Helical poly(phenylacetylene)s containing schiff-base and amino groups: Synthesis, secondary structures, and responsiveness to benzoic acid

Article abstract of DOI:10.1002/pola.26955

Novel acetylenic monomers containing Schiff-base and amino groups, (S)-N-(4-ethynylbenzylidene)-1-phenylethanamine (1a), (R)-N-(4- ethynylbenzylidene)-1-phenylethanamine (1b), N-(4-ethynylbenzylidene)-1- phenylethanamine (1c), (R)-N-(4-ethynylbenzyl)-1-ph

Full text of DOI:10.1002/pola.26955

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Helical Poly(phenylacetylene)s Containing Schiff-Base and Amino Groups:
Synthesis, Secondary Structures, and Responsiveness to Benzoic Acid
Yu Zhang,1 Keke Gao,1 Zhongfu Zhao,1 Dongmei Yue,2 Yanming Hu,1 Toshio Masuda3,4
1State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering, School of Chemical Engineering,
Dalian University of Technology, Dalian 116024, China
2State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
3Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-8505, Japan
4Research Center for Environmentally Friendly Materials Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho,
Muroran 050–8585, Japan
Correspondence to: Y. Hu (E-mail: ymhu@dlut.edu.cn)
Received 15 May 2013; accepted 18 September 2013; published online 5 October 2013
DOI: 10.1002/pola.26955
ABSTRACT: Novel acetylenic monomers containing Schiff-base
and amino groups, (S)-N-(4-ethynylbenzylidene)-1-phenylethan-
amine (1a), (R)-N-(4-ethynylbenzylidene)-1-phenylethanamine
(1b), N-(4-ethynylbenzylidene)-1-phenylethanamine (1c), (R)-N-
(4-ethynylbenzyl)-1-phenylethanamine (1d), and (R)-N-(4-ethy-
nylbenzyl)-1-phenylethanamine (1e) were synthesized and poly-
merized with [(nbd)RhCl]2/Et3N catalyst to afford the
corresponding polymers 2a-e with moderate molecular weights
(Mn 5 9000–60,000) in high yields (85–97%). All the polymers
were soluble in common organic solvents including toluene,
CHCl3, CH2Cl2, THF, and DMF. Large optical rotations and
strong CD signals demonstrated that 2a, 2b, 2d, and 2e take
sense. The effects of solvents and temperature revealed that
these polymers took dynamic helical structure based on the
steric effect of side groups. The CD patterns of 2d and 2e con-
taining free amino moieties were completely inverted by the
addition of benzoic acid. Upon further addition of NaOH, the
CD pattern returned to the original one, indicating the reversi-
ble conformational change of these polymers according to pH.
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KEYWORDS: conjugated polymers; helix; polyacetylenes; stim-
helical structures with
a
predominantly one-handed screw
uli-sensitive; polymers
ties, which are derived from their conjugated main chain and
secondary structures. Since the pioneering work by Ciardelli
on helical poly(1-alkyne)s,6 a variety of helical polyacetylene
derivatives based on optically active side groups have been
successfully synthesized; for example, poly(alkyl propio-
late)s,13 poly(N-propargylalkylamide)s,7 poly(N-propargylcar-
bamate)s,14 and poly(methylpropargyl ester)s.15 As examples
of dynamic helical polymers, some polyacetylenes undergo
helix/random coil transition and/or helix inversion by exter-
nal stimuli such as temperature, light, and change of medium
conditions including polarity and pH.16–18 These properties
are attractive in a wide variety of fields including life science,
pharmacy, and chemical engineering.
