Chiral oxazoline substituted optically active poly(m-phenylene)s: Synthesis and coupling polymerizations of (S)-4-benzyl-2-(3,5-dihalidephenyl) oxazoline using transition metal catalysts

Article abstract of DOI:10.1002/pola.26499

Optically active poly(m-phenylene)s substituted with chiral oxazoline derivatives have been synthesized by the nickel-catalyzed Yamamoto coupling reaction of optically active (S)-4-benzyl-2-(3,5-dihalidephenyl)oxazoline derivatives (X = Br or I). The structures and chiroptical properties of the polymers were characterized by spectroscopic methods and thermal gravimetric analyses. The polymers showed higher absolute optical specific rotation values than their corresponding monomer, and showed a Cotton effect at transition region of conjugated main chain. The optical activities of the polymers should be attributed to the higher order structure such as helical conformations. Moreover, the helical conformation could be induced by addition of metal salts into polymer solutions. The polymers showed good thermal stabilities, which was attributable to the oxazoline side chains.

Full text of DOI:10.1002/pola.26499

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Chiral Oxazoline Substituted Optically Active Poly(m-Phenylene)s:
Synthesis and Coupling Polymerizations of (S)-4-Benzyl-2-(3,5-
dihalidephenyl) Oxazoline Using Transition Metal Catalysts
Poompat Rattanatraicharoen,* Yoko Tanaka, Keiko Shintaku, Takuro Kawaguchi,
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
Correspondence to: K. Onimura (E-mail: onimura@yamaguchi-u.ac.jp)
Received 10 August 2012; accepted 27 November 2012; published online 18 December 2012
DOI: 10.1002/pola.26499
ABSTRACT: Optically active poly(m-phenylene)s substituted
with chiral oxazoline derivatives have been synthesized by the
nickel-catalyzed Yamamoto coupling reaction of optically active
(S)-4-benzyl-2-(3,5-dihalidephenyl)oxazoline derivatives (X ¼ Br
or I). The structures and chiroptical properties of the polymers
were characterized by spectroscopic methods and thermal
gravimetric analyses. The polymers showed higher absolute
optical specific rotation values than their corresponding mono-
mer, and showed a Cotton effect at transition region of conju-
gated main chain. The optical activities of the polymers should
be attributed to the higher order structure such as helical
conformations. Moreover, the helical conformation could be
induced by addition of metal salts into polymer solutions. The
polymers showed good thermal stabilities, which was attribut-
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able to the oxazoline side chains. 2012 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1315–1322
KEYWORDS: chiral; conjugated polymers; helical polymer; opti-
cally active polymer; oxazoline; polyaromatics; Yamamoto
reaction
INTRODUCTION Functional p-conjugated polymers have
attracted much considerable interest during the past deca-
des. These functional polymers exhibit unique structure and
interesting properties owing to the p-conjugation in the
backbone chains, which allow them to be served for many
applications including optical and electronic devices.1–3
Meanwhile, the artificial helical polymers have been also one
of the major research subjects of polymer chemistry because
of their secondary structure and backbone conformation,
which are similar to the structure of natural macromolecules,
that is, DNAs and proteins. There have been many reports
on the sophisticated design and synthesis of artificial helical
polymers.4–8
vophobic effects and bended linkage. These were demonstrated
by the previous studies, namely poly(m-phenylene)s11,12
and poly(m-phenylene ethynylene)s.13,14 In addition, an
interaction between chiral side groups is sometime required to
induce an excess screw sense for the helical polymers. We have
previously reported the induction of a prevailing screw sense of
helical polyacetylene by complexation of chiral oxazoline side
chain with metal ions in a solvent, resulted in the formation of
chiral supramolecular aggregation as evidenced by induced
Cotton effects.15
To the best of our knowledge, there are relatively few
reports on the synthesis of helical poly(m-phenylene)s bear-
ing an optically active side chain. Therefore, the present
study is devoted to the design and synthesis of a series of
optically active m-phenylene substituted with chiral oxazo-
line derivatives, which were polymerized by Yamamoto cou-
pling reaction to obtain the optically active poly(m-phenyl-
ene)s bearing chiral oxazoline at the side chains (Scheme 1).
