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Synthesis and characterization of novel colored
polymers based on lawsone natural compound
a
a
a
Mehrdad Mahkam , Hadieh Rahbar Kafshboran & Mehdi Nabati
a
Faculty of Science, Chemistry Department, Azarbaijan Shahid Madani University, Tabriz,
Iran
Published online: 20 May 2014.
To cite this article: Mehrdad Mahkam, Hadieh Rahbar Kafshboran & Mehdi Nabati (2014) Synthesis and characterization
of novel colored polymers based on lawsone natural compound, Designed Monomers and Polymers, 17:8, 784-794, DOI:
10.1080/15685551.2014.918017
To link to this article: http://dx.doi.org/10.1080/15685551.2014.918017
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Designed Monomers and Polymers, 2014
Vol. 17, No. 8, 784–794, http://dx.doi.org/10.1080/15685551.2014.918017
Synthesis and characterization of novel colored polymers based on lawsone natural compound
Mehrdad Mahkam*, Hadieh Rahbar Kafshboran and Mehdi Nabati
Faculty of Science, Chemistry Department, Azarbaijan Shahid Madani University, Tabriz, Iran
Downloaded by [Dalhousie University] at 00:47 13 November 2014
(Received 28 January 2014; accepted 20 March 2014)
The purpose of this paper is the expansion of new colored polymers based on lawsone. Polymers should be highly
thermally stable in order to be colored materials. Also, their solubility and color intensity should be good enough. First, the
hydroxyl group of lawsone was covalently linked to acryloyl chloride, abbreviated as LA. Free radical copolymerization of
polymerizable derivative of lawsone with methacrylic acid, acrylic acid, and acrylonitrile was carried out using 2,
2′-azobisisobutyronitrile (AIBN) as initiator at the temperature 80 °C. The colored polymers were then characterized by the
UV/vis, FT-IR, and 1H NMR spectroscopies; and DSC and TGA studies. These colored polymers exhibit a high degree of
thermal stability.
Keywords: colored polymers; lawsone; acryloyl chloride; natural compound; colored materials
Introduction
The concept of dyeing started in 415 BC with dyeing of
wool by natural materials. In 1856, the first synthetic
dye Mauve was discovered.[1] The use of eco-friendly
and non-toxic natural dyes has become a matter of significant importance because of increased environmental
awareness in order to avoid some toxic of synthetic
dyes.[2] This issue has led to an increasing attitude
towards the natural dyes as a substitute of synthetic ones
in last few years, especially according to their useful
effects on biological systems.[3,4]
One of the most famous natural dyes is lawsone. The
lawsone (2-hydroxyl- 1,4- naphthoquinone) is the main
coloring element of henna that exists in dried leaves with
a concentration of 1–1.5% w/w.[5–7] The lawsone is a
red-orange dye with optical absorption maximum of
452 nm in the UV–vis analysis. The properties of henna
are connected with the presence of this natural compound. It is practically insoluble in water and soluble in
methanol, dichloromethane, acetone, chloroform, ethyl
acetate, isopropyl alcohol, diethyl ether, dimethylformamide (DMF), and dimethylsulfoxide (DMSO).[8] The
dyeing, antispasmodic, antibacterial, UV absorption, corrosion inhibitor properties have been attributed to the
presence of lawsone.[9] The solubility of these dyes is
limited and this problem resolved as dye polymers. The
polymeric dyes are chromophores that are attached to
polymeric scaffolds. The dyes can either incorporate into
polymer backbones or attached as side chains.[10] They
are classified as graft and block types according to their
structures. Either of graft or block polymeric dyes offers
*Corresponding author. Email: [email protected]
© 2014 Taylor & Francis
the advantage of allowing a range of many physical
properties, such as absorption, viscosity, solubility, and
migratory, that are tunable. The range of color and polymer chemistry is actually endless.[11,12] Now, they are
being applied in hair dyes,[13,14] fiber,[15,16] jet-printing,
[17,18] solid-state polymeric dye lasers.[19,20] The major
classes of synthetic dyes are toxic or even carcinogenic
with long turnover times.[21] The natural colored polymers with acrylate scaffolds are biodegradable and their
solubility is not restricted and color intensity is higher
than others.[22]
This paper deals with an effective preparation and
characterization of the colored polymers lawsone. The
structure of polymers was characterized with the FT-IR
and 1H NMR spectroscopy methods. The λmax and color
intensity of polymers were determined by a UV–vis
spectrophotometer in DMSO solvent. The solubility of
products was identified in various solvents. Thermal
properties of polymers were characterized by DSC and
TGA studies.
