Synthesis and chiroptical properties of helical poly(phenylacetylene) bearing optically active chiral oxazoline Pendants

Article abstract of DOI:10.1016/j.polymer.2012.04.026

The novel optically active (S)-4-benzyl-2-(ethynylphenyl)-oxazoline (BnEPhOx) was successfully prepared and polymerized using rhodium catalyst ([Rh(nbd)Cl]₂) to obtain the moderate molecular weight poly(phenylacetylene)s bearing chiral oxazolin

Full text of DOI:10.1016/j.polymer.2012.04.026

Polymer 53 (2012) 2567e2573
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and chiroptical properties of helical poly(phenylacetylene) bearing
optically active chiral oxazoline Pendants
*
Poompat Rattanatraicharoen, Keiko Shintaku, Kazuhiro Yamabuki, Tsutomu Oishi, Kenjiro Onimura
Department of Applied Chemistry, Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755 8611, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The novel optically active (S)-4-benzyl-2-(ethynylphenyl)-oxazoline (BnEPhOx) was successfully
prepared and polymerized using rhodium catalyst ([Rh(nbd)Cl]2) to obtain the moderate molecular
weight poly(phenylacetylene)s bearing chiral oxazoline derivatives with high yields (ꢀ90%). The 1H NMR
spectra demonstrated that the resulting polymers had high stereoregular structures. Moreover, the
poly(phenylacetylene)s bearing chiral oxazoline exhibited better thermal stability than poly(-
phenylacetylene). The resulting polymers showed higher absolute values of optical specific rotation than
Received 17 January 2012
Received in revised form
6 April 2012
Accepted 16 April 2012
Available online 21 April 2012
the monomer. The polymers also exhibited intense CD signal in the region of the
p-p
* band of the
Keywords:
Polyacetylene
Conjugated polymer
Optically active polymer
conjugated polyacetylene backbone in chloroform solution. The results of specific rotation and CD
spectroscopy indicated that all the polymers adopted higher-order structure with predominantly one-
handed screw sense.In addition, the resulting polymers emitted fluorescence under UV irradiation.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
backbone, which exhibits simultaneous changes in both optical and
chiroptical properties [18]. The helical polyacetylenes are inter-
In recent years, the design and synthesis of artificial helical
polymers have been extensively studied in the area of macromo-
lecular science. This is not only to mimic the nature materials, but
also to produce the sophisticated functions based on the regulated
structure which is similar to the biological polymers such as DNAs
and proteins [1e3]. The progress in macromolecular science allows
us to synthesize the artificial helical polymers with rather complex
and elegant structures, such as polymethacrylates, polyisocyanides
and polyisocyanates [4]. The helical polymers have been used in
a wide range of applications, for example, catalysis for asymmetric
syntheses, enantiomeric separation, and chiral sensing [5e8]. Apart
from the helical polymers, the conjugated polymers have also
attracted a large number of attention over the past decades due to
their interesting properties and they become a candidate for the
next generation active materials for organic-based devices such as
chemical and biological sensors, or polymeric light-emitting diodes
[9e11]. Polyacetylenes, one of the typical conjugated polymers,
have been extensively studied for several decades by several
esting not only in the fundamental viewpoints regarding of
synthesis and their properties, but also in their practical applica-
tions, for example, semi-conductivity, nonlinear optical properties
and chiral recognition. These properties arise from their regulated
secondary structure [19,20].
Chiral oxazolines are of great interest in scientific research and
technological innovation because they can be easily obtained from
amino alcohols, and have unique structures and significant appli-
cations. The chiral oxazolineemetal complexes were shown to be
very efficient catalysts for many enantioselective organic reactions
including aldol, Diels-Alder, allylicalkylation, aziridination, cyclo-
propanation reactions and polymerizations [21]. The advantage of
the polymer-supported catalysts is that they can be easily extracted
from those reactions by simple methods such as solvent precipita-
tion [22,23]. We have previously reported a series of polymerizations
of acetylene [24] and diidobenzene [25] bearing chiral oxazoline
group. The polymers adopted the helical structures in a solution,
evidenced by their unique induced circular dichroism (ICD) in the
p-
research groups [12e17]. Furthermore, the
p
-conjugated poly-
p
* transition regions of the polymer backbones. Interestingly, the
acetylenes are known to form the helical structure with
a predominantly one-handed screw sense by the introduction of
the appropriate chiral groups substituted into the polymer
Cotton effects could be induced upon the complexation of chiral
oxazoline side chain with metal ions, resulting in the formation of
chiral supramolecular aggregations. The resulting polymer-metal ion
complexes were proposed to be used as the novel chiral polymeric
catalysts in the asymmetric reactions.
