filtered and
vacuum-dried to afford polymer 3b (65% yield, yellow powder).
1H NMR (D2O):
d
¼ 8.44e8.30 (br, 2 H), 8.28e7.96 (br, 2 H),
7.95e7.40 (br, 5 H), 7.34e6.95 (br, 3 H), 6.91e6.52 (br, 2 H), 5.39
(d, 1 H), 5.14e4.97 (br, 2 H). IR (KBr): 1640, 1385, 1203 cmꢁ1. UV
(H2O): lmax ¼ 342 nm. Anal. Calcd for C27H17N6Na3O10S2: C, 45.13;
H, 2.38; N, 11.70; S, 8.93. Found: C, 45.05; H, 2.36; N, 11.85; S, 8.96.
1H NMR (CDCl3):
(s, 3 H), 2.58 (s, 1 H). 13C NMR (CDCl3):
d
¼ 7.99 (d, 2 H), 6.98 (d, 2 H), 4.73 (s, 2 H), 3.87
d
¼ 166.9, 161.3, 131.8, 123.6,
114.7, 78.0, 76.4, 56.0, 52.2. IR (KBr): 3176 (s, CspeH), 2120 (CspeCsp),
1709 (carbonyl), 1281 (CeO) cmꢁ1. LC-MS: m/z 190.06 [M]þ.
2.6. Preparation of a quaternary ammonium bromide 4a
2.3. Preparation of 1b (R ¼ CH3) and 1b (R ¼ H or Na)
Methyl 3,5-bis(20-propynyloxy)benzoate 2a (2.44 g, 10.0 mmol)
was dissolved in THF (30 mL) and treated with LiAlH4 (0.19 g) for
1 h. Methanol (5 mL) and water (100 mL) were added to decompose
any excess LiAlH4. The resulting mixture was extracted with EA,
dried over MgSO4, filtered, and concentrated to afford a crude
alcohol. Triphenylphosphine (2.62 g, 10 mmol) was dissolved in
anhydrous dichloromethane (30 mL) and bromine (0.26 mL) was
slowly added at 0 ꢂC. The milky bromophosphonium bromide
solution was mixed with the crude alcohol under nitrogen atmo-
sphere at 0 ꢂC. After being stirred for 1 h, the mixture was
concentrated and mixed with diethyl ether (50 mL). The resulting
mixture was filtered to remove triphenylphosphonium oxide and
organic layer was concentrated. A column chromatographic sepa-
ration using EA/hexane (1/7, v/v) afforded 1-bromomethyl-3,5-
bis(20-propynyloxy)benzene in overall 68% yield.
1a (0.382 g, 2.00 mmol) and DASS (0.536 g, 1.00 mmol) were
placed in a 25 mL-round bottom flask charged with tetrahydrofuran
(THF)/water (4.5 mL/1.5 mL). To the reaction was added CuSO4
(50 mg) and sodium ascorbate (80 mg) at room temperature. The
reaction mixture was stirred for 3 days at room temperature. After
removing the volatile solvents, the residue was treated with
methanol (10 mL) and then the insoluble solid was filtered out
using a filter paper. The resulting mixture was purified by a chro-
matographic separation using EA/methanol (4/1, v/v)to afford 1b
(R ¼ CH3, yellow powder, 45%). 1a (2.00 mmol) was treated with
NaOH (2.4 mmol) in water (6 mL) for 4 h and then reacted with
DASS (0.536 g, 1.00 mmol) in the presence of CuSO4 (50 mg) and
sodium ascorbate (80 mg) for 12 h at 60 ꢂC to yield 1b (R ¼ H or Na).
1H NMR (R ¼ CH3, CD3OD):
d
¼ 8.77 (s, 2 H), 8.46 (s, 2 H), 8.32
(s, 2 H), 8.23 (s, 2 H), 8.03 (s, 2 H), 8.01 (s, 2 H), 7.18 (s, 2 H), 5.36 (s,
4 H), 3.87 (s, 6 H). IR (KBr): 1707, 1605, 1509, 1437, 1250 cmꢁ1. UV
(H2O): lmax ¼ 327, 253 nm. ESIeMS (negative matrix), m/z: 800.77
[M ꢁ 2Na]þ. Anal. Calcd for C36H28N6Na2O12S2: C, 51.06; H, 3.33; N,
9.93, S, 7.57. Found: C, 51.00; H, 3.29; N, 9.88; S, 7.62.