INTRODUCTION The design and synthesis of artificial helical
polymers have attracted considerable attention.1 Such stud-
ies aim at not only the imitation of elegant and sophisticated
structures of biomacromolecules such as DNA and protein in
nature but also the development of novel functional materi-
als with unique properties. Several synthetic helical poly-
mers
including
polyisocyanides,
polymethacrylates,
polyisocyanates, polysilanes, and polyacetylenes have been
successfully synthesized so far. As a category of typical con-
jugated polymers, polyacetylene and its derivatives have
been extensively studied because of their interesting physical
and chemical properties.2–5 Substituted polyacetylenes can
form helical structures with a predominantly one-handed
screw sense by the introduction of appropriate chiral sub-
stituents into the side chain or helix-sense-selective polymer-
ization of achiral monomers.6–9 Such polymers are of great
interest for potential applications as functional materials in
molecular recognition,10 asymmetric catalysis,11 and chemi-
cal sensors12 based on their electronic and optical proper-
Meanwhile, Schiff-bases are an important class of ligands in
scientific research because they can be easily synthesized by
the condensation of a variety of amines and aldehydes and
form complexes with various metal ions.19,20 Besides the
extensively studied low molecular weight Schiff-bases,
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthesis of Schiff-base- and amine-containing polyacetylene derivatives.
various polymers containing Schiff-base moieties have been
synthesized, and they exhibit interesting and important
properties such as metal ion recognition, antibacterial and
antifungal activities, high thermal stability, catalytic activity,
and photoluminescence.21–25 For instance, vertically standing
nanowalls of polyazomethines on graphite are synthesized
by polycondensation of diamine and dialdehyde/diketone,
and the morphology and luminescent properties can be well-
controlled.26 Recent examples of polymers containing Schiff-
base type groups include polystyrene,27 polyacrylate,28 and
polynorbornene.29,30 However, only a few polyacetylenes
containing achiral Schiff-base groups have been reported by
Balcar’s and our groups so far,31,32 and the studies are
restricted to the polymerizability of the corresponding mono-
mers. Recently, the research interest is focused on the chiral
Schiff-bases because their metal complexes are efficient in
asymmetric catalysis for a variety of enantioselective reac-
tions such as Diels–Alder cyclization, aziridination, and
hydrogenation.19,33 The combination of functional Schiff-base
group as a chiral auxiliary and conjugated main chain of pol-
yacetylene will afford functional helical polymer materials,
which may find potential application in asymmetric catalysis.
On the other hand, the Schiff-base group can be easily
changed into amino group by reduction. By using this reac-
tion, incorporation of free amino groups into the polymer is
possible, which may lead to acid-responsive helical polymers.
spectropolarimeter. Thermogravimetric analyses (TGA) were
conducted under nitrogen at a heating rate of 10 ꢀC/min
with a Perkin-Elmer TGA7 thermal analyzer.
Materials
THF used for polymerization was distilled over CaH2 prior to
use. Triethylamine was distilled and dried over potassium
hydroxide. 4-Bromobenzaldehyde (Aladdin Co.), (R)-(1)-1-
phenylethylamine
(Aldrich),
(S)-(1)-1-phenylethylamine
(Aldrich), trimethylsilylacetylene (Acros), bis(triphenylphos-
phine)palladium(II) dichloride (Aldrich), and [(nbd)RhCl]2
(Aldrich) were used as received. Other solvents such as
methanol, toluene, and CHCl3 were of high purity and used
without further purification. 4-Ethylnylbenzaldehyde was
synthesized referring to the literature.34
Monomer Synthesis
(R)-N-(4-Ethynylbenzylidene)-1-Phenylethanamine (1a)
A 100 mL three-necked flask was equipped with a three-way
stopcock and a magnetic stirring bar. After the flask was flushed
with nitrogen, (R)-1-phenylethanamine (1.21 g, 10 mmol) and
4-ethylnylbenzaldehyde (1.3 g, 10 mmol) were dissolved in tol-
uene (70 mL). The reaction mixture was heated at reflux tem-
perature for 10 h. The generated water was separated with a
Dean-Stark apparatus. After that, toluene in the reaction mixture
was evaporated off, and the crude product was purified by
recrystallization in ethanol to give light yellow crystals; yield
54%. [a]D 5 –154ꢀ (c 5 0.1 g/dL, CHCl3, room temperature).