Experimental setup
Materials
The all of reagent and solvents were purchased from
Merck. The acrylic acid, methacrylic acid, and acrylonitrile were distilled under reduced pressure to remove
inhibitors, before use. The initiator α, α-azobis (isobutyronitrile) (AIBN) was purified by crystallization from
methanol. All the solvents were distilled and stored over
a drying agent. Tetrahydrofuran (THF) was dried by a
Designed Monomers and Polymers
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standard method before use. Thin layer chromatography
was carried out with silica gel 60 GF (Merck). Synthesis
of monomer and copolymerization were carried out
under dry argon to exclude oxygen and moisture from
the reaction systems.
Measurements
Infrared spectra were recorded with a Shimadzu FT-IR408 spectrophotometer as KBr pells. 1H NMR spectra
were recorded on a Brucker 250 AC spectrometer in
dimethyl sulfoxide (d6) as a solvent at room temperature.
The DSC and TGA curves were obtained on a TGA/
SDTA 851 calorimeter at heating and cooling rates of
10 °C/min under N2. The λmax and color intensity of
products were determined on a Philips PU 8620 UV spectrophotometer in DMSO solvent using a 1-cm quartz cell.
Synthesis of monomer from lawsone and acryloyl
chloride: LA
For preparing of monomer (LA), 0.4 g lawsone was dissolved in 10-mL dried THF in a two-necked flask. The
flask was degassed under argon gas with stirring at room
temperature. After 10 min, 0.12 mL acryloyl chloride
was added to this solution under stirring, followed by
0.35-mL pure triethylamine. A deep red color appeared
immediately. After 4 h, the solvent removed by rotary
evaporation, then 30 mL of toluene was added to mixture
followed the solution was washed with 10% sulfuric
acid. For removing of remaining sulfuric acid on solution, the 20% NaHCO3 solution was used. After removing impurities from solution, toluene was removed by
rotary evaporation. The product was chromatographed
over silica-gel by CH2Cl2:CHCl3 in a ratio of 40:1. The
dark red product (LA) was dried in an evacuated desiccator. The yield of final product was above 95%
(Scheme 1). 1H NMR (DMSO-d6, ppm): 0.86–1.4
(CH=CH2 of vinyl ester), 4.1 (–C=CH of lawsone), 7.7–
7.8 (Ar–H) (Figure 1). FT-IR (KBr, cm−1): 2964–3074
(aromatic C–H), 1754 (C=O), 1594–1681 (aromatic
C=C), 1405–1458 (vinyl C=C), 1260 (C–O).
Polymerization of LA monomer: HLA
For preparing of homopolymer (HLA), the monomer LA
was dissolved in 10 mL of toluene and was mixed with
AIBN (1% molar) as a radical initiator, in a Pyrex glass
ampoule. The ampoule was degassed under argon gas,
sealed under vacuum, and maintained at 80 ± 1 °C in a
water bath, with stirring for about 72 h. The polymerization temperature was well controlled in a water bath.
After reacting for 72 h, the ampoule was cooled rapidly
to room temperature. Then the solutions were poured
from ampoules into cooled methanol. The dark orange
precipitates were collected and washed with methanol
and dried under vacuum to yield (approximately 90%) of
HLA (Scheme 2). 1H NMR (DMSO-d6, ppm): 1–2.3
(aliphatic C–H), 2.8–3.3 (–C=CH of lawsone), 6.45–8
(Ar–H) (Figure 2). FT-IR (KBr, cm−1): 2961 (aromatic
C–H), 2850–2919 (aliphatic C–H), 1688–1760 (C=O),
1538–1592 (aromatic C=C), 1462 (vinyl C=C), 1261
(C–O).
Copolymerization of LA monomer with methacrylic
acid: PMLA
For preparing of copolymer (PMLA), a mixture of LA
monomer and methacrylic acid with molar ratio of 1:1
was dissolved in 10 mL of toluene and was mixed with
AIBN (1% molar) as a radical initiator, in a Pyrex glass
ampoule. The ampoule was degassed under argon gas,
sealed under vacuum, and maintained at 80 ± 1 °C in a
water bath, with stirring for about 72 h. After this time,
the ampoule was cooled rapidly to room temperature.