1H NMR (CDCl3):
d
¼ 6.64 (s, 2 H), 6.56 (s, 1 H), 4.68 (s, 1 H), 4.42
(s, 2 H), 2.58 (s, 2 H). 13C NMR (CDCl3):
d
¼ 154.2, 138.6, 120.8, 108.0,
70.0, 69.7, 57.6, 34.8. LCeMS, m/z: 278.01 [M]þ.
1-Bromomethyl-3,5-bis(20-propynyloxy)benzene (1.39 g) was
dissolved in triethylamine (10.0 mL) and refluxed for 24 h. The
resulting mixture was concentrated, rinsed with anhydrous ether,
and dried under vacuum in an oven at 50 ꢂC to afford 4a (88% yield,
hydroscopic solid).
2.4. Preparation of 3a
A 500 mL-round bottom flask was charged with methyl 3,5-
dihydroxybenzoate (5.04 g, 30 mmol) and acetone (100 mL). After
being completely dissolved, propargyl bromide (8.88 mL, 80%),
K2CO3 (8.29 g), and tetrabutylammonium bromide (TBAB, 0.97 g)
were successively added. The reaction mixture was refluxed for 48 h
and then cooled to room temperature. The resulting mixture was
poured into water (100 mL) and extracted with EA (300 mL). The
organic layer was dried over MgSO4, filtered, and concentrated to
give crude 2a (methyl 3,5-bis(20-propynyloxy)benzoate). The crude
(2a) was re-dissolved in THF (40 mL) and mixed with aqueous NaOH
solution (3.0 M, 15 mL) and methanol (10 mL) at room temperature.
1H NMR (D2O):
d
¼ 6.80 (s, 2 H), 6.75 (s, 1 H), 4.80 (s, 4 H), 4.35
(s, 2 H), 3.22 (q, 6 H), 3.09 (s, 2 H), 1.38 (t, 9 H). 13C NMR (D2O):
d
¼ 158.5, 130.0, 112.6, 105.1, 100.2, 78.7, 77.5, 56.5, 52.9, 46.9, 7.4. IR
(KBr): 3176 (s, CspeH), 2120 (CspeCsp), 1599 (C]C) cmꢁ1. LCeMS,
m/z: 300.20 [M ꢁ Br]þ. Anal. Calcd for C19H25BrNO2: C, 60.16; H,
6.64; N, 3.69. Found: C, 60.13; H, 6.53; N, 3.58.
2.7. Preparation of water-soluble polymer 4b
A 25 mL-round bottom flask was charged with 4a (0.758 g,
2.00 mmol), DASS (1.08 g, 2.00 mmol), and water (8 mL). After

B. Jang et al. / Dyes and Pigments 94 (2012) 217e223
219
SO Na
3
O
O
OH
N
O
N
N
(i)
N
(ii)
N
N
NaO S
3
or (iii)
CO R
2
R=CH
=H or Na
CO CH
2
3
CO CH
3
2
3
CO R
2
1a
1b
Fig. 1. Click reaction for water-soluble molecule 1b: (i) propargyl bromide and K2CO3 in DMF, (ii) DASS/CuSO4/sodium ascorbate in water/THF (r.t.), and (iii) NaOH and then DASS/
CuSO4/sodium ascorbate in water (60 ꢂC).
adding CuSO4 (52 mg) and sodium ascorbate (80 mg), the mixture
was warmed to 60 ꢂC and agitated for 24 h, yielding a phase-
separated mass. The solid was washed with water (30 mL) and
methanol (30 mL) and then vacuum-dried to afford insoluble
polymer 4b (90% yield).
IR (supplementary data). Anal. Calcd for C33H34BrN7Na2O8S2: C,
46.81; H, 4.05; N, 11.58; S, 7.57. Found: C, 46.66; H, 4.01; N, 11.44; S,
7.49.
column chromatography using EA/hexane (1/2, v/v) to afford pure
5a (red powder) in 44% yield.
1H NMR (CDCl3):
d
¼ 8.14 (d, 2 H), 7.88 (m, 4 H), 6.81 (d, 2 H),
4.38 (m, 4 H), 4.18 (d, 4 H), 3.75 (m, 4 H), 2.45 (s, 2 H), 1.41 (t, 3 H).