The obtained polymers are expected to fold into helical con-
formation due to incurvated linkage backbones and chiral
oxazoline groups at the side chain. This includes an induc-
tion of helical sense of the polymers by the complexation
It is well understood that a prevailing screw sense of poly-
mers could be achieved by the introduction of appropriate
chiral substitute groups into the polymer backbone. The
secondary structures of those polymers are generated and
stabilized by interactions such as steric repulsion, hydrogen
bonds, and van der Waals forces.9,10 Interestingly, p-conju-
gated polymers constructed from the monomer with molecu-
lar shape like m-phenylene are known to fold into helical
conformations in a process thermodynamically driven by sol-
*Present address: UBE Technical Center (Asia) Limited, 140/10 Moo 4 Tapong, Muang Rayong, Rayong 21000, Thailand.
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2012 Wiley Periodicals, Inc.
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bis(1,5-cyclooctadiene) nickel (0) were received from Kanto
R
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Chemical. Celite 545 was purchased from Sigma-Aldrich Ja-
pan. 1,5-Cyclooctadiene were obtained from Tokyo Chemical
Industry. 2,20-Bipyridyl (a,a0-) were received from Kishida
Reacgents Chemicals (Osaka, Japan). 3,5-Diiodobenzoic acid
(5b)16 and (S)-phenylalaninol (2)17 were synthesized accord-
ing to the literatures. n-Hexane, toluene, tetrahydrofuran,
ethyl acetate, N,N-dimethylformamide, dimethylsulfoxide, 1,2-
dichloroethane, CH2Cl2, and CHCl3 were dried according to
the standard procedure and distilled under nitrogen atmos-
phere. Column chromatography was performed with silica
gel 60 (0.063–0.200, MERCK). Analytical thin-layer chroma-
SCHEME 1 Polymerization of 3,5-dihalidebenzene derivatives
bearing chiral oxazoline group.
tography was performed on Merck silica gel plate 60F254
.
Monomer Synthesis
with metal ions in a solvent. All the obtained polymers are
then also characterized by spectroscopies and thermogravi-
metric analysis (TGA).
The typical synthesis procedures of optically active (S)-4-ben-
zyl-2-(3,5-dihalidephenyl)oxazoline derivatives (DXPhBnOx, X
¼ Br or I) were given according to strategy outline in Scheme 2.
EXPERIMENTAL
(S)-Phenylalaninol (3)
To a stirred suspension of NaBH4, (10.0 g, 264.4 mmol) in
THF (100 mL) was added L-phenylalanine (16.53 g, 100.0
mmol). The flask was immersed in an ice-water bath, and a
solution of concentrated H2SO4, (6.6 mL, 120 mmol) in
diethyl ether (20 mL) was added dropwise (addition time ca.
3 h). Stirring of the reaction mixture was continued at room
temperature overnight. The excess borane was quenched by
slow addition of MeOH (70 mL) until gas bubble stopped
carefully. The mixture was concentrated to about 70 mL and
5N NaOH (100 mL) was added. After removing the organic
solvent, the mixture was heated at reflux for 6 h. The result-
ing two-phase mixture was cooled and filtered through a
thin pad of Celite which was washed with water and
CH2Cl2.17 The filtrate was extracted three times with CH2Cl2
(30 mL), and then washed with saturated NaCl (aq) (30 mL
ꢁ 3). The combined organic phase was dried over anhydrous
sodium sulfate, followed by evaporation of the solvent left
solid phenylalaninol, which was recrystallized from ethyl ac-
etate and hexane to yield 12.64 g (84.6 mmol, 84.6% yield)
as colorless needle crystal.
