Accordion-like oscillation of contracted and stretched helices of polyacetylenes synchronized with the restricted rotation of side chains

Article abstract of DOI:10.1021/ja4004987

A chiral substituted acetylene, (s)-2-octyl propiolate, was stereoregularly polymerized using a catalyst, [Rh(nbd)Cl]₂, at 40 C in methanol to give the corresponding helical polymer, Ps2OcP. The changes of ¹H and ¹³C NMR spectra in line shapes and splitting patterns were consistently interpreted in terms of restricted rotation around the ester O-*C bond, ～O-*C^εH^ε(R)～, R = a branched CH^ε₃ in the ester side chains rather than the helix inversion with the aid of a 3-site jump model. Three peaks due to the branched methyl H^ε proton and its C^η carbon observed at 0 C suggested the formation of three rotamers called A, B, and C, based on the presence of the contracted helix and stretched helix forms that have an intrinsic helical pitch. Furthermore, an accordion-like helix oscillation (HELIOS) along the main chain axis was proposed to explain the temperature dependence spectral changes observed in ¹H and ¹³C NMR, UV-vis, and circular dicromism (CD) spectra. The temperature dependence UV-vis and CD spectra of Ps2OcP corroborate the presence of contracted and stretched one-handed helix sense polymers in solution in which the helical pitches and their persistence lengths depend on the temperature.

Full text of DOI:10.1021/ja4004987

Article
pubs.acs.org/JACS
Accordion-like Oscillation of Contracted and Stretched Helices of
Polyacetylenes Synchronized with the Restricted Rotation of Side
Chains
Yoshiaki Yoshida,† Yasuteru Mawatari,†,‡ Asahi Motoshige,† Ranko Motoshige,† Toshifumi Hiraoki,§
Manfred Wagner,∥ Klaus Mullen,∥ and Masayoshi Tabata†,‡,*
̈
†Department of Applied Chemistry, Graduate School of Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho,
Muroran, Hokkaido 050-8585, Japan
‡Research Center for Environmentally Friendly Materials Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho,
Muroran, Hokkaido 050-8585, Japan
§Department of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
∥Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
S
* Supporting Information
ABSTRACT: A chiral substituted acetylene, (s)-2-octyl
propiolate, was stereoregularly polymerized using a catalyst,
[Rh(nbd)Cl]2, at 40 °C in methanol to give the corresponding
1
helical polymer, Ps2OcP. The changes of H and 13C NMR
spectra in line shapes and splitting patterns were consistently
interpreted in terms of restricted rotation around the ester
O−*C bond, ∼O−*CεHε(R)∼, R = a branched CHε3 in the
ester side chains rather than the helix inversion with the aid of
a 3-site jump model. Three peaks due to the branched methyl
Hε proton and its Cη carbon observed at 0 °C suggested the
formation of three rotamers called A, B, and C, based on the presence of the contracted helix and stretched helix forms that have
an intrinsic helical pitch. Furthermore, an accordion-like helix oscillation (HELIOS) along the main chain axis was proposed to
1
explain the temperature dependence spectral changes observed in H and 13C NMR, UV−vis, and circular dicromism (CD)
spectra. The temperature dependence UV−vis and CD spectra of Ps2OcP corroborate the presence of contracted and stretched
one-handed helix sense polymers in solution in which the helical pitches and their persistence lengths depend on the
temperature.
