α-Propargyl amino acid-derived optically active novel substituted polyacetylenes: Synthesis, secondary structures, and responsiveness to ions

Article abstract of DOI:10.1002/pola.25975

Novel optically active substituted acetylenes HCi￡ CCH ₂CR¹(CO₂CH₃)NHR² [(S)-/(R)-1: R¹ = H, R² = Boc, (S)-2: R¹ = CH₃, R² = Boc, (S)-3: R¹ = H, R² = Fmoc, (S)-4: R¹ = CH₃, R² = Fmoc (Boc = tert-butoxycarbonyl, Fmoc = 9-fluorenylmethoxycarbonyl)] were synthesized from α-propargylglycine and α-propargylalanine, and polymerized with a rhodium catalyst to provide the polymers with number-average molecular weights of 2400-38,900 in good yields. Polarimetric, circular dichroism (CD), and UV-vis spectroscopic analyses indicated that poly[(S)-1], poly[(R)-1], and poly[(S)-4] formed predominantly one-handed helical structures both in polar and nonpolar solvents. Poly[(S)-1a] carrying unprotected carboxy groups was obtained by alkaline hydrolysis of poly[(S)-1], and poly[(S)-4b] carrying unprotected amino groups was obtained by removal of Fmoc groups of poly[(S)-4] using piperidine. Poly[(S)-1a] and poly[(S)-4b] also exhibited clear CD signals, which were different from those of the precursors, poly[(S)-1] and poly[(S)-4]. The solution-state IR measurement revealed the presence of intramolecular hydrogen bonding between the carbamate groups of poly[(S)-1] and poly[(S)-1a]. The plus CD signal of poly[(S)-1a] turned into minus one on addition of alkali hydroxides and tetrabutylammonium fluoride, accompanying the red-shift of λ_max. The degree of λ_max shift became large as the size of cation of the additive.

Full text of DOI:10.1002/pola.25975

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a-Propargyl Amino Acid-Derived Optically Active Novel Substituted
Polyacetylenes: Synthesis, Secondary Structures, and
Responsiveness to Ions
Hiromitsu Sogawa, Masashi Shiotsuki,* Fumio Sanda
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura Campus, Nishikyo-ku,
Kyoto 615-8510, Japan
Correspondence to: F. Sanda (E-mail: sanda.fumio.5n@kyoto-u.ac.jp)
Received 13 October 2011; accepted 30 January 2012; published online 5 March 2012
DOI: 10.1002/pola.25975
ABSTRACT: Novel optically active substituted acetylenes HCB
of Fmoc groups of poly[(S)-4] using piperidine. Poly[(S)-1a]
and poly[(S)-4b] also exhibited clear CD signals, which were
different from those of the precursors, poly[(S)-1] and poly[(S)-
4]. The solution-state IR measurement revealed the presence of
intramolecular hydrogen bonding between the carbamate
groups of poly[(S)-1] and poly[(S)-1a]. The plus CD signal of
poly[(S)-1a] turned into minus one on addition of alkali hydrox-
ides and tetrabutylammonium fluoride, accompanying the red-
shift of kmax. The degree of kmax shift became large as the size
CCH2CR1(CO2CH3)NHR2 [(S)-/(R)-1: R1 ¼ H, R2 ¼ Boc, (S)-2: R1
¼
¼
CH3, R2 ¼ Boc, (S)-3: R1 ¼ H, R2 ¼ Fmoc, (S)-4: R1 ¼ CH3, R2
Fmoc (Boc ¼ tert-butoxycarbonyl, Fmoc ¼ 9-fluorenylmethoxy-
carbonyl)] were synthesized from a-propargylglycine and
a-propargylalanine, and polymerized with a rhodium catalyst to
provide the polymers with number-average molecular weights
of 2400–38,900 in good yields. Polarimetric, circular dichroism
(CD), and UV–vis spectroscopic analyses indicated that
poly[(S)-1], poly[(R)-1], and poly[(S)-4] formed predominantly
one-handed helical structures both in polar and nonpolar sol-
vents. Poly[(S)-1a] carrying unprotected carboxy groups was
obtained by alkaline hydrolysis of poly[(S)-1], and poly[(S)-4b]
carrying unprotected amino groups was obtained by removal
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of cation of the additive. 2012 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 50: 2008–2018, 2012
KEYWORDS: chiral; conjugated polymers; conformational analy-
sis; polyacetylenes; stimuli-sensitve polymers
INTRODUCTION The helix is one of the most common
higher-order structures of macromolecules. Many sophisti-
cated and intricate functions of biomacromolecules such as
proteins and DNA largely depend on their well-defined heli-
cal structures. Various types of helical polymers have been
synthesized so far including polymethacrylates,1 polyisocya-
nides,2 polysilanes,3 poly(phenyleneethynylene)s,4 and polya-
cetylenes,5 dating back to the discovery of isotactic polypro-
pylene by Natta et al.6 Monosubstituted polyacetylenes
synthesized by the polymerization with rhodium catalysts
feature a highly cis-stereoregurlar structure.7–9 Introduction
of appropriate chiral substituents into the side chain leads to
the formation of a helical structure with predominantly one-
handed screw sense. We have reported that rhodium-based
cis-stereoregular polyacetylene derivatives such as poly(N-
ular hydrogen bonds together with the steric repulsion
between the side chains in the biomimetically same way as
peptides and proteins. They undergo reversible helix–helix
or helix–coil transitions by external stimuli such as heat and
the addition of polar solvents, wherein the formation and de-
formation of hydrogen bonds are key factors for the confor-
mational transition.
Amino acid is a useful chiral building block for synthetic hel-
ical polymers, because the amino and carboxy groups are
transformable into wide variety of functional groups, making
versatile molecular design possible. When amide groups are
introduced in amino acid-based helical polymers, they are
usable for stabilizing the helical structure due to the strong
nature of forming hydrogen bonding in a manner similar to
a-helix of peptide. Various helical polyacetylenes functional-
ized with amino acids have been also synthesized so far,18–42
some of which change the conformation according to exter-
nal stimuli such as temperature,18,19 solvent,20–23 acid/
base,24–27 electricity,28 and photo-irradiation.29,30 Amino
propargylamide)s,10–12
poly(N-propargylcarbamate)s,13,14
poly(N-butynylamide)s,15,16 poly(1-methylpropargyl-N-alkyl-
carbamate)s17 form helical structures with predominantly
one-handed screw sense, which are stabilized by intramolec-
*Present address: Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka, Fukuoka 820-8555, Japan.
Additional Supporting Information may be found in the online version of this article.
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standards at 40 ꢁC. CD and UV–vis absorption spectra were
recorded on a JASCO J-820 spectropolarimeter.