The present study deals with the synthesis and polymerization
of novel phenylacetylenes having chiral Schiff-base moieties
(1a–c) as well as their hydrogenated analogues with second-
ary amino groups (1d and 1e) (Scheme 1). The secondary
structure of the resultant polymers was investigated in detail.
Moreover, chiroptical responsiveness of the polymer possess-
ing secondary amino groups to benzoic acid was examined.
1H NMR (400 MHz, CDCl3): d 8.32 (s, 1H, CH5N), 7.72–7.23
(m, 9H, ArH), 4.53 (s, 1H, N-CH), 3.15 (s, 1H, HCꢁC), 1.58
(m, 3H, CH3). 13C NMR (100 MHz, CDCl3): d 158.7 (CH5N),
145.1, 136.8, 132.4, 128.6, 128.3, 127.1, 126.8, 124.3, 83.5,
79.0, 70.0, 25.0. IR (cm21, KBr): 3377, 3298, 3080, 3027,
2967, 2849, 1646 (C5N), 1638, 1599, 1491, 1449, 1377,
837, 763, 700. ANAL. CALCD. for C17H15N: C, 87.52; H, 6.48; N,
6.00. Found: C, 87.32; H, 6.40; N, 5.87.
EXPERIMENTAL
Measurements
1H and 13C NMR spectra were recorded on a Varian Unity-
400 spectrometer. IR spectra were measured on a BRUKER
Vertex-70 spectrophotometer. The number- and weight-
average molecular weights (Mn and Mw) of polymers were
determined with a gel permeation chromatograph equipped
with a Waters 515 HPLC pump and a Waters 2414 differen-
tial refractometer using THF as eluent at a flow rate of 1.0
mL/min, calibrated with polystyrene standard. Elemental
analysis of monomers was carried out at the Analytical Cen-
ter of Dalian University of Technology. Specific rotations
([a]D) were measured on a JASCO DIP-1000 digital polarime-
ter. CD and UV-vis spectra were recorded on a JASCO J-810
(S)-N-(4-Ethynylbenzylidene)-1-Phenylethanamine (1b)
This monomer was prepared by the same method as for
monomer 1a using (S)21-phenylethanamine instead of
(R)21-phenylethanamine to give light yellow crystals. Yield
50%. [a]D 5 1131ꢀ (c 5 0.1 g/dL, CHCl3, room temperature).
1H NMR (400 MHz, CDCl3): d 8.32 (s, 1H, CH5N), 7.73–7.24
(m, 9H, ArH), 4.54 (s, 1H, N-CH), 3.16 (s, 1H, HCꢁC), 1.58
(m, 3H, CH3). 13C NMR (100 MHz, CDCl3): d 158.7 (CH5N),
145.1, 136.8, 132.4, 128.6, 128.3, 127.1, 126.8, 124.3, 83.5,
79.0, 70.0, 25.0. IR (cm21, KBr): 3376, 3297, 3078, 3028,
2966, 2850, 1645 (C5N), 1638, 1600, 1491, 1450, 1378,
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838, 763, 699. ANAL. CALCD. for C17H15N: C, 87.52; H, 6.48; N,
6.00. Found: C, 87.42; H, 6.21; N, 5.88.
and Et3N were added to a monomer solution under nitrogen,
and the resulting solution ([M]0 5 0.25 mol/L, [Rh]/
[M]0 5 50, [Et3N]/[Rh] 5 10) was kept at 30 ꢀC for 4 h. The
formed polymers were isolated by precipitation into a large
amount of methanol or hexane, and dried to constant weight
under reduced pressure; the polymer yields were deter-
mined by gravimetry.
N-(4-Ethynylbenzylidene)-1-Phenylethanamine (1c)
This monomer was prepared by the same method as for mono-
mer 1a using racemic 1-phenylethanamine instead of (R)21-
phenylethanamine to give light yellow crystals. Yield 50%.
