Synthesis of highly refractive poly(phenylene thioether) derived from 2,4-dichloro-6-alkylthio-1,3,5-triazines and aromatic dithiols

Article abstract of DOI:10.1021/ma2020003

A highly refractive, transparent, and totally colorless poly(phenylene thioether) (PPT) containing a triazine unit has been developed. A new aromatic dithiol, thianthrene-2,7-dithiol, with a high sulfur content was designed and synthesized in order to dev

Full text of DOI:10.1021/ma2020003

ARTICLE
pubs.acs.org/Macromolecules
Synthesis of Highly Refractive Poly(phenylene thioether) Derived
from 2,4-Dichloro-6-alkylthio-1,3,5-triazines and Aromatic Dithiols
Yu Nakagawa, Yasuo Suzuki, Tomoya Higashihara, Shinji Ando, and Mitsuru Ueda*
Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
S Supporting Information
b
ABSTRACT: A highly refractive, transparent, and totally colorless
poly(phenylene thioether) (PPT) containing a triazine unit has been
developed. A new aromatic dithiol, thianthrene-2,7-dithiol, with a
high sulfur content was designed and synthesized in order to develop
PPTs with high refractive indices and high glass transition tempera-
tures (Tgs). These PPTs were prepared by the phase-transfer
catalyzed polycondensation of 2,4-dichloro-6-alkylthio-1,3,5-
triazines with aromatic dithiols such as 4,40-thiobisbenzenethiol,
benzene-1,3-dithiol, and thianthrene-2,7-dithiol. The PPTs obtained
from the 2,4-dichloro-6-methylthio-1,3,5-triazines and aromatic dithiols showed good thermal stabilities such as relatively high Tgs
over 120 °C and 5% weight-loss temperatures (T5%) around 350 °C. The optical transmittance of the polymer films with a 1 μm
thickness is around 98% at 400 nm. The combination of the triazine and sulfur-containing thianthrene units provides PPTs with high
refractive indices up to 1.7720 at 633 nm and low birefringences (<0.005).
’ INTRODUCTION
oxadiazole substituents, have been developed for optical appli-
cations.19ꢀ21 Although these polymers showed a high thermal
stability and excellent optical transparency with a low birefrin-
gence, the refractive indices of these polymers are still in the
range of 1.66ꢀ1.72 at the sodium-D line (589 nm) because of the
low sulfur content and the presence of sterically bulky substitu-
ents, such as fluorene and sulfone groups, which endow large free
volumes among the polymer chains. To enhance the refractive
index, heteroaromatic rings containing ꢀCdNꢀ bonding in
place of the phenyl rings would be effective because the ꢀCꢀ
NdCꢀ bonds possess a higher molar refraction (4.10) as
compared to the ꢀCdCꢀ bond (1.73).22 On the basis of this
finding, we were interested in triazine dichloride having an
alkylsulfanyl side chain as a monomer, and briefly reported a
highly refractive and transparent poly(phenylene thioether)
(PPT) containing a triazine unit, which was obtained by the
polycondensation of 4,40-thiobisbenzenethiol (TBT) and 2,4-
dichloro-6-methylthio-1,3,5-triazine (DMT). The polymer ex-
hibited a high refractive index of 1.7492 at 633 nm and excellent
transmittance higher than 80% at 400 nm.23 In order to clarify the
potential of the triazine-containing PPTs as optical materials,
2,4-dichloro-6-butylthio-1,3,5-triazine (DBT) was chosen to
improve the solubility of the polymers, and aromatic dithiols,
such as benzene-1,3-dithiol (BDT) and thianthrene-2,7-dithiol
(TDT), were chosen as monomers in addition to DMT and TBT
to increase the sulfur content in the repeating unit.
A high refractive index (n) and low birefringence (Δn)
combined with a good thermal stability and high optical trans-
parency are the basic concerns in designing optical polymers for
camera lens, antireflective coatings, and optical waveguide
systems.1ꢀ6 According to the LorentzꢀLorenz equation, the
introduction of substituents with a high molar refraction and low
molar volume efficiently increases the refractive indices of the
polymers.7 Among the various substituents, the sulfur atom with
a high atomic refraction, is one of the most effective candidates,
and many sulfur-containing polymers, such as epoxy,8 poly-
urethane,9 polymethacrylate,4 and poly(arylene thioether)s,10
have been reported to increase their refractive indices with a
high transparency for optical device applications. However, the
refractive indices of these polymers are relatively low in the range
of 1.50ꢀ1.70 at 589 nm (sodium-D line) or 633 nm.
