Helix-sense inversion of poly(phenylacetylene) derivatives bearing an optically active substituent induced by external chiral and achiral stimuli

Article abstract of DOI:10.1021/ma025726p

A study of helix-sense inversion of poly(phenylacetylene) derivatives was carried out. The poly(R)-1 and poly(S)-1 undergo a helix-helix transition by response to external, chiral, and achiral interaction with small molecules. The results show that the poly-1's have a predominant one-handed helical conformation in solution, and the helix-sense undergoes a transition from one helix to another in response to external, chiral, and achiral stimuli, such as temperature, solvent, and chiral interaction with small molecules.

Full text of DOI:10.1021/ma025726p

1480
Macromolecules 2003, 36, 1480-1486
Helix-Sense Inversion of Poly(phenylacetylene) Derivatives Bearing an
Optically Active Substituent Induced by External Chiral and Achiral
Stimuli
Ka zu h id e Mor in o, Ka tsu h ir o Ma ed a , a n d Eiji Ya sh im a *
Department of Molecular Design and Engineering, Graduate School of Engineering,
Nagoya University, Chikusa-ku, Nagoya 464-8603, J apan
Received October 9, 2002; Revised Manuscript Received December 31, 2002
ABSTRACT: Novel, optically active, cis-transoidal poly(phenylacetylenes), poly((R)-(-)- or (S)-(+)-
(4-((1-(1-naphthyl)ethyl)carbamoyl)phenyl)acetylene) (poly(R)-1 and poly(S)-1), were prepared, and their
chiroptical properties were investigated by means of circular dichroism spectroscopy. We have found that
the poly-1’s have a predominant one-handed helical conformation in solution, and the helix-sense undergoes
a transition from one helix to another in response to external, chiral, and achiral stimuli, such as
temperature, solvent, and chiral interaction with small molecules.
In tr od u ction
rare. Only a few optically active poly(phenylacetylenes)
bearing a chiral amine, i.e., (1R,2S)-norephedrine8 or
cyclodextrin residues,9 exhibited a helix-helix transi-
tion induced by external chiral stimuli through diaster-
eomeric acid-base interactions or complexations with
chiral guest molecules, respectively.
Optically active helical polymers have aroused con-
siderable interest from many viewpoints including
syntheses, structures, and functions.1 We recently found
that stereoregular cis-transoidal poly(phenylacetylene)
derivatives bearing various functional groups such as
poly((4-carboxyphenyl)acetylene) form an induced one-
handed helical structure upon complexation with opti-
cally active compounds capable of interacting with the
functional groups of the poly(phenylacetylenes).2 The
complexes show an induced circular dichroism (ICD) in
the UV-vis region, probably due to the prevailing one-
handed helix formation of the polymer backbone upon
complexation with optically active compounds. The
Cotton effect signs can be used as a probe for the
chirality assignment of the chiral compounds. More
interestingly, the macromolecular helicity induced on
poly((4-carboxyphenyl)acetylene) (PCPA) with chiral
amines, for instance (R)-(+)-1-(1-naphthyl)ethylamine
((R)-2), was found to be “memorized” when the chiral
amine was replaced by various achiral amines.3 To
elucidate the mechanism of this unique helicity induc-
tion and memory process, novel, optically active poly-
(phenylacetylenes), poly((R)-(-)- or (S)-(+)-(4-((1-(1-
naphthyl)ethyl)carbamoyl)phenyl)acetylene) (poly(R)-1
and poly(S)-1) (Chart 1), were prepared, and their
chiroptical properties were investigated using circular
dichroism (CD) spectroscopy. Poly-1’s are considered as
model polymers for the helicity induction of PCPA with
(R)-2 or (S)-2 because poly-1 has an optically active (R)-
or (S)-2 unit as the substituent through covalent bond-
ing. We report here that poly(R)-1 and poly(S)-1 undergo
a helix-helix transition by response to external, chiral,
and achiral stimuli, such as temperature, solvent, and
chiral interaction with small molecules. Although sev-
eral synthetic polymers as well as biopolymers4 are
known to exhibit helix inversion between right- and left-
handed helical conformations by changing the external
conditions, such as solvent,5 temperature,6 or by the
irradiation of light,7 helicity inversion of a macromo-
lecular helicity responding to chiral stimuli is still quite
Exp er im en ta l Section
Ma ter ia ls. Tetrahydrofuran (THF) was dried over sodium
benzophenone ketyl and distilled onto calcium hydride, fol-
lowed by vacuum distillation onto LiAlH4 under nitrogen.