Measurements
1H and 13C nuclear magnetic resonance (NMR) spectra were
recorded on a JNM-LA500 apparatus (JEOL) spectrometer
using tetramethylsilane (TMS) (1H NMR, d 0.00 ppm) or CDCl3
(
13C NMR, d 77.0 ppm) as internal reference peaks at room
temperature. The number- and weight-average molecular
weight (Mn and Mw) of polymers were determined by gel per-
meation chromatographic (GPC) on a LC-10AS and CHROMA-
TOPAC C-R7A plus (Shimadzu) 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) on the basis of standard polystyrene
samples. Thermo gravimetric analyses were carried out using
an MS-Tg/DTA220 (JEOL) at a scanning rate of 10 ꢀC/min
under nitrogen (100 mL/min). Melting points (mp) were
measured on a Yanagimoto micromelting point apparatus.
Specific rotations were measured on the concentration of
0.1–1.0 g/dL in THF at 25 ꢀC using a quartz cell (1.0 cm)
with a JASCO DIP-1030 (JASCO). Circular dichroism (CD) spec-
tra were mꢀeasured on the concentration of 0.01–0.10 g/dL in
THF at 25 C using a quartz cell of 1 mm with a JASCO J-805
(JASCO). Infrared (IR) spectra were performed on a FTIR Jasco
4100 (JASCO) spectrophotometer. Thermo gravimetric analy-
ses were carried out using an MS-Tg/DTA220 (JEOL) at a scan-
ning rate of 10 ꢀC/min under nitrogen (100 mL/min). Elemen-
tal analysis was done MICRO CORDER JM10 (J-SCIENCE LAB)
at the Collaborative Center for Engineering Research Equip-
ment, Faculty of Engineering, Yamaguchi University.
Mp: 90–91 ꢀC, [a]D ꢂ18.0ꢀ (1.0 g/dL, l ¼ 10 cm, THF). 1H
NMR (CDCl3) d(ppm from TMS): 1.57 (br, 5H, NH2), 2.54
(dd, 1H, CH2AOH), 2.81 (dd, 1H, CH2AOH), 3.13 (m, 1H,
CH), 3.38 (dd, 1H, CH2-Ph), 3.64 (dd, 1H, CH2-Ph), 7.18–7.34
(m, 5H, Ph).
3,5-Diiodobenzoic Acid (5b)
To an ice-cooled, stirred suspension of 3,5-diaminobenzoic
acid (3.00 g, 19.7 mmol) in a mixture of concentrated sulfu-
ric acid (30 mL) and water (15 mL) was added sodium ni-
trite powder (3.26 g, 47.2 mmol) with the temperature con-
trolled between ꢂ5 and 0 ꢀC. After 1 h, urea (0.251 g, 4.18
mmol) was added, and then a cold solution of potassium
iodide (32.8 g, 198 mmol) in 30 mL of water was added
dropwise. The black–brown mixture was stirred for another
3 h with the temperature between ꢂ5 and 0 ꢀC and heated
Materials
All reagents were used without any further purification.
Sodium nitrite, triethylamine, toluenesulfonyl chloride (TsCl),
and anhydrous magnesium sulfate were received from Naca-
lai Tesque (Kyoto, Japan). 3,5-Aminobenzoic acid was
obtained from Merck (Darmstadt, Germany). L-Phenylalanine
(2), 3,5-dibormobenzoic acid (5a), sulfuric acid, urea, cupper
iodide, potassium iodide, N,N0-dicyclohexylcarbodiimide
(DCC) and 4-dimethylaminopyridine (DMAP) were purchased
from Wako Pure Chemical Industries. Sodium sulfate and
ꢀ
to 60 C for 30 min. Then the warm suspension was poured
into 150 mL of ice-cold water. The precipitate was washed
six times with water by decantation. The resulting brown
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SCHEME 2 Synthetic route of monomer.
precipitate was collected by vacuum filtration, dried, and
then dissolved in diethyl ether (150 mL).16 The resulting
dark brown diethyl ether solution was washed with aqueous
Na2S2O3 until a pale yellow color was observed. The organic
phase was dried over anhydrous magnesium sulfate, fol-
lowed by evaporation of the solvent to give orange crystal
3.44 g (7.03 mmol, 35.7% yield).