We found in the previous study that polymerizability of acetylene
bearing chiral oxazoline group (BnEOx) was low. This was due to the
* Corresponding author. Tel.: þ81 836 85 9283; fax: þ81 836 85 9201.
E-mail address: onimura@yamaguchi-u.ac.jp (K. Onimura).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.04.026

2568
P. Rattanatraicharoen et al. / Polymer 53 (2012) 2567e2573
lower electron density of the acetylenic moiety by oxazoline side
groups, which is unfavorable to the coordination with the rhodium
catalyst. We expected that an introduction of a phenyl moiety into
the BnEOx would increase the polymerizability, lead to obtain the
high molecular weight polymers with high yield. In the continuation
of our study described herein, we synthesized and polymerize
a novel phenylacetylenes substituted with the chiral oxazoline
derivatives (BnEPhOx) using rhodium/triethyl amine (Et3N) catalyst
to obtain optically active helical poly(phenylacetylene)s bearing
chiral oxazoline in a side chain (Scheme 1). The obtained functional
polymers formed the helical structure with predominantly one-
handedness screw sense in a solution. Their properties were char-
acterized by spectroscopic and thermal gravimetric methods.
(DMAP) were purchased from Wako Pure Chemical Industries, Ltd.
Bis(triphenylphosphine)palladium(II) dichloride was obtained
from Tokyo Chemical Industry Co., Ltd. Triethylamine (Et3N), p-
toluenesulfonyl chloride (TsCl), ammonium chloride and anhy-
drous magnesium sulfate were received from NACALAI TESQUE,
INC. Sodium sulfate was purchased from Kanto Chemical Co. Ltd.
(Bicyclo [2.2.1]hepta-2,5-diene)chloro rhodium (I) dimer ([Rh(nbd)
Cl]2) and CeliteÒ 545 were purchased from SigmaeAldrich Japan.
Trimethylsilylacetylene was obtained from Junsei Chemical Co. Ltd.
(S)-phenylalaninol was synthesized according to the literature [26].
n-Hexane, ethyl acetate, tetrahydrofuran (THF), toluene, N,N’-
dimethylformamide (DMF), dimethyl sulfoxide (DMSO), 1,2-
dichloroethane, CH2Cl2 and CHCl3 were dried according to the
standard procedure and distilled under nitrogen atmosphere.
Analytical thin-layer chromatography (TLC) was performed on
Merck silica gel plate 60F254. Column chromatography was per-
formed with silica gel 60 (0.063e0.200, MERCK).
2. Experimental section
2.1. Measurements
1H and 13C nuclear magnetic resonance (NMR) spectra were
recorded on a JNM-LA500 apparatus (JEOL, Ltd.) spectrometer
3.1. Monomer synthesis
using tetramethylsilane (TMS) (1H NMR, 0.00) or CDCl3 (13C NMR,
d
The typical synthesis procedure of the novel (S)-2-(3,5-
diiodophenyl)-oxazoline (BnEPhOx) was given according to the
strategy outline in Scheme 2.
d
77.0) as internal reference peaks at room temperature. Infrared
(IR) spectra were obtained on a FT-IR Jasco 4100 (JASCO Corpora-
tion) spectrophotometer. The number- and weight-average
molecular weight (Mn and Mw) of polymers were determined by
gel permeation chromatographic (GPC) on a LC-10AS and CHRO-
MATOPAC C-R7A plus (Shimadzu Corporation) using HSG-40H,
HSG-20H, HSG-15H and HSG-10H columns (THF as an eluent at
flow rate of 1.0 mL/min, 50 ꢁC) equipped with an ultraviolet (UV)
detector SPD-10A (Shimadzu Corporation). Polystyrene samples
were used as standards. Specific rotations were measured at
concentration of 0.1e1.0 g/dL in CHCl3 at 25 ꢁC using a quartz cell
(1.0 cm) with a JASCO DIP-1030 (JASCO Co., Ltd.). Circular dichroism
(CD) spectra were measured at concentration of 0.01e0.10 g/dL in
CHCl3 at 25 ꢁC using a quartz cell of 1.0 mm with a JASCO J-805
(JASCO Co., Ltd.). Melting points (m.p.) were measured on a YANA-
GIMOTO micro melting-point apparatus. Thermogravimetric anal-
yses were carried out using an MS-Tg/DTA220 (JEOL) at a scanning
rate of 10 ꢁC minꢂ1 under nitrogen (100 mL minꢂ1). The UVevisible
spectra and photoluminescence were obtained from a Shimadzu
UV-1650 PC spectrophotometer and a Jasco FP-6300 spectro-
fluorophotometer, respectively. Elemental analysis (JMA 10 YA:
J-SCIENCE LAB CO., Ltd) was carried out at Collaborative Center for
Engineering Reserch Equipment, Faculty of Engineering, Yamaguchi
University.