alkoxy substituted poly(phenylacetylene) can be controlled
using external stimuli, e.g., thermal treatment or exposure to
solvent vapor in the solid phase where the cis−transoid as a
stretched helix rearranged to cis−cisoid as a contracted helix
accompanied with a drastic change in color from blight yellow
to dark red.10 Thus, the detailed geometrical and spatial helical
structures of SPA polymers should be explicitly and
unequivocally determined to identify their functional properties
such as their oxygen permeability,7 nonlinear optical behavior,11
electrical conductivity,12 memory effects of spin glass
materials,13 and responsiveness to external stimuli.14 Many
helical polymers, including SPAs and polypeptides, have been
used for effective separation of enantiomers.15 Recently, the
helix inversion of various SPAs with chiral side chains has been
proposed based on CD spectral data.5,6 In this report, we
demonstrate a restricted rotation about an ester O−*C bond in
the chiral side chain of helical poly((s)-2-octyl propiolate)
INTRODUCTION
■
During the past two decades, a few rhodium complexes, e.g.,
[Rh(nbd)Cl]2-cocatalysts (nbd: norbornadiene), also called
bidentate Rh catalysts,1 including zwitterionic Rh complexes2
and cationic Rh+BF3 complexes,3 have been developed as
−
important stereoregular polymerization catalysts for various
substituted acetylene (SA) monomers to obtain helical
substituted polyacetylenes (SPAs) with cis−transoid geometrical
structures. Furthermore, it has been observed that a
monodentate Rh complex (Rh catalyst) is generated in situ
from the corresponding bidentate Rh complex when triethyl-
amine, alcohol, or aqueous alcohol are used as either the
cocatalyst, or the polymerization solvent which generates the
propagation species of the Rh catalyst.1,4 SPAs with helical cis−
transoid main-chain structures are associated with chirotropic
properties5,6 and form pseudohexagonal crystals, called
columnars,1,4 which are driven by van der Waals forces in the
solid state. Previously, poly(alkyl propiolate)s (PAPs) were
reported to be promising oxygen-permeable materials7 with
neither siloxyl nor trialkylsilyl groups.8,9 The helical pitch of an
Received: January 16, 2013
Published: February 12, 2013
© 2013 American Chemical Society
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23.1, 19.8, 14.3. Anal. Calcd. for C11H18O2: C, 72.49; H, 9.95. Found:
C, 72.17; H, 9.95 (see Figure S1 of the Supporting Information, SI).
(Ps2OcP). We also propose that a 3-site jump rotation about
the ester bond produces contracted and stretched helices
accompanied by an Accordion-like Helix Oscillation (HELIOS).
The conformational structures corresponding to the 3-site jump
were determined together with their helical pitches by using
RESULTS AND DISCUSSION
■
Polymerization. The polymerization of 1 was performed
using the catalyst, [Rh(nbd)Cl]2, at 40 °C for 4 h in MeOH to
obtain the corresponding polymer, Ps2OcP, as shown in
Scheme 1 and Table 1. The polymer was produced in a
1
solution dynamic H and 13C NMR, infrared (IR), absorption
(UV−vis), and circular dichromism (CD) techniques.
EXPERIMENTAL SECTION
■
Measurements. Number-average and weight-average molecular
weights (Mn and Mw, respectively) of Ps2OcP polymers were
measured using a JASCO GPC 900−1 equipped with two Shodex
K-806L columns and an RI detector. Chloroform was used as an
eluent at 40 °C, and poly(styrene) standards (Mn = 800−1 090 000)
Table 1. Synthesis of Poly((s)-2-octyl propiolate)s by a
a
Catalyst [Rh(nbd)Cl]2 in MeOH
b
c
c
d
polymer
yield (%)
Mn (x10−4
)
Mw/Mn
cis (%)
Ps2OcP
33
5.7
3.9
79
1
were employed for calibration. H NMR (500 MHz) and 13C NMR
a
b
[M]0 = 2.0 mol/L, [M]0/[cat.] = 100. Insoluble fraction in
(125 MHz) spectra were recorded on a JEOL JNM-ECA500 using
chloroform-d1 (CDCl3) and methylene dichloride-d2 (CD2Cl2) as
solvents at various temperatures. Tetramethyl silane (TMS) (Aldrich)
was used as standard. Solution UV−vis and IR spectra of polymers
were recorded on JASCO V570 and FT/IR 230 spectrophotometers,
respectively, in methylene dichloride (CH2Cl2) at various temper-
atures. Solution CD spectra of polymers were taken on a JASCO J-
720WI spectrometer in CH2Cl2 at various temperatures. Energetically
optimized dynamic conformations of Ps2OcP were deduced from
molecular mechanics calculations using the MMFF94 force field
program (Wavefunction, Inc., Spartan ’10 Windows version 1.1.0).16
Spectral simulation of 1H and 13C NMR spectra and calculation of the
restricted rates of rotation about the ester O−*C bond were
performed using the gNMR program.17
c
d
methanol. Estimated by GPC analysis (PSt, CHCl3). Determined by
1H NMR analysis (CD2Cl2).