(2.14 g, 5.72 mmol), NHS (0.82 g,7.15 mmol), DCC (1.42 g,
6.87 mmol), and (S)-phenylalaninol (0.95 g, 6.30 mmol) in 50
mL of dry THF. The crude product was purified by silica gel
column chromatography eluted with hexane/ethyl acetate (7/
3, v/v) to give a white solid (1.62 g, 3.31 mmol) in 58% yield.
1H NMR (CDCl3) d (ppm from TMS): 7.10–7.00 (m, 5H, Ph),
6.22 (d, 1H, HAN), 4.35(m, 1H), 3.70 (m, 2H, CH2AOH), 2.69
(d, 2H, CH2-Ph).
1H NMR (DMSO-d6): d (ppm from TMS): 8.33 (s, 1H, 4-H),
8.15 (s, 2H, 2, 6-H).
(S)-4-Benzyl-2-(3,5-dibromophenyl)oxazoline
N-((S)-1-Benzyl-2-hydroxyethyl)-3,5-dibromobenzamide
(6a)
(1a: DBrPhBnOx)
To a stirred solution of hydroxyamide 6a (2.90 g, 7.02 mmol),
triethylamine (3.80 g, 37.65 mmol) and DMAP (0.0149 g, 0.116
mmol) in 25 mL of 1,2-dichloroethane was added TsCl (1.996 g,
10.47 mmol) at 0 ꢀC. The reaction mixture was stirred at reflux
temperature for 12 h. After that, the mixture was washed twice
with saturated NaCl(aq), dried over MgSO4, and removed solvent
in vacao. The crude product was purified by column chromato-
graphy eluted with hexane/ethyl acetate (10/1, v/v) to give
1.50 g (3.81 mmol) of yellowish oil in 54% yield.
To a stirred solution of 3,5-dibromobenzoic acid (2.00 g, 7.14
mmol) in 25 mL of dry THF was added N-hydroxysuccinimide
(NHS) (1.02 g, 8.91 mmol) and DCC (1.77 g, 8.56 mmol)
sequentially. The mixture was continued to stir at room temper-
ature under nitrogen atmosphere for 3 h. Then, a solution of
(S)-phenylalaninol (1.19 g, 7.87 mmol) in 25 mL of dry THF
was added to the mixture via syringe. After being stirred for
overnight at room temperature, the mixture was then filtrated
using aspirator and the solvent was removed out at reduced
pressure. The residue was taken up with EtOAc, washed twice
with NaCl(aq), dried over sodium sulfate (Na2SO4), and filtration.
The organic layer was removed on a rotary evaporator. The
product was purified by column chromatography eluted with
hexane/ethyl acetate (7/3, v/v) to obtain the product as white
solid in 2.90 g (7.02 mmol, 98% yield).
1H NMR (CDCl3) d (ppm from TMS): 8.30 (s, 1H, 4-H), 8.17
(s, 2H, 2,6-H), 7.21–7.33 (m, 5H, Ph), 4.58 (m, 1H, CH), 4.36
(t, 1H, CH2AO), 4.12 (t, 1H, CH2AO), 3.20 (dd, 1H, CH2-Ph),
2.74 (dd, 1H, CH2-Ph). 13C NMR (CDCl3) d (ppm from TMS):
161.58, 137.51, 136.69, 129.93, 129.27, 128.44, 126.63,
122.84, 72.38, 67.93, 41.71. [a]435 ¼ ꢂ11.9ꢀ, [a]D ¼ ꢂ5.0ꢀ
(c ¼ 1.0 g/dL, l ¼ 10.0 cm, THF). ELEM. ANAL: Calcd. for
1H NMR (CDCl3) d (ppm from TMS): 7.10–7.00 (m, 5H, Ph),
6.22 (d, 1H, HAN), 4.35(m, 1H), 3.70 (m, 2H, CH2AOH), 2.69
(d, 2H, CH2-Ph).
