Highly refractive polymer resin derived from sulfur-containing aromatic acrylate

Article abstract of DOI:10.1002/pola.24040

New sulfur-containing polymers with high-refractive indices and low birefringences have been developed as UVcurable high-refractive polymer resins. The polymers derived from 2,7-bis[(2-acryloylethyl)sulfanyl]thianthrene (2,7-BAST) and 4,4'-bis[(acryloylox

Full text of DOI:10.1002/pola.24040

Highly Refractive Polymer Resin Derived from Sulfur-Containing
Aromatic Acrylate
NAM-HO YOU, TOMOYA HIGASHIHARA, SHINJI ANDO, MITSURU UEDA
Department of Organic and Polymeric Materials, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1-H-120, O-okayama, Meguro-Ku, Tokyo 152-8552, Japan
Received 24 February 2010; accepted 14 March 2010
DOI: 10.1002/pola.24040
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: New sulfur-containing polymers with high-refractive
indices and low birefringences have been developed as UV-
curable high-refractive polymer resins. The polymers derived
from 2,7-bis[(2-acryloylethyl)sulfanyl]thianthrene (2,7-BAST)
and 4,40-bis[(acryloyloxyethylthio)diphenylsulfide (4,40-BADS)
were prepared by photopolymerization under UV irradiation.
Transparent UV-cured films were obtained in both cases. Both
polymers showed good thermal stability, such as a 5% weight-
loss temperature at 355 ꢀC under nitrogen and glass transition
temperatures (Tg) in the range of 94–143 ꢀC. They also showed
high-refractive indices of 1.6531 and 1.6645 at 632.8 nm and
low birefringences of 0.0039 and 0.0069 in addition to high
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transparency in the visible region.
2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 48: 2604–2609, 2010
KEYWORDS: optics; resins; synthesis
INTRODUCTION In recent years, much attention has been
focused on high-refractive index materials for their practical
application, such as lenticular lenses, arrays of prisms,
graded-index optical cables, antireflective (AR) coatings, and
telecommunication systems.1–5 The high-refractive index,
combined with low birefringence and high-optical transpar-
ency in the visible region, are the most mainly important fac-
tors for these applications. The general approach to increase
the refractive indices of polymers is the introduction of sub-
stituents with high-molar refraction, low-molar volume, or
high density, according to the Lorentz–Lorenz equation.6
Among the various substituents, the sulfur (S) atom is one of
the most effective candidates because of its high-atomic
refraction. Recently, sulfur-containing polymers with thio-
ether linkages have been developed for optical applica-
tions.7–15 Although all of the polymers showed high-refrac-
tive indices, high transparency and low birefringence, the
polymer films were prepared by thermal conversion at high
temperatures for a long time and solution processes using
organic solvents were necessary, which led to deep colors
and strong odors. Moreover, long production cycles owing to
the prolonged curing times were required for maintaining
optical homogeneity.
pollution. As one of the important requirements, photocura-
ble materials should be in the liquid state at room
temperature.16
Recently, UV-curable high-refractive polymer resins have
been developed for optical application in both academia and
industry.17–21 Bis(acrylate or methacrylate) monomers,
which can produce crosslinking polymers under UV irradia-
tion, are one of the most important classes of advanced opti-
cal materials possessing various outstanding properties, such
as excellent optical clarity and rapid curing time via photo-
induced radical polymerization.22 Moreover, the photocuring
process can lead to an the improvement in the refractive
index because of an increase in molecular polarizability orig-
inating from volume shrinkage and molecular orientation.23
Quite recently, Khudyakov et al. developed sulfur-containing
UV-curable acrylate resins using several kinds of photoinitia-
tors for manufacturing highly refractive coatings. Although
the resulting acrylate resins show relatively high-refractive
indices ranging from 1.521–1.620, these values are not suffi-
cient for real applications.4
In this study, we report the synthesis of 2,7-bis[(2-acryloyle-
thyl)sulfanyl]thianthrene (2,7-BAST) and 4,40-bis[(acryloyloxy-
ethylthio)diphenylsulfide (4,40-BADS) as a new class of photo-
curable acrylate monomer. In particular, the acrylate resin
derived from 2,7-BAST exhibited a very high-refractive index
(1.6645), which is one of the highest refractive photocurable
polymer resins developed so far, with a high-glass transition
temperature (>143 ꢀC), high transparency in the visible
There are several methods for preparing optical plastics,
such as photocuring, injection molding, compression mold-
ing, and solution casting. Among these processes, the photo-
curing process is one of the most important techniques
because of the absence of a solvent, low demand on equip-
ment, low consumption of energy, and low-environmental
Correspondence to: M. Ueda (E-mail: ueda.m.ad@m.titech.ac.jp)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2604–2609 (2010) 2010 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of 2,7-BAST and 4,40-BADS.
region, and low birefringence (0.0069). The structure-prop-
erty relationships of the polymers, such as the thermal and
optical properties, and the refractive indices of the UV-cured
resins are investigated.