1H NMR (400 MHz, CDCl3): d 8.32 (s, 1H, CH5N), 7.73–7.24
(m, 9H, ArH), 4.54 (s, 1H, N-CH), 3.16 (s, 1H, HCꢁC), 1.58 (s,
3H, CH3). 13C NMR (100 MHz, CDCl3): d 158.7 (CH5N),
145.1, 136.8, 132.4, 128.6, 128.3, 127.1, 126.8, 124.3, 83.5,
79.0, 70.0, 25.0. IR (cm21, KBr): 3375, 3300, 3080, 3029,
2969, 2847, 1646 (C5N), 1638, 1598, 1490, 1450, 1378,
837, 763, 700. ANAL. CALCD. for C17H15N: C, 87.52; H, 6.48; N,
6.00. Found: C, 87.59; H, 6.55; N, 5.73.
Spectroscopic Data of the Polymers
2a: H NMR (400 MHz, CDCl3): d 7.83 (br, 1H, CH5N), 7.52–
1
6.60 (br, 9H, Ar), 5.81 (br, 1H, HC5C), 4.32 (br, 1H, NCH),
1.56 (br, 3H, HCCH3). IR (cm21, KBr): 3080, 3027, 2967,
2849, 1646 (HC5N), 1638, 1599, 1491, 1449, 1377, 837,
763, 700. 2b: 1H NMR (400 MHz, CDCl3): d 7.82 (br, 1H,
CH5N), 7.49–6.58 (br, 9H, Ar), 5.82 (br, 1H, HC5C), 4.31 (br,
1H, NCH), 1.54 (br, 3H, HCCH3). IR (cm21, KBr): 3297, 3078,
3028, 2966, 2850, 1645 (C5N), 1638, 1600, 1491, 1450,
1378, 838, 763, 699. 2c: 1H NMR (400 MHz, CDCl3): d 7.81
(br, 1H, CH5N), 6.58–7.51 (br, 9H, Ar), 5.83 (br, 1H, HC5C),
4.30 (br, 1H, NCH), 1.53 (br, 3H, HCCH3). IR (cm21, KBr):
3080, 3029, 2969, 2847, 1646 (C5N), 1638, 1598, 1490,
1450, 1378, 837, 763, 700. 2d: d 7.81–6.20 (br, 9H, Ar), 5.82
(br, 1H, HC5C), 3.80 (br, 2H, H2CNH), 2.82 (br, 1H, HCNH),
1.56 (br, 3H, HCCH3). 2e: d 7.80–6.18 (br, 9H, Ar), 5.83 (br,
1H, HC5C), 3.80 (br, 2H, H2CNH), 2.82 (br, 1H, HCNH), 1.56
(br, 3H, HCCH3).
(R)-N-(4-Ethynylbenzyl)-1-Phenylethanamine (1d)
A 100 mL three-necked flask was equipped with a three-way
stopcock and a magnetic stirring bar. After the flask was
flushed with nitrogen, 1a (1.0 g, 4.3 mmol) was dissolved in
dry methanol, and sodium borohydride (NaBH4, 0.49 g, 12.9
mmol) was added. The reaction mixture was stirred under
nitrogen at room temperature for 72 h. After the solvent was
removed by rotary evaporation, the residue was dissolved in
diethyl ether, washed with aqueous NaOH solution (1.0 mol/
L; 50 mL) and water (50 mL 3 3), and dried over MgSO4.
The crude product was purified by silica gel chromatography
with hexane/diethyl ether (1/1, v/v) as eluent to give the
desired product as a colorless liquid. Yield 85%. [a]D 5 –26ꢀ
(c 5 0.1 g/dL, CHCl3, room temperature).
RESULTS AND DISCUSSION
Monomer Synthesis and Polymerization
Acetylenic monomers 1a–c containing Schiff-base groups
were synthesized by condensation of 4-ethynylbenzaldehyde
with the corresponding amine according to Scheme 2. The
hydrogenation of 1a and 1b with sodium borohydride in dry
methanol gave secondary amine-containing monomers 1d
and 1e. The structures of monomers were identified by 1H
NMR, 13C NMR, and IR spectroscopies as well as elemental
analysis.