[1] Preparation of N-((S)-1-Benzyl-2-hydroxyethyl)-4-iodobenz-
amide (3)
To a stirred solution of (S)-phenylalaniol (2) (0.85 g, 5.64 mmol)
and triethylamine (0.95 mL, 6.77 mmol) in THF (10 mL), a solution of
4-iodobenzoyl chloride (1) (1.51 g, 5.64 mmol) in THF (15 mL) was
added dropwise at 0 ꢁC. The reaction mixture was stirred for 3 h.
After completion of the reaction, the NaHCO3(aq) (15 mL) was added,
and then extracted with CH2Cl2. The organic layers were washed
twice with NaCl(aq), dried over sodium sulfate (Na2SO4), and fil-
trated. The filtrate was concentrated on a rotary evaporator to give
white solid (1.96 g, 5.13 mmol) of hydroxyamide (3) with 91% yield.
The product was used for the next reaction without any purification.
1H NMR (CDCl3)
d (ppm from TMS): 7.76 (d, 2H, aromatics), 7.37
(d, 2H, aromatics), 7.24e7.31 (m, 5H, aromatics), 6.22 (d, 1H, CO-
HN), 4.35(m, 1H, CH), 3.70-3.81 (m, 2H, CH2-OH), 2.98 (d, 2H, CH2-
Ph). 13C NMR (CDCl3)
d (ppm from CDCl3): 167.6, 137.8, 137.3, 129.2,
128.6, 128.4, 98.3, 64.2, 53.1, and 36.8.
[2] preparation of (S)-4-Benzyl-2-(4-iodophenyl)oxazoline (4)
To a stirred solution of hydroxyamide (3) (1.96 g, 5.13 mmol),
triethylamine (2.80 g, 27.69 mmol) and DMAP (18.8 mg, 0.15 mmol)
in 1,2-dichloroethane (25 mL), toluenesulfonyl chloride (TsCl)
(1.47 g, 7.69 mmol) was added. The reaction mixture was refluxed
for overnight. The mixture was washed twice with NaCl(aq), dried
over MgSO4 and the solvent was removed in vacao. The crude
product was purified by column chromatography eluted with
hexane/ethyl acetate (6/1, v/v) to give yellowish oil 4 (1.67 g,
4.59 mmol) with 90% yield.
3. Materials
All reagents were used as received without any further purifi-
cation. L-Phenylalanine, cupper(I) iodide, 4-iodobenzoyl chloride,
sodium hydrogencarbonate and N,N’-dimethylaminopyridine
n
1H NMR (CDCl3)
d (ppm from TMS): 7.79 (d, 2H, aromatics), 7.70
[Rh(nbd)Cl]2, Et3N
(d, 2H, aromatics), 7.22e7.46 (m, 5H, aromatics), 4.58 (m,1H, CH-N),
4.36 (t, 1H, CH2-O), 4.12 (t, 1H, CH2-O), 3.20 (dd, 1H, CH2-Ph), 2.74
O
anhydrous solvent
N
(dd, 1H, CH2-Ph). 13C NMR (CDCl3)
d (ppm from CDCl3): 163.5, 137.9,
O
50 ºC, 24 h
N
137.4, 129.7, 129.2, 128.5, 127.2, 126.5, 98.0, 72.1, 67.8, and 41.7.
Ph
BnEPhOx
Ph
[3] Preparation of (S)-4-Benzyl-2-(4-trimethylsilylethynylphenyl)
oxazoline (5)
poly(BnEPhOx)
To a solution of (S)-4-benzyl-2-(4-iodophenyl)oxazoline (4)
Scheme 1. Polymerization of phenylacetylene derivatives bearing chiral oxazoline
group.
(1.67 g, 4.59 mmol), bis(triphenylphosphine)palladium dichloride

P. Rattanatraicharoen et al. / Polymer 53 (2012) 2567e2573
2569
Ph
O
HN
(3)
Et3N
I
COCl
+
I
Ph
OH
H2N
THF
Ph
OH
(1)
(2)
TMSA, PdCl2(PPh3)2, CuI, Et3N
THF
TsCl, DMAP, Et3N
C2H4Cl2
O
N
I
(4)
O
N
O
K2CO3
MeOH, r.t., 1 h
TMS
H
Ph
Ph
N
(5)
BnEPhOx
Scheme 2. Synthetic route of monomer.