Then the solutions were poured into cooled methanol. The
brown precipitates were collected and washed with methanol and dried under vacuum to yield (approximately 95%)
of copolymer (Scheme 3). 1H NMR (DMSO-d6, ppm):
0.93–1.56 (aliphatic C–H), 3.18 (–C=CH of lawsone),
7.72–7.96 (Ar–H), 12.36 (–COOH) (Figure 3). FT-IR
(KBr, cm−1): 2600–3446 (–COOH), 2985 (aromatic
C–H), 2875 (aliphatic C–H), 1700–1716 (C=O),
1637–1653 (aromatic C=C), 1473–1488 (vinyl C=C),
1180–1271 (C–O).
O
O
OH
Cl
+
O
O
Scheme 1.
Preparation of LA.
785
O
THF, Et3N
O
Ar , RT, 4h
O
Downloaded by [Dalhousie University] at 00:47 13 November 2014
786
Figure 1.
M. Mahkam et al.
1
H NMR spectrum of LA in DMSO-d6.
Copolymerization of LA monomer with acrylic acid:
PALA
For preparing of copolymer (PALA), a mixture of LA
monomer and acrylic acid with molar ratio of 1:1 was
dissolved in 10 mL of toluene and was mixed with
AIBN (1% molar) as a radical initiator, in a Pyrex
glass ampoule. The ampoule was degassed under argon
gas, sealed under vacuum, and maintained at 80 ± 1 °C
in a water bath, with stirring for about 72 h. After 72 h,
the ampoule was cooled rapidly to room temperature.
Then the solutions in ampoule were poured into cooled
methanol. The red precipitates were collected and
washed with methanol and dried under vacuum to yield
(approximately 45%) of copolymer (Scheme 4). 1H
NMR (DMSO-d6, ppm): 2.31– 3.24 (aliphatic C–H),
3.54–3.94 (–C=CH of lawsone), 5.5–6.1 (Ar–H)
(Figure 4). FT-IR (KBr, cm−1): 2450–3424 (–COOH),
2965 (aromatic C–H), 2880 (aliphatic C–H), 1654–1719
(C=O), 1542–1611 (aromatic C=C), 1457 (vinyl C=C),
1097–1261 (C–O).
O
O
m
Toluene, AIBN
O
O
Scheme 2.
Preparation of HLA.
O
O
Ar , 80 C , 72 h
O
O
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Designed Monomers and Polymers
Figure 2.
1
787
H NMR spectrum of HLA in DMSO-d6.
Polymerization of Acrylonitrile: PAN
For preparing of homoacrylonitrile (PAN), the acrylonitrile (1 mL) was dissolved in 10 mL of toluene and was
mixed with AIBN (1% molar) as a radical initiator, in a
Pyrex glass ampoule. The ampoule was degassed under
argon gas, sealed under vacuum, and maintained at
CH3
O
O
m
O
O
Scheme 3.
Preparation of PMLA.
OH
Toluene, AIBN
O
+
O
Ar , 80 C , 72 h
O
O
C
O
O
n
OH
Downloaded by [Dalhousie University] at 00:47 13 November 2014
788
Figure 3.
M. Mahkam et al.
1
H NMR spectrum of PMLA in DMSO-d6.
80 ± 1 °C in a water bath, with stirring for about 24 h.
The polymerization temperature was well controlled in a
water bath. After reacting for 24 h, the ampoule was
cooled rapidly. Then the solutions were poured into
cooled methanol. The light yellow precipitates were
collected and washed with methanol and dried
under vacuum to yield (approximately 95%) of PAN
(Scheme 5). 1H NMR (DMSO-d6, ppm): 2.05–2.09
(CH2), 3.15–3.19 (C–H) (Figure 5). FT-IR (KBr, cm−1):
2938 (aliphatic C–H), 2243 (CN), 1454 (bending C–H).
Copolymerization of LA monomer with acrylonitrile:
PANLA
For preparing of copolymers (PANLA1 and PANLA2), a
mixture of LA monomer and acrylonitrile with molar
ratios of 1:10 and 1:20 was dissolved in 10 mL of toluene and was mixed with AIBN (1% molar) as a radical
initiator, in a Pyrex glass ampoule. The ampoules were
degassed under argon gas, sealed under vacuum, and
maintained at 80 ± 1 °C in a water bath, with stirring for
about 24 h. After 24 h, the ampoules were cooled rapidly
O
m
O
O
O
Scheme 4.
