Strictly Alternating Sequences When Copolymerizing Racemic and Chiral Acetylene Monomers with an Organo-Rhodium Catalyst

Article abstract of DOI:10.1021/acs.macromol.6b02508

A racemic mixture and two chiral monomers of 2-methyl-1-butyl propiolate, i.e., rac1, R1, and S1, were stereoregularly polymerized with a catalyst, [Rh(norbornadiene)Cl]₂, in methanol at 40 °C to obtain the corresponding helical racemic and two chiral polymers, Prac1, PR1, and PS1, and a copolymer, Pco. The ¹H and ¹³C NMR spectra of the racemic and chiral polymers differed, although the NMR spectra of their monomers were the same. The structures of the Pco copolymers with different chiral monomer ratios were analyzed using 1D and 2D NMR, optical rotation, circular dichroism (CD), UV-vis, and computational methods to elucidate the stereochemical effect of the chiral monomers together with the polymerization mechanism. The temperature dependence of ¹H and ¹³C NMR spectra in line shape and intensity indicated that the helical main chain undergoes restricted rotation around the ester methylene bonds ?O-CH₂- through a three-site jump exchange called an accordion-like helix oscillation (HELIOS). The energetically preferred structures of the helical-sense polymers PR1 or PS1 were simulated using the MMFF94 program. The dependence of the NMR spectral line shapes, optical rotations, and calculated structures on the monomer feed clearly indicated that the copolymers alternatively incorporate R1 and S1 to generate one-handed helical-sense chains. Based on these results, a polymerization mechanism is proposed, explaining a strictly alternating copolymerization that yields helical chains.

Full text of DOI:10.1021/acs.macromol.6b02508

Article
pubs.acs.org/Macromolecules
Strictly Alternating Sequences When Copolymerizing Racemic and
Chiral Acetylene Monomers with an Organo-Rhodium Catalyst
†
,†,‡
,§,#
†
∥
⊥
Yoshiaki Yoshida, Yasuteru Mawatari,* Takahiro Sasaki, Toshifumi Hiraoki, Manfred Wagner,
⊥
Klaus Mu
̈
llen, and Masayoshi Tabata*
†
‡
§
Graduate School of Engineering, Research Center for Environmentally Friendly Materials, and Center of Environmental Science
and Disaster Mitigation for Advanced Research, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran, Hokkaido
0
50-8585, Japan
∥
Graduate School of Engineering, Department of Applied Physics, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
⊥
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
#
Faculty of Science and Technology, Department of Applied Chemistry and Bioscience, Chitose Institute of Science and Technology,
Bibi 65-758, Chitose, Hokkaido 066-8655, Japan
*
S Supporting Information
ABSTRACT: A racemic mixture and two chiral monomers of 2-methyl-1-butyl propiolate, i.e., rac1, R1, and S1, were
stereoregularly polymerized with a catalyst, [Rh(norbornadiene)Cl] , in methanol at 40 °C to obtain the corresponding helical
2
1
13
racemic and two chiral polymers, Prac1, PR1, and PS1, and a copolymer, Pco. The H and C NMR spectra of the racemic and
chiral polymers differed, although the NMR spectra of their monomers were the same. The structures of the Pco copolymers with
different chiral monomer ratios were analyzed using 1D and 2D NMR, optical rotation, circular dichroism (CD), UV−vis, and
computational methods to elucidate the stereochemical effect of the chiral monomers together with the polymerization
1
13
mechanism. The temperature dependence of H and C NMR spectra in line shape and intensity indicated that the helical main
chain undergoes restricted rotation around the ester methylene bonds −O−CH − through a three-site jump exchange called an
2
accordion-like helix oscillation (HELIOS). The energetically preferred structures of the helical-sense polymers PR1 or PS1 were
simulated using the MMFF94 program. The dependence of the NMR spectral line shapes, optical rotations, and calculated
structures on the monomer feed clearly indicated that the copolymers alternatively incorporate R1 and S1 to generate one-
handed helical-sense chains. Based on these results, a polymerization mechanism is proposed, explaining a strictly alternating
copolymerization that yields helical chains.
1
3,18−46
21−37
INTRODUCTION
(PPAs)
and its analogues,
and poly-
■
47,48
1
(naphthylacetylene).
linear (R = (CH ) CH ) or branched (R = CH(CH )-
CH ) CH ) alkyl chains, which were proven by H and
PAPs are (−HCCCOOR−) with
n
Helical synthetic polymers, e.g., poly(alkyl methacrylate)s,
2
3
4
poly(isocyanide)s, poly(isocyanate)s, poly(silane)s, and
2 6
3
3
13
1
5
−10
(
C
others,
have been prepared using various catalysts, e.g.,
2 5
3
NMR spectroscopy in solution to possess a helical contracted
cis−cisoid structure and stretched cis−transoid structure.