Recently, we have developed a series of sulfur-containing
aromatic polyimides (PIs) for optical applications because the
PIs have several merits, such as high thermal, oxidative, chemical,
and mechanical stabilities.11ꢀ18 These PIs showed very high
refractive indices in the range of 1.74ꢀ1.77 and a low birefrin-
gence of less than 0.0093, but the optical transparency at around
400 nm of ca.10 μm-thick films is lower than 80% due to the
coloration originating from the charge transfer (CT) interactions
between the electron-donating and electron-accepting moieties
in the PIs. This may cause problems with the optical applications
of the aromatic PIs.
To remedy this problem, well-known high performance
engineering thermoplastics, such as poly(thioether ketone)s
and poly(aryl thioether)s containing fluorene, sulfone, and
Received: September 1, 2011
Revised:
October 23, 2011
Published: November 10, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of TDT
This article reports the synthesis and properties of the triazine-
containing PPTs prepared from 2,4-dichloro-6-alkylthio-1,3,5-
triazines and aromatic dithiols. The obtained PPTs exhibited
high refractive indices in the range of 1.699ꢀ1.772, a low
birefringence of less than 0.005, and a high optical transparency
over 400 nm.
chloroform and poured into hexane and dichloromethane (10:1 vol %).
The precipitate was filtered and dried in vacuum. Yield: 0.220 g (71.5%);
1H NMR (300 MHz, CDCl3, ppm): 7.54, 7.44, 2.78, 1.43, 1.26, 0.84. 13
C
NMR (CDCl3, ppm): 180.8, 180.7, 179.4, 141.5, 136.4, 129.5, 128.1,
30.8, 30.2, 21.8, 13.7. Anal. Calcd for C13H13N3S3 (1 mol % H2O): C,
50.78; H, 4.26; N, 13.67. Found: C, 50.75; H, 4.27; N, 13.66.
PPT-1. 1H NMR (300 MHz, CDCl3, ppm): 7.38, 7.27, 2.21. 13C NMR
(CDCl3, ppm): 180.0, 137.7, 136.3, 131.6, 126.8, 13.77
PPT-2. H NMR (300 MHz, CDCl3, ppm): 7.41, 7.30, 2.77, 1.43, 1.26,
0.86. 13C NMR (CDCl3, ppm): 180.7, 179.6, 137.3, 136.0, 131.2, 126.5,
31.1, 30.1, 21.8, 13.7.
’ EXPERIMENTAL SECTION
Materials. TBT and BDT were purchased from Aldrich Japan.
DMT and DBT were kindly donated by Sankyo Chemical Ltd. 2,7-
Difluorothianthrene (DFT) was prepared according to a reported
procedure.24 Other commercially available reagents and solvents were
used as received.
PPT-3. 1H NMR (300 MHz, CDCl3, ppm): 7.70, 7.56, 7.43, 7.27, 2.22.
13C NMR (CDCl3, ppm): 184.2, 182.5, 181.2, 179.8, 168.7, 148.5, 142.0,
137.6, 136.8, 130.2, 128.4, 14.1.
PPT-5. 1H NMR(300 MHz, CDCl3, ppm):7.49 (2H), 7.21 (4H), 2.73
(2H), 1.35 (2H), 1.15 (2H), 0.76 (3H). 13C NMR (CDCl3, ppm):
179.8, 176.3, 148.8, 148.7, 135.1, 134.9, 129.5, 129.2, 31.2, 31.0,
22.1, 14.0.
Synthesis of 2,7-Dibenzylthiothianthrene (DBTT). A 30 mL
round flask was charged with potassium tert-butoxide (4.85 g, 43.0
mmol) and N,N-dimethylformamide (DMF) (15 mL) under a nitrogen
atmosphere. Benzylmercaptan (5.1 mL, 42 mmol) and DFT (3.98 g,
15.8 mmol) were slowly added into the mixture cooled with an
ice bath. The solution was stirred at 60 °C for 24 h and was poured
into water. The precipitate was filtered and dried in vacuum. The
crude product was recrystallized from 2-methoxyethanol to give 2,7-
dibenzylthiothianthrene (DBTT) as a yellow crystal (65% yield). Mp:
116ꢀ117 °C.
1
PPT-6. H NMR(300 MHz, CDCl3, ppm)): 7.48, 7.22, 7.18, 2.73,
2.22, 1.35, 1.14, 0.75.