(0.16 g, 0.23 mmol), cupper(I) iodide (43.26 mg, 0.23 mmol) in THF
(15 mL) and Et3N (0.77 mL, 5.51 mmol) was added trimethylsily-
lacetylene (0.78 mL, 5.51 mmol). The reaction mixture was stirred
under nitrogen atmosphere at 50 ꢁC for overnight. After that, the
mixture was filtrated, diluted with ethyl acetate, washed by satu-
rated NH4Cl(aq) and NaCl(aq) and dried over MgSO4. The filtrate was
concentrated on a rotary evaporator to give 5 (1.42 g, 4.26 mmol) of
yellowish crude oil with 93% yield. The crude product was used for
the next reaction without any purification.
atmosphere according to Scheme 1. A typical experimental proce-
dure for polymerization of BnEPhOx is given as follow;
The BnEPhOx (0.30 g, 1.15 mmol) was placed in a dry schlenk
glass tube, which was then evacuated on a vacuum line and flushed
with N2. After this evacuation-flush procedure was repeated for
three times, and anhydrous solvent (1 mL) and Et3N (0.08 mL,
0.57 mmol) were added with a syringe. To this was added a solution
of [Rh(nbd)Cl]2 (10.58 mg, 22.96
mmol) in solvent (1.5 mL). The
reaction mixture became brown from light yellow immediately.
The concentrations of the monomer and the rhodium catalyst were
0.46 M and 9.18 mM, respectively. After being stirred at 50 ꢁC for
24 h under N2, the resultant solution was added dropwise to a large
amount of n-hexane. The appearing precipitate was collected by
filtration. Purification of the polymer was achieved from re-
precipitation from CHCl3 - henxane, and dried in vacuo at room
temperature until weight constant.
1H-NMR (CDCl3)
d (ppm from TMS): 7.88 (d, 2H, aromatics), 7.55
(d, 2H, aromatics), 7.22e7.46 (m, 5H, aromatics), 4.58 (m,1H, CH-N),
4.36 (t, 1H, CH2-O), 4.12 (t, 1H, CH2-O), 3.20 (dd, 1H, CH2-Ph), 2.74
(dd, 1H, CH2-Ph), 0.24, (s, 9H, CH3-Si). 13C NMR (CDCl3)
d (ppm from
CDCl3): 163.5, 138.1, 131.9, 129.3, 128.6, 128.2, 127.6, 126.6, 126.2,
64.2, 53.1, and 36.8.
[4] Preparation of (S)-4-Benzyl-2-(ethynylphenyl)oxazoline (BnEPhOx)
Spectroscopic data of the polymers. 1H NMR (CDCl3)
d (ppm from
TMS): 7.45e7.63 (br, 2H, aromatics) 7.00e7.23 (br, 5H, aromatics),
6.61e6.74 (br, 2H, aromatics) 5.78e5.92 (br, 1H, C]CH), 4.28e4.40
(br, 1H, CH-N), 3.98e4.17 (br, 1H, CH2-O), 3.80e3.94 (br, 1H, CH2-
O), 3.02e3.17 (br, 1H, CH2-Ph), 2.40e2.58 (br, 1H, CH2-Ph).
The
(S)-4-Benzyl-2-(4-trimethylsilylethynylphenyl)oxazoline
(5) (1.42 g, 4.26 mmol) was stirred in the presence of methanol
(15 mL) and potassium carbonate (0.44 g, 3.20 mmol) at room
temperature for 1 h. To the resulting solution was added dieth-
ylether (100 mL), washed 3 times with water and then dried over
MgSO4. The filtrate was concentrated on a rotary evaporator. The
product was purified by column chromatography eluted with
hexane/ethyl acetate (6/1, v/v) to obtain the product as a pale
yellow solid (0.62 g, 2.37 mmol) with yield of 67%.
4. Results and discussion
4.1. Monomer synthesis
[
a]
¼
þ23.5ꢁ in CHCl3 (c
¼
1.0 g/dL). Melting-point:
The synthetic route of the novel optically active phenylacetylene
bearing oxazoline derivative (BnEPhOx) is illustrated in Scheme 2.
The monomer was obtained from very mild conditions, starting
from coupling reaction of 4-iodobenzoyl chloride (1) with (S)-
phenylalaninol (2) in anhydrous THF with Et3N, to obtain hydrox-
yamide (3). Subsequently, ring-closure reaction of hydroxyamide 3
was carried out by refluxing 1,2-dichloroethane in the presence of
TsCl, Et3N and DMAP, yielding the 4-iodobenzene which has chiral
oxazoline (4). The intermediate 4 was then converted to the cor-
D
56.5e58.8 ꢁC, Anal. Calcd for C18H15NO: C 82.73%, H 5.79%, N
5.36%. Found: C 82.50%, H 5.97%, N 5.36%. 1H NMR (CDCl3)
d (ppm
from TMS): 7.89 (d, 2H, aromatics), 7.59 (d, 2H, aromatics), 7.22e7.42
(m, 5H, aromatics), 4.58 (m,1H, CH-N), 4.36 (t,1H, CH2-O), 4.12 (t,1H,
CH2-O), 3.20 (dd, 1H, CH2-Ph), 3.18 (s, 1H, H-Ch), 2.74 (dd, 1H,
CH2-Ph). 13C NMR (CDCl3)
d (ppm from CDCl3): 163.8, 137.8,
132.0, 129.2, 128.5, 128.1, 127.9, 126.5, 125.0, 83.0, 79.3, 71.9, 67.9,
and 41.7.