Furthermore, these helical polymers exhibited a unique
(
−)-(sparteine), Ni(CN-t-C H ) (ClO ) , TiCl (OCH CF ),
4
9 4
4 2
3
2
3
1
−10
or sodium in the presence of 15-crown-5.
organo-rhodium complex catalyst, e.g., [Rh(nbd)Cl] (nbd =
Recently, an
2
norbornadiene), and alcohol or amine as a cocatalyst or
1
1−14
solvent
were used to stereoregularly prepare helical-
Received: November 19, 2016
Revised: January 17, 2017
substituted polyacetylenes (HSPAs), e.g., poly(alkyl
propiolate)s (PAPs),
1
5−17
poly(phenylacetylene)s
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.6b02508
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accordion-like helix oscillation called a HELIOS mode in
solution, although they possessed hexagonal or tetragonal
Scheme 1. Syntheses of (a) (R)-2-Methyl-1-butanol, 1b, (b)
rac1, R1, S1, (c) Prac1, PR1, PS1, and (d) Pco Using a
15,49−54
packing in their solid phases.
The helical aromatic
Catalyst [Rh(nbd)Cl] in MeOH for 4 h at 40 °C
2
polyacetylenes (HAPAs) are remarkable polymers because they
exhibit two different modes of π-conjugation, i.e., through bond
conjugation within the helical main chains composed of
approximately three helical monomer units and through space
π-conjugation along the fairly long helical axis with
approximately 3.4 Å as the π-stacking distance between side
1
8−27,38−42,52−54
chains.
HAPAs are quite different, e.g., pale yellow,
red,
Therefore, the colors of the resulting
21−27
25,27
orange,
and can be controlled by
experimental conditions, such as temperature and sol-
2
1−27
24,26
21−27
violet,
and black,
1
9,21−27
vent.
A helix inversion of SPAs with chiral or achiral
side chains in solution has been proposed based on CD and
3
5,55−59
UV−vis spectral data,
but further evidence to support
this dynamic process is needed. Nevertheless, SPAs have
proven valuable as advanced polymer materials, e.g., as a chiral
stationary phase for optical resolution columns of racemic small
1
8−27,38−42
molecules.
Detailed structural information on HSPAs
is needed to rationalize the intrinsic functional properties. It is
still not known whether the copolymers possess alternating,
random, and/or block sequences, although knowledge of their
microtacticity has been considered crucial for various types of
60−64
vinyl or styrene polymers.
Using various pieces of
spectroscopic evidence, we reveal strictly alternating monomer
sequences in poly((rac)-2-methyl-1-butyl propiolate) (R =
CH CH(CH )CH CH ) composed of R1 and S1 monomer
2
3
2
3
units in a ratio of 1:1. This structural information can be used
to propose a prevailing polymerization mechanism.
EXPERIMENTAL SECTION
■
(
Measurements. Number- and weight-averaged molecular weights
M and M ) of Prac1, PR1, and PS1 were measured using a GPC
n
w
900-1 (JASCO, Tokyo, Japan) equipped with two Shodex K-806L
columns and an RI detector. Chloroform was employed as an eluent at
Synthesis of Monomers. DL-2-Methyl-1-butyl propiolate (rac1),
R)-2-methyl-1-butyl propiolate (R1), and (S)-2-methyl-1-butyl
propiolate (S1) were prepared from propiolic acid (Tokyo Chemical
4
0 °C, and poly(styrene) standards (M = 800−1 090 000) were used
n
(
1
for calibration. H NMR (500 MHz) spectra were measured on a
JNM-ECA500 (JEOL, Tokyo, Japan) using chloroform-d (CDCl ) at
1
3
Industry) and the corresponding alcohol (1a, 1b, 1c) under acid
13
3
0 °C and p-xylene-d10 at 30, 80, 100, and 130 °C. C NMR (125
49−51
catalysis according to the literature (Scheme 1a).
As a typical
MHz) spectra were also measured in CDCl at 0, 30, 40, and 60 °C.