Synthesis of 2,7-Bis[(2-acryloylethyl)sulfanyl]thianthrene
To a solution of 2,7-bis[(2-hydroxyethyl)sulfanyl]thianthrene
(2,7-BHST) (1.84 g, 5.0 mmol) in dry THF (30 mL), N,N-
dimethylaniline (1.82 g, 15.0 mmol) was added under N2
atmosphere. Then, the solution was cooled at 0 ꢀC, and the
solution of acryloyl chloride (1.81 g, 20.0 mmol) in dry THF
(10 mL) was added dropwise. After the solution was stirred
for 24 h at room temperature, the reaction was quenched
with saturated aqueous NaHCO3 solution. The mixture was
diluted with distilled water and extracted with chloroform.
The organic layer was dried over anhydrous MgSO4, filtered,
and concentrated. The crude product was purified by column
chromatography (eluent, dichloromethane/hexane ¼ 1/1) to
EXPERIMENTAL
Materials
Mercaptoethanol and acryloyl chloride were purchased from
TCI, Japan. N,N-Dimethylformamide (DMF), potassium t-but-
oxide, and N,N-dimethylaniline were obtained from Wako,
Japan and used as received. Diphenyl (2,4,6-trimethylben-
zoyl)phosphineoxide (BASF) as a photoinitiator was obtained
from JSR (Japan Synthetic Rubber) corporation. 2,7-Difluoro-
thianthrene (2,7-DFT) and 4,40-bis(hydroxyethylthiophenyl)-
sulfide (4,40-BHTS) were prepared according to the lite-
ratures.23,24
1
give a colorless oil. Yield: 1.56 g (66 %); H NMR (300 MHz,
DMSO-d6, ppm): d ¼ 7.59 (s, 2H), 7.48 (d, 2H), 7.35 (d, 2H),
6.25 (d, 2H), 6.08 (m, 2H), 5.87 (d, 2H), 4.28 (t, 4H), 3.32 (t,
4H). 13C NMR (300 MHz, CDCl3/TMS, ppm): d ¼ 166.2,
136.9, 135.9, 133.8, 131.8, 129.9, 129.5, 129.4, 128.3, 63.3,
and 32.8. Anal. Calcd. for C22H20O4S4 (476.65): C, 55.44; H,
4.21. Found: C, 55.65; H, 4.23.
Synthesis of 2,7-Bis[(2-hydroxyethyl)sulfanyl]thianthrene
Mercaptoethanol (4.25 g, 54.5 mmol), potassium t-butoxide
(6.12 g, 54.5 mmol), DMF (50 mL) were placed in a 200 mL
three-necked flask equipped with a magnetic stirrer, a nitro-
gen inlet, and a reflux condenser. After the solution was
stirred for 1 h at room temperature, 2,7-DFT (5.5 g, 21.8
mmol) was added. The mixture was heated to 120 ꢀC and
stirred for 12 h then the reaction mixture was poured into
water. The white precipitate was filtrated and washed with
water. The white solid was purified by recrystallization from
toluene to give white crystals. Yield: 6.0 g (75 %); m.p. 103
ꢀC (DSC peak temperature). 1H NMR (300 MHz, DMSO-d6,
ppm): d ¼ 7.40 (s, 2H), 7.36 (d, 2H), 7.17 (d, 2H), 3.44 (m,
4H), 2.96 (t, 4H). 13C NMR (300 MHz, DMSO-d6/TMS, ppm):
d ¼ 138.2, 136.6, 131.6, 129.8, 128.2, 127.9, 60.5, 35.6. Anal.
Calcd. for C16H10O2S4 (368.56): C, 52.14; H, 4.38. Found: C,
51.77; H, 4.27.
Synthesis of 4,40-Bis[(acryloyloxyethylthio)diphenyl]
sulfide (4,40-BADS)
This compound was synthesized by a similar procedure
according to synthesis of 2,7-BAST. Yield: (66 %); 1H NMR
(300 MHz, DMSO-d6, ppm): d ¼ 7.26 (d, 4H), 7.14 (d, 4H),
6.15 (d, 2H), 5.99 (m, 2H), 5.81 (d, 2H), 4.15 (t, 4H), 3.16 (t,
4H). 13C NMR (300 MHz, DMSO-d6/TMS, ppm): d ¼ 166.1,
135.9, 132.9, 132.8, 132.2, 130.0, 128.8, 63.4, 31.6. Anal.
Calcd. for C22H22O4S3 (446.60): C, 59.17; H, 4.97. Found: C,
59.13; H, 4.89.