Triethylamine (Et3N) was dried over KOH pellets and distilled
onto KOH under nitrogen. These solvents were distilled under
high vacuum just before use. Dimethylformamide (DMF) was
dried over calcium hydride and distilled under reduced pres-
sure. Chloroform (CHCl3) was dried over calcium hydride,
distilled, and stored under nitrogen. 4-Ethynylbenzoic acid was
prepared according to the previously reported method.2d 1-Hy-
droxybenzotriazole monohydrate (HBT) and N,N′-dicyclohex-
ylcarbodiimide (DCC) were purchased from Wako (Osaka,
J apan) and Nacalai Tesque (Kyoto, J apan), respectively.
N-Methylmorpholine (NMM) was obtained from Kishida
(Osaka, J apan). Anhydrous dimethyl sulfoxide (DMSO, water
content <0.005%),anhydrous methanol(water content <0.002%),
and bis[(norbornadiene)rhodium(I) chloride] {[Rh(nbd)Cl]2}
were purchased from Aldrich. Benzylamine and (RS)-1-(1-
naphthyl)ethylamine were from Tokyo Kasei (Tokyo, J apan).
Both (R)-(+)- and (S)-(-)-1-(1-naphthyl)ethylamine were kindly
supplied from Yamakawa Chemical (Tokyo, J apan).
(R)-(-)-(4-((1-(1-Na p h t h yl)et h yl)ca r b a m oyl)p h en yl)-
a cetylen e ((R)-1). 4-Ethynylbenzoic acid (500 mg, 3.42 mmol)
and HBT (525 mg, 3.43 mmol) were dissolved in DMF (25 mL),
and to this was added DCC (712 mg, 3.45 mmol) at 0 °C. The
reaction mixture was stirred at 0 °C for 1 h and then at room
temperature for 1 h. To the solution was added (R)-(+)-1-(1-
naphthyl)ethylamine (0.66 mL, 4.1 mmol) and NMM (0.38 mL,
3.5 mmol). The reaction mixture was stirred at room temper-
ature for 24 h. After evaporating the solvent, the residue was
dissolved in THF, and the solution was poured into aqueous
HCl. After the dispersion solution was stirred at room tem-
perature for 14 h, the precipitate was collected by filtration.
The crude product was purified by silica gel chromatography
with CHCl3 as the eluent and then recrystallization from
n-hexane-ethyl acetate (1/1, v/v) to give 570 mg of monomer
(R)-1 as a white crystal in 56% yield (mp 184.0-184.5 °C). IR
(Nujol): 3287 (νtCH), 1626 (amide I), 1525 cm-1 (amide II). 1H
NMR (DMSO-d6, 300 MHz): δ 1.61 (d, CH3, J ) 7.2 Hz, 3H),
4.39 (s, tCH, 1H), 5.91-6.01 (m, CH, 1H), 7.47-8.21 (m,
* Corresponding author: e-mail yashima@apchem.nagoya-
u.ac.jp.
10.1021/ma025726p CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/07/2003

Macromolecules, Vol. 36, No. 5, 2003
Poly(phenylacetylene) Derivatives 1481
Ch a r t 1
aromatic, 11H), 9.08 (d, NH, J ) 7.5 Hz, 1H). 13C NMR
(DMSO-d6, 75 MHz): δ 21.5, 44.8, 82.8, 82.9, 122.4, 123.0,
124.3, 125.3, 125.4, 126.1, 127.1, 127.6, 128.5, 130.2, 131.4,
133.2, 134.3, 140.1, 164.4. Anal. Calcd for C21H17NO‚1/8H2O:
C, 83.62; H, 5.76; N, 4.64. Found: C, 83.62; H, 5.83; N, 4.65.