moderate yield of 33%, a number-average molecular weight
(Mn) of 5.7 × 104, a molecular weight dispersity (Mw/Mn) of
3.9, and the ratio of helical cis−transoid structure of 79%.1,4
Conformational Rotamers A, B, and C Detected by 1H
NMR. The proton signals at 7.15, 4.78, 1.63−1.47, 1.29, 1.20,
and 0.88 ppm were assigned to Hα−CC, O*CHε, CHζ ,
2
(CH2)4, OCH(CHη ), and CH3, respectively (Figure 1a).
3
Figure 1b shows the temperature-dependence 1H NMR spectra
Materials. Synthesis of (s)-2-octyl propiolate(s2OcP) 1. A mixture
of 100 mL of toluene, 19.5 g (0.15 mol) of (s)-2-octanol (Tokyo
Chem. Ind.), 7.0 g (0.10 mol) of propiolic acid (Aldrich), and 1.9 g
(10.0 mmol) of p-toluenesulfonic acid (Tokyo Chem. Ind.) was
refluxed for 8 h in a Dean−Stark apparatus. The resulting mixture was
washed with saturated sodium hydrogen carbonate aqueous solution
and distillated water. The organic layer was dried over anhydrous
sodium sulfate and the solvent was removed by evaporation. The crude
product was purified by distillation (90 °C/7 mmHg) to obtain a
colorless liquid 1, 12.2 g in 67% yield. 1H NMR (CDCl3): δ 5.01 (sext,
J = 6.3 Hz, 1H, OCH(CH3)CH2), 2.85 (s, 1H, HCC), 1.72−1.46
(m, 2H, OCH(CH3)CH2), 1.36−1.31 (m, 6H, (CH2)3), 1.27 (d, J =
6.3 Hz, 3H, OCH(CH3)), 0.88 (t, J = 6.3 Hz, 3H, CH3); 13C NMR
(ppm): δ 152.9, 74.8, 74.6, 66.5, 31.7, 28.9, 28.4, 25.8, 22.6, 14.1.
Polymerization. Poly((s)-2-octyl propiolate), Ps2OcP, was obtained
upon polymerization of (s)-2-octyl propiolate monomer (s2OcP) 1
using the catalyst, [Rh(nbd)Cl]2, as shown in Scheme 1. In a typical
Scheme 1. Preparation of Ps2OcP with Rh Complex Catalyst
procedure, 1.0 g (5.5 mmol) of the monomer and a calculated amount,
25 mg (5.5 × 10−2 mmol), of the catalyst were dissolved in MeOH
(2.8 mL). The mixture was added to a specially designed U-shaped
ampule1a and was stirred for 4 h at 40 °C. The resulting reaction
mixture was poured into excess methanol with stirring. The resulting
pale yellow polymer was washed with methanol and dried under
dynamic vacuum, approximately 10−2 Torr, for 12 h at room
temperature.
Figure 1. 1H NMR spectra of Ps2OcP prepared with a catalyst,
[Rh(nbd)Cl]2, in MeOH at 40 °C. (a) observed at 30 °C, (b)
expanded temperature-dependence spectra in the range of 7.5−4.0
ppm, and (c) computer-simulated spectra in the range of 7.5−4.0 ppm
using a gNMR program.