13C NMR (CDCl3):
d
¼ 166.5, 155.9, 151.1, 143.9, 130.7, 126.0, 122.2,
111.8, 79.7, 75.2, 67.5, 61.3, 58.7, 51.4, 14.6. IR (KBr): 3180 (s, CspeH),
2120 (CspeCsp), 1700 (carbonyl), 1280 (CeO) cmꢁ1. LCeMS, m/z:
434.18 [M þ 1]þ. Anal. Calcd for C25H27N3O4: C, 69.27; H, 6.28; N,
9.69. Found: C, 69.26; H, 6.28; N, 9.65.
2.8. Preparation of 5
2.10. Preparation of 5b
In
a 100 mL-round bottom flask were placed N-phenyl-
diethanolamine (3.82 g, 95%), propargyl bromide (80%, 8.88 mL),
KOH (3.37 g), and TBAB (1.29 g) and dissolved in dimethyl sulfoxide
(DMSO, 35 mL). The reaction mixture was stirred for 24 h at 60 ꢂC
and poured into 100 mL of water. The mixture was extracted with
EA, dried over MgSO4, and filtered. The concentrated crude was
purified bya chromatographic separation using EA/hexane (1/4, v/v)
to afford 5 (55% yield, viscous liquid).
Ethyl 4-aminobenzoate (98%, 1.03 g, 6.00 mmol) was dissolved
in acetic acid (2.4 mL) and aqueous HCl (35%, 0.7 mL). Aqueous
NaNO2 (7.50 mmol) was added at 0 ꢂC to yield a yellow solid. After
additional stirring for 30 min, a methanol solution of N,N-bis(2-
hydroxyethyl)aniline (95%, 0.95 g) was slowly added and reacted
for 2 h at room temperature. The mixture was poured into water
(50 mL) under vigorous agitation and filtered to afford a reddish
solid. Further purification was performed by column chromatog-
raphy using EA/hexane (2/1, v/v). The solid (1.07 g, 3.00 mmol) was
dissolved in THF (3 mL) and mixed with an aqueous NaOH solution
(3.0 M, 2 mL) and methanol (3 mL). After stirring for 2 h at 50 ꢂC, the
resulting mixture was cooled to room temperature, neutralized
with aqueous HCl solution (3.0 M, 2.0 mL), and precipitated with
additional water (15 mL). The solid was filtered and vacuum-dried
at 60 ꢂC to afford 5b (overall 42% yield, red powder).
1H NMR (CDCl3):
(s, 4 H), 3.67 (m, 4 H), 3.58 (m, 4 H), 2.44 (s, 2 H). IR (KBr): 3180 (s,
d
¼ 7.28 (d, 2 H), 6.60 (d, 2 H), 6.31 (s, 1 H), 4.15
C
speH), 2120 (CspeCsp) cmꢁ1
.
13C NMR (CDCl3):
d
¼ 146.9, 132.2,
113.8, 108.2, 79.9, 75.1, 67.4, 58.7, 54.2. LCeMS, m/z: 257.18 [M]þ.
2.9. Preparation of 5a
Ethyl 4-aminobenzoate (98%, 1.05 g, 6.22 mmol) was dissolved
in acetic acid (2.4 mL) and aqueous HCl (35%, 0.7 mL). Aqueous
NaNO2 (7.46 mmol) was added at 0 ꢂC to yield a yellow solid. After
being stirred for 30 min, a methanol solution of 5 was slowly added
and reacted for 2 h at room temperature. The mixture was poured
into water (50 mL) under vigorous agitation and filtered to afford
a reddish yellow solid. Further purification was performed by
1H NMR (DMSO-d6):
3.75 (m, 4 H, CHeO), 3.65 (m, 4 H, CHeN), 3.55 (br, 2 H, OeH). 13C
d
¼ 8.16 (d, 2 H), 7.82 (m, 4 H), 6.91 (d, 2 H),
NMR (DMSO-d6):
d
¼ 168.1, 157.1, 152.8, 143.4, 131.9, 131.8, 127.0,
122.5,114.8, 59.0, 53.5. LCeMS, m/z: 330.12 [M þ 1]þ. Anal. Calcd for
C17H19N3O4: C, 62.00; H, 5.81; N, 12.76. Found: C, 61.95; H, 5.77; N,
12.74.