13C NMR (CDCl3, ppm): 181.1, 179.8, 160.0, 158.8, 157.6, 148.4,
137.4, 135.9, 135.1, 134.7, 129.2, 127.2, 31.2, 22.8, 22.1, 14.1, 13.9.
Film Preparation. All of the polymer films were prepared by solution
casting in TCE, followed by heating on a hot plate from 25 to 250 °C in
nitogen.
1H NMR (300 MHz, CDCl3, ppm): 4.10 (s, 4H), 7.13 (d, 2H), 7.27
(m, 12H), 7.39 (d, 2H). 13C NMR (CDCl3, ppm): 148.7, 148.6, 137.3,
137.1, 136.8, 133.5, 129.7, 129.3, 129.2, 129.0, 127.8, 39.4.
Anal. Calcd for C26H20S4: C, 67.78; H, 4.38. Found: C, 67.59;
H, 4.53.
Measurements. The NMR spectra were recorded on a BRUKER
DPX-300S spectrometer at the resonant frequencies at 300 MHz for 1H
and at 75 MHz for 13C nuclei using CDCl3 as the solvents. The 13C
DEPT sequence was used to detect and assign protonated carbons. The
FT-IR spectra were obtained by a Horiba FT-120 Fourier transform
spectrophotometer. The ultravioletꢀvisible (UVꢀvis) spectra were
performed on a Hitachi U-3210 spectrophotometer. The optical trans-
mittance of PPT films was evaluated in the wavelength range of
250ꢀ800 nm at 1 μm film thickness. Elemental analyses were carried
out on a Yanaco MT-6 CHN recorder elemental analysis instrument.
Thermogravimetric analysis (TGA) was estimated by a Seiko TG/DTA
6300 under a nitrogen atmosphere at a heating rate of 10 °C/min.
Differencial scanning calorimetry (DSC) was estimated by using a
Synthesis of Thianthrene-2,7-dithiol (TDT). A 50 mL two-
necked round flask was charged with DBTT (1.38 g, 3.0 mmol),
Cp2TiCl2 (0.078 g, 0.31 mmol), and diglyme (10 mL) under a nitrogen
atmosphere at 0 °C. Dibutylmagnesium (Bu2Mg, 20 mL, 2.5 M) was
slowly added, and then the solution was stirred for 3 h. The aqueous
solution of Na2CO3 (20 mL) was slowly added to the reaction mixture at
0 °C. The reaction mixture was extracted with dichloromethane and
water. To the aqueous layer was added hydrochloric acid and the
solution was stirred for 1 h. The precipitate was filtered and dried in
vacuum to give TDT as an ivory-colored powder (0.517 g, 61% yield).
Mp: 139 °C.
Seiko DSC 6300 at a heating rate of 20 °C/min. The in-plane (nTE
)
and out-of-plane (nTM) refractive indices of PPT films were carried
out using a prism coupler (Metricon, model PC-2000) equipped with
a HeꢀNe laser (wavelength: 633 nm) and a half-waveplate in the
light-path. The in-plane/out-of-plane birefringences (Δn) were esti-
1H NMR (300 MHz, CDCl3, ppm): 3.48 (s, 2H), 7.13 (d, 2H), 7. 31
(d, 2H), 7.40 (s, 2H). 13C NMR (CDCl3, ppm): 136.7, 132.3, 131.0,
129.1, 129.0, 128.5. Anal. Calcd for C12H8S2: C, 51.06; H, 2.96. Found:
C, 51.39; H, 2.88.
Synthesis of Poly(phenylene thioether) (PPT): PPT-4. To a
5 mL round flask was charged with BDT (0.145 g, 1.02 mmol) and
2.1 mL of 1 M aqueous sodium hydroxide. CTMAB (30 mol %, 0.220 g)
was added to the solution, and the mixture was stirred for 1 h. To the
mixture was added a solution of BTD (0.243 g, 1.02 mmol) in chloro-
form (2 mL). The two-phase solution was vigorously stirred at room
temperature for 24 h and poured into methanol. The precipitate was
filtered and dried in vacuum. Then, the precipitate was dissolved in
mated as a difference between nTE and nTM, and the average refractive
2
indices were calculated according to the equation: nav = [(2nTE
+
nTM2)/3]1/2
.
DFT Calculation. The wavelength-dependent refractive indices of
models for PPTs were calculated based on the LorentzꢀLorenz theory.
The 6-311G(d) basis set was used for geometry optimizations under no
constraints, and the 6-311++G(d,p) was used for the calculations of
frequency-dependent linear polarizabilities. The three-parameter Becke-
style hybrid functional (B3LYP) was adopted as the functional and all
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Figure 1. 1H NMR spectrum of TDT in CDCl3.