1
Poly((s)-2-octyl propiolate). H NMR (CDCl3): δ 7.15 (br., 1H,
HCC), 4.78 (br., 1H, OCH), 1.63, 1.47 (br. d, 2H, CH(CH3)CH2),
1.29 (br., 8H, (CH2)4), 1.20 (br., 3H, CH(CH3)), 0.88 (br., 3H, CH3);
13C NMR (ppm): δ 165.2, 139.1, 131.4, 72.6, 36.3, 32.3, 29.8, 25.9,
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expanded in the range of 7.5−4.0 ppm. A fairly broad and
distorted peak due to the methine Hε roton was observed at
4.78 ppm at 30 °C, and the line shape of the peak gradually
changed to give a triple line peak at 4.95, 4.84, and 4.53 ppm
with a relative intensity of approximately 1:1:1 at 0 °C. Most
notably, the line width of the Hε signal decreased with
decreasing temperature. The narrowing of the line width can be
explained by the chemical site exchange phenomenon within
Ps2OcP and not by the helix inversion of the helical main chain
of Ps2OcP.5 Hence, the restricted rotation about the ester
O−*C bond through the 3-site jump in the side chain of
helix, then the vinyl proton of rotamer A is located above the
plane of the carbonyl CO π bond at every n+3 monomer
unit of the main-chain. Thereby, the narrow helical pitch causes
a shielding of the vinyl proton (see Figure 2) as compared to
rotamers B and C. Therefore, the ratio of the contracted helix,
A, and the stretched helices, B and/or C, as formed in solution,
can be estimated from Figure 1b as 1:2. The restricted ester
O−*C rotation is also confirmed from the fact that no changes
in chemical shifts and line widths of the remaining methyl
branched hexyl side chain moieties occur, even when the
temperature is increased from 0 to 30 °C (see Figure S2 of the
SI).
1
Ps2OcP occurs on the H NMR time scale. Each conformer
corresponding to the chemical site exchange is depicted using a
Newman-like model, and referred to as rotamers A, B, and C, as
seen in Figure 2. Therefore, the intensity of the triple line peak
The restricted ester O−*C bond rotation can be explained
by the increasing double bond order of the ester O−*C single
bond as a result of the extended conjugation. The observed
three line peaks coalesce at 30 °C in CD2Cl2, in which a
relatively fast ester O−*C bond rotation occurs, as shown in
Figures 1b and 2. To the best of our knowledge, this is the first
reported case of such a restricted ester O−*C bond rotation in
synthetic polymers, although restricted rotation is well-known
in the case of low-molecular amides.18,19
Rate of Restricted Rotation and Energy Barrier about
the O−*C Bond. 1H NMR spectral simulations were
performed with the gNMR program17 to determine the energy
barrier of rotation around the ester O−*C bond, Gibbs’ free
energy, ΔG‡, together with the rotational jump rate, τ,
assuming a 3-site jump model,18 i. e., the equal jump rate
and equal population. The observed spectra reasonably agree
with the simulated spectra at each temperature with respect to
line shape and intensity (Figure 1c). The peak separation due
to the vinyl proton was experimentally estimated to be 283 Hz
at 0 °C (Figure 1b). The exchange jump rate between the peaks
at 7.27 ppm and 6.70 ppm was determined as 20 Hz, which is
associated with a free energy of the rotation of ΔG‡ = 13.9 kJ/
mol.
Rotamers A, B, and C Detected by 13C NMR. The signals
at 165.2, 139.1, 131.4, 72.6, and 19.8 ppm as well as the other 6
peaks at higher field in the 13C NMR spectrum of Ps2OcP at 30
°C (Figure 3a) were ascribed to a carbonyl carbon, γ; two vinyl
carbonyl carbons, α and β; a methine carbon, ε; a branched
methyl carbon, η; and six n-alkyl chain carbons (see Figure S3
of the SI), respectively. The temperature-dependent spectra
expanded in the ranges of 170−125 ppm and 22−18 ppm are
shown in Figure 3b. It appears that the degree of steric
hindrance in the proposed rotamers A, B, and C takes the
following order (see Figures 2).
Figure 2. Perspective views showing helical structures including the
vinyl proton and carbonyl moiety of rotamers A, B, and C.
observed at 0 °C indicates that rotamers A, B, and C are
generated in an approximately equal amount. Conversely, the
vinyl proton due to Hα−C was also separately observed as
two peaks (7.27 and 6.70 ppm, relative intensity of
approximately 2:1 at 0 °C) reflecting the different dihedral
angle between the CO and C−Hα bonds.18,19 If rotamer A
has an anti conformation between the main chain and the n-
hexyl moiety in order to decrease steric hindrance, then a
relatively narrow helix called a contracted helix is generated.