C16H13NOBr2: C, 48.64%; H, 3.32%; N, 3.55%. Found: C,
49.10%; H, 3.43%; N, 3.49%.
(S)-4-Benzyl-2-(3,5-diiodophenyl)oxazoline (1b: DIPhBnOx)
This compound was synthesized according the general pro-
cedure described above, starting from hydroxyamide 6b
(1.62 g, 3.31 mmol), triethylamine (1.75 g, 17.25 mmol),
N-((S)-1-Benzyl-2-hydroxyethyl)-3,5-diiodobenzamide (6b)
This compound was synthesized according the general proce-
dure described above, starting from 3,5-diiodobenzoic acid
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TABLE 1 Yamamoto Homocoupling-Polymerizations of DXPhBnOxa
b)
Solvent (mL)
d)
d)
e)
[a]435 (ꢀ)
Run
DXPhBnOx
Temp. (ꢀC)
Time (h)
Yieldc) (%)
Mn
Mw/Mn
1
2
3
4
5
6
7
a
1a
1a
1b
1b
1a
1a
1a
Tol/DMF (3.0/3.0)
Tol/DMF (3.0/3.0)
Tol/DMF (3.0/3.0)
Tol/DMF (3.0/3.0)
Tol (6.0)
80
80
80
80
80
80
80
24
48
24
48
48
48
48
c
33.0
54.7
21.9
37.7
31.0
19.3
20.0
1250
1080
1310
1090
760
1.19
1.14
1.24
1.22
1.24
1.24
1.24
þ313.2f
þ258.8
þ301.0
þ240.6
þ54.3f
þ791.7
þ27.3
DMF (6.0)
950
DMSO (6.0)
840
(S)-4-Benzyl-2-(3,5-dibromophenyl)oxazoline(DBrPhBnOx),
[a]435
¼
Et2O insoluble part.
d
e
f
ꢂ11.9ꢀ, [a]D ¼ ꢂ5.0ꢀ in THF (1a), (S)-4-benzyl-2-(3,5-diiodophenyl)oxazoline
(DIPhBnOx), [a]435 ¼ ꢂ3.2ꢀ, [a]D ¼ ꢂ1.23ꢀ in THF (1b), conditions: 1a ¼ 1b ¼
0.8 mmol, bpy ¼ 0.90 mmol, COD ¼ 0.90 mmol, Ni(COD)2 ¼ 1.2 mmol.
By GPC (eluent: THF) with poly(styrene) standard.
c ¼ 0.01 g/dL, l ¼ 10.0 cm in THF.
c ¼ 0.1 g/dL, l ¼ 10.0 cm in THF.
b
Solvents; Tol, toluene, DMF: N,N-dimethylformamide, DMSO: di-
methyl sulfoxide.
DMAP (0.0117 g, 0.10 mmol), and TsCl (0.9136 g, 4.79
mmol) in 25 mL of 1,2-dichloroethane. The crude product
was purified by silica–gel column chromatography eluted
with hexane/ethyl acetate (15/1, v/v) to give 0.9396 g (1.92
mmol) of yellowish solid in 58% yield.
sequence of reactions described in Scheme 2. The reaction
took place under very mild conditions, starting from cou-
pling reaction of 3,5-dibromobenzoic acid (5a) or 3,5-diiodo-
benzoic acid (5b) with L-phenylalaninol (3), followed by a
ring closure reaction of hydroxyamide (6a) and (6b) to
obtain the monomer (1a) and (1b). The structure of mono-
mers was confirmed by 1H NMR, 13C NMR, and FTIR
spectroscopy.