Preparation of PALA.
OH
+
O
O
Toluene, AIBN
Ar , 80 C , 72 h
O
O
n
HO
O
O
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Designed Monomers and Polymers
Figure 4.
1
H NMR spectrum of PALA in DMSO-d6.
to room temperature. Then the solutions in ampoules
were poured into cooled methanol. The brown precipitates were collected and washed with methanol and dried
under vacuum to yield (approximately 95%) of copolymers (Scheme 6). 1H NMR (DMSO-d6, ppm): 1.4–2.31
(aliphatic C–H), 3.06–3.18 (–C=CH of lawsone),
7.15–7.3 (Ar–H) (Figure 6). FT-IR (KBr, cm−1): 2983
(aromatic C–H), 2939 (aliphatic C–H), 2243 (CN),
1716–1771 (C=O), 1558–1636 (aromatic C=C), 1455
(vinyl C=C), 1079–1210 (C–O).
Scheme 5.
789
Preparation of PAN.
Result and discussion
Preparation of colored polymers with natural dyes is very
interesting in organic synthesis field. There are various
methods for these compounds preparation. In the present
work, first we synthesized monomer (LA) by treatment
of lawsone with acryloyl chloride and triethylamine as a
moderate base. The monomer is prepared by SN2 mechanism and is stable under heat and moisture. Then, the
LA polymerized with various vinyl compounds by FRP
(Free Radical Polymerization) mechanism. The structure
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790
Figure 5.
M. Mahkam et al.
1
H NMR spectrum of PAN in DMSO-d6.
of products is characterized with FT-IR and 1H NMR
analyses.
Interestingly, in compound LA because of magnetic
anisotropy effect, vinyl protons appear in the region
below 2 ppm.
Scheme 6.
Preparation of PANLA.
The molar compositions of LA and methacrylic acid
in copolymer PMLA were calculated from the ratio integrated intensities of the peaks around 7.7–8 ppm, corresponding to four protons of benzene ring in LA units to
the total area between 0.93 and 1.56 ppm, which were
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Designed Monomers and Polymers
Figure 6.
1
791
H NMR spectrum of PANLA in DMSO-d6.
attributed to eight protons corresponding to three protons
in LA and five protons in methacrylic acid. The molar
compositions of LA and methacrylic acid were calculated
from below two equations where m and n were the mole
fractions of LA and methacrylic acid, respectively.
m þ n ¼ 100
3m þ 5n 6:557
¼
4m
0:923
m ¼ 10:56% n ¼ 89:44%
The molar compositions of LA and acrylic acid in
copolymer PALA were calculated from the ratiointegrated intensities of the peaks around 5.5–6.1 ppm,
corresponding to four protons of benzene ring in LA
units to the total area between 2.31 and 3.24 ppm, which
were attributed to six protons corresponding to three protons in LA and three protons in acrylic acid. The molar
compositions of LA and acrylic acid were calculated
from below two equations where m and n were the mole
fractions of LA and acrylic acid, respectively.
m þ n ¼ 100
3m þ 3n
3:41
¼
4m
4:012
m ¼ 88:24% n ¼ 11:76%
The same approach, the molar compositions of LA and
acrylonitrile in copolymer PANLA1 were calculated from
the ratio-integrated intensities of the peaks around
7.15–7.3 ppm, corresponding to four protons of benzene
ring in LA units to the total area between 1.4 and
2.31 ppm, which were attributed to six protons corresponding to three protons in LA and three protons in
acrylonitrile. The molar compositions of LA and acrylonitrile were calculated from below two equations where
m and n were the mole fractions of LA and acrylonitrile,
respectively.
792
M. Mahkam et al.
m þ n ¼ 100
The products solubility was checked in eight solvents
inclusive of water, ethyl acetate, toluene, diethyl ether,
methanol, DMF, DMSO, and acetonitrile. The products
solubility results are presented in Table 2. These results
showed all compounds are insoluble in water and soluble
in two solvents DMF and DMSO.
3m þ 3n 22:06
¼
4m
1:97
m ¼ 6:7% n ¼ 93:3%
A similar method was used to calculate the molar compositions of monomers in copolymer PANLA2.