Materials
Unless stated otherwise, reagents and solvents were pur-
chased and used without purification. (nbd)Rhþ[g6-
C6H5Bꢂ(C6H5)3] (nbd ¼ 2,5-norbornadiene) was prepared
according to the literature.45 a-Propargyl amino acid deriva-
tives [(S)-a-propargylglycine, (R)-a-propargylglycine, (S)-a-
propargylalanine, (S)-N-Fmoc-a-propargylalanine (Fmoc ¼ 9-
fluorenylmethoxycarbonyl) (ee ꢃ 99%)] and di-tert-butylcar-
bonate [(Boc)2O] (Boc ¼ tert-butoxycarbonyl) were gifted
from Nagase & Co., LTD. and Tokuyama. CHCl3, THF, and N,N-
dimethylformamide (DMF) used for polymerization were dis-
tilled prior to use.
SCHEME 1 Polymerization of monomers (S)-/(R)-1–4.
acid-based helical polymers also show useful properties
including chemical sensing,31–34 chiral recognition,35 and
asymmetric induction36–39 originated from the conjugated
backbone and regularly ordered functional groups at the
side chain strands.
Monomer Synthesis
(S)-N-Boc-a-propargylglycine Methyl Ester [(S)-1]
(Boc)2O (3.72 g, 15.0 mmol) and K2CO3 (1.59 g, 15.0 mmol)
were added to a solution of (S)-a-propargylglycine (ꢁ1.13 g,
10.0 mmol) in 1,4-dioxane/H2O (30 mL/50 mL) at 0 C, and
the resulting mixture was stirred at room temperature over-
night. 1,4-Dioxane was evaporated off, and the residual solu-
tion was carefully acidified with 0.5 M HCl to pH ¼ 3. The
resulting solution was extracted with CH2Cl2. The organic
layer was washed with water and saturated NaCl aq., dried
over anhydrous MgSO4, and then filtered. The filtrate was
concentrated to obtain (S)-N-Boc-a-propargylglycine [(S)-1a]
as a viscosity liquid. After that, 1-(3-dimethylaminopropyl)-
3-ethylcarbodiimide hydrochloride (EDCꢄHCl, 2.30 g, 12.0
mmol), N,N-dimethyl-4-aminopyridine (DMAP, 0.140 g, 1.20
mmol), and MeOH (2.00 mL, 49.4 mmol) were added to a so-
lution of (S)-1a in CH2Cl2 (40 mL) at 0 ꢁC, and then the
resulting mixture was stirred at room temperature overnight.
It was washed with 1.0 M HCl, saturated NaHCO3 aq., and
saturated NaCl aq., dried over anhydrous MgSO4, and then
filtered. The filtrate was concentrated, and the residual mass
was purified by preparative HPLC to obtain (S)-1 as a vis-
cous liquid in 44%. [a]D þ23ꢁ (c ¼ 0.14 g/dL, THF). IR (in
CHCl3): 3436, 3308, 2982, 1746, 1709, 1502, 1368, 1222,
Recently, the synthesis of highly optically pure non-natural
amino acids becomes possible by the enantioselective alkyla-
tion of a prochiral protected glycine derivative utilizing a chi-
ral phase transfer catalyst (Maruoka catalyst) having
binaphthyl group.43 Various optically active amino acids
bearing olefinic pendants are now commercially synthesized
with Maruoka catalyst, and find application to hydrocarbon
stapling (ring-closing metathesis reaction) of helical peptides,
which provides a useful strategy for experimental and thera-
peutic modulation of protein–protein interactions in many
signaling pathways.44 Non-natural optically active amino
acids bearing acetylenic pendants such as a-propargylglycine
are synthesized by Maruoka catalyst as well. In the course of
our study on amino acid-based helical polyacetylenes, we
have decided to utilize a-propargyl amino acid derivatives as
the monomers. The present article deals with the synthesis
of novel polyacetylenes from a-propargylglycine and alanine
(Scheme 1), and examination of the secondary structures. As
far as we know, this is the first example regarding the syn-
thesis of a-propargyl amino acid-derived helical polyacety-
lenes. We further disclose the removal of protecting groups
from the amino and carboxy groups, and ion-responsiveness
of the polymers having unprotected amino/carboxy groups.
1162, 1064, 655 cmꢂ1. H NMR (400 MHz, CDCl3): d 1.45 [s,
1
9H, AC(CH3)3], 2.07 (t, J ¼ 2.4 Hz, 1H, ACBCH), 2.71–2.74
(m, 2H, ACH2A), 3.78 (s, 3H, AOCH3), 4.45–4.50 (m, 1H,
ACHA), 5.45 (d, J ¼ 8.4 Hz, 1H, ANHA). 13C NMR (100
MHz, CDCl3): d 22.56 (ACH2A), 28.06 [AC(CH3)3], 51.76
(AOCH3), 52.38 (ACHA), 71.46 (ACBCH), 78.38 (ACBCH),
79.89 [AC(CH3)3], 154.89 (ANHCOA), 170.93 (ACOOCH3).
High-resolution mass spectra (HRMS, m/z): [M þ Na]þ calcd
for C11H17NO4Na, 250.1055; found, 250.1042.
EXPERIMENTAL
Measurements
1H (400 MHz) and 13C (100 MHz) NMR spectra were
recorded on a JEOL EX-400 or a JEOL AL-400 spectrometer.
IR spectra were measured on a JASCO FT/IR-4100 spectro-
photometer. Melting points (mp) were measured on a Yanaco
micro melting point apparatus. Mass spectra were measured
on a Thermo Scientific Exactive mass spectrometer. Specific
rotations ([a]D) were measured on a JASCO DIP-1000 digital
polarimeter. Number- and weight-average molecular weights
(Mn and Mw) of polymers were determined by size-exclusion
column chromatography (SEC, Shodex columns KF805 ꢀ 3)
eluted with tetrahydrofuran (THF) calibrated by polystyrene
(R)-N-Boc-a-propargylgylcine Methyl Ester [(R)-1]
The title compound was synthesized from (R)-a-propargyl-
glycine in a manner similar to (S)-1. Yield 43% (viscous liq-
uid). [a]D ꢂ21ꢁ (c ¼ 0.09 g/dL, THF). IR (in CHCl3): 3436,
3308, 3019, 2981, 2123, 1747, 1709, 1503, 1368, 1215,
1162, 1064, 668 cmꢂ1. H NMR (400 MHz, CDCl3): d 1.45 [s,
1
9H, AC(CH3)3], 2.07 (t, J ¼ 2.8 Hz, 1H, ACBCH), 2.71–2.74
(m, 2H, ACH2A), 3.78 (s, 3H, AOCH3), 4.44–4.50 (m, 1H,
ACHA), 5.44 (d, J ¼ 8.4 Hz, 1H, ANHA). 13C NMR (100
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MHz, CDCl3): d 22.56 (ACH2A), 28.07 [AC(CH3)3], 51.78
(AOCH3), 52.37 (ACHA), 71.45 (ACBCH), 78.39 (ACBCH),
79.89 [AC(CH3)3], 154.89 (ANHCOA), 170.92 (ACOOCH3).
HRMS. (m/z): [M þ Na]þ calcd for C11H17NO4Na, 250.1055;
found, 250.1050.