Alternatively, rotamers B and C do not adopt these
conformations due to steric reasons, and their pitch widths
are expanded. From the chemical shifts of the two rotamers, it
can be deduced that the rotamer absorbing at 6.70 ppm does
not have the same pitch width as the rotamer absorbing at 7.27
ppm. The signal at 6.70 ppm is attributed to the contracted
helices of rotamer A, and the one at 7.27 ppm to the stretched
helices present in rotamers B and/or C. The latter have wider
helical pitches than rotamer A. If rotamer A has a contracted
(1)
A < B ≤ C
In rotamers B and C, the n-hexyl chains stay more or less on
the same side as the polymer main chain, which produces a
relatively large steric hindrance, even though the chain direction
is opposite. In rotamer A, the hexyl chain and the main chain
adopt the anti-zigzag conformation. As a consequence, the
dihedral angle between the main chain CC bond and the
ester moiety, ∼O−CO∼, becomes small compared to that of
rotamers B and C because of the narrow helical pitch. The alkyl
side chain including the methyl group increases the electron
density on the carbonyl carbon, although the CC bond
decreases the electron density on the ester carbon to some
extent. Therefore, the peak observed at 166.6 ppm is assigned
to the contracted helix of rotamer A. Alternatively, the helical
pitch of rotamers B and C is wider than that of rotamer A, and
thus, the dihedral angle between the CC bond and the ∼O−
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generated: one contracted helix and two stretched helices. Two
broad signals due to the α and β carbons in the main-chain C
C bond at 30 °C were also split at 0 °C into three peaks:
namely 142.6, 139.3, and 136.7 for the α carbon and 133.5,
131.9, and 127.2 ppm for the β carbon. Rotamer A has a trans
zigzag plane, including a CαCβ−Cγ−O−*CεR1 (R1 = n-
hexyl) bond that increases the electron density of both the α
and β carbons. Therefore, the vinyl carbons observed at 136.7
and 127.2 ppm were attributed to rotamer A. Rotamer C does
not adopt such a trans zigzag conformation between the methyl
carbon Cη and n-hexyl carbon chain moiety, and therefore, the
peaks at 142.6 and 133.5 ppm were attributed to rotamer C
(Figures 2 and 3b). The peaks at 139.6 and 131.9 ppm were
assigned to rotamer B. The simulated temperature-dependent
13C NMR spectra agreed well with the observed spectra (see
Figure 3c). The rotational jump rate, τ, at each carbon is shown
in Figure 3c. Three peaks due to the branched methyl carbon,
Cη, were also observed at 0 °C, and only a broad singlet peak
was observed at 30 °C. These signal changes were also
interpreted in terms of the generation of the rotamers A, B, and
C. In rotamer B, the carbonyl carbon, Cγ, and methyl carbon,
Cη, in the −O−Cε−CηH3 moiety exist in a trans zigzag
structure; thus, the methyl carbon Cη peak should be observed
at a lower magnetic field, 20.5 ppm. In contrast, the peak at
19.0 ppm was attributed to rotamer C because no such zigzag
conformation was observed. Therefore, the peak 20.0 ppm, was
attributed to rotamer A.
The negative charges on each carbon calculated by the
Hartree−Fock 3-21G method were determined to be rotamer B
= 0.574, rotamer A = 0.602, and rotamer C0.617,
respectively,24 whose charge order agreed with the observed
chemical shift of their carbonyl groups. Thus, it clearly follows
from the temperature-dependent 1H and 13C NMR spectra that
rotamers A, B, and C, which have an intrinsic pitch width and
helical main chain length, or persistence length, are
undoubtedly generated with a dynamically and spatially
favorable conformational structure on the prevailing time
scales. Every pitch length of the helical rotamers is dynamically
oscillating, or synchronized, with the rotation around the ester
O−*C bond. Therefore, we propose that this phenomenon is
called an Accordion-like Helix Oscillation (HELIOS) of the PAP
Main Chains. Conformational and spatial information regarding
the ester carbonyl moiety of each rotamer was also obtained
from the IR spectra (Figure 4). Only one strong and broad
peak was observed at 1714 cm−1, which is an average value of
Figure 3. 13C NMR spectra of Ps2OcP. (a) observed at 30 °C, (b)
expanded temperature-dependence spectra in the ranges of 170−125
and 22−18 ppm, and (c) computer-simulated spectra using a gNMR
program.