CO R
2
CO R
2
CO CH
2
3
(i) or
(i)&(ii)
(iii)
SO Na
3
sonic
O
O
N
O
O
N
HO
OH
radiation
N
N
n
N
N
NaO S
3
2a (R=CH3)
3a (R=H)
R=CH
2b
Oligomer
3
3b R=H or Na Polymer
Br-
Br-
+
N
+
N
(iv),(v)
(iii)
2a
SO Na
3
O
O
N
O
O
N
N
N
N
n
N
NaO S
3
4a
4b insoluble polymer
Fig. 2. Preparation of water-soluble click polymers: (i) propargyl bromide/K2CO3/TBAB in refluxing acetone, (ii) NaOH and then HCl, (iii) NaOH and then DASS/CuSO4/sodium
ascorbate in water (60 ꢂC), (iv) LiAlH4 in THF and then Br2/PPh3 in CH2Cl2, and (v) refluxing in Et3N.

220
B. Jang et al. / Dyes and Pigments 94 (2012) 217e223
SO Na
3
1
1
1
1
N
R O
OR
R O
OR
O
O
N
N
N
(ii),(iii)
N
(i)
N
NaO S
3
N
N
N
N
N
n
N
N
(R = propargyl)
5
1
2
CO R
1
2
CO R
2
2
(R = propargyl, R =ethyl)
(R ,R =H)
5a
(R=H or Na)
5c
1
2
5b
Fig. 3. Preparation of water-soluble polymer dye: (i) ethyl 4-aminobenzoate, HCl(aq), NaNO2, and acetic acid/water, (ii) NaOH in THF/water, (iii) and DASS/CuSO4/sodium ascorbate
in water/THF (60 ꢂC).
2.11. Preparation of water-soluble polymer dye 5c
of the polymer proved the triazole formation in agreement with
the result of the preliminary study. When the reaction was irra-
diated with ultrasound at 20 kHz [23], the polymerization was
accelerated to completion in 10 h. The sonic wave radiation was
ineffective for the polymerization without the copper catalyst and
unreacted monomers were almost recovered over 10 h. Because
the carboxylate salt of alkyne, 3a, was soluble in basic water,
a quaternary ammonium salt was considered for neutral pH
media. The LiAlH4 reduction and bromination of 2a provided an
ammonium bromide, 4a, after refluxing in triethylamine. The
polymerization of 4a and DASS proceeded faster than that of 3b in
the catalytic condition. Monitoring of the azide band in the IR
spectrum revealed its disappearance after 24 h. Unfortunately, the
separated polymeric product dissolved in neither water nor
organic solvents, which was attributed to the formation of zwit-
terions by losing NaBr from 4b.
5a (0.866 g, 2.00 mmol) was dissolved in THF (4 mL) and stirred
for 2 h with aqueous NaOH solution (3 M, 0.67 mL) at 50 ꢂC. To the
resulting mixture were added DASS (1.07 g, 2.00 mmol), CuSO4
(52 mg), and sodium ascorbate (80 mg). The reaction mixture was
agitated at 60 ꢂC for 48 h and then filtered and concentrated under
reduced pressure. The resulting crude polymer was purified by
precipitation using methanol to afford 5c (75% yield, red powder).
1H NMR (D2O)
d
¼ 8.17 (m, 2 H), 8.05 (s, 2 H), 7.85 (d, 2 H),
7.38e7.67 (m, 2 H), 7.23 (d, 2 H), 6.42 (br m, 2 H), 4.69 (d, 2 H),
4.44e4.56 (m, 6 H), 3.41e3.78 (m, 6 H), 1.83 (s, 1 H). IR (KBr): 1599,
1552, 1496, 1385, 1203 cmꢁ1. UV (H2O): lmax ¼ 325, 462 nm. PL
(H2O, radiated at 462 nm) lmax: 367, 401, 577, 643 nm. Anal. Calcd
for C37H30N9Na3O10S2: C, 49.72; H, 3.38; N, 14.10; S, 7.18. Found: C,
49.58; H, 3.32; N, 14.03; S, 7.10.
These water-soluble polymers led us to focus on the synthesis
of a water-soluble polymer dye. MR is an azo-coupled, dipolar
dye that is used as a pH indicator and coloring agent for bacteria
detection [24]. MR analogue 5a was prepared through a para-
azotization from a dipropargyl compound 5. Alkali-hydrolysis
of 5a and successive polymerization with equimolar DASS
afforded polymer dye 5c after neutralization as described in
Fig. 3. The polymer exhibited an excellent water solubility of
ca.120 g/100 mL (25 ꢂC, yellow color in water). An IR absorption
spectrum showing no azide absorption indicated successful
polymerization.