Figure 2. 13C NMR spectrum of TDT in CDCl3.
the calculations were performed using the software package of Gaussian-
09 (Rev.B01). A packing coefficient (Kp) of 0.625 was used for
evaluating the intrinsic molecular volumes of the models. The details
of the calculation method were reported elsewhere.25
Recently, Akao et al. reported a highly effective and mild deprotec-
tion method of the benzyl thioethers using Bu2Mg in the presence
of a catalytic amount of titanocene dichloride.26 Thus, DBTT was
treated with Cp2TiCl2 and Bu2Mg in diglyme at 0 °C. This
reaction smoothly proceeded to give TDT in 65% yield.
The structure of TDT was characterized by FT-IR and NMR
spectroscopy. The IR spectrum of TDT showed the character-
istic absorption at 2534 cmꢀ1 due to the SꢀH stretching
’ RESULTS AND DISCUSSION
As described in the Introduction, the refractive index (n) of a
polymer generally increases with the increasing sulfur content in
a repeating unit. Thus, a new aromatic dithiol, TDT, with the
high sulfur content of 45.7% was designed and synthesized in
order to develop novel PPTs with a high-n and high Tg.
Synthesis of TDT. TDT was synthesized by a two-step pro-
cedure with DFT as the starting material (Scheme 1). In the first
step, DFT was reacted with benzylmercaptan in the presence of
potassium tert-butoxide in DMF to afford DBTT. The deprotec-
tion of the benzyl thioether was then attempted using general
reagents, such as strong acids or bases (alkali metals or stannyl
hydride). All these trials, however, did not produce TDT.
1
of a thiol group (Supporting Information, Figure S1). The H
and 13C NMR spectra of TDT are shown in Figures 1 and 2 along
with assignments of all the signals. The characteristic proton of
the thiol group is observed at 3.47 ppm. In the 13C NMR
spectrum, the six carbon signals, which are consistent with the
expected structure, are observed. The structure of TDT was
further confirmed by elemental analysis, in which the measured C
and H composition agreed well with the calculated values.
Synthesis and Characterization of Triazine-Containing
PPTs. The synthetic route for the triazine-containing PPTs is
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Scheme 2. Synthesis of PPTs
Table 1. Molecular Weights and Thermal Properties of PPTs
Table 2. Solubility of PPTsa
a
b
c
dichlorideꢀdithiol
PPT-1 DMT-TBT
Mw
Mw/Mn Tg
T5%
CHO
CHCl3
TCE
DMPU
37 000
29 000
24 500
27 000
26 000
7300
2.2
4.1
2.2
2.8
5.2
3.7
2.8
127
367
PPT-1
PPT-2
PPT-3
PPT-4
PPT-5
PPT-6
PPT-7
ꢀ
+
++
++
++
++
++
+
++
++
++
++
++
+
++
++
++
++
++
+
PPT-2 DBT-TBT
77.0 335
PPT-3 DMT-BDT
129
377
75.0 342
93.1 327
+
PPT-4 DBT-BDT
++
ꢀ
ꢀ
ꢀ
PPT-5 DBT-TDT
PPT-6 DMT (70%) DBT (30%) -TDT
185
170
343
PPT-7 DMT (50%) DBT (50%) -TDT 16 000
a PSt standard. b Glass transition temperature measured by DSC. 5%
364
+
++
++
c
a Key: (ꢀ) partially dissolved; (+) dissolved on heating; (++) dissolved
weigh loss temperature.
at room temperature.
respectively. Furthermore, PPT-6 and PPT-7 are soluble in
chloroform, TCE, and DMPU upon heating.
shown in Scheme 2. All the triazine-containing PPTs were
prepared by the phase-transfer catalyzed polycondensation of
the 2,4-dichloro-6-akylthio-1,3,5-triazines, DMT and DBT with
aromatic dithiols, such as TBT, BDT, and TDT. The phase-
transfer-catalyzed polycondensation was carried out in a chloro-
form/aqueous alkaline solution system in the presence of
cetyltrimethylammonium bromide (CTMAB) as a quaternary
ammonium salt at room temperature. The results are summar-
ized in Table 1.