ꢂ45ꢁ (c ¼ 0.09 g/dL, THF). IR (KBr): 3357, 3292, 3065,
3040, 2951, 2120, 1719, 1524, 1509, 1450, 1276, 1231,
1118, 1077, 974, 739 cmꢂ1 1H NMR (400 MHz, CDCl3): d
.
1.57 (s, 3H, ACH3), 2.01 (s, 1H, ACBCH), 2.89–3.05 (m, 2H,
ACH2A), 3.73 (s, 3H, AOCH3), 4.20 (t, J ¼ 6.8 Hz, 1H,
>CHA), 4.38 (br, 2H, ACH2OA), 5.76 (br, 1H, ANHA), 7.27
(t, J ¼ 7.6 Hz, 2H, Ar), 7.35 (t, J ¼ 7.2 Hz, 2H, Ar), 7.57 (d, J
¼ 7.2 Hz, 2H, Ar), 7.71 (d, J ¼ 7.6 Hz, 2H, Ar). 13C NMR
(100 MHz, CDCl3): d 23.03 (ACH3), 26.73 (ACH2A), 47.02
(>CHA), 52.78 (AOCH3), 58.52 (ACHA), 66.57 (ACH2OA),
71.25 (ACBCH), 79.08 (ACBCH), 119.74, 124.85, 126.81,
127.44, 141.04, 143.56 (Ar), 154.46 (ANHCOA), 172.96
(ACOOCH3). HRMS. (m/z): [M þ H]þ calcd for C22H22NO4,
364.1549; found, 364.1532.
(S)-N-Boc-a-propargylalanine Methyl Ester [(S)-2]
The title compound was synthesized from (S)-a-propargylala-
nine in a manner similar to (S)-1. Yield 38% (white solid).
Mp 72–73 ꢁC. [a]D ꢂ63ꢁ (c ¼ 0.10 g/dL, THF). IR (KBr):
3382, 3331, 3318, 2985, 2937, 2120, 1732, 1711, 1666,
1517, 1457, 1385, 1251, 1174, 1064, 624 cmꢂ1 1H NMR
.
(400 MHz, CDCl3): d 1.43 [s, 9H, AC(CH3)3], 1.54 [s, 3H,
ACH3], 2.06 (t, J ¼ 2.4 Hz, 1H, ACBCH), 2.88–2.96 (m, 2H,
ACH2A), 3.76 (s, 3H, AOCH3), 5.37 (br, 1H, ANHA). 13C
NMR (100 MHz, CDCl3): d 22.93 (ACH3), 26.69 (ACH2A),
28.04 [AC(CH3)3], 52.45 (AOCH3), 57.97 (ACHA), 71.08
(ACACH), 79.20 [AC(CH3)3], 79.66 (ACACH), 154.16
(ANHCOA), 173.41 (ACOOCH3). HRMS. (m/z): [M þ H]þ
calcd for C12H20NO4, 242.1392; found, 242.1386.
Polymerization
All the polymerizations were carried out in a glass tube
equipped with a three-way stopcock under nitrogen. A typi-
cal experimental procedure for polymerization is given
below.
(S)-N-Fmoc-a-propargylglycine Methyl Ester [(S)-3]
A
solution of (nbd)Rhþ[g6-C6H5Bꢂ(C6H5)3] (5.1 mg, 10
A solution of N-Fmoc-succinimide (1.01 g, 3.00 mmol) in
1,2-dimethoxyethane (DME) (10 mL) was added to a solu-
tion of (S)-a-propargylglycine (0.226 g, 2.00 mmol) in 10 wt
% Na2CO3 aq. (10 mL) at 0 ꢁC, and the resulting mixture
was stirred at room temperature overnight. Precipitates
formed were filtered off, and then DME was removed from
the filtrate by evaporation. The residual solution was care-
fully acidified by 0.1 M HCl to adjust the pH neutral, and
then extracted with EtOAc. The organic phase was washed
with 0.1 M HCl, and saturated NaCl aq., dried over anhy-
drous MgSO4, and then filtered. The filtrate was concentrated
to obtain (S)-N-Fmoc-a-propargylglycine [(S)-3a]. After that,
(S)-3 was synthesized from (S)-3a and MeOH in a manner
similar to (S)-1, and purified by silica gel column chromatog-
raphy eluted with hexane/CHCl3 ¼ 1/1 (v/v). Yield 32%
(white solid). Mp 136–137 ꢁC. [a]D ꢂ9ꢁ (c ¼ 0.08 g/dL,
THF). IR (KBr): 3320, 3273, 3065, 3020, 2951, 2116, 1734,
lmol) in CHCl3 (2.5 mL) was added to a solution of a mono-
mer (1.0 mmol) in CHCl3 (2.5 mL) under dry nitrogen, and
the resulting solution was kept at 30 ꢁC for 24 h. The reac-
tion mixture was poured into a large amount of hexane to
precipitate a polymer. It was separated by filtration using a
membrane filter (ADVANTEC H100A047A) and dried under
reduced pressure.
Alkaline Hydrolysis of Ester Groups of Poly[(S)-1]
Aqueous NaOH (1.00 M, 1.00 mL) was added to a solution of
poly[(S)-1] (0.113 g, 0.500 unit mmol) in THF/MeOH/H2O
(5 mL/5 mL/10 mL) at room temperature, and then the
resulting mixture was stirred at 60 ꢁC for 8 h. THF and
MeOH were evaporated from the mixture, and the residual
solution was carefully acidified by citric acid, and then
extracted with EtOAc. The organic phase was washed with
saturated NaCl aq., and then dried over anhydrous MgSO4.
EtOAc was evaporated off to obtain poly[(S)-1a] as a yellow
solid in 76%. [a]D þ245ꢁ (c ¼ 0.07 g/dL, THF).
1691, 1543, 1450, 1296, 1014, 746 cmꢂ1 1H NMR (400
.
MHz, CDCl3): d 2.05 (s, 1H, ACBCH), 2.75–2.78 (m, 2H,
ACH2A), 3.75 (s, 3H, AOCH3), 4.21 (t, J ¼ 6.8 Hz, 1H,
>CHA), 4.38 (d, J ¼ 7.2 Hz, 2H, ACH2OA), 4.53–4.55 (m,
1H, ACHA), 5.74 (d, J ¼ 6.8 Hz, 1H, ANHA), 7.28 (t, J ¼ 7.6
Hz, 2H, Ar), 7.37 (t, J ¼ 7.6 Hz, 2H, Ar), 7.59 (d, J ¼ 6.8 Hz,
2H, Ar), 7.73 (d, J ¼ 7.6 Hz, 2H, Ar). 13C NMR (100 MHz,
CDCl3): d 22.56 (ACH2A), 46.93 (>CHA), 52.23 (AOCH3),
52.26 (ACHA), 67.08 (ACH2OA), 71.74 (ACBCH), 78.21
(ACBCH), 119.86, 124.97, 126.93, 127.59, 141.13, 143.57
(Ar), 155.50 (ANHCOA), 170.64 (ACOOCH3). HRMS. (m/z):
Removal of Fmoc Groups from Poly[(S)-4]
Piperidine (2.00 mL) was added to a solution of poly[(S)-4]
(0.108 g, 0.300 unit mmol) in CHCl3 (10.0 mL). The resulting
mixture was stirred at room temperature for 8 h, and then
poured into hexane to precipitate the produced polymer. It
was collected by filtration using a membrane filter (ADVAN-
TECH H100A047A) and dried under reduced pressure to
obtain poly[(S)-4b] as a yellow solid in 83%.