1H NMR (CDCl3) d (ppm from TMS): 8.30 (s, 1H, 4-H), 8.17
(s, 2H, 2,6-H), 7.21–7.33 (m, 5H, Ph), 4.58 (m, 1H, CH), 4.36
(t, 1H, CH2AO), 4.12 (t, 1H, CH2AO), 3.20 (dd, 1H, CH2-Ph),
2.74 (dd, 1H, CH2-Ph). 13C NMR (CDCl3) d (ppm from TMS):
160.76, 147.34, 137.12, 135.90, 130.72, 128.89, 128.28,
126.30, 93.98, 71.87, 67.60, 41.20. FTIR (/cm): 2970, 2910,
1650, 1540, 1350, 1300, 955, 710, 555. mp: 72.3–73.7 ꢀC,
[a]435 ¼ ꢂ3.2ꢀ, [a]D ¼ ꢂ1.23ꢀ (c ¼ 1.0 g/dL, l ¼ 10.0 cm,
THF). ELEM. ANAL: Calcd for C16H13NOI2: C, 39.29%; H, 2.68%;
N, 2.86%. Found: C, 39.61%; H, 2.78%; N, 2.84%.
Polymerization
Table 1 summarized the conditions and results of the poly-
merization. Monomers were used in Yamamoto coupling
reactions to prepare poly(m-phenylene) bearing a chiral oxa-
zoline. The reactions were carried out at 80 ꢀC for 24 and
48 h in toluene, DMF, DMSO, and a mixture of toluene/DMF
using a nickel catalyst in the presence of 2,20-bipyridyl and
1,5-cyclooctadiene, in accordance with our method reported
previously.18 Powdery polymers were obtained by precipita-
tion of polymer solution into an excess of diethyl ether. The
polymers were obtained in 20–55% yields, having number
average molecular weight (Mw) range from 760 to 1310
relative to polystyrene standard. As shown in the table, the
polymers obtained from mixed solvent systems and longer
polymerization times were obtained in higher yields, and
had higher Mw than otherwise. These indicated that yields
and Mw were influenced by the polymerization conditions
such as solvents and reaction time.
Polymerization
The polymerizations were carried out in a Schlenk glass
tube equipped with three-way stopcock under an inert nitro-
gen atmosphere according to Scheme 1. A typical experimen-
tal procedure for Yamamoto homocoupling polymerization of
1a is given as follow:
A mixture of 1a (0.32 g, 0.80 mmol), 2,20-bipyridyl (0.14 g, 0.90
mmol), 1,5-cyclooctadiene (0.97 g, 0.90 mmol), and Ni(COD)2
(0.33 g, 1.2 mmol) in anhydrous solvent 6 mL was replaced with
N2. The mixture was continued to stir at 80 ꢀC for 12 and 48 h
under nitrogen atmosphere. After cooling to room temperature,
the mixture was diluted with CH2Cl2 (100 mL). The organic
layer was washed with 1N HCl (3 ꢁ 100 mL) and H2O (3 ꢁ 100
mL), and then dried over MgSO4, and the solvent was removed
solvent at reduced pressure. The residue distilled with THF (0.5
mL) was slowly added dropwise into a large amount of diethyl
ether. Purification of the polymer was performed with reprecipi-
tation from a THF-diethyl ether system and dried under reduced
pressure at room temperature until constant weight.
The low polymerizability could be explained by two possible
reasons. First, it is due to the solubility of polymers. The
polymers showed low solubility, resulting in the precipitation
during polymerizations. Coupling reactions of aryl com-
pounds substituted at o- or m-position yield cyclic
compounds such as porphyrin, phthalocyanine, calixarene,
and cyclic [n] m-phenyleneacetylenes. Cyclic phenylene deriv-
atives direct-coupled by aryl units bonded to an o-posi-
tion.19,20 To our knowledge, macrocyclic compounds synthe-
sized by direct-coupling reaction of m-aryldihalide derivatives
have not been reported previously. The second reason should
be attributed to the steric hindrance of bulky substitutes.