MHz for 1H or a Varian VXR-500 (500 MHz for 1H) spectrom-
eter with TMS (for CDCl3 and DMF-d7) or a solvent residual
peak (for DMSO-d6) as the internal standard. The absorption
and CD spectra were measured in a 0.1 cm quartz cell unless
otherwise noted using a J asco V-570 spectrophotometer and
a J asco J -725 spectropolarimeter, respectively. The tempera-
ture was controlled with a J asco PTC-348WI apparatus. Size
exclusion chromatography (SEC) was performed with a J asco
PU-980 liquid chromatograph equipped with a UV-vis (300
nm, J asco UV-970), a RI (J asco RI-930) detector, and a column
oven (J asco CO-965). The molecular weight (Mn) was deter-
mined at 40 °C using Tosoh TSK-GEL R-3000 (30 cm) and
R-5000 (30 cm) SEC columns connected in series, and DMF
containing 10 mM LiCl was used as the eluent at a flow rate
of 0.5 mL/min. The molecular weight calibration curve was
obtained with poly(ethylene oxide) and poly(ethylene glycol)
standards (Tosoh). Laser Raman spectra were measured on a
J asco NRS-1000 spectrophotometer. Atomic force microscopy
(AFM) measurements were performed on a Nanoscope IIIa
microscope (Digital Instruments, Santa Barbara, CA) in air
using standard silicon tips (NCH-10V) in the tapping mode.
Height and phase images were simultaneously measured at
the resonance frequency of the tips with 125 µm long canti-
levers (235-240 kHz). All the images were collected with the
maximum available number of pixels (512) in each direction
(2 µm). Scanning speed was at a line frequency of 1.0 Hz.
Dynamic light scattering (DLS) measurements were performed
25
[R]D -170° (c 0.5, methanol).
(S)-1 was prepared in the same way using (S)-(-)-1-(1-
naphthyl)ethylamine in 79% yield; mp 186.2-187.0 °C. IR
(Nujol): 3287 (νtCH), 1626 (amide I), 1526 cm-1 (amide II). 1H
NMR (DMSO-d6, 300 MHz): δ 1.60 (d, CH3, J ) 7.2 Hz, 3H),
4.36 (s, tCH, 1H), 5.92-6.01 (m, CH, 1H), 7.47-8.21 (m,
aromatic, 11H), 9.06 (d, NH, J ) 7.5 Hz, 1H). 13C NMR
(DMSO-d6, 75 MHz): δ 21.4, 44.8, 82.8, 82.9, 122.5, 123.1,
124.4, 125.5, 125.6, 126.2, 127.3, 127.7, 128.7, 130.4, 131.6,
133.2, 134.4, 140.2, 164.4. Anal. Calcd for C21H17NO: C, 84.25;
H, 5.30; N, 4.91. Found: C, 83.97; H, 5.35; N, 4.80. [R]D25 +170
° (c 0.5, methanol).
P olym er iza tion . Polymerization was carried out in a dry
glass ampule under a dry nitrogen atmosphere using [Rh(nbd)-
Cl]2 as a catalyst. A typical polymerization procedure is
described below (see Chart 1).
Monomer (R)-1 (200 mg, 0.668 mmol) was placed in a dry
glass ampule, which was then evacuated on a vacuum line and
flushed with dry nitrogen. After this evacuation-flush proce-
dure was repeated three times, a three-way stopcock was
attached to the ampule and dry THF (2.3 mL) and Et3N (0.088
mL) were added with a syringe. To this was added a solution
of [Rh(nbd)Cl]2 (0.005 M) in THF at 30 °C. The concentrations
of the monomer and the rhodium complex were 0.2 and 0.002
M, respectively. The polymerization rapidly proceeded, and a
red polymer was precipitated within a few seconds. After 0.5
h, the resulting polymer was precipitated into a large amount
of methanol, collected by filtration, and dried in vacuo at 50
°C for 3 h. The obtained polymer was dissolved in a small
amount of DMF, and the DMF solution was reprecipitated into
methanol. The resulting yellow polymer was collected by
centrifugation and dried in vacuo at 50 °C for 3 h.
on
a DLS-6600HK (Otsuka Electronics Co. Ltd., J apan)
equipped with a 10 mW He-Ne Laser (632.8 nm) at a fixed
scattering angle of 90° at 25 °C.
CD Mea su r em en ts: Effect of Solven t on ICD of P oly-
1. A typical experimental procedure is described below. A stock
solution of poly(R)-1 (5 mg/mL) in DMF was prepared in a 2
mL flask equipped with a stopcock. A 100 µL aliquot of the
poly(R)-1 solution was transferred to four 1 mL flasks equipped
with a stopcock. To each flask was added DMF (900, 860, 800,
and 700 µL) using a Hamilton microsyringe, and the solutions
were diluted with methanol so as to keep the poly(R)-1
concentration at 0.5 mg/mL. The absorption and CD spectra
were taken for each flask. In a similar manner, effects of other
poor solvents (CHCl3 and pyridine) on ICD of poly(R)-1 were
investigated. The same procedure was performed in the CD
measurements for poly(S)-1.