1H NMR (400 MHz, CDCl3): d 7.22–7.44 (m, 9H, ArH), 3.83
(m, 3H), 3.09 (1H, HCꢁC), 1.69 (1H, NH), 1.41 (s, 3H, CH3).
13C NMR (100 MHz, CDCl3): d 145.5, 141.7, 132.3, 128.7,
128.2, 127.2, 126.8, 120.6, 83.8, 77.5, 57.6, 51.4, 24.6. IR
(cm21, CHCl3): 3423, 3286, 2933, 2780, 2485, 2417, 2105,
1658, 1577, 1498, 1453, 1384, 1084, 1026, 920,824, 766,
701, 559. ANAL. CALCD. for C17H17N: C, 86.77; H, 7.28; N, 5.95.
Found: C, 86.41; H, 7.59; N, 5.73.
Rhodium catalysts are known to be tolerant of polar and
functional groups, and polymerize monosubstituted acety-
lenes to afford the corresponding polyacetylenes.35–37 The
polymerizations of monomers 1a-e were carried out in THF
with [(nbd)RhCl]2/Et3N catalyst, whose results are listed in
Table 1. Monomers 1a-c containing Schiff-base groups gave
polymers 2a-c as methanol-insoluble products with fairly
high molecular weights (Mn 5 4.7 3 104 to 6.0 3 104) in
high yields (91–97%). The polymerization of 1d and 1e hav-
ing secondary amine moieties also proceeded smoothly,
affording the corresponding polymers 2d and 2e in good
yields (ca. 85%). Because 2d and 2e were partly soluble in
methanol, which might be attributable to the affinity of polar
amino groups to methanol, these polymers were isolated as
hexane-insoluble products. The apparent molecular weights
of 2d and 2e (Mn ꢂ1.0 3 104) were somewhat lower than
those of 2a–c bearing Schiff-base moieties. This might be
due to the presence of amino groups in the polymer struc-
ture, which commonly extends the retention time of GPC.
(S)-N-(4-Ethynylbenzyl)-1-Phenylethanamine (1e)
This monomer was prepared by the same method as for
monomer 1d using 1b instead of 1a to give a colorless liq-
uid. Yield 87%. [a]D 5 124ꢀ (c 5 0.1 g/dL, CHCl3, room
temperature).
1H NMR (400 MHz, CDCl3): d 7.22–7.44 (m, 9H, ArH), 3.83
(m, 3H), 3.09 (1H, HCꢁC), 1.69 (1H, NH), 1.41 (s, 3H, CH3).
13C NMR (100 MHz, CDCl3): d145.5, 141.7, 132.3, 128.7,
128.2, 127.2, 126.8, 120.6, 83.8, 77.5, 57.6, 51.4, 24.6. IR
(cm21, CHCl3): 3423, 3286, 2933, 2780, 2485, 2417, 2105,
1658, 1577, 1498, 1453, 1384, 1084, 1026, 920,824, 766,
701, 559. ANAL. CALCD. for C17H17N: C, 86.77; H, 7.28; N, 5.95.
Found: C, 87.12; H, 6.90; N, 5.68.
Polymerization
Polymerizations were carried out in a glass tube equipped
with a three-way stopcock under dry nitrogen. [(nbd)RhCl]2
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SCHEME 2 Synthesis of acetylenic monomers 1a–e.
Polymers 2a-e showed good solubility in common organic
solvents such as toluene, CH2Cl2, CHCl3, THF, and DMF.