Preparation of UV-Cured Films
The acrylate monomers containing 1.5 wt % photoinintiator
were stirred at 60 ꢀC for 20 min to obtain viscous liquids.
To control the film thickness, the acrylate solutions were
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FIGURE 1 NMR spectra of 2,7-BAST (a) 1H NMR; (b) 13C NMR
FIGURE 2 NMR spectra of 4,40-BADS (a) 1H NMR; (b) 13C NMR
without/with DEPT-135 pulse sequence.
without/with DEPT-135 pulse sequence.
rate of 10 ꢀC/min. Differencial scanning calorimetry (DSC)
was estimated by using a Seiko DSC 6300 at a heating rate
of 10 ꢀC/min. The in-plane (nTE) and out-of-plane (nTM) re-
fractive indices of polymer films were measured with a
prism coupler (Metricon, model PC-2000) equipped with a
half-waveplate in the light-path and a He-Ne laser light
source (wavelength: 632.8 nm). The in-plane (nTE)/out-of-
plane (nTM) and birefringence (Dn) were calculated as a dif-
ference between nTE and nTM. The average refractive indices
were calculated according to equation:
spin-coated on silicon wafers and fused silica substrates. As
shown in Scheme 2, the UV-cured films were prepared by UV
irradiation for 30 min and then the films were dried in a
vacuum. High-pressure mercury lamp (Miksa mercury lamp
Model M-1S) was used as a light source.
Measurements
NMR spectra were recorded on a BRUKER DPX-300S spec-
1
trometer at the resonant frequencies at 300 MHz for H and
at 75 MHz for 13C nuclei, using CDCl3 or DMSO-d6 as a sol-
vent. The FTIR spectra were measured by a Horiba FT-120
Fourier transform spectrophotometer. Ultraviolet–visible
(UV–vis) spectra were recorded on a Hitachi U-3210 spectro-
photometer in the wavelengths range of 250–800 nm. Ele-
mental analyses were performed on a Yanaco MT-6 CHN re-
corder elemental analysis instrument. Thermogravimetric
analysis (TGA) of the acrylate resin was performed on a
Seiko TG/DTA 6300 under nitrogen atmosphere at a heating
1=2
nAV ¼ ½ð2n2TE þ nT2MÞ=3ꢁ
:
RESULTS AND DISCUSSION
Synthesis of Monomers
Sulfur-containing aromatic acrylates were prepared by three-
and two-step procedures, as shown in Scheme 1. First,
SCHEME 2 Synthesis of UV-cured aromatic polymer resins.
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TABLE 1 Thermal Properties of 3SMEA and APMEA Films
a
b
c
Polymer
T5% (ꢀC)
T10% (ꢀC)
Tg (ꢀC)
3SMEA
APMEA
a
355
355
365
365
94
143
5% weight loss temperature.
10% weight loss temperature.
Glass transition temperature.
b
c
lated values. These results clearly show the formation of 2,7-
BHST and 4,40-BADS.
UV-curing Behavior of Acrylate Monomers
Scheme 2 shows the preparation of UV-cured aromatic poly-
mer resins. The relative conversion percentage of unsatu-
rated bonds during UV-curing process was measured by
FTIR spectroscopy using the stretching vibration of the acry-
late carbon-carbon double bond at 1635 cmꢂ1, as shown in
Figure 3. The IR peaks were normalized with the carbonyl
peak at 1725 cmꢂ1 as the internal standard.20 The unsatu-
rated bond content (A0) of an uncured sample containing a
photoinitiator was defined as 100%. If the sample is irradi-
ated for time t, the unsaturated bond content can be defined
as At, and the conversion of the unsaturated bond can be
calculated as follows:
FIGURE 3 Changes of FTIR spectra of 2,7-BAST monomer on
silicon wafer under UV irradiation.
intermediate 2,7-DFT was prepared according to a reported
procedure and purified by sublimation under reduced pres-
sure.19 Second, 2,7-DFT was reacted with mercaptoethanol
to afford 2,7-BHST with a yield of 75%. Finally, 2,7-BAST
was synthesized by an acylation reaction of 2,7-BHST with
acryloyl chloride. Then, 4,40-BADS was prepared by a similar
procedure, according to the literature.18 The structures of
the monomers were confirmed by IR and NMR spectroscopy.