Hashimoto et al.21 reported the synthesis and polymerization
RESULTS AND DISCUSSION
Monomer Synthesis
A series of optically active monomers containing an oxazo-
line derivative were obtained from two main steps via the
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FIGURE 1 1H NMR spectra of (A) DIPhBnOx and (B) poly(PhBnOx) (run 4 in Table 1) in CDCl3.
of a series of substituted 2,5-dichlorobenzoates. They found
that polymerization of monomers substituted with the bulky
groups proceeded to give polymers with relative low Mw and
yield. The steric hindrance of bulky substitutes definitely
affected the degree of polymerizations. Therefore, low poly-
merizability of current study should be resulted from the
bulky chiral oxazoline derivative as well.
at 2.74 and 3.18 ppm. It should be pointed out that reso-
nance peaks in the 1H NMR spectra of the polymer were
broader than the monomer, indicating that the monomer was
successfully polymerized.
Figure 2 shows 13C NMR of the polymer and monomer. The
polymer showed all important peaks when compared with
that of monomer. The quaternary carbons to iodide in mono-
mer at 93.98 ppm shifted to a low magnetic field (139–143
ppm) in the spectrum of the polymer. In addition, the poly-
mer showed characteristic peak of the quaternary carbon of
the poly(m-phenylene) backbone at around 141 ppm.22
These confirmed that the monomer was successfully con-
verted to polymer as expected.
Polymer Structure
The structures of the polymers were characterized and con-
1
firmed by H NMR, 13C NMR, and IR spectra. These spectro-
scopic methods usually provide valuable information about
1
the monomer and polymer structures. H NMR charts of the
polymer and monomer measured in CDCl3 are given in Fig-
ure 1. The polymers and monomer gave satisfactory analysis
data corresponding to their expected molecular structure.
Signals of chiral oxazoline derivatives were detected for both
a monomer and a polymer, that is, the signals of CHAN was
found at 4.58 ppm and the signal of CH2-Ph were appeared
FTIR was also carried out to confirm the structure of the
monomer and polymers. Figure 3 indicates the FTIR spectra.
The spectrum of polymer was almost similar to the mono-
mer. The C¼¼N stretching bands of oxazoline groups at
1543/cm were detected for both polymer and its monomer.
FIGURE 2 13C NMR spectra of (A) DIPhBnOx and (B) poly(PhBnOx) (run 4 in Table 1) in CDCl3.
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([a]435) range from þ27.3ꢀ to þ791.7ꢀ. The larger value of
specific rotation of the polymer, compared with monomer is
commonly accepted as evidence of the presence of a chiral
higher order structure such as helical conformations with a
predominantly one-handed screw sense.25–27 The lower val-
ues of optical specific rotation of the polymers obtained in
run 5 and run 7 should signify the less higher order struc-
tures, compared with the other polymers, which agreed with
those results of CD measurement and GPC.
Figure 4 showed CD and UV spectra of polymers along with
those of monomers measured in THF at room temperature.
The polymers showed a UV absorption band over a wave-
length range from 220 to 300 nm, having maximum absorp-
tion (emax) at around 250 nm. These could be assigned to
the conjugation of the poly(m-phenylenes) backbones, which
was in agreement with those of reported previously.28–30 In
addition, the monomers showed almost no induced Cotton
effects, while almost the polymers showed intense CD signal
in the transition area of the conjugated polymer backbone.
This could be concluded that the polymers possess a higher-
order structure, a partial helical structure, in view of their
comparatively low specific rotations and low molecular
weights.
FIGURE 3 FT IR spectra of (A) DIPhBnOx and (B) poly(PhBnOx)
(run 3 in Table 1).