1, the polymers showed much larger [a]D values than those
of the corresponding monomers, suggesting that the poly-
mers formed helices with predominantly one-handed screw
sense. The signs of [a]D of 2a and 2b as well as 2d and 2e,
enantiomerically isomeric polymers, were opposite to each
other and the absolute values were almost the same, as
expected. Figure 2 depicts the CD and UV–Vis spectra of 2a–
c measured in DMF. 2a and 2b showed split-type intense
Cotton effects at a UV-Vis region of 260–450 nm, where
absorptions due to p-conjugated structure of the polymers
typically appear. In contrast, the corresponding monomers
showed rather weak CD signals and a strong UV absorption
only in the range of 250–280 nm. These results indicate that
the polymers take helical structures with predominantly
one-handed screw sense based on higher-order structures of
polyacetylene backbone. Polymers 2a and 2b having pendant
groups with opposite absolute configurations exhibited
mirror-image CD spectra at 260–450 nm and their optical
rotations possessed practically the same absolute values
with opposite signs (see Table 1; 2a: 2412ꢀ and 2b:
1402ꢀ). Thus, it is concluded that these polymers have heli-
cal structures with opposite screw senses and that the helix
sense is determined by the chirality of the Schiff-base
groups.
The polymer structures were examined by IR and 1H NMR
spectroscopies. The monomers exhibited absorption bands at
3377 and 2100 cm21 assignable to the ꢁC–H and –CꢁC–
stretching vibrations, respectively, which disappeared in the
1
IR spectra of the polymers. The H NMR spectra of the poly-
mers were compatible with the IR data (see, e.g., Fig. 1).
Namely, the ethynyl proton peak of the monomers was
absent in the polymers, and meanwhile the azomethine pro-
ton signal at 7.8 ppm remained intact in the polymers. Thus,
these results clearly indicate both conversion of the triple
bond into the double bond during polymerization and no
contamination with the monomer in the polymeric product.
In general, Rh catalysts efficiently polymerize monosubsti-
tuted acetylenes by the insertion mechanism to give cis-ster-
eoregular polyacetylenes.38,39 The geometric structure of the
polymers in the present study is also the case; that is, 2a–e
possessed cis-rich structures (82–97%) in the main chain,
which were determined by the integration ratio of cis vinyl
proton and the other proton signals.
Chiroptical Properties of the Polymers Containing Schiff-
Base Groups
Figure 3 illustrates the CD and UV-Vis spectra of 2a meas-
ured in various solvents. 2a showed CD profiles similar to
The chiroptical properties of the polymers were examined by
polarimetry, CD, and UV–Vis spectroscopy. As shown in Table
TABLE 1 Polymerizations of Monomers 1a–ea
Mn 3 1024e
Mw/Mn
cis Contentf (%)
[a]D (deg)
b
e
g
[a]D (deg)
Yield (%)
1a
1b
1c
1d
1e
2154
1131
0
96c
91c
97c
85d
87d
4.7
4.8
6.0
0.9
1.0
3.2
4.9
4.7
4.5
4.8
96
96
97
82
83
2412
1402
0
226
124
2108
1115
a
e
Polymerization conditions: in THF, 30 ꢀC, 4 h, [M]0 5 0.25 mol/L, [M]0/
Determined by GPC using THF as eluent with
a
polystyrene
[Rh] 5 50.
calibration.
b
f
Measured by polarimetry at 20 ꢀC, c 5 0.10 g/dL in CHCl3.
Determined by 1H NMR.
c
g
Methanol-insoluble part.
Measured by polarimetry at 20 ꢀC, c 5 0.10 g/dL in DMF.
d
Hexane-insoluble part.
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FIGURE 1 1H NMR spectra of 1a and 2a measured in CDCl3.
FIGURE 3 CD and UV-Vis spectra of 2a measured in various sol-
vents at 20 ꢀC (c 5 0.05 mg/mL). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
one another in these solvents. The intensity of the CD signals
increased in the order of CHCl3 < THF < CH2Cl2 < DMF < to-
luene, while the molar absorptivity (e) of UV-Vis spectra
slightly increased in the same order. This order of solvents is
not related with the polarity and basicity of solvents. These
findings suggest that the present helical structure originates
from steric effects and not from hydrogen bonding. Further,
the variation of Cotton effects with solvents indicates that
the helix is a dynamic one.