responding precursors
5 through the Sonogashira-Hagihara
3.2. Polymerization
coupling reaction with (trimethylsilyl)acetylene (TMSA) using
palladium/copper(I) catalyst system in anhydrous THF. Finally, the
BnEPhOx was achieved in moderate yield from the treatment of 5
with K2CO3 in MeOH for deprotecting the trimethylsilyl group. The
The polymerizations were carried out in a schlenk glass tube
equipped with three-way stopcock under an inert nitrogen

2570
P. Rattanatraicharoen et al. / Polymer 53 (2012) 2567e2573
structures of all the intermediates and the monomer were
an acetylenic proton at around 3.18 ppm. Moreover, the signals of
the cis-transoidal proton of main chain were observed at around
5.82 ppm [30,31]. All these results clearly indicate that the acety-
lene polymerization takes place to form the polymers composed of
alternating single and double bonds. The cis contents of the pol-
y(phenylacetylene)s main chain were also determined by
comparing the integration ratio of cis vinyl proton signal (-CH ¼ C-;
Ha) with the other proton signals in the 1H NMR spectra, which was
widely used to determine cis content (%) of the polyacetylenes.
Their cis contents of polymers were listed in Table 1. Almost the
polymers possessed highly stereoregular cis structure (cis
contents > 67.5%), like the other polymers of the same types
[32e34].
Commonly, the polymerization of an acetylene monomer using
rhodium catalyst provides cis-rich polyacetylene. There have been
reported that the cis-trans isomerization during the acetylene
polymerization is dependent on three possible factors, i.e.,
temperature [35], stereo-hinder of the functional groups side chain
[36] and polarity of solvents [37]. In this study, the BnEPhOx was
polymerized at the same temperature. Hence, the low cis content of
the polymer obtained in DMSO should be resulted from the high
polarity of the solvent.
confirmed by 1H NMR, 13C NMR and FT-IR.
4.2. Polymerization
A novel type of poly(phenylacetylene) bearing functional oxa-
zoline Pendant groups (poly(BnEPhOx)) was successfully synthe-
sized by catalytic polymerization of BnEPhOx using rhodium
catalyst in the presence of Et3N under mild conditions, in accor-
dance with a previously reported method [24]. The yellow powdery
polymers were obtained from precipitation of polymeric solution
into an excess of n-hexane. The results of the polymerizations of
BnEPhOx are summarized in Table 1. Poly(BnEPhOx) with Mn
ranging from 1.11 to 3.51 ꢃ 103 were obtained with 90e94% yield.
Anyway, the different appearance of the polymers should be closely
related to their different number-averaged molecular weight
(Mn).The polymerizations were carried out under the same condi-
tions except only the kind of solvents. Therefore, it can be proposed
that the solvents had the influence on the polymerization of this
type of monomer.
Here, it is important to point out that, when polymerization was
carried out in DMSO as the solvent, precipitation was quite difficult
to achieve the resulting polymer from the solution by directly
pouring the polymer solution into a large amount of n-hexane as it
is poor solvents (Run 3 in Table 1). Instead, a small amount of CHCl3
should be added in the polymeric solution first before the normal
procedure of the precipitation. This is to transfer the DMSO system
to a CHCl3 system.
The FT-IR measurement, a versatile method to determine the
steroregularity of the polyacetylene, was carried out to determine
the structure of the resulting polymers. Fig. 2 showed the FT-IR
spectrum of the polymers. As can be seen in the Fig. 2, BnEPhOx
clearly showed a strong absorption band at 3250 cmꢂ1 and a weak
absorption band at 2150 cmꢂ1 respectively assignable to the C-H
and C^C stretching vibration in the monosubstituted acetylene. No
absorption bands were observed at wave number area above
stretching vibration in the polymers. Moreover, the polymers
4.3. Polymer structure
showed characteristic peaks at around 1340 and 960 cmꢂ1
,
The polymers were characterized by NMR and IR spectrometers.