Effect of Op tica lly Active Am in es on ICD of P oly-1. A
stock solution of poly(R)-1 (2 mg/mL) in DMF was prepared
in a 5 mL flask equipped with a stopcock. A 500 µL aliquot of
the polymer solution was transferred to six 1 mL flasks
equipped with a stopcock and (R)- and (S)-2 (54, 220, and 270
µL) were directly added to the flasks. The solutions were then
diluted with DMF to keep the poly(R)-1 concentration at 1.0
mg/mL. The absorption and CD spectra were then taken for
each flask. In the same way, the effect of (R)- and (S)-2 on
ICD of poly(S)-1 in DMF was investigated.
Spectroscopic data of poly(R)-1. IR (Nujol): 1639 (amide I),
1
1525cm-1 (amide II). H NMR (DMF-d7, 80 °C, 500 MHz): δ
1.54 (d, CH3, 3H), 5.88 (s, dCH, 1H), 5.99 (s, CH, 1H), 6.74 (s,
aromatic, 2H), 7.18-8.15 (m, aromatic, 9H), 8.39 (s, NH, 1H).
Anal. Calcd for (C21H17NO‚2/3H2O)n: C, 81.00; H, 5.93; N, 4.50.
25
Found: C, 81.18; H, 5.92; N, 4.44. [R]D +105° (c 0.5, DMF).
Spectroscopic data of poly(S)-1. IR (Nujol): 1638 (amide I),
1
1525 cm-1 (amide II). H NMR (DMF-d7, 80 °C, 500 MHz): δ
1.55 (d, CH3, 3H), 5.88 (s, dCH, 1H), 5.98 (s, CH, 1H), 6.74 (s,
aromatic, 2H), 7.19-8.15 (m, aromatic, 9H), 8.35 (s, NH, 1H).
Anal. Calcd for (C21H17NO‚5/9H2O)n: C, 81.53; H, 5.90; N, 4.53.
25
Found: C, 81.78; H, 5.74; N, 4.53. [R]D -113° (c 0.5, DMF).
In str u m en ts. Melting points were measured on a Bu¨chi
melting point apparatus and are uncorrected. IR spectra were
recorded using a J asco Fourier transform IR-620 spectropho-
tometer (Hachioji, J apan). Optical rotation was measured in
a 2 or 5 cm quartz cell on a J asco P-1030 polarimeter. NMR
spectra were taken on a Varian Mercury 300 operating at 300
Con cen tr a tion Effect of P oly-1 on ICD. A stock solution
of poly(R)-1 (1 mg/mL) in DMF was prepared in a 2 mL flask

1482 Morino et al.
Macromolecules, Vol. 36, No. 5, 2003
Ta ble 1. P olym er iza tion of (R)-1 w ith [Rh (n bd )Cl]2
a t 30 °Ca
c
run
solvent
time (h)
yield (%)b
Mn × 10-5 c
Mw/Mn
1d
2
THF
0.5
3
2
2
2
87
2
100
100
90
3.0
1.8
DMSO
DMF
DMF
DMF
3
6.5
6.0
2.1
1.3
1.2
1.7
4e
5f
a
Polymerized under nitrogen; [(R)-1] ) 0.5 M, [(R)-1]/[Rh] )
200, [Et3N]/[Rh] ) 200. Methanol-insoluble fraction. c Deter-
mined by SEC (poly(ethylene oxide) and poly(ethylene glycol)
standards) with DMF containing LiCl (10 mM) as the eluent.
b
[(R)-1] ) 0.2 M, [(R)-1]/[Rh] ) 100, [Et3N]/[Rh] ) 100. e (S)-1 was
d
used instead of (R)-1. f [(R)-1] ) 0.5 M, [(R)-1]/[Rh] ) 10, [Et3N]/
[Rh] ) 50.
F igu r e 2. CD and absorption spectra of poly(R)-1 (run 1 in
Table 1) in DMF (a) and DMF-CHCl3 (1/1, v/v) (b). The CD
and absorption spectra were measured in a 0.1 cm quartz cell
at ambient temperature (ca. 25-26 °C) with a poly(R)-1
concentration of 0.5 mg/mL. The CD spectrum of poly(S)-1 (run
4 in Table 1) in DMF is also shown in (c).
main chain protons, which can be assigned to the cis-
1
transoidal main chain’s olefinic protons.11 A similar H
NMR spectrum was observed in DMSO-d6-CDCl3 (1/1,
v/v). In addition, the laser Raman spectra of the polymer
measured in the solid state showed characteristic peaks
at 1354 and 977 cm-1 due to the C-C and C-H bond
vibrations in the cis-polyacetylenes, respectively,12 while
those in the trans-polyacetylene were not observed.