The inherent viscosity of the polymer was in the range
0.27e0.28 dL/g at 25 ꢂC and was stable up to 267 ꢂC by a thermal
gravimetric analysis. Aqueous GPC analysis was performed to give
relative molecular weights calibrated by poly(acrylic acid) stan-
dards. The GPC results of three polymers are summarized in Table 1
3. Results and discussion
A water-soluble polymer dye was synthesized via the click
polymerization of a water-soluble segment and a dye. The Huisgen
cycloaddition reaction of azide and alkyne is a representative click-
reaction with a high reaction conversion required for polymeriza-
tion [22]. Aqueous Cu(I) was used to catalyze the reaction and guide
selective 1,4-triazole formation from the click reaction. A prelimi-
nary study was performed for water-media click polymerization
with water-soluble DASS and an alkyne. Methyl 4-hydroxybenzoate
was derived with a propargyl group and treated with DASS under
the copper catalytic condition. A co-solvent system of THF and
water (3/1, v/v) was used in the reaction due to an organic soluble
alkyne, 1a, as illustrated in Fig. 1. A single product, 1b, was gener-
ated over 3 days at room temperature and confirmed with 1H-NMR
analysis using deuterium oxide. A peak at
d 5.36 was assigned to
protons from two triazole rings. IR analysis revealed the complete
disappearance of the strong azide band (2200 cmꢁ1) and terminal
alkyne CeH band (3300 cmꢁ1). The reaction with a sodium salt of
1a was completed within 12 h.
Table 1
Analytic data of the prepared polymers.
Polymers
GPC analysisa
Mn (kg/mol)
TGAc
Based on this result, click polymerization was investigated with
DASS. Methyl 3,5-dihydroxybenzoate was derived with two
propargyl groups. The click polymerization with organic soluble 2a
afforded oligomers as shown in Fig. 2. The absorption band of the
azide was observed from the IR analysis of the oligomeric product
and still remained for a long reaction period of 5 days. Thus, 2a
was hydrolyzed to enhance solubility in water before polymeri-
zation. An equimolar mixture of 3a and DASS was agitated under
the same click reaction condition for 48 h, when resulting polymer
showed no azide band in the IR spectrum. The 1H-NMR spectrum
Mw (kg/mol)
PDIb
5% loss (ꢂC)
2b
3b
3bd
5c
3.8
16.2
10.7
13.4
5.3
25.9
19.3
21.4
1.4
1.6
1.8
1.6
294
320
e
267
a
The number average molecular weight (Mn) and the weight average molecular
weight (Mw) were calibrated by poly(acrylic acid) standards.
b
Polydispersity.
Thermal gravimetric analysis.
Polymerization irradiated with ultrasound (20 kHz).
c
d

B. Jang et al. / Dyes and Pigments 94 (2012) 217e223
221
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
pH 2
pH 3
pH 4
pH 5
pH 6
pH 8
pH 10
pH 2
pH 3
pH 4
pH 5
pH 6
pH 8
pH 10
0.8
0.6
6~10
2
2
3
0.4
5
6~10
4
3
0.2
5
4
0.0
300
400
500
600
700
300
400
500
Wavelength (nm)
600
700
Wavelength (nm)
Fig. 4. UVeVis absorption spectra of dye 5b (left) and polymer dye 5c (right) and their pH dependence.
along with thermal gravimetric analysis. Neutral alkyne monomer
2a generated oligomer 2b of low molecular weight and anionic
monomer 3a evidently led to the formation of polymer 3b of high
molecular weight. The ultrasound-assisted polymerization
provided the polymer of lower molecular weights than other
polymerization.
point was retained. Surprisingly, the color transition of the polymer
dye was observed at pH 5 with the broad band combining two
absorptive bands. The protonation on an azo nitrogen seems to be
accelerated in the polymer. Thus, the following mechanism of
a structure presented in Fig. 5 can be proposed with reasonable
assurance: protonation preferentially proceeds at triazole groups
[27] of the polymer backbone and the resultant triazolium proton
may induce hydrogen bonding with a neighboring azo group. This
triazole-induced hydrogen bond was considered to have a similar
mechanistic effect of ortho-carboxylic acid in OMR yielding
a quinonoid structure, which has a transition pH range of 4.2e6.3
[20,21,25]. The apparent color change of the polymer dye was
observed at a pH range of 5e6, in marked contrast to the pH range
of 3e4 for dye 5b. Fig. 5 shows the color change of the polymer dye
according to pH. The purple color at pH 5 was similar to that of dye
5b at pH 3.