[M
372.1207.
þ
Na]þ calcd for C21H19NO4Na, 372.1212; found,
Methyl Esterification of Poly[(S)-1a]
A solution of trimethylsilyldiazomethane in hexane (0.6 M,
1.0 mL) was added to a solution of poly[(S)-1a] (0.010 mg,
0.027 unit mmol) in THF/MeOH (5 mL/5 mL). The resulting
mixture was stirred at room temperature for 6 h, and then
concentrated to obtain methyl esterificated poly[(S)-1a]. [a]D
þ107ꢁ (c ¼ 0.09 g/dL, THF). The 1H NMR spectroscopic
data were almost the same as those of poly[(S)-1].
(S)-N-Fmoc-a-propargylalanine Methyl Ester [(S)-4]
The title compound was synthesized from (S)-N-Fmoc-a-
propargylalanine containing 27% of methyl tert-butyl ether
and MeOH in a manner similar to (S)-1, and purified by
silica gel column chromatography eluted with hexane/CHCl3
¼ 1/1 (v/v). Yield 53% (white solid). Mp 54–56 ꢁC. [a]D
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TABLE 1 Polymerization of (S)-/(R)-1–4a
regular polymers.7,8 Thus, the polymerization of monomers
(S)-/(R)-1–(S)-4 was carried out using (nbd)Rhþ[g6-
ꢂ
ꢁ
Polymer
C6H5B (C6H5)3] in CHCl3 and THF at 30 C for 24 h (Scheme
1). The formed polymers were isolated as hexane- or Et2O-
insoluble parts. The polymerization of (S)-1, (R)-1, and (S)-4
proceeded homogeneously to give polymers with Mn’s of
2400–38,900 in good yields (Table 1). These polymers were
soluble in common organic solvents such as CHCl3 and THF.
On the other hand, yellow polymeric masses precipitated out
of the solution onto the sides of the glass tube in a little
while after initiating the polymerization of (S)-2 and (S)-3.
The molecular weights of isolated poly[(S)-2] and poly[(S)-3]
could not be determined by SEC because they were insoluble
in CHCl3, THF, DMF, and H2O. We further tried the polymer-
ization of (S)-2 and (S)-3 under the conditions different from
those in Table 1, but failed to obtain solvent-soluble
polymers.46
c
c
d
Run
Monomer
Yieldb (%)
Mn
Mw/Mn
[a]D (ꢁ)
1
2
3
4
5
6g
a
(S)-1
(R)-1
(S)-2
(S)-3
(S)-4
(S)-4
83
76
38,900
30,700
1.9
1.8
þ113
ꢂ106
e
e
e
e
–
–
–
–
e
e
e
e
–
–
–
–
f
61
83
2,400
7,900
1.6
1.7
–
þ662
Conditions: catalyst (nbd)Rhþ[g6-C6H5Bꢂ(C6H5)3], nbd ¼ 2,5-norborna-
diene, [M]0 ¼ 0.20 M, [M]0/[Rh] ¼ 100, in CHCl3, at 30 ꢁC for 24 h.
b
Hexane-insoluble part {poly[(S)-1] and poly[(R)-1]} and Et2O-insoluble
part {poly[(S)-4]}.
c
Determined by SEC eluted with THF, polystyrene calibration.
d
Measured by polarimetry at room temperature, c ¼ 0.09–0.14 g/dL in
THF. [a]D of monomers: (S)-1, þ23ꢁ; (R)-1, ꢂ22ꢁ; (S)-4, ꢂ45ꢁ.
e
Not determined due to insolubility.
f
Not determined.
Chiroptical Properties of the Polymers
g
[M]0 ¼ 0.50 M, in THF. Gelation occurred at this concentration in
As shown in Table 1, poly[(S)-1], poly[(R)-1], and poly[(S)-4]
displayed |[a]D| values 5–14 times larger than those of the
corresponding monomers, indicating the presence of chirally
regulated higher-order structures. The signs of [a]D of
poly[(S)-1] and poly[(R)-1], enantiomerically isomeric poly-
mers, were opposite, and the absolute values were almost
the same as predicted. The polymers exhibited intense CD
signals at the absorption region ranging from 250 to 400 nm
in CHCl3, THF, THF/MeOH ¼ 1/1, THF/DMF ¼ 1/1, and
DMF as shown in Figures 1 and 2. It is considered that the
CD signals originate from chirally arranged chromophors
CHCl3.
Addition of Alkali Hydroxides and TBAF to Poly[(S)-1]
A solution of an alkali hydroxide or tetrabutylammonium flu-
oride (TBAF) in THF, MeOH, or H2O (5.0 mL, 0.25–4.0 mM)
was added to a solution of poly[(S)-1a] in THF or MeOH (5.0
mL, 1.0 mM). The CD and UV–vis spectra of the resulting
mixture were measured. Alkali hydroxides and TBAF were
used as purchased from commercial suppliers.
Spectroscopic Data of the Polymers Poly[(S)-1]
IR (KBr): 3368, 2978, 2933, 1747, 1717, 1509, 1367, 1166,
1
1057, 1024, 860, 781 cmꢂ1. H NMR (400 MHz, CDCl3): d 1.42
[br, 9H, (CH3)3CA], 2.71 (br, 2H, ACH2A), 3.70 (br, 3H,
AOCH3), 4.30 (br, 1H, ACHA), 5.50–6.64 (br, 2H, AC¼¼CHA,
ANHA). Poly[(R)-1]: IR (KBr): 3368, 2978, 2933, 1746, 1718,
1
1509, 1366, 1167, 1057, 1024, 856, 781 cmꢂ1. H NMR (400
MHz, CDCl3): d 1.42 [br, 9H, (CH3)3CA], 2.82 (br, 2H, ACH2A),
3.70 (br, 3H, AOCH3), 4.33 (br, 1H, ACHA), 5.60–6.42 (br, 2H,
AC¼¼CHA, ANHA). Poly[(S)-1a]: IR (KBr): 3412, 2980, 2935,
2623, 1704, 1509, 1395, 1369, 1251, 1163, 855 cmꢂ1. 1H NMR
(400 MHz, CD3OD): d 1.45 [br, 9H, (CH3)3CA], 2.68 (br, 2H,
ACH2A), 4.23 (br, 1H, ACHA), 6.02–6.48 (br, 2H, AC¼¼CHA,
ANHA). Poly[(S)-4]: IR (KBr): 3407, 3017, 2949, 1721, 1500,
1450, 1233, 1107, 1075, 739 cmꢂ1 1H NMR (400 MHz,
.