These results indicated that the poly(R)-1 possesses a
highly cis-transoidal stereoregular structure. Other
polymers including poly(S)-1 also showed similar 1H
NMR and Raman spectra, indicating high stereoregu-
larity.
F igu r e 1. 1H NMR spectrum of poly(R)-1 (run 1 in Table 1)
in DMF-d7 at 80 °C. The X denotes protons from DMF.
equipped with a stopcock, and the initial CD spectrum was
recorded with a 0.05 cm quartz cell. A 300 µL aliquot of the
polymer solution was transferred to a 3 mL flask with a
Hamilton microsyringe, and the solution was diluted with
DMF, giving a 0.1 mg/mL solution of poly(R)-1. The CD
spectrum of this solution was taken with a 0.5 cm quartz cell,
and the above similar dilution procedure was repeated to
prepare dilute poly(R)-1 solutions (0.05 and 0.005 mg/mL).
Similarly, the concentration effect of poly(R)-1 on ICD in
DMSO/CHCl3 (1/1, v/v) was also investigated.
Ch ir op tica l P r op er ty of P oly-1 a n d Helix-Sen se
In ver sion . To characterize the chiroptical properties
of the optically active polyacetylenes bearing a chiral
substituent, CD spectroscopy is suitable for detecting
macromolecular asymmetry. The CD and absorption
spectra of poly(R)-1 and poly(S)-1 were then measured.
Figure 2a shows the typical CD and absorption spectra
of poly(R)-1 in DMF (run 1 in Table 1). The polymer
exhibited an intense, split-type ICD in the long absorp-
tion region of the conjugated polyene backbone. Poly-
(S)-1 also showed similar Cotton effects with complete
mirror images to those of poly(R)-1 (Figure 2c). The
monomers (R)-1 and (S)-1 showed no CD band at
wavelengths greater than 320 nm. This indicates that
poly(R)-1 and poly(S)-1 have a predominantly one-
handed helical conformation induced by the covalent-
bonded chiral pendants. Unfortunately, the poly-1’s are
not soluble in DMSO in which PCPA forms an induced
helix with chiral amines, but poly-1’s were soluble in
DMF and exhibited ICDs. The Cotton effects are similar
in pattern to those of the cis-transoidal PCPA com-
plexed with optically active amines including (R)-22 and
poly(phenylacetylenes) with an optically active substitu-
ent at the para position, as previously reported,13 but
the sign of the PCPA-(R)-2 complex was opposite
compared with that of poly(R)-1 in DMF, although the
CD intensity of the PCPA-(R)-2 complex was much
weaker than that in DMSO (Table 2). This implies that
the PCPA complexed with (R)-2 has a slightly excess of
a single-handed helical conformation and the helix-sense
Resu lts a n d Discu ssion
Syn th esis an d Str u ctu r al Ch ar acter istics of P oly-
(R)-1 a n d P oly(S)-1. Poly(R)-1 and poly(S)-1 were
prepared by polymerization of the corresponding mono-
2,3,10
mers with [Rh(nbd)Cl]2
in various solvents. The
results of the polymerization are summarized in Table
1. In THF, the polymerization rapidly proceeded, and a
polymer precipitated within a few seconds, whereas the
polymerization proceeded homogeneously in DMF to
quantitatively afford high molecular weight polymers.
The resulting polymers were soluble in DMF and a
DMSO-CHCl3 mixture (1/1, v/v) but insoluble in THF,
CHCl3, acetonitrile, DMSO, and acetone. However, the
lower molecular weight poly(R)-1 (run 5 in Table 1) was
partially soluble in CHCl3. On the other hand, the
polymerization hardly occurred in DMSO, and only a
trace amount of a methanol insoluble polymer was
obtained (run 2 in Table 1).