It has been reported that rhodium catalysts can commonly provide
the substituted polyacetylene with high cis-transoidal [27,28]. The
1H NMR was performed to confirm the microstructure of the
constitution units in the resulting polymers.The 1H NMR spectra of
poly(BnEPhOx) and BnEPhOx were shown in the same Fig. 1, for
comparison. All signals could be assigned, no other unexpected
signals were observed.Aromatic signals of the ethynylphenyl
protons upfield shifted to 6.7 ppm after polymerization. The reso-
nance peaks of the oxazoline moieties protons also upfield shift by
w0.25 ppm, which should be due to the enhanced electronic
interaction between the oxazoline groups in the polymer. These are
consistent with those from another study [29]. The signals of the
oxazoline group of monomer were observed, i.e., signals of CH2-Ph
appeared at 2.74 and 3.18 ppm and the signal of the CH-N was also
found at 4.58 ppm.The polymers displayed no signal assignable to
respectively, which were assigned to C]C and C-H vibrations in the
cis-polyacetylene [38]. While the characteristic peak of the trans
isomer at 1200 cmꢂ1 was not observed in this spectrum. These
results indicated that the monomer was completely conversed to
poly(phenylacetylene) substituted with the functional chiral oxa-
zoline groups, which possessed highly stereoregular cis structure.
The solubilities of the polymers were investigated to find out
that all the resulting polymers were soluble in only CHCl3 and
partially insoluble in THF, DMSO, DMF and toluene, while insoluble
in methanol, n-hexane and diethyl ether.
4.4. Absorption and emission properties of the polymers
We investigated optical properties of the polymers obtained.
The absorption and emission properties of poly(BnEPhOx)s along
with BnEPhOx were investigated in chloroform. The UVevis
absorption spectra and photoluminescence (PL) spectra in chloro-
form solution are illustrated in Fig. 3. The BnEPhOx showed a strong
absorption attributed to the oxazoline group in the range of
270e330 nm (Fig. 3(A)). On the other hand, the absorption of all
polymers were observed at 270e520 nm. The broad absorption
peaks of the polymers were assigned by comparing to the
absorption spectra of the corresponding monomer. The strong
absorption peaks of the polymer at 280e340 nm were definitely
the signatures of the absorption of the oxazoline pendants. All the
polymers showed the absorption in the longer wavelength region
while the monomer did not. These were the signatures of the p-p
*
interband transition of the main chain of the conjugated poly-
acetylene [39].
The relative photoluminescence spectra of the poly(BnEPhOx)s
measured in chloroform were shown in Fig. 3(B). As depicted in
Fig. 3(B), the poly(BnEPhOx) polymerized in DMSO exhibited the
strongest characteristic emission peak at around 400 nm upon
Table 1
Asymmetric polymerization of BnEPhOx using rhodium catalysta.
Run Solvent Yieldb(%) Mn ꢃ 10ꢂ4 Mw/Mn
[a]
d(deg.) cis contentse (%)
D
c
c
1
2
3
4
THF
Tol
DMSO 90
DMF 94
90
91
1.36
3.51
1.11
2.01
3.10
3.18
1.78
3.16
ꢂ244.4
ꢂ325.1
ꢂ147.3
ꢂ200.2
99
100
68
99
Abbreviations: DMF, N,N-dimethylformamide; DMSO: dimethyl sulfoxide; THF,
tetrahydrofuran; Tol, toluene.
a
(S)-4-Benzyl-2-(4-ethynylphenyl)-oxazoline (BnEPhOx) [
a
]
D
¼ þ23.5ꢁ in CHCl3.
Conditions: [BnEPhOx] ¼ 1.15 mmol, [BnEPhOx]/[Rh cat.] ¼ 50/1, polymerized at
50 ꢁC for 24 h in solvent (2.5 mL).
b
Insoluble part in n-hexane.
THF-soluble part, determined by GPC eluted with THF calibrated by polystyrene
c
standards.
d
c ¼ 0.1 g/dL, l ¼ 1 cm, CHCl3.
e
Determined by comparing the intregation ratio of cis vinyl proton signal (around
5.82 ppm) and the other proton signals in the 1H NMR spectra.

P. Rattanatraicharoen et al. / Polymer 53 (2012) 2567e2573
2571
Fig. 1. 1H NMR spectra of (A) BnEPhOx and (B) poly(BnEPhOx) (run 2 in Table 1) in CDCl3.
excitation at around 300 nm. In contrast, the poly(BnEPhOx)s
4.5. Chiroptical properties of the polymer
obtained from toluene, DMF and THF showed only negligible weak
emissions. These emission bands should be due to the stereoregular
chains of the polyacetylene [40,41]. The difference in PL intensity
was related to the cis concentrations of the resulting polymers. This
result indicates that the non-radiative decay rate increases by
increasing the cis content. The PL intensities of the polymers
decrease with increasing of cis content; these results are consistent
with those from the previous studies [41].