The difference was what of only the monomer showed a
strong absorption band at 500/cm due to the C-I bending
vibrations absorption bands while the polymer did not,
implying that the nickel catalyst is effective to polymerize
this type of monomers.
Chiroptical Properties of Polymers
It is well understood that the helical structure of the poly-
mer could be induced by introduction of suitable chiral
groups into the polymer backbones. There are evidences that
the presence of optically active chiral oxazoline derivatives
side group could induce the polymer main chain with an
enantiomeric excess in the helical conformation.23,24 This
study expects that the main chain of the poly(m-phenylene)s
could be endowed to form higher order structure with a pre-
vailing helical handedness as well. The chiroptical activities
of the polymers were then examined by means of optical
specific rotation and CD.
Next, we further investigated the responsiveness of the poly-
mer to metal ions in a solution. As shown in Figure 5, the
addition of Zn(II) salt to a polymer solution brought to the
dramatically changes in both CD and UV spectra patterns.
The formation of Zn(II)-induced complexes has been evi-
denced by the CD. The [h]max of the polymer at 288 nm were
found to be increased with an increase of Zn(II) contents,
whereas almost no change was observed in the case of
monomer. There have been evidences that addition of metal
salts into the solution of chiral oxazoline substituted poly-
mers produced chiral aggregations with a handedness.31,32
In this study, the increase in CD intensities observed in the
p–p* transition region of the polymer backbones may also
cause the formation of chiral supramolecular aggregates with
handedness likewise.
As given in Table 1, the values of optical specific rotation
([a]435) of DBrPhBnOx and DIPhBnOx are ꢂ11.9ꢀ and ꢂ3.2ꢀ,
respectively. In contrast to the monomers, their correspond-
ing polymers have positive optical specific rotation value
FIGURE 4 CD and UV–vis spectra of DXPhBnOx and poly(PhBnOx); (A): (a) DBrPhBnOx, (b) DIPhBnOx, (c) run 1, (d) run 2, (e) run
3, (f) run 4, (g) run 5, (h) run 6, and (i) run 7 in Table 1; (B) expanded CD spectra at 240–360 nm.
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Thermal Properties of the Polymer
Thermal stability of polymer was investigated using TGA
under nitrogen atmosphere, and compared with its monomer
(Fig. 6). The first slight weight loss of the polymer and its
monomer were observed at below 100 ꢀC, which should be
assigned to the loss of absorbed moisture. Another noticea-
ble feature is that the degradation profile of the polymer is
not similar to that of monomer. The significant weight loss
(5%) of the polymer began around 275 ꢀC, and gradually
decrease up to 500 ꢀC while the monomer showed poor
thermal stability profile. The temperature for which the
weight loss is 5%, 10%, and maximum weight loss tempera-
ture of polymer were observed at higher value than that of
the monomer. It is notable to conclude that poly(m-phenyl-
ene) bearing chiral oxazoline derivatives show good thermal
stability, attributed from the oxazoline pendants.11,13
FIGURE 6 TGA curves of (A) DIPhBnOx and (B) poly(PhBnOx)
(run 3 in Table 1) at heating rate of 10 ꢀC/min.
CONCLUSIONS
ACKNOWLEDGMENTS
We have successfully prepared the novel poly(m-phenylene)
bearing chiral oxazoline derivatives at the side chain
from (S)-4-benzyl-2-(3,5-dihalidephenyl)oxazoline derivatives
(DXPhBnOx, X ¼ Br or I) through the Yamamoto coupling
reaction using Ni(COD)2 and 2,20-bipyridyl as catalytic sys-
tem. The yields, molecular weight, specific rotation of the
polymers were influenced by polymerization conditions such
as solvent and temperature. The polymers exhibit good ther-
The authors are thankful to S. Fujii at Faculty of Engineering,
Yamaguchi University, for elemental analysis measurement.
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