The stereoregularity of the poly-1’s was investigated
1
by 1H NMR spectroscopy. The H NMR spectrum of the
poly(R)-1 (run 1 in Table 1) in DMF-d7 (Figure 1)
showed a sharp singlet centered at 5.88 ppm due to the

Macromolecules, Vol. 36, No. 5, 2003
Poly(phenylacetylene) Derivatives 1483
Ta ble 2. CD a n d Absor p tion Sp ectr a l Da ta of P oly(R)-1 (Ru n 1 in Ta ble 1), P oly(S)-1 (Ru n 4 in Ta ble 1), a n d P CP A-(R)-2
Com p lex in Va r iou s Solven tsa
poly(R)-1
poly(S)-1
PCPA-(R)-2b
run
solvent
ꢀ400
[θ]2nd × 10-4 (λ)
ꢀ400
[θ]2nd × 10-4 (λ)
ꢀ400
[θ]2nd × 10-4 (λ)
1
2
3
4
5
6
DMF
3150
3280
3240
3220
3440d
e
2.74 (374)
-2.83 (377)
-1.97 (378)
-3.21 (378)
-1.93 (378)d
e
3270
3480
3460
3270
3410d
e
-2.87 (375)
2.77 (378)
2.17 (379)
3.24 (377)
1.96 (378)d
e
3020
3000c
2940
3010c
2870c
2940
-0.13 (372)
-0.05 (376)c
-0.65 (375)
-0.16 (375)c
-0.61 (378)c
-3.11 (376)
DMF-CHCl3 (1/1)
DMF-methanol (8/2)
DMF-pyridine (1/9)
DMSO-CHCl3 (1/1)
DMSO
a
CD and absorption spectra were measured in a 0.1 cm quartz cell at ambient temperature (ca. 25-26 °C) with a poly-1 concentration
of 0.5 mg/mL; ꢀ400 (cm-1 M-1), [θ] (deg cm2 dmol-1) of the second Cotton, and λ (nm). CD and absorption spectra were measured in a 0.05
b
cm quartz cell with a PCPA concentration of 1 mg/mL (6.8 mM). The concentration of (R)-2 was 340 mM. c [PCPA] ) 0.25 mg/mL. CD
d
and absorption spectra were measured in a 0.05 cm quartz cell with a poly-1 concentration of 1 mg/mL. e Insoluble in DMSO.
is opposite to that of poly(R)-1 bearing the same (R)-2
residue covalently at least in DMF.
However, interestingly, the ICD pattern of the poly-
1’s is almost completely inverted in DMF containing a
poor solvent, such as CHCl3 (DMF-CHCl3 (1/1, v/v)) at
ambient temperature (ca. 25-26 °C), resulting in an
almost mirror image (Figure 2b). This suggests that the
predominant helix-sense of the polymer may be opposite
in DMF and DMF-CHCl3 (1/1, v/v). The changes in the
ICDs were accompanied by a negligible change in their
absorption spectra. Similar helix-helix transitions also
occurred for other helical polyacetylenes, polyisocya-
nates, and polysilanes in response to solvent,5 temper-
ature,6 and chirality,8,9 and such transitions are con-
sidered to originate from the difference in entropy
between the right- and left-handed helices generated by
the changes in solvation, structures of the side chains
and the main chains, and intra- and intermolecular
F igu r e 3. Changes in CD intensity of poly(R)-1 (run 1 in
Table 1) (a (9), b (b)) and poly(S)-1 (run 4 in Table 1) (c (0),
interactions, although the exact mechanism is still
obscure.5-9 Similar inversion of the Cotton effect sign
was also observed in other solvent mixtures and also
for poly(S)-1 (Table 2). PCPA complexed with (R)-2 did
not show such CD changes in DMSO in the presence of
poor solvents such as toluene, CHCl3, and acetonitrile2d
and in DMF containing CHCl3, methanol, and pyridine
(see Table 2); therefore, it is concluded that the helix-
helix transition of macromolecules accompanied by
inversion of the optical activity (CD and optical rotation)
is a characteristic feature for helical polymers in which
the optically active pendants are covalently bonded.
Another possible explanation for the helix inversion
of the poly-1’s accompanied by the Cotton effect inver-
sion depending on the solvent may be due to aggrega-
tions of the polymer main chains in the presence of poor
solvents.14 It is well-known that aggregations are highly
sensitive to the concentration of polymers. However, the
magnitudes of the ICDs of poly(R)-1 in DMF, DMF-
pyridine (1/9, v/v), and DMSO-CHCl3 (1/1, v/v) were
hardly changed over the concentration range of poly-
(R)-1 from 1 to 0.005 mg/mL, indicating that the
formation of aggregates could be excluded.9,15 Filtration
experiments16 of a poly(R)-1 solution also support this
speculation. The DMF, DMF-pyridine (1/9, v/v), and
DMSO-CHCl3 (1/1, v/v) solutions of poly(R)-1 (1 mg/
mL, run 1 in Table 1) were filtered through the
membrane filter with a pore size of 0.02 µm, and the
CD and absorption spectra of the filtrates were mea-
sured, but the spectra did not change after the filtration.