The Pendant chiral oxazoline moiety is known as a potential
group to endow the main chain of polymers with a prevailing
helical handedness structure [24]. So, it is anticipated that the
introduction of optically active chiral oxazoline as the Pendant
groups of poly(phenylacetylene) could induce an excess one
handedness helical conformation in the main chain of the polymer.
Subsequently, chiroptical properties of the polymers obtained were
Fig. 2. FT-IR spectra of (A) BnEPhOx and (B) poly(BnEPhOx) (run 2 in Table 1).

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P. Rattanatraicharoen et al. / Polymer 53 (2012) 2567e2573
Fig. 3. UVevis Absorption (A) and Emission (B) spectra of BnEPhOx and poly(BnEPhOx) measured in CHCl3.
investigated by means of optical specific rotation ([a]D) and circular
dichroism (CD). The optical specific rotations of BnEPhOx and
poly(BnEPhOx)s measured in CHCl3 were listed in Table 1. The
optical specific rotation of BnEPhOx was þ23.5ꢁ. On the other hand,
poly(BnEPhOx)s showed the values range from ꢂ147.3ꢁw325.1ꢁ.
Poly(BnEPhOx) obtained in DMSO had the lowest absolute optical
specific rotation among the other polymers. These observations
were consistent with the results of the cis contents of polymer. The
polymers which possessed highly stereoregular cis structure are
known to have higher value of optical specific rotation.
As discussed above, the large [a]D values of the polymers imply
that they take a helical conformation. We conducted CD spectro-
scopic analysis to confirm whether the polymer chains are really
spiraling in a helical sense. As shown in Fig. 4, the CD spectra of
poly(BnEPhOx)s exhibited a positive Cotton effect at around 340 nm
and a negative one at 270 nm. In addition, BnEPhOx showed a strong
UV absorption only in the range of 270e330 nm while the polymer
showed strong UV absorption over the range of 270e520 nm.
Judging from the comparison between the CD and UVevis absorp-
tion spectra of the monomers and polymers, it seems that the
configurational chirality of the optically active oxazoline groups
induced the Cotton effects at around 270 nm, and the polyacetylene
main chain induced the ones at around 340 nm. It is considered that
the CD signal of poly(BnEPhOx) arises from not only the
Fig. 5. Thermogravimetric curves of (A) poly(BnEPhOx) (run 3 in Table 1) and (B)
poly(phenylacetylene) at heating rate of 10 ꢁC/min under nitrogen.
configurational chirality of the optically active oxazoline groups but
also the conformational one based on higher-order structures of
polyacetylene backbone, i.e., partial helical conformation. The
results of chiroptical properties obviously indicated that these
polymers possess a helical structure with an excess single screw
sense induced by chiral oxazoline groups in the side chain.
4.6. Thermal analysis of polymers
Thermal property of poly(phenylacetylene)s having chiral oxa-
zoline was examined by thermogravimetric analysis (TGA) as
shown in Fig. 5. The polymer showed high thermal stability. As
shown in Fig. 5, the polymer showed a very small weight loss below
100 ꢁC. This should be assigned to the loss of absorbed water. The
significant weight loss of 10% was observed at around 310 ꢁC. The
char residue at 500 ꢁC was calculated to be 62%. Poly(BnEPhOx) had
higher thermal stability than poly(phenylacetylene). The polymers
having an oxazoline group in the side chain are known as ther-
mostable polymers [24]. Thermal stabilities of poly(BnEPhOx) were
improved by the introduction of oxazoline group to the acetylene
backbone.
5. Conclusion
In this paper, we have demonstrated the preparation of the
novel poly(phenylacetylene)s bearing a chiral oxazoline group at
a side chain, using rhodium catalyst ([Rh(nbd)Cl]2) in various
Fig. 4. CD spectra of BnEPhOx and poly(BnEPhOx)s measured in CHCl3.

P. Rattanatraicharoen et al. / Polymer 53 (2012) 2567e2573
2573
[16] Ishiwari F, Nakazono K, Koyama Y, Takata T. Chem Commun 2011;47:
11739e41.
[17] Lam JWY, Tang BZ. Acc Chem Res 2005;38:745e54.
[18] Akagi K, Mori T. Chem Rec 2008;8:395e406.
[19] Yashima E, Huang S, Okamoto Y. Chem Commun 1994;15:1811e2.
[20] Aoki T, Kaneko T, Teraguchi M. Polymer 2006;47:4867e92.
[21] Ghosh AK, Mathivanan P, Cappiello J. Tetrahedron Asymmetry 1998;9:1e45.
[22] Fan QH, Ren CY, Yeung CH, Hu WH, Chan ASC. J Am Chem Soc 1999;121:
7407e8.