Molecular absorption of colored polymers
Absorption of photons in the ultraviolet/visible range
(UV/vis) is a result of excitation of ground state valence
or bonding electrons into higher energy orbitals. UV/vis
spectrophotometry takes advantage of these electronic
transitions to identify and quantify chemical substances.
Chemical identification is possible by recording a spectrum. The UV/vis/NIR spectral range on the Spectra
m þ n ¼ 100
3m þ 3n 16:09
¼
4m
1
m ¼ 4:66% n ¼ 95:34%
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The compositions of all copolymers are presented in
Table 1.
Table 1.
Molar composition of copolymers.
Copolymer
Molar composition of
monomers in the feed (%)
Calculated from the 1H-NMR (% mole)
m:n
50:50
50:50
9.1:90.9
4.8:95.2
10.56:89.44
88.24:11.76
6.7:93.3
4.66:95.34
PMLA (LA)m (methacrylic acid)n
PALA (LA)m (acrylic acid)n
PANLA1 (LA)m (acrylonitrile)n
PANLA2 (LA)m (acrylonitrile)n
Table 2.
Solubility study of colored compounds.
Compound
Lawsone
LA
HLA
PALA
PMLA
PAN
PANLA1
PANLA2
Figure 7.
H2O
Ethyl acetate
Toluene
Diethyl ether
Methanol
DMF
DMSO
Acetonitrile
–
–
–
–
–
–
–
–
+
+
+
–
–
+
–
–
–
+
+
–
–
–
–
–
+
+
+
–
–
–
–
–
+
+
+
+
+
–
–
–
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
–
–
–
+
+
Coloured compounds: (A) Lawsone, (B) LA, (C) HLA, (D) PALA, (E) PMLA, (F) PAN, (G) PANLA1, (H) PANLA2.
Downloaded by [Dalhousie University] at 00:47 13 November 2014
Designed Monomers and Polymers
Figure 8.
793
Molecular absorption of colored polymers.
extends from 200 to 1000 nm (UV 200–400 nm, VIS
400–650 nm, NIR = near infrared 650–1000 nm). For this
reason, absorbance spectra were obtained from 200 to
700 nm with 2 nm intervals. The all colored compounds
(Figure 7) absorption was measured with 4.9 g/l concentration in DMSO solvent (Figure 8). The commonly
observed transitions are n–π* or π–π*. We saw conjugation causes absorption signatures shift to longer wavelengths because the π–π* transitions are more intense
than n–π* transitions. The wavelengths of maximum
absorption of compounds are showed in Table 3.
Thermal properties of the polymers
The thermal properties of the polymers were summarized
in Table 4, including the initial decomposition temperature of the polymer (IDT), temperature of 50% weight
loss of the polymer (PDT) and the temperature at which
the maximum decomposition rate occurred for the
Table 3.
Molecular absorption study of colored compounds.
Compound
Color
Lawsone
LA
HLA
PALA
PMLA
PAN
PANLA1
PANLA2
Orange
Dark red
Dark orange
Red
Brown
Yellow
Dark brown
Light brown
Maximum transitions (nm)
296-339-416-448
296-337-414-453
310-337-415-451
312-339-409-492
312-339-417-497
296-329
296-337-416-434
296-334-416-449
polymer (PDTmax). Comparison of the polymers shows
that the PANLA copolymers are more stable than other
polymers. Comparing PANLA copolymers with PAN or
PALA and PMLA copolymers with HLA has shown the
homopolymers to the copolymers are stable. All
polymers decompose completely by 900 °C leaving no
residue at 950 °C.
794
Table 4.
M. Mahkam et al.
Thermal properties of colored polymers.
Polymer
IDT(°C)
PDT(°C)
PDTmax(°C)
Tg(°C)
HLA
PALA
PMLA
PAN
PANLA1
PANLA2
139
142
143
147
146
146
591
325
323
720
624
624
863
783
775
912
911
911
77.17
–
70.31
–
–
–
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Conclusions
The colored polymers were synthesized in two steps. In
step 1, LA monomer was prepared by the reaction of
lawsone and acryloyl chloride with good yield. In subsequent step, the LA was polymerized with itself and other
monomers. All products were soluble in DMF and
DMSO. The UV/vis spectrophotometry study has shown
the conjugation causes of absorption signatures shift to
longer wavelengths. Study of the compounds’ thermal
behavior showed that the PANLA copolymers are stable
and suitable for applications as colored materials.
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