CDCl3): d 1.46 (br, 3H, ACH3), 2.26 (br, 2H, ACH2A), 3.56 (br,
3H, AOCH3), 4.01–4.38 (br, 3H, >CHA, ACH2OA), 5.71–5.91
(br, 1H, ANHA), 6.29 (br, 1H, AC¼¼CHA), 7.12–7.72 (br, 8H,
Ar). Poly[(S)-4b]: IR (in CHCl3): 3438, 3308, 1747, 1714,
1610, 1508, 1424, 1046, 928 cmꢂ1 1H NMR (400 MHz,
.
CDCl3): d 1.66 [br, 3H, CH3A], 2.32 (br, 2H, ACH2A), 3.75 (br,
3H, AOCH3), 6.11 (br, 1H, AC¼¼CHA).
RESULTS AND DISCUSSION
Polymerization
FIGURE 1 CD and UV–vis spectra of poly[(S)-1] and poly[(R)-1]
measured in CHCl3, THF, THF/MeOH ¼ 1/1, and THF/DMF ¼ 1/1
(c ¼ 0.50 mM) at 20 ꢁC. A color version of this figure is avail-
able in the Supporting Information.
Rhodium catalysts tolerate a wide variety of functional
groups including carbamate and ester, and polymerize mono-
substituted acetylenes to afford the corresponding cis-stereo-
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effective to induce a helix stabilized by steric repulsion
between the side chains, in a fashion similar to poly(1-meth-
ylpropargyl alcohol)s,53 poly(1-methylpropargyl ester)s,53,54
and poly(1-methylpropargyl-N-alkylcarbamate)s.17 Poly[(S)-
1] and poly[(R)-1], having side chains with different absolute
configurations, exhibited mirror-image CD spectra at 250–
500 nm. Together with the results of optical rotations listed
in Table 1 {poly[(S)-1] þ113ꢁ, poly[(R)-1] ꢂ106ꢁ}, it is con-
cluded that these polymers form helical structures with op-
posite screw sense mutually, and the helix sense is deter-
mined by the amino acid chirality. As depicted in Figure 2,
poly[(S)-4] also showed intense CD signals in both nonpolar
and polar solvents, CHCl3, THF, and DMF. The UV–vis absorp-
tion maximum at the region of main chain chromophore was
33 nm shorter in DMF than those in THF and CHCl3. It is
presumed that poly[(S)-4] form a tightly twisted helix (i.e.,
smaller pitch/diameter) in DMF than the latter two solvents,
causing blue-shift due to the reduced conjugation.55
We further examined the thermal stability of helical confor-
mation of the polymers. Figure 3 shows the Kuhn dissymme-
try factor (g ¼ De/e, in which De ¼ [h]/3298) at [h]max of
poly[(S)-1] and poly[(S)-4] in CHCl3, THF, and DMF at vari-
ous temperatures. The g value ꢁof poly[(S)-1] decreased 24%
ꢁ
FIGURE 2 CD and UV–vis spectra of poly[(S)-4] measured in
CHCl3, THF, and DMF (c ¼ 0.50 mM) at 20 ꢁC.
by raising temperature from 0 C to 60 C in THF, while that
of poly[(S)-4] decreased only 2% under the same tempera-
ture range. Since the g values give quantitative information
associated with the degree of preferential screw sense,56 the
present results indicate that the helical structure of poly[(S)-
4] is more stable than that of poly[(S)-1] to thermo-driven
screw sense reversal probably due to the larger steric repul-
sion between the side chains originated from the methyl
groups at the chiral centers and bulky fluorenyl groups. The
g values of poly[(S)-4] were smaller and more temperature-
sensitive in DMF than those in CHCl3 and THF. In DMF,
poly[(S)-4] may be more flexible than in the latter two sol-
vents, because of the less conjugated main chain as men-
tioned above. Compared to CHCl3 and THF, it seems that the
including conjugated polyacetylene main chain. Fluorine-
derived CD signals are negligibly small, presumably because
the fluorene moieties are positioned apart from the main
chain, which makes their regulated arrangement difficult.47
After membrane filtration (pore size ¼ 0.45 lm) of the sam-
ple solutions, the CD and UV–vis signals were still observed
with the same intensities as those before filtration,48 and the
sample concentration did not affect the signal intensity at a
range of 0.25–1.00 mM. These results indicate that the CD
signals do not originate from chiral aggregates like the case
of poly(thiophene)s49,50 and poly(p-phenylenevinylene)s50,51
bearing chiral substituents but unimolecular helical confor-
mations of the polymers with predominantly one-handed
screw sense.
Poly(N-butynylamide)s15,16 and poly(N-propargylamide)s10,15
efficiently form helices stabilized by intramolecular >NAH
ꢄꢄꢄ O¼¼C< hydrogen bonds between the amide groups at
the side chains in nonpolar solvents such as CHCl3. On the
other hand, the polymers hardly form helices in polar sol-
vents such as MeOH and DMF, because these solvents disturb
the formation of the intramolecular hydrogen bonding. Inter-
estingly, poly[(S)-1] showed intense CD signals even in THF/
MeOH ¼ 1/1 and THF/DMF ¼ 1/1,52 and the CD intensity of
poly[(S)-1] was larger in THF/DMF ¼ 1/1 than that in less
polar THF. Poly[(S)-1] has high helix-forming ability even in
these polar solvents unlike poly(N-butynylamide)s and
poly(N-propargylamide)s. The remarkable helix induction is
probably due to the location of stereogenic centers close to
the main chain. Namely, it is considered that the presence of
chiral groups in close proximities to the main chain is quite
FIGURE 3 Plots of g values of poly[(S)-1] and poly[(S)-4] at
[h]max versus temperature measured in CHCl3, THF, and DMF (c
¼ 0.50 mM).
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SCHEME 2 Synthesis of poly[(S)-1a].
solvation ability of DMF with poly[(S)-4] is high because
dipole–dipole interaction between the carbonyl moieties of
DMF and the polymer is possibly present along with hydro-
gen-bonding interaction, resulting in high flexibility of con-
formation as well.
Removal of Protecting Groups of Poly[(S)-1] and
Poly[(S)-4]
The ester groups of poly[(S)-1] were hydrolyzed using NaOH
aq. to obtain the corresponding polymer {poly[(S)-1a]} bear-
ing unprotected carboxy groups (Scheme 2).57 The proceed-
ing of the reaction was confirmed by 1H NMR spectroscopy,
i.e., a signal around 3.6–3.8 ppm corresponding to the
methyl groups mostly disappeared after hydrolysis.58 The Mn
and Mw/Mn of poly[(S)-1a] were determined as 30,500 and
1.9 by SEC,59 respectively, which were almost the same as
those of poly[(S)-1] listed in Table 1. Decomposition during
hydrolysis seems to be negligibly small.