[23] Yu HB, Hu QH, Pu L. J Am Chem Soc 2000;122:6500e1.
[24] Onimura K, Shintaku K, Rattanatraicharoen P, Yamabuki K, Oishi T. Chirality
2011;23(E1):E43e51.
solvents. The results from optical specific rotation and CD spec-
troscopy implied that all poly(BnEPhOx)s adopted helical struc-
tures with predominantly one-handed screw sense in chloroform
solution, induced by chiral oxazoline group at the side chain. The
molecular weights, yields and optical specific rotations were
influenced by the polymerization solvents. The polymers exhibited
good thermal stability. Poly(BnEPhOx) obtained in DMSO showed
the highest PL intensity.
[25] Rattanatraicharoen P, Yamabuki K, Oishi T, Onimura K. Polym J 2012;44:
224e31.
References
[26] Abiko A, Masamune S. Tetrahedron Lett 1992;33:5517e8.
[27] Tabata M, Yang W, Yakota K. J Polym Sci Part A: Polym Chem 1994;32:
1113e20.
[28] Yang W, Tabata M, Kobayashi S, Yakota K, Shimizu A. Polym J 1991;23:
1135e8.
[1] Okamoto Y, Nakano T. Chem Rev 1994;94:349e72.
[2] Isobe Y, Onimura K, Tsutsumi H, Oishi T. Macromolecules 2001;34:7617e23.
[3] Yashima E, Maeda K, Iida H, Furusho Y, Nagai K. Chem Rev 2009;109:
6102e211.
[29] Lam JWY, Dong Y, Kwok HS, Tang BZ. Macromolecules 2006;39:6997e7003.
[30] Furlani A, Napoletano C, Russo MV, Feast WJ. Polym Bull 1986;16:311e7.
[31] Simionescu CI, Percec V. Prog Polym Sci 1982;8:133e214.
[32] Kakuchi R, Tago Y, Sakai R, Satoh T, Kakuchi T. Macromolecules 2009;42:
4430e5.
[33] Fukushima T, Takachi K, Tsuchihara K. Macromolecules 2006;39:3103e5.
[34] Saeed I, Khan FZ, Shiotsuki M, Masuda T. J Polym Sci Part A: Polym Chem
2009;47:1853e63.
[35] Goto H, Dai X, Ueoka T, Akagi K. Macromolecules 2004;37:4783e93.
[36] Wang X, Guan S, Xu H, Su X, Zhu X, Li C. J Polym Sci Part A: Polym Chem 2010;
48:1406e14.
[37] Masuda T, Sasaki N, Higashirnura T. Macromolecules 1975;8:717e21.
[38] Tabata M, Tanaka Y, Sadahiro Y, Sone T, Yokota K, Miura I. Macromolecules
1997;30:5200e4.
[4] Yashima E, Maeda K, Furusho Y. Acc Chem Res 2008;41:1166e80.
[5] Okamoto Y. J Polym Sci Part A: Polym Chem 2009;47:1731e9.
[6] Itsuno S. Prog Polym Sci 2005;30:540e58.
[7] Tamura K, Miyabe T, Iida H, Yashima E. Polym Chem 2011;2:91e8.
[8] Gao H, Isobe Y, Onimura K, Oishi T. Polym J 2007;39:764e76.
[9] Burroughes JH, Bradley DDC, Brown AR, Marks RN, Mackay K, Friend RH, et al.
Nature 1990;347:539e41.
[10] Zhu Y, Gibbons KM, Kulkarni AP, Jenekhe SA. Macromolecules 2007;40:
804e13.
[11] McQuade DT, Pullen AE, Swager TM. Chem Rev 2000;100:2537e74.
[12] Aoki T, Fukuda T, Shinohara K, Kaneko T, Teraguchi M, Yagi M. J Polym Sci Part
A: Polym Chem 2004;42:4502e17.
[13] Teraguchi M, Mottate K, Kim S, Aoki T, Kaneko T, Hadano S, et al. Macro-
molecules 2005;38:6367e73.
[39] Zhao H, Sanda F, Masuda T. Macromolecules 2004;37:8893e6.
[40] Lam JWY, Luo J, Dong Y, Cheuk KKL, Tang BZ. Macromolecules 2002;35:
8288e99.
[14] Wu ZQ, Nagai K, Banno M, Okoshi K, Onitsuka K, Yashima E. J Am Chem Soc
2009;131:6708e18.
[15] Perec V, Aqad E, Peterca M, Rudick JG, Lemon L, Ronda JC, et al. J Am Chem Soc
2006;128:16365e72.
[41] Lee CW, Wong KS, Lam WY, Tang BZ. Chem Phys Lett 1999;307:67e74.