These results indicated that the CD inversion of poly-
(R)-1 between the DMF, DMF-pyridine (1/9, v/v), and
DMSO-CHCl3 (1/1, v/v) solutions was not due to ag-
gregation of the polymer chains.17 AFM analyses of poly-
(R)-1 and poly(S)-1 on a freshly cleaved mica surface
d (O)) at 374 nm in DMF-methanol (a (9), c (0)) and DMF-
CHCl3 (b (b), d (O)) mixtures at ambient temperature (ca. 25-
26 °C). The concentration of poly-1 was 0.5 mg/mL.
also support this speculation; individual poly(R)-1 and
poly(S)-1 chains can be directly visualized on mica
prepared from a solution of poly(R)-1 and poly(S)-1
(0.025 mg/mL) in DMF and poly(R)-1 (0.005 mg/mL) in
DMF-pyridine (1/9, v/v) (see Supporting Information).18
The CD titrations of poly(R)-1 and poly(S)-1 in DMF
with increasing volumes of a poor solvent such as CHCl3
and methanol were then performed to obtain an insight
into the helix inversion of poly-1 depending on the
solvent composition. Figure 3 shows the changes in the
[θ] value at 374 nm in DMF-poor solvent mixtures at
ambient temperature. The ICD intensity significantly
changed with an increase in the composition of metha-
nol and CHCl3, and the sign became inverted from the
positive to negative direction for poly(R)-1 and from
negative to positive for poly(S)-1 with clear isosbestic-
like points at 430 and 351 nm to yield almost mirror
images in the presence of 10 and 20 vol % methanol and
CHCl3, respectively. The CD spectral changes are
independent of the poly(R)-1 concentration (for instance,
0.5-0.025 mg/mL in DMF containing 4, 10, and 20 vol
% of methanol), indicating that the formation of ag-
gregates by the addition of poor solvents can be negli-
gible. These results demonstrate that the poly-1’s
undergo a transition from one helix to another in DMF-
methanol or DMF-CHCl3 mixtures. These significant
changes in the ICDs were accompanied by a slight
change in their absorption spectra; the absorption
spectra exhibited a small red shift indicating that the
poly-1’s may have a slightly loose helical conformation
in the solvent mixtures.

1484 Morino et al.
Macromolecules, Vol. 36, No. 5, 2003
F igu r e 4. CD and absorption spectral changes of poly(R)-1
(run 1 in Table 1) (1 mg/mL) in DMF with temperature.
Inset: plots of the molar ellipticity at 374 nm ([θ]374) for poly-
(R)-1 vs temperature.
Besides solvents, poly-1’s were found to respond to
temperature and exhibited a similar temperature-driven
helix inversion in DMF. Figure 4 shows the changes in
the CD and absorption spectra of poly(R)-1 in DMF with
temperature as an example. The CD pattern dramati-
cally changed at high temperatures, and the sign
inverted through a transition temperature ([θ] of the
Cotton effects ≈ 0 at around Tm ) 37 °C) with clear
isosbestic points (([θ] of the Cotton effects ≈ 0 at 352
and 430 nm) to give an almost mirror image at 60 °C.
The right- and left-handed helices of poly(R)-1 are not
exactly enantiomers, but diastereomers due to the
presence of chiral naphthylethylcarbamoyl residues of
poly(R)-1, and therefore, their CD spectra differ from
one another.19 These ICD changes were accompanied
by a slight red shift in the absorption spectra with
isosbestic points at 323 and 381 nm. These CD and
absorption spectral changes are reversible and inde-
pendent of the poly(R)-1 concentration (0.025-1.0 mg/
mL) and time, indicating that the formation of aggre-
gates depending on the temperature could be excluded.
Poly(S)-1 also showed the same CD changes with
temperature. These results suggest that the poly-1’s
undergo a thermodriven helix-helix transition.