CO∼ moiety is fairly large compared to that of rotamer A. In
other words, the ester moiety is out of plane with the CC
bond plane. In rotamer B, the carbonyl carbon and the methyl
moiety adopt a trans zigzag conformation, and the electron
density on the carbonyl carbon is increased because of an I
effect due to the methyl moiety. In rotamer C, the electron
density on the carbonyl carbon remains unaffected because the
methyl and the n-hexyl moieties do not possess an anti
conformation (Figure 2).
Therefore, the peak observed at 165.0 ppm was attributed to
rotamers B and/or C as the stretched helices. Thus, the CγO
carbon signal was observed at 166.6 and 165.0 ppm, and the
approximate intensity ratio of 1:2 can reasonably be explained.
This finding indicates that three types of helices have been
Figure 4. IR spectrum of the carbonyl group of Ps2OcP observed in
CH2Cl2 at room temperature.
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the cis and trans vinyl carbonyl absorptions (e.g., 1729 and 1704
cm−1 of the model compounds)20,21 (see Figure S4 of the SI).
The presence of a broad peak suggests conformational
exchanges of the carbonyl carbon among rotamers A, B, and
C, as mentioned above.
UV and CD spectra of Contracted and Stretched
Helices. Two absorption maxima, λmax, were observed at 30 °C
(Figure 5). The absorption at 371 nm was red-shifted, i.e., it
helix pitch width as discussed below. Thus, this limitation is the
reason why the absorption position at 290 nm is kept even
when observed at 0 °C as shown in Figure 5a. Therefore, the
absorption at 290 nm can be reasonably assigned to that of the
contracted helix, although the correct absorption coefficients of
each helix are not known.
Thus, two absorptions at 290 and 371 nm were reasonably
attributed to the contracted helix, rotamer A, and stretched
helices, rotamers B and/or C, of Ps2OcP, respectively. Further,
the temperature-dependent UV−vis spectral changes could be
explained by assuming that the helical chain rapidly oscillated,
exchanging the helical pitch width between the contracted and
stretched sequences within a limited helical main chain length
(see Figure 6). Previously, the λmax observed at a shorter
Figure 6. Proposed image model for the contracted helix and stretched
helix main chain.
wavelength was attributed to the random coil of a one-handed
helix sense polymer,23 where an inversion of the helix sense of
the main chain was assumed. However, it seems that 1H or 13
C
Figure 5. Temperature-dependence UV−vis and CD spectra of
Ps2OcP. (a) UV−vis spectra in CH2Cl2 and (b) CD spectra of Ps2OcP
observed in CH2Cl2.
NMR spectral data do not support the existence of such a
random coil, even at 30 °C.23
CD spectra of Ps2OcP were observed in order to determine
the helicity and degree (Figure 5b). The CD spectra showed
the Cotton effect at 325 nm whose intensity greatly increased at
lower temperature. Therefore, the helicity of the main chain
was increased at lower temperature, and the ratio of a one-
handed helix with a more tightly wound main chain is
increased. If helix inversion for Ps2OcP occurred rapidly along
the main chain helix, then inversion of the Cotton effect sign
should be observed at 30 °C. At this temperature, the ester
proton together with the vinyl proton of 1H NMR coalesced to
symmetrical and unsymmetrical broad peaks at 30 °C,
respectively.5 However, no such signal inversion was detected
in the CD spectra,22 even at 30 °C, as shown in Figure 5b. This
result strongly indicates that rather a restricted ester O−*C
bond rotation occurs. Thus, the temperature-dependence UV−
vis and CD spectra allow us to conclude that the main chain of
the one-handed helix sense polymer, Ps2OcP, never undergoes
helix inversion under these experimental conditions.