We further examined the thermal stability of the helical
conformation of the polymers. Figure 4 shows the tempera-
ture dependence of the CD and UV-vis spectra of 2a meas-
ured in DMF. The CD intensity decreased as the
temperature was raised from 0 to 80 ꢀC, and the molar
ellipticity value [h] became 50% of the value of 0 ꢀC at 80 ꢀC. In
general, the helicity of polymer chain tends to decrease with
increasing temperature, because the thermal perturbation acti-
vates the randomization of chain conformations, and the pres-
ent temperature dependence of helix agrees with this tendency.
The slight decrease in UV-Vis absorption with increasing
temperature is also explicable in terms of thermal perturbation
of polymer conformation. The magnitude of the CD signal
reversibly recovered the original one by lowering the tempera-
ture. Similar results were observed with 2b and other
helical polyacetylene derivatives including poly(1-methyl-
propargyl-N-alkylcarbamate)s,40 poly(ethynylcarbazole),41 and
FIGURE 2 CD and UV-Vis spectra of 1a and 2a-c measured in
DMF at 20 ꢀC (c 5 0.05 mg/mL). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 5 CD and UV-Vis spectra of 2d measured in various
solvents (c 5 0.05 mg/mL). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 4 CD and UV-Vis spectra of 2a in DMF measured at vari-
ous temperatures (c 5 0.05 mg/mL). [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
poly(phenylacetylene)s.42 This is
a characteristic of the
dynamic helical conformation in polyacetylene derivatives.1
Chiroptical Properties of the Polymers Containing
Secondary Amino Groups
As aforementioned, polymers containing free secondary
amino groups, 2d and 2e, were successfully synthesized by
the polymerization of the corresponding monomers. Polymer
2d exhibited intense Cotton effects in THF, CH2Cl2, CHCl3,
and DMF (Fig. 5), and moderate solvent effects were
observed. The molar absorptivity of UV-Vis spectra increased
in the order of THF < CH2Cl2 < CHCl3 < DMF, and the inten-
sity of CD signal increased in the same order. Polymers 2d
and 2e exhibited mirror-image CD spectra at 250–430 nm to
each other (Fig. 6), which is consistent with their [a]D values.
It is hence concluded that these polymers bearing amino
groups also adopt helical conformations with an excess of
predominantly one-handed screw sense in the solvents. Like
the case of 2a and 2b, the CD intensity of 2d decreased by
ꢀ
raising the temperature from 0 to 80 C (Fig. 7), which indi-
cates that the degree of predominance of one-handedness of
screw sense decreased. A similar result was observed also
with 2e. When the CD intensities of polymers are compared,
the degrees of predominance of one-handedness of screw
sense are somewhat lower in 2d and 2e than those in 2a
and 2b.
FIGURE 6 CD and UV-Vis spectra of 2d and 2e measured in
DMF at 20 ꢀC (c 5 0.05 mg/mL). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 7 CD and UV-Vis spectra of 2d in DMF measured at vari-
ous temperatures (c 5 0.05 mg/mL). [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 8 CD and UV-Vis spectra of 2d upon addition of ben-
zoic acid measured in CHCl3 at 20 ꢀC (c 5 0.05 mg/mL; the
curve of 5.0 equiv NaOH denotes that 5.0 equivalents of NaOH
was added to the polymer solution containing 5.0 equivalents
of benzoic acid). [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
Responsiveness of 2d and 2e to Benzoic Acid
Polyacetylenes bearing free carboxy or amino groups in side
chains change the conformation according to pH.43–46 For
instance, the helix content of poly(N-propargylamides) with
free amino groups decreases upon addition of o-phthalic
acid,45 and poly[(1R,2S)-N-(4-ethynylbenzyl)norephedrine]
exhibits a unique helix-helix transition upon complexation
with chiral acids.46 Thus, it is anticipated that the conforma-
tional structure of 2d and 2e with free amino groups is
affected by the addition of acids. In fact, as seen in Figure 8,
a drastic change occurred in the CD spectrum of 2d in the
presence of benzoic acid in CHCl3. With increasing amount
of benzoic acid, the CD signals at about 325 and 380 nm
gradually decreased, and they almost disappeared upon addi-
tion of an equimolar amount of acid. Further addition of ben-
zoic acid gave rise to an opposite-signed CD signals with the
same shapes. These results suggest that helix inversion pro-
gressively proceeded by complexation of the side-chain
amino groups with relatively small amounts of benzoic acid.