Controlling or switching the helix-sense of macromol-
ecules using chiral stimuli is quite interesting but still
a very difficult task.8,9 We found that the poly(R)-1 and
poly(S)-1 also underwent the helix-helix transition in
DMF in the presence of a chiral amine, (R)-(+)- and (S)-
(-)-1-(1-naphthyl)ethylamine ((R)- and (S)-2). Figure 5
shows the CD spectra of poly(R)-1 in the presence of
(R)- and (S)-2 in DMF. The ICD changed with an
increase in the concentration of (R)-2; the CD intensity
at 374 nm decreased, and the sign inverted to give an
almost mirror image at [2]/[poly(R)-1] ) 400 (see Figure
5B). These changes in the ICDs were accompanied by
only minor changes in the UV-vis absorption spectra;
a peak around 400 nm in the UV-vis spectrum of poly-
(R)-1 slightly shifted to a longer wavelength by ca. 5
nm in the presence of 500-fold 2. The ICD of poly(R)-1
also changed in the presence of excess (S)-2, but the
changes with (R)-2 are remarkable. Thus, 2 can be used
to regulate the helicity of poly(R)-1. Similar CD changes
F igu r e 5. Plots of CD intensity at 374 nm for poly(R)-1 (run
1 in Table 1) (1 mg/mL) vs the concentration of (R)-2 (a) and
(S)-2 (b) (A) in DMF at ambient temperature (ca. 25-26 °C).
The CD spectra of poly(R)-1 at [2]/[poly(R)-1] ) 0 and 400 are
also shown in (B).
were also observed for poly(S)-1 in the presence of the
same chiral amines ([θ]2nd × 10-4 (deg cm2 dmol-1) of
poly(S)-1 with (R)-2 and (S)-2 ([2]/[poly(S)-1] ) 400) )
-1.06 and +0.82, respectively) (see also Supporting
Information).
For the poly(phenylacetylene) with a chiral norephe-
drine group, the binding with a small amount of (R)-
mandelic acid induced a dramatic change in the ICD of
the polymer in solution due to the helix-helix transition
through a rather strong acid-base interaction.8 On the
other hand, the main interaction of poly-1’s with 2 may
be hydrogen bonding, and such interaction is not strong
enough so that excess (R)-2 or (S)-2 should be necessary
for the helix inversion of the polymers. The helical
structures of the poly-1’s are sensitive to temperature
as well as to the chirality of the chiral amines as
mentioned above. We then measured the changes in the
CD spectra of the poly(R)-1-2 complexes ([2]/[poly(R)-
1] ) 400, see Figure 5) in DMF at various temperatures
(Figure 6). As expected, the poly(R)-1-(S)-2 complex
showed completely reversible inversion of the Cotton
effect sign at high temperature and exhibited a similar
CD as the poly(R)-1-(R)-2 complex.20 At lower temper-
atures, however, both complexes showed the same
Cotton effect signs. The helix-senses of the complexes
are determined in a delicate fashion that is balanced
by temperature, additives, and solvents.
We conclude that optically active poly(phenylacet-
ylenes), poly(R)-1 and poly(S)-1, exhibit a unique helix-
helix transition by response to achiral and chiral

Macromolecules, Vol. 36, No. 5, 2003
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Ack n ow led gm en t. We thank Professor K. Koba-
yashi (Nagoya University) for permission to use the DLS
apparatus. We also acknowledge T. Asari (Nagoya
University) for his experimental assistance. This work
was partially supported by Grant-in-Aid for Scientific
Research from J apan Society for the Promotion of
Science and the Ministry of Education, Culture, Sports,
Science, and Technology, J apan.
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Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures for AFM and DLS measurements, AFM images of poly-
(R)-1, plots of the CD intensity of poly(R)-1 vs temperature in
the absence and presence of (R)-2, (S)-2, (RS)-2, and benzyl-
amine in DMF, and CD spectral data of poly(R)-1 and poly-
(S)-1 in the presence of (R)- and (S)-2 in DMF. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
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(19) In this case, loss of helicity or incomplete inversion of the
helix sense should be taken into consideration for the
observed weak Cotton effects at high temperatures compared
with those at low temperatures.
(20) In this case, chiral solvation effect should be also taken into
consideration for the observed CD changes, since poly-(R)-1
showed inversion of the Cotton effect sign at high tempera-
ture even in the presence of racemic 2 ((RS)-2) and achiral
benzylamine (see Supporting Information). However, the
difference in the ICD changes of poly-(R)-1 with (R)- and (S)-2
at different temperature indicates that chirality of 2 contrib-
utes to the inversion to some extent.
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(17) We also measured DLS of poly-(S)-1 (run 4 in Table 1) in
DMF and DMF-pyridine (1/9, v/v). The estimated hydrody-
namic radius (Rh) value of the polymer in DMF-pyridine
(1/9) (28 nm) was almost the same as that measured in DMF
(33 nm). These DLS results also support this conclusion (see
Supporting Information).
MA025726P