was at 381 nm when observed at 0 °C, and the absorption
intensity increased from 30 to 0 °C. The absorption intensity at
290 nm was greatly enhanced, keeping the absorption position
at 0 °C. If the pitch width of the stretched helix within a limited
main-chain length is thermally and reversibly oscillating, then
its average pitch width becomes smaller because the degree of
the stretched helix component is decreased. Therefore, it can be
deduced that the effective conjugation length due to the
stretched helix in the main chain of Ps2OcP is decreased. This
decrease is the reason why λmax at 381 nm is blue-shifted,
decreasing the absorption intensity when heated. It is assumed
that the λmax of a rotamer having a wider helical pitch is red-
shifted compared with a rotamer with a narrow helical pitch.22
On the contrary, the pitch width of the contracted helix also
thermally oscillates, then its average pitch width become wider
because the degree of the contracted helix, i.e., helicity is
decreased, although the oscillation degree of the contracted
helix is relatively small and limited compared with those of the
stretched helices. Therefore, it is easily deduced that the
helicity, i.e., the absorption intensity due to the contracted helix
is decreased when observed at 30 °C as seen in Figure 5a. This
also indicates that the resulting contracted helix has a narrow
coherent and limited pitch width unlike that of the stretched
Calculated Pitch for Contracted and Stretched
Helices. Dihedral angles between the two double bonds, the
helical pitches, and the strain energies of Ps2OcP were
calculated using the MMFF94 force field program (Table
2).16 The most stable rotamer among the three rotamers, A, B,
and C was identified as A, assuming the 3-site jump model and
an equal distribution ratio at each site. The calculation showed
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and the Cη carbon observed at 0 °C suggested the formation of
three rotamers, A, B, and C, due to the presence of a contracted
helix and two stretched helix forms. All of them possessed an
intrinsic helical pitch, as confirmed by the MMFF94 and gNMR
programs. Furthermore, an accordion-like helix oscillation
(HELIOS) along the main-chain axis was proposed to explain
Table 2. Dihedral Angles between C−C and CC Bonds,
a
Helical Pitches, And Strain Energies of Ps2OcP
dihedral angle
(deg)
pitch
(Å)
strain energy (kJ/
polymer rotamer
A
mol·unit)
70
3.6
5.1
5.3
230
243
311
Ps2OcP
B
110
140
1
the temperature-dependent changes observed in H and 13C
C
NMRs, UV−vis, and CD spectra (Figure 7). The two UV
absorption peaks also supported three such helices which were
identified from temperature-dependent CD spectra as
contracted and stretched one-handed helix sense polymers.5,6,22
The smallest helical pitch width, 3.6 Å for Ps2OcP, was close to
that of graphite, a typical conductive and layered material. The
present study will be very useful for the fabrication of a single
helical molecule polymer device. We are currently studying the
preparation of related helical polymers and their conventional
spectroscopic features, such as electron spin resonance (ESR),
to understand the contracted and stretched helix structures
showing the HELIOS phenomena. The results will be
published elsewhere soon.
a
Calculated by Molecular Mechanics method (MMFF94).
that rotamer A determined as a contracted helix, has the
smallest pitch width, 3.6 Å, and strain energies, 230 kJ/
mol·units. Rotamers B and C are classified as stretched helices
and have pitch widths of 5.1 Å and 5.3 Å and strain energies of
243 kJ/mol·units and 311 kJ/mol·units, respectively (see Figure
6 and Table 2).
Importance of PAPs with a Contracted Helical Pitch.
SPAs, including Ps2OcP, are classified as some of the stiffest
helical polymers because they are composed of alternating C−
C and CC conjugated bonds and present additional π-
conjugation due to π-stacking, which is generated in the
direction of the molecular axis.10 The calculated helical pitch
width of the rotamers is approximately 3.6−5.3 Å (see Table 2),
and the smallest pitch is comparable to the layer distance of
graphites, 3.35 Å, which are electrically conductive materials.25
In the case of poly(ethyl propiolate), the electrically current
was measured to be approximately 10−8 A without doping.26
ASSOCIATED CONTENT
* Supporting Information
Full scale spectral data of materials. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
tabata@mmm.muroran-it.ac.jp
■
CONCLUSIONS
■
(S)-2-octyl propiolate, s2OcP (1), a chiral alkylpropiolate, was
stereoregularly polymerized by using the catalyst [Rh(nbd)Cl]2
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support of Dr. Y. Yoshida from
the Muroran Institute of Technology. Thanks are extended to
the Instrument Center at the Institute for Molecular Science for
collecting the CD spectra measurements.
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