FIGURE 4 CD and UV–vis spectra of poly[(S)-1a] measured in
THF, MeOH, and DMF (c ¼ 0.50 mM) at 20 ꢁC.
unprotected carboxy and amino groups also adopt helical
conformations with an excess of predominantly one-handed
screw sense in the solvents. It should be noted that CD and
UV–vis spectroscopic patterns of poly[(S)-1a] and poly[(S)-
4b] were totally different from those of the precursors,
poly[(S)-1] and poly[(S)-4] bearing protected carboxy and
amino groups, implying that these polymers form different
helical structures before and after removal of the protecting
groups. This is predictable because it is likely that the car-
boxy groups interact with the functional groups at the side
chains strongly and intramolecularly, and also with solvent
molecules by hydrogen bonding, both of which largely affect
the conformation and helicity. This is also the case for amino
groups. Specifically, poly[(S)-4b] exhibited the kmax at a
wavelength 30–60 nm shorter than that of poly[(S)-4]. Re-
moval of bulky Fmoc groups probably reduced the steric
repulsion between the side chains, leading to tightening of
the helix accompanying the inversion of screw sense as a
more stable conformation.
Further, removal of the Fmoc groups from poly[(S)-4] was
attempted to obtain poly[(S)-4b] bearing unprotected amino
groups according to Scheme 3. The transformation from
poly[(S)-4] to poly[(S)-4b] was confirmed by the disappear-
ance of the 1H NMR signals assignable to the Fmoc proton
signals. The olefinic proton signal at the main chain was
intact.60 The Mn of poly[(S)-4b] became low (2100) com-
pared with the precursor {poly[(S)-4], 7900}. It is considered
that this is not due to the decomposition of the main chain
but removal of fluorenyl groups, because the Mn of poly[(S)-
4b] calculated based on the MW decrease of fluorenyl
groups was 3100, almost the same as the observed one.
Another reason may be the formation of amino groups,
which commonly extends the retention time of SEC.
Figures 4 and 5 depict the CD and UV–vis spectra of
poly[(S)-1a] and poly[(S)-4b] measured in THF, MeOH, and
DMF. Poly[(S)-1a] exhibited intense Cotton effects around
300 nm in THF, while only small peaks in MeOH and DMF as
shown in Figure 4. Meanwhile, poly[(S)-4b] exhibited a large
minus CD signal around 280 nm both in THF and DMF as
shown Figure 5. It is concluded that these polymers bearing
As aforementioned, intramolecular hydrogen bonds possibly
exist between the pendent side chains and stabilize the heli-
cal structures in a fashion similar to poly(N-propargylcarba-
mate)s13 and poly(N-butynylamide)s.15,16 Solution-state IR
spectroscopic study was carried out under diluted conditions
to determine the presence/absence of intramolecular hydro-
gen bonding. Table 2 summarizes the results of IR measure-
ment for solutions (40 mM)61 of poly[(S)-1] and poly[(S)-
1a], and the corresponding monomers (S)-1 and (S)-1a.
Monomer (S)-1 and poly[(S)-1] exhibited two strong absorp-
tion peaks assignable to C¼¼O stretching of ester and carba-
mate groups, and a peak assignable to NAH bending of the
SCHEME 3 Synthesis of poly[(S)-4b].
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FIGURE 5 CD and UV–vis spectra of poly[(S)-4b] measured in
THF and DMF (c ¼ 0.50 mM) at 20 ꢁC.
FIGURE 6 CD and UV–vis spectra of poly[(S)-1a] upon addition
of KOH measured in THF/MeOH ¼ 1/1 (c ¼ 0.50 mM) at 20 ꢁC.
carbamate groups. Monomer (S)-1a and poly[(S)-1a] exhib-
ited C¼¼O peaks of carboxy groups instead of ester groups.
The carbamate C¼¼O peaks of poly[(S)-1] were observed at
11 cmꢂ1 lower, and NAH peaks at 12 cmꢂ1 higher wave-
number regions than those of (S)-1. Compared to the differ-
ence of the carbamate C¼¼O peak positions between the
monomer and polymer, the difference of ester C¼¼O peak
positions was smaller, i.e., 4 cmꢂ1. On the other hand, the
difference of carboxy C¼¼O peak positions between (S)-1a
and poly[(S)-1a] was as large as 16 cmꢂ1. The difference of
carbamate C¼¼O peak was 1 cmꢂ1 between the monomer
and polymer, and that of NAH was 16 cmꢂ1. These results
confirm the presence of intramolecular hydrogen bonds in
the both polymers, but the patterns seem to be largely dif-
ferent. Namely, it is assumed that poly[(S)-1] forms intramo-
lecular hydrogen bonds between the C¼¼O and NAH of car-
bamate groups. The participation of the ester groups in
hydrogen bonding of poly[(S)-1] seems to be negligibly small
judging from the trace difference of the ester C¼¼O absorp-
tion from that of (S)-1 as mentioned earlier. Meanwhile,
poly[(S)-1a] presumably forms hydrogen bonds between the
C¼¼O of unprotected carboxy and NAH of carbamate groups.
It is considered that this difference of hydrogen-bonding pat-
terns causes the different helical structures between
poly[(S)-1] and poly[(S)-1a].
Ion Sensing Properties of Poly[(S)-1a]
Figure 6 shows the change of CD and UV–vis spectra of
poly[(S)-1a] upon addition of KOH measured in THF/MeOH
¼
1/1. The CD signal around 250–350 nm gradually
decreased by raising the amount of KOH up to 0.75 equiv,
and disappeared at 1.00 equiv. Further addition of KOH
induced a minus peak around 312 nm at 1.50 equiv, and a
red-shift to 330 nm at 4.00 equiv. The analogous spectral
change was also observed in MeOH (Supporting Information
Fig. S3). It is suggested that poly[(S)-1a] varied its helical
conformation (preference of screw sense and pitch/diame-
ter) in response to KOH in these solvents. The other alkali
metal hydroxides were also added to a solution of poly[(S)-
1a]. Figure 7 depicts the CD and UV–vis spectra of poly[(S)-
1a] in the absence and presence of 4 equiv of LiOH and
NaOH in THF/MeOH ¼ 1/1, along with the data of KOH. A
plus-signed CD signal was observed at 304 nm without an
alkali hydroxide, while minus-signed ones were observed
around 310–330 nm with all alkali hydroxides. The kmax
order was non < LiOH (þ23 nm) < NaOH (þ2 nm) < KOH
(þ5 nm).62 The kmax was more shifted as the size of alkali
metal became larger.63 These results suggest that poly[(S)-
TABLE 2 Solution-State IR Spectroscopic Data (C¼O and NAH
Absorption Peaks) of the Monomers and Polymers
Wavenumber (cmꢂ1
)
C¼O
Compound
Ester or carboxy
Carbamate
NAH
(S)-1a
1,746
1,742
1,747
1,731
1,709
1,698
1,718
1,717
1,506
1,518
1,506
1,522
poly[(S)-1]a
(S)-1ab
poly[(S)-1a]b
a
Measured in CHCl3 (c ¼ 40 mM).