The H-bonding contribution of a quinonoid canonical structure
was further investigated with a simple molecule through NMR
analysis. Dye 6 was prepared and treated with deuterium chloride
(supplementary data) to induce intra-molecular H-bonding. Fig. 6
compares 13C-NMR spectra of 6 and 6a. Three peaks assigned to
aromatic carbons (C14, C15, C16) of 6 were strongly affected by H-
bonding. The acid treatment of 6 led to color change similar to 5c
from yellow to purple. Protonated 6a showed a red-shifted
absorption spectrum as being compared with 6, which is attrib-
uted to the formation of a quinonoid structure.
MR has a tri-basic activity due to three nitrogen atoms (neutral-
MR, monoprotonated-HMR, diprotonated-H2MR) and a specific
color change according to the equilibrium state in pH media [20].
HMR has a strong absorption at around 520 nm due to quinonoid
canonical structures while neutral or anionic MR has a strong
absorption at around 430 nm. The color transition of methyl red
occurs over a pH range of 4.2e6.3 [25]. Ortho-substituted carbox-
ylic acid on the MR (OMR) participates in intra-molecular hydrogen
bonding between the carboxylic proton and the azo group, thereby
yielding a stable planar structure [21]. Para-substituted carboxylic
acid on the MR (PMR) has a similar color change at lower pH
because of the absence of intra-molecular hydrogen bonding.
Water-soluble dye 5b, being analogous to PMR, was analyzed with
UVeVis spectroscopy at the various buffered pHs. A color transition
of 5b was observed in the pH range of 3e4 and an isosbestic point
at 485 nm indicated a protonation-deprotonation equilibrium in
Fig. 4. The pH behavior indicates that 5b is more basic than previous
PMR having pKa of ca. 2 [26]. The UVeVis absorption of polymer
dye 5c was similarly compared under pH conditions. A strong
absorption at 330 nm was definitively attributed to a DASS unit by
the similar observation with a simple polymer 3b and was used as
an internal standard for the pH comparison. As the pH was lowered,
the band intensity at 489 nm decreased and a new band was
developed at 540e560 nm by the protonation at an azo nitrogen of
the dye. The absorption bands of the polymer were red-shifted by
ca. 20 nm from the similar bands of the dye 5b but the isosbestic
The transition range of the polymer dye was similar to that of
OMR as a pH indicator but the color was different. The transition
was reproducible during successive up/down pH cycles. The
distinct color change and water solubility of the polymer dye were
considered useful as much as OMR to detect an end point during
the analytic titration.
Fig. 5. The proposed hydrogen bonding domination at pH <6 and color change of 5c at various pHs.

222
B. Jang et al. / Dyes and Pigments 94 (2012) 217e223
O
N
N
5
13
14
15
6
N N
16
C6H13
N
N
6
17
18
19
20
O
OC2H5
21
N
13
O
14
15
6
5
+
N C6H13
16
N
N
N
6a
N
D
17
18
19
20
O
OC2H5
21
Fig. 6. 13C-NMR spectral shift of a dye (6) by hydrogen bonding.
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A water-soluble polymer dye was developed via click polymer-
ization. 1,2,3-Triazole was formed by the reaction between
a difunctional dye and DASS in which two sulfonate groups
provided the water solubility of the polymer dye. A PMR analogue
in the polymer showed a color transition pH similar to that of OMR
at pH 5e6. Triazole groups were proposed to be involved in intra-
molecular H-bonding with an azo nitrogen, leading to the prefer-
ential formation of a quinonoid canonical structure. The polymer
dye exhibited a purple color at pH below 5 and a yellow color at pH
above 6, and this color transition was critical and reproducible
toward successive pH change.
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Acknowledgement
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (2010-
0009455) and by a grant from the Fundamental R&D Program for
Core Technology of Materials funded by the Ministry of Knowledge
Economy, Korea.
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Appendix. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.dyepig.2012.01.012.
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