The polymerizations of the 2,4-dichloro-6-akylthio-1,3,5-tria-
zines with TBT or BDT afforded the PPTs with the weight-
average molecular weights (Mws) of around 25 000 (PSt
standard). On the other hand, the PPT prepared from DMT
and TDT has a limited molecular weight due to the low solubility
of the resulting PPT. The polycondensation of DBT with TDT
smoothly proceeded to give PPT-5 with the Mw of 26 000,
indicating that a butyl group in DBT effectively to improves
the solubility of the resulting PPT. To obtain PPTs with a higher
sulfur content, PPT-6 and PPT-7 were prepared by the poly-
condensation of TDT with the mixture of DMT and DBT
(7:3 and 1:1 respectively, mol %). The solubilities of the PPTs
are listed in Table 2. The PPTs prepared from TBT and BDT,
except for TDT, are soluble in chloroform, 1,1,2,2-tetrachloro-
ethane (TCE), and N,N0-dimethylpropyleneurea (DMPU) at
room temperature. In particular, PPT-4 and PPT-5 are soluble in
cyclohexanone (CHO) at room temperature and upon heating,
The structures of the PPTs were characterized by 1H and 13
C
NMR and FT-IR spectroscopies. The characteristic IR peaks
were observed at 1473 and 1245 cmꢀ1, which are attributable to
the ꢀCdNꢀ groups in the triazine and thioether units, respec-
tively. In the 1H NMR spectrum of PPT-4 (Figure 3), the signals
that resonated at 7.54, 7.44, 2.78, 1.43, 1.26, and 0.84 ppm are
assignable to the aromatic and butyl protons. Additionally, in the
13C NMR spectrum (Figure 4), 11 carbon signals, which agree
well with the expected structure, are observed. Among these
signals, three quaternary carbon signals are assigned based on
the DEPT-90 13C NMR experiment (Supporting Information,
Figure S2).
Thermal Properties. The thermal stability, i.e., the glass
transition temperatures (Tg) and thermal decomposition tem-
peratures of the high-n polymers, plays a critical role in the optical
device fabrication. The thermal requirement mainly comes from
the consideration of the servicing circumstance of the optical
devices and the thermal-releasing problem caused by the minia-
turization of the opto-integrated assembly.27,28 The thermal
stabilities of the PPTs investigated by TGA and DSC are
summarized in Table 1. As shown in Figure 5 (TG curves for
PPT-2, PPT-4, PPT-5, and PPT-7), PPTs show a good thermal
stability, such as a 5% weight loss temperature (T5%) above
325 °C in a nitrogen atmosphere. Figure 6 shows the DSC
thermograms of the PPTs. The PPTs prepared using DMT show
high Tgs in the range of 127ꢀ185 °C. On the other hand, the
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Figure 3. 1H NMR spectrum of PPT-4 in CDCl3.
Figure 4. 13C NMR spectrum of PPT-4 in CDCl3.
Table 3. Optical Properties of PPTs
S (wt %)a db (μm) nTE nTM
nav
Δnf ncalc λoffh (nm)
c
d
e
g
PPT-1 34.2
PPT-2 30.8
PPT-3 36.3
PPT-4 31.3
PPT-5 36.0
PPT-6 38.4
PPT-7 37.7
5.2 1.7535 1.7486 1.7518 0.0049 1.762
22.3 1.7223 1.7207 1.7222 0.0023 1.733
13.5 1.7391 1.7371 1.7384 0.0020 1.714
33.5 1.6999 1.6972 1.6990 0.0027 1.683
12.5 1.7533 1.7482 1.7516 0.0051 1.749
7.0 1.7724 1.7711 1.7720 0.0013 ꢀ
345
350
330
325
350
345
340
25.8 1.7604 1.7591 1.7600 0.0013 1.762
a Sulfur content. b Film thickness. c The in-plane refractive index. d The
out-of-plane refractive index. e Average refractive index at 633 nm.
f Birefringence. g Calculated refractive index at 633 nm. h Cut-off
wavelength.
Figure 5. TG curves of PPTs.
Figure 7. UVꢀvis spectra of PPTs.
PPTs derived from DBT exhibit lower Tgs, which are derived
from the flexible long alkyl chains.
Optical Properties. The optical properties of the PPT films
are summarized in Table 3. Figure 7 displays the representative
UVꢀvisible transmission spectra of the PPT films (PPT-2, PPT-4,
Figure 6. DSC curves of PPTs.
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(ꢀSꢀ) linkage in diphenylsulfide (0.44 kJ/mol)29 is lower than
that of the freely rotatable ether (ꢀOꢀ) linkage of diphenylether
(0.67 kJ/mol),30 the smaller number (two) of meta-linked ꢀSꢀ
linkages in the main chains of PPT-3 and PPT-4 can generate
more compact aggregation structure compared with PPT-1 and
PPT-2 having two para- and one meta-linked ꢀSꢀ linkages.