Measured in THF (c ¼ 40 mM).
b
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FIGURE 9 Photograph of solutions of poly[(S)-1a]. From left to
right: without additive, with 4 equiv of LiOH, NaOH, KOH
measured in THF/MeOH ¼ 1/1, and TBAF measured in THF
(c ¼ 0.50 mM). A color version of this figure is available in the
Supporting Information.
larger, resulting in a loosely twisted helix (larger pitch/diam-
eter and enhanced conjugation) showing the kmax at a longer
wavelength region. Addition of TBAF,64 which has larger cati-
onic species than the alkali metals,65,66 was examined in
THF. As shown in Figure 8, the CD and UV–vis spectra of
poly[(S)-1a] showed trends similar to those of the case of
KOH addition (Fig. 6). The kmax was red-shifted largely by
TBAF (51 nm) compared to those by the alkali metal hydrox-
ides (25–30 nm), which supports the assumption of helix-
loosening induced by ionic repulsion as mentioned earlier.67
After the addition of 4 equiv of alkali metal or TBAF to a
polymer solution, excess equivalents of HCl was added to the
resulting solution. Then the CD and UV–vis spectral patterns
completely returned to the original ones. It was confirmed
that poly[(S)-1a] recovered the original helical structure
without an alkali metal or TBAF reversibly.68
FIGURE 7 CD and UV–vis spectra of poly[(S)-1a] in the absence
and presence of 4 equiv of LiOH, NaOH, and KOH measured in
THF/MeOH ¼ 1/1 (c ¼ 0.50 mM) at 20 ꢁC.
1a] changes the structure by the addition of an alkali metal
hydroxide due to the ionic interactions between the carboxy
groups and cationic species. Since the cations seem to exist
close to the carboxy groups of the polymer in THF/MeOH, it
is likely that the ionic repulsion between the side chains gets
Figure 9 shows the pictures of poly[(S)-1a] solutions before
and after addition of 4 equiv of alkali metal hydroxides and
TBAF. Since the color of each polymer solution is distinguish-
able by the naked eye, the present system may be applicable
to an ionic sensor. It is apparent that the ionic interaction
between unprotected carboxy groups and additives is the
key importance to induce such sensing abilities, because no
spectral change was observed upon addition of TBAF to a
solution of poly[(S)-1] bearing protected carboxy groups.
Conformational Analysis
As described earlier, it is considered that the present poly-
mers take predominantly one-handed helical structures. The
molecular mechanics calculation (MMFF9469) was carried
out to gain knowledge on the conformation of the polymers.
The conformers of poly[(S)-1] (18-mer) were optimized with
the dihedral angles / at the single bonds in the main chain
varying by the increment of 10ꢁ. The polymer formed two
different patterns of hydrogen bonding according to the
value of /, intramolecular hydrogen bonding between ith
and (iþ2)th units, and that between ith and (iþ3)th units.
As shown in Figure 10, a left-handed helical conformer with
/ ¼ ꢂ80ꢁ was the most stable one. The right-handed
FIGURE 8 CD and UV–vis spectra of poly[(S)-1a] upon addition
of TBAF measured in THF (c ¼ 0.50 mM) at 20 ꢁC.
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and poly[(S)-4b] were largely different from those of the pre-
cursor polymers, presumably due to the participation of car-
boxy and amino groups into intramolecular hydrogen-bonding
strands at the side chains, which play an important role for he-
lix formation and stabilization. The presence of intramolecular
hydrogen bonding is supported by the shifts of C¼¼O and NAH
IR absorption to lower and higher wavenumber regions from
monomers to polymers. The different patterns of hydrogen
bonding of poly[(S)-1] and poly[(S)-1a] is also suggested by IR
spectroscopy. The helical structure of poly[(S)-1a] became
extended upon addition of alkali hydroxides (LiOH, NaOH, and
KOH) and TBAF, and the degree of extension agreed with the
order of the cation size, which were confirmed by the red kmax
shift of the CD and UV–vis peaks. A conceivable idea for
explaining this phenomenon is the change of ionic and steric
repulsion between the side chains. Namely, the repulsion
becomes large due to the cations existing close to the carboxy-
late moieties, resulting in helix loosening accompanying the
extension of conjugation of the polyacetylene backbone, and
the order of the red-shift agrees with that of cation size.
Poly[(S)-1a] has potential as a cation sensor since the ion-re-
sponsive conformational transition is reversible and causes the
color change of the polymer solution detectable by the naked
eye. Molecular mechanics calculation suggested that the most
stable conformer of poly[(S)-1] was a left-handed helix stabi-
lized by intramolecular hydrogen bonds between the carba-
mate groups, whose presence was confirmed by IR spectros-
copy as mentioned. Further investigation on the mechanistic
aspects of conformational change of poly[(S)-1a] upon addition
FIGURE 10 Relationship between the dihedral angle at the sin-
gle bond in the main chain of poly[(S)-1] (18-mer) and the
energy calculated by MMFF94.
counterpart with / ¼ þ80ꢁ was 11.7 kJ/mol unit less stable
than the left-handed one. This energy difference between the
left- and right-handed helical conformers is explainable by
the steric factor. Namely, the van der Waals surface areas of
the left- and right-handed conformers with / ¼ ꢂ80ꢁ and
þ80ꢁ are 4663 and 4639 Å2, and volumes are 3813 and
3813 Å3, respectively.70 These data indicate that the left-
handed one is more extended, and therefore sterically more
favorable than the right-handed one, resulting in the higher
stability. The most stable conformer with / ¼ ꢂ80ꢁ forms
regulated intramolecular hydrogen-bonding strands between
the carbamate groups at ith and (iþ3)th monomer units as
depicted in Figure 11. The existence of this pattern of intra-
molecular hydrogen bonds is supported by the solution-state
IR spectroscopic data listed in Table 2.
The conformers of poly[(S)-4] and poly[(S)-4b] were also an-
alyzed in a similar fashion to those of poly[(S)-1]. It is
revealed that the most stable conformers of poly[(S)-4] and
poly[(S)-4b] are left- (/ ¼ ꢂ80ꢁ) and right-handed (/ ¼
þ80ꢁ) helices (Supporting Information Fig. S8). These molec-
ular mechanics calculation results well explain the CD signals
with opposite sign of these two polymers.
CONCLUSIONS
In the present study, we have demonstrated the synthesis and
polymerization of novel optically active substituted acetylenes
(S)-/(R)-1–(S)-4 derived from a-propargylglycine and a-propar-
gylalanine using a rhodium zwitterionic catalyst. Poly[(S)-1],
poly[(R)-1], and poly[(S)-4] exhibited optical rotations 5–14
times larger than those of the corresponding monomers. These
polymers showed intense CD signals at the absorption region
of the conjugated polyacetylene backbone. Since the CD pat-
terns and intensities were not affected by membrane filtration
and sample concentration, the chiroptical properties are not
explainable by the formation of chiral aggregates but predomi-
nantly one-handed helical structures. As far as we know, this
is the first example regarding the synthesis of a-propargyl
amino acid-derived helical polyacetylenes. Removal of the pro-
tecting groups from poly[(S)-1] and poly[(S)-4] provided
poly[(S)-1a] and poly[(S)-4b] bearing unprotected carboxy and
amino groups, respectively. The helical natures of poly[(S)-1a]
FIGURE 11 The most stable conformer of poly[(S)-1] optimized
by MMFF94. The dihedral angles / at the single bonds in the
main chain are ꢂ80ꢁ. The green dotted lines represent hydro-
gen bonds between the carbamate groups at ith and (iþ3)th
monomer units. The polyacetylene backbone is colored in yel-
low. A color version of this figure is available in the Support-
ing Information.