Thus, an achiral benzoic acid can be used to control the helix
sense of 2d. It is noted that the addition of two equivalents
of benzoic acid brought about a practically mirror-imaged
CD pattern to that of 2d alone and that five equivalents of
benzoic acid resulted into even more intense CD signals. The
[h] value of 2d at 380 nm varied from 25000 to 15000 deg
cm2 dmol21, when five equivalents of benzoic acid was
added. The detailed mechanism of the helix inversion is not
clear, but it is speculated that complexation of benzoic acid
FIGURE 9 TGA curves of the polymers (in nitrogen, heating
rate 10 ꢀC/min).
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ARTICLE
7 R. Nomura, Y. Fukushima, H. Nakako, T. Masuda, J. Am.
Chem. Soc. 2001, 122, 8830–8836.
with the amino groups of the polymer changes the shape
and bulkiness of the side groups, leading to helix inversion.
When an equimolar amount of NaOH to the acid was added
to the solution of 2d containing five equivalents of benzoic
acid, the CD spectroscopic pattern completely returned to
the one before the acid addition. Similar results were also
observed with 2e. Thus, we could confirm the reversible
conformational change of 2d and 2e according to pH.
8 T. Aoki, T. Kaneko, N. Maruyama, A. Sumi, M. Takahashi, T.
Sato, M. Teraguchi, J. Am. Chem. Soc. 2003, 125, 6346–6347.
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10 K. Zhou, L. Tong, J. Deng, W. Yang, J. Mater. Chem. 2010,
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11 Z. Tang, H. Iida, H. Hu, E. Yashima, ACS Macro. Lett. 2012,
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Thermal Stability of the Polymers
12 R. Sakai, N. Sakai, T. Satoh, W. Li, A. Zhang, T. Kakuchi,
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The thermal stability of the polymers was examined by TGA
in air (Fig. 9). The onset temperatures of weight loss of 2a
and 2d were approximately 260 ꢀC, which is about same as
that of poly(phenylacetylene). The weight residue at 800 ꢀC
for 2a was 68%, whereas those of 2d and poly(phenylacety-
lene) were approximately 10%. This indicates that all these
polymers begin to be oxidized at similar temperatures, while
certain ꢀinvolatile materials remain at high temperatures up
to 800 C in 2a.
13 (a) H. Nakako, R. Nomura, M. Tabata, T. Masuda,
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CONCLUSIONS
16 H. Sogawa, M. Shiotsuki, F. Sanda, J. Polym. Sci. Part A:
Polym. Chem. 2012, 50, 2008–2018.
In this study, we have demonstrated the polymerization of
novel acetylenic monomers containing Schiff-base and sec-
ondary amino groups with a Rh catalyst. The polymerization
satisfactorily proceeded to afford the corresponding poly-
mers 2a–e in good yields. These polymers were soluble in
common organic solvents. Polymers 2a, 2b, 2d, and 2e
exhibited large optical rotations and intense CD signals, indi-
cating that they formed helical structure with predominantly
one-handed screw sense. The effects of solvents and temper-
ature revealed that these polymers took dynamic helical
structure based on the steric effect of side groups. Polymers
bearing free amino groups 2d and 2e exhibited a helix-helix
transition upon complexation with an achiral benzoic acid,
and their helical senses underwent reversible transformation
of helicity upon further addition of NaOH.
17 J. Qu, F. Jiang, H. Chen, F. Sanda, T. Masuda, J. Polym. Sci.
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26 R. Higuchi, R. Tanoue, K. Sakaguchi, K. Yanai, S. Uemura,
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The authors acknowledge the financial support from the
National Natural Scientific Foundation of China (No.
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