Although PPT-5 possesses four ꢀSꢀ linkages, two of them in
the thianthrene unit are not rotatable. Moreover, the molecular
polarizability per unit volume, which corresponds to ‘molar
refraction’, and the sulfur content of thianthrene (0.146, 29.6
wt %) are significantly higher than those of diphenylsulfide
(0.138, 17.2 wt %). As a consequence, the PPT-6 which contains
one thianthrene unit and two rotatable ꢀSꢀ linkages with the
highest sulfur content (38.5 wt %) showed the highest nav of
1.7720. These nav values are much higher than those of the
already reported poly(arylene sulfide)s (nD < 1.72).19ꢀ21,31
Furthermore, the flexible thioether linkages in the PPT chains
endow a birefringence (Δn) lower than 0.005. All these results
indicate that the introduction of the triazine moiety together with
multiple sulfur atoms is an effective way to afford PPTs with a
high refractive index and high transparency in the visible region.
Figure 8. Wavelength dispersion of calculated refractive indices of
PPTs.
’ CONCLUSIONS
A new aromatic dithiol, TDT, with a high sulfur content was
designed and synthesized in order to develop high-n PPTs with a
high thermal stability. The PPTs were prepared by the phase-
transfer-catalyzed polycondensation of 2,4-dichloro-6-alkylthio-
1,3,5-triazines, DMT and DBT with aromatic dithiols, such as
TBT, BDT, and TDT. The PPTs obtained from the DMT and
aromatic dithiols showed good thermal stabilities, such as Tgs
higher than 110 °C and a T5% around 350 °C. All the PPTs
exhibited refractive indices higher than 1.69, in particular, the
PPT-6 derived from TDT showed the highest nav of 1.772 at
633 nm. It can be concluded that the high refractive indices are
caused by high sulfur contents and minimal alkyl chains, while
keeping the solubility of polymers in organic solvents. Moreover,
the optical transmittance of the PPT films with a 1 μm thickness
is around 98% at 400 nm. The synthesis of the TTPs is more
facile than the other monomers reported for sulfur-containing
polymers. Thus, the TTPs are promising candidate polymers
for optical applications, such as optical wave guides for CMOS
image sensor.
Figure 9. Relation between calculated and experimental refractive
indices of PPTs.
PPT-5, and PPT-7), in which the absorbance was normalized to a 1
μm thickness. All films exhibit the cutoff wavelengths (λoff) of
325ꢀ350 nm, and the values of the optical transmittance are as high
as 98% at 400 nm. The excellent colorlessness is probably due to the
meta-substituted triazine and thioalkyl groups in the triazine unit,
which efficiently suppresses the localization of the π-electrons and
CT interactions in PPTs in addition to the prevention of interchain
packing. These results clearly indicate that the optical transparency is
not reduced by the introduction of a triazine unit into the polymer
chains. Moreover, the colorlessness of the films maintained after
thermal treatment up to 250 °C.
The in-plane (nTE) and out-of-plane (nTM) refractive indices
of the PPT films range from 1.6999ꢀ1.7724 and 1.6972ꢀ
1.7711, respectively. The facts that the values of nTE are slightly
higher than those of nTM for all the PPT films reflect the
preferential orientation of the main chains parallel to the film
plane. The average refractive indices (nav) range between 1.699
and 1.772. Figure 8 shows the wavelength-dependent calculated
refractive indices (ncalc) of monomeric models for PPTs, and
Figure 9 shows the relation between the calculated and refractive
indices at 633 nm. As shown in Figure 9 and Table 3, the DFT
calculations well reproduce the experimental trend of nav, in
which the refractive indices increase with increasing the sulfur
content. In particular, the replacement of the ꢀCH3 side chain
with ꢀC4H9 lowers the refractive indices of PPT-1 and PPT-3 to
those of PPT-2 and PPT-4, respectively, by ca. 0.03. The notably
higher experimental nav of PPT-3 and PPT-4 compared to their
ncalc values could be due to the dense molecular packing of these
polymers. Since the rotational barrier around the thioether
’ ASSOCIATED CONTENT
S
Supporting Information. IR spectra of DBTT and TDT
b
and NMR spectra of the polymers, This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Telephone: + 81-3-5734-2127. Fax: +81-3-5734-2127. E-mail:
ueda.m.ad@polymer.titech.ac.jp.
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