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24 Maeda, K.; Kamiya, N.; Yashima, E. Chem.—Eur. J. 2004, 10,
4000–4010.
of alkali metal and TBAF with considering solvent effect, and
responsiveness of poly[(S)-4b] bearing unprotected amino
groups to various acids are now under progress.
25 Sanda, F.; Terada, K.; Masuda, T. Macromolecules 2005, 38,
8419–8154.
This research was supported by the Kyoto University Global
COE Program ‘‘International Center for Integrated Research
and Advanced Education in Materials Science’’ from the Minis-
try of Education, Culture, Sports, Science, and Technology,
Japan, and The Sumitomo Foundation, and Nagase & Co., Ltd.
26 Liu, R.; Sanda, F.; Masuda, T. Polymer 2007, 48, 6510–6518.
27 Chan, K. H.; Lam, J. W.; Wong, K. M.; Tang, B. Z.; Yam, V.
W. Chem.—Eur. J. 2009, 15, 2328–2334.
28 Zheng, Y.; Cui, J.; Zheng, J.; Wan, X. J. Mater. Chem. 2010,
20, 5915–5922.
29 Sanda, F.; Teraura, T.; Masuda, T. J. Polym. Sci. Part A:
Polym. Chem. 2004, 42, 4641–4647.
30 Zhao, H.; Sanda, F.; Masuda, T. Polymer 2006, 47,
2596–2602.
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46 The more detail about the polymerization results is listed in
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16 Suzuki, Y.; Tabei, J.; Shiotsuki, M.; Inai, Y.; Sanda, F.;
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47 In the case of poly[(S)-4], fluorine-derived absorption peaks
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17 Shirakawa, Y.; Suzuki, Y.; Terada, K.; Shiotsuki, M.; Masuda,
T. Macromolecules 2010, 43, 5575–5581.
48 It has been reported that membrane filtration of solutions
of conjugated polymers is effective to remove aggregates. See
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H.; Asakawa, N.; Inoue, Y.; Kubota, K.; Sasaki, S.; Fukuda, T.;
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51 Peeters, E.; Janssen, R. A. J.; Meijer, E. W. Synth. Met.
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22 Sanda, F.; Araki, H.; Masuda, T. Macromolecules 2005, 38,
10605–10608.
52 Poly[(S)-1] and poly[(R)-1] were insoluble in MeOH and
DMF.
23 Cheuk, K. K. L.; Li, B. S.; Lam, J. W. Y.; Xie, Y.; Tang, B. Z.
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53 Suzuki, Y.; Shiotsuki, M.; Sanda, F.; Masuda, T. Macromole-
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66 Mo, H.; Pochapsky, T. C. J. Phys. Chem. B 1997, 101,
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67 No red-shift of kmax was observed upon TBAF addition in
THF/MeOH ¼ 1/1 and THF/H2O¼ 1/1 (Supporting Information
Fig. S6). The ionic interaction between unprotected carboxy
groups and TBAF does not seem preferable in highly polar sol-
vents, presumably due to the presence of hydrophobic alkyl
groups.
55 Percec, V.; Obata, M.; Rudick, J. G.; De, B. B.; Glodde, M.;
Bera, T. K.; Magonov, S. N.; Balagurusamy, V. S. K.; Heiney, P.
A. J. Polym. Sci. Part A: Polym. Chem. 2002, 40, 3509–3533.
56 Fujiki, M. Macromol. Rapid Commun. 2001, 22, 539–563.
68 See the CD and UV–vis spectra of poly[(S)-1a] with 100
equiv of HCl after addition of 4 equiv of KOH in THF/MeOH
(Fig. S7). We could not examine the possibility of proton
extraction from the polymer main chain or side groups by 1H
NMR spectroscopy after alkali addition, because the polymer
concentration was too low to measure the NMR spectra. Pro-
ton extraction reactions from vinylene and carbamate are com-
monly carried out using alkyl lithium and NaH, whose
basicities are much higher than those of alkali hydroxides.
TBAF also strongly affected the CD and UV–vis spectra of the
polymer in a fashion similar to alkali hydroxides, although the
basicity of TBAF is much lower than those of alkali hydroxides.
It is therefore likely that the interaction between the carboxy
groups and ions caused the conformational changes as men-
tioned. We can deny the cis-trans isomerization of the double
bonds at the main chain after the addition of alkali hydroxides
since the CD and UV–vis spectral patterns completely returned
to the original ones by the addition of HCl. We assume that the
chirotopic change is mainly brought about by the change of
population of right and left handed helices as well as change
of degree of twisting.
57 We also tried to obtain poly[(S)-1a] by the polymerization of
(S)-1a bearing unprotected carboxy groups, but failed under the
conditions in Table 1, presumably due to the deactivation of the
rhodium catalyst by the carboxy groups. See Saito, M. A.; Maeda,
K.; Onouchi, H.; Yashima, E. Macromolecules 2000, 33, 4616.
58 See the 1H NMR spectra of the polymers before and after
hydrolysis (Fig. S1).
59 The carboxy groups were transformed into methyl ester
groups using trimethylsilyldiazomethane, because poly[(S)-1a]
was not eluted out.
60 See the 1H NMR spectra of the polymers before and after
deprotection (Fig. S2).
61 This concentration is low enough to eliminate the effect of
intermolecular hydrogen bonding.
62 RbOH was hardly soluble in THF/MeOH ¼ 1/1. CsOH (4
equiv) shifted the kmax but induced no clear CD signal unlike
the others. See Figure S4.
þ
63 Ionic radius (A): Li 0.9, Naþ 1.2, Kþ 1.5. See Shannon, R. D.
˚
Acta. Cryst. 1976, A32, 751–767.
64 Addition of tetrabutylammonium hydroxide was also exam-
ined but the trends of CD and UV–vis spectra were different
from those with alkali metal hydroxides (Fig. S5).
69 Halgren, T. A. J. Comput. Chem. 1996, 17, 490–519. The mo-
lecular mechanics calculation was carried out with Wavefunc-
tion Inc., Spartan ’10 Macintosh.
65 The ionic radius of Bu4Nþ ¼ 4.8 A. See Watanabe, Y.;
˚
70 Calculated by Winmostar Ver. 3.8. http://winmostar.com/;
Senda, N. Idemitsu Tech. Rep. 2006, 49, 106–111.
Ohnaka, K.; Fujita, S.; Kishi, M.; Yuchi, A. Anal Chem 2011, 83,
7480–7485.
2018
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