For example, the IR spectrum of 2,7-BHST shows characteris-
tic absorptions of carbonyl groups and acylate carbon-carbon
double bonds at 1635 cmꢂ1. The 1H- and 13C NMR spectra
of 2,7-BHST and 4,40-BADS are shown in Figure 1 and 2 with
the assignments of all peaks. For example, the 1H NMR sig-
nals at 6.16, 5.99, and 5.81 ppm in Figure 2(a) are assigned
to the vinyl groups of acrylate. On the other hand, aromatic
protons are observed at 7.26 and 7.13 ppm. In the 13C NMR
spectrum [Fig. 2(b)], nine carbon signals, which are consist-
ent with the expected structure, are observed. Among these
peaks, six quaternary carbon signals are assigned by DEPT-
135 measurement. In addition, the measured elemental com-
positions of the C and H elements agree well with the calcu-
Conversion ð%Þ ¼ ½ðA0 ꢂ AtÞ=A0ꢁ ꢃ 100
Figure 3 shows the changes in the FTIR spectra of the 2,7-
BAST monomer on a silicon wafer upon UV irradiation. The
IR absorption peak of the C¼¼C bond at 1635 cmꢂ1 decreases
by extending the irradiation time. Figure 4 shows the con-
version of the monomer to a polymer measured by FTIR
spectroscopy, and the conversions of 2,7-BAST and 4,40-BADS
reach 85 and 70%, respectively, after 60 s.
Thermal Properties
The thermal properties of the polymers were evaluated by
TGA and DSC measurements under a nitrogen atmosphere.
The results are summarized in Table 1. As shown in Figure
5, both polymers show good thermal stability with a 5%
weight-loss temperature (T5%
)
at 355 ꢀC. The glass
FIGURE 4 Conversion of 2,7-BAST monomer to polymer versus
FIGURE 5 TGA curves of 3SMEA and APMEA films (under
nitrogen atmosphere, 10 ꢀC/min).
irradiation time.
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TABLE 2 Optical Properties of 3SMEA and APMEA Films
Refractive Indices and Birefringences
at 632.8 nmb
Monomer
da (lm)
nTE
nTM
nav
Dn
3SMEA
APMEA
a
25
11
1.6544
1.6669
1.6504
1.6599
1.6531
1.6645
0.0039
0.0069
Film thickness for refractive index measurement.
Measured at 632.8 nm, see Measurements.
b
Table 2. All resulting polymers exhibit significantly high-re-
fractive indices compared with conventional UV-curable ther-
moset polymers.4,18 The nD values of 3SMEA and APMEA are
1.6531 and 1.6645, respectively. Especially, the APMEA-con-
taining thianthrene unit exhibits a higher refractive index
than 3SMEA because of two factors. One is that APMEA has
a higher sulfur content (31.9 wt %) than 3SMEA (21.3
wt %). The other is the higher molecular packing of APMEA
compared with that of 3SMEA because of the rigid thian-
threne unit. These two factors could be the reasons for the
high nD value of APMEA. Moreover, the polymers show low
birefringence (Dn) in the range of 0.0039–0.0069, which is
closely related to the highly flexible alkyl chains and rotata-
ble Ph-S-Ph linkages in the main chains. These results indi-
cate that a thianthrene unit, which has a high-sulfur content
and a bent structure, is effective to improve the refractive
indices of the resulting polymer while maintaining low bire-
fringences and high-optical transparency.
FIGURE 6 DSC curves of 3SMEA and APMEA films (under
nitrogen atmosphere, 10 ꢀC/min).
transition temperature (Tg) of the polymers, which is one of
the most important key parameters for optical device fabri-
cation, was determined by DSC, as shown in Figure 6. Both
of the cured acrylate resins show Tgs in the range of 94–
ꢀ
143 C measured by DSC. The APMEA-containing thianthrene
group shows a higher Tg than 3SMEA. The significantly
higher Tg of the former is mainly attributable to the rigid thi-
anthrene unit.
Optical Properties
Figure 7 shows the UV–vis absorption spectra of the APMEA
and 3SMEA films with a thickness of about 10 lm. Both
polymers show high transparency in the visible region (k ¼
400–800 nm) with transmittance over 80% at 400 nm, indi-
cating that the introduction of a rigid thianthrene unit does
not deteriorate the optical transparency of the films. The
bent structure of the thianthrene ring and thioether unit
may suppress packing between the polymer chains, which is
required for high transparency and low birefringence. The
polarization-dependent refractive indices and in-plane/out-
of-plane birefringences (Dn) for the polymers are listed in
CONCLUSIONS
New UV-curable sulfur-containing aromatic acrylates, such as
2,7-BAST and 4,40-BADS, were synthesized for optical appli-
cations. The acrylate monomers were polymerized rapidly
under UV irradiation. The UV-cured 4,40-BADS and 2,7-BAST
polymer films showed high-refractive indices and low bire-
fringences, whereas maintaining high-optical transparency in
the visible region. At the same time, both of the polymers
show high glass transition temperatures (Tg). In particular,
the APMEA with a high-sulfur content in the molecular
chains (31.9%), showed a very high-refractive index (1.6645)
with a low birefringence of 0.0069, which is one of the high-
est refractive photocurable polymer resins thus far
developed.
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