3
procedure, a mixture of 100 mL of toluene, 9.4 g (0.11 mol) of
prepared 1b, 11.9 g (0.17 mol) of propiolic acid (Aldrich), and 3.2 g
17 mmol) of p-toluenesulfonic acid monohydrate (Tokyo Chemical
Industry) was refluxed for 8 h in a Dean−Stark apparatus. The
resulting mixture was washed with saturated aqueous sodium
hydrogen carbonate solution and distilled water. After the organic
layer had been dried over anhydrous sodium sulfate, the solvent was
removed by evaporation. The crude product was purified by
Rotating frame Overhauser effect spectroscopy (ROESY) and nuclear
Overhauser effect spectroscopy (NOESY) spectra were recorded on a
(
JNM-ECA 920 (JEOL, Tokyo, Japan) spectrometer using CDCl at 40
3
°C. All 2D NMR spectra were collected over 1024 complex points in
t2 and into 256 points in t1 with a sweep width of 11.5 kHz at a mixing
time of 100 ms. Optical rotation of monomers and polymers was
measured on a P-1010 (JASCO, Tokyo, Japan) in CHCl at room
3
temperature. Enantiomer excess (ee) was calculated according to the
distillation to give a colorless liquid R1, 7.7 g in 50% yield (Scheme
65
literature. Solution UV−vis and CD spectra of polymers were
1
1
(
b and Figure S1). rac1: 44% yield, bp 74 °C/24 mmHg. H NMR
recorded on V570 and J-720WI (JASCO, Tokyo, Japan) spectropho-
ppm): δ 4.09, 4.00 (d, J = 10.8 Hz, 2H, OCH ), 2.88 (s, 1H, HC
2
tometers, respectively, in CHCl at 30 °C. Energetically optimized
3
C), 1.77 (oct, J = 6.8 Hz, 1H, CH(CH )), 1.46, 1.22 (m, 2H,
3
dynamic conformations of polymers were simulated using a MMFF94
CH CH ), 0.95 (d, J = 6.8 Hz, 3H, CH(CH )), 0.92 (t, J = 7.5 Hz, 3H,
2
3
3
13
force field program (Wavefunction, Inc., Spartan ’14 Windows version
CH CH ). C NMR (ppm): δ 152.6, 74.4, 70.68, 33.9, 25.8, 16.2,
2
3
6
6
1
13
1
.1.0). Spectral simulation of H7 and C NMR spectra was
1
1.1. R1: 53% yield, [α]D = −4.71 (c 1.00, CHCl ). S1: 50% yield,
3
6
performed using a gNMR program.
[α] = +4.62 (c 1.00, CHCl ).
D
3
Synthesis of (R)-2-Methyl-1-butanol, 1b. (R)-2-Methyl-1-
butanol (1b) was prepared using (S)-methyl 3-hydroxy-2-methyl-
propionate (1b-1) (Tokyo Chemical Industry) as a starting material in
Polymerization. Poly(DL-2-methyl-1-butyl propiolate) (Prac1),
poly((R)-2-methyl-1-butyl propiolate) (PR1), and poly((S)-2-methyl-
1-butyl propiolate) (PS1) were obtained by polymerization of the
corresponding monomers, rac1, R1, and S1, using the [Rh(nbd)Cl]2
catalyst as shown in Scheme 1c. In a typical procedure, 1.0 g (7.1
mmol) of the monomer and the calculated amount of the catalyst, 33
68
five steps according to the literature (Scheme 1a). (R)-2-Methyl-1-
butanol, 1b: 66% yield, bp 85 °C/196 mmHg, [α] = +8.00 (c 1.00,
D
1
CHCl ). H NMR (CDCl ): δ 3.52, 3.43 (d, J = 10.5 Hz, 2H, OCH ),
3
3
2
−2
1
2
3
.31 (br, 1H, −OH), 1.55 (oct, J = 6.7, 1H, CH(CH )), 1.45, 1.15 (m,
mg (7.1 × 10 mmol), were dissolved in MeOH (3.5 mL), and the
3
H, CH CH ), 0.92 (d, J = 6.7 Hz, 3H, CH(CH )), 0.91 (t, J = 7.4 Hz,
mixture was stirred in a specially designed U-shaped ampule for 4 h at
2
3
3
13
11−14
H, CH CH ). C NMR (ppm): δ 67.8, 37.5, 25.9, 16.2, 11.4.
40 °C.
The resulting reaction mixture was poured into excess
2
3
B
DOI: 10.1021/acs.macromol.6b02508
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Article
methanol with stirring. The resulting pale yellow polymer was washed
with methanol and dried under vacuum, ca. 10−2 Torr, for 12 h at
room temperature. Copolymers composed of monomers of R1 and S1
were prepared using the calculated monomer ratio (mol %) of R1 to
S1 according to the above method. The polymerization results are
summarized in Table 2.
1
Prac1: H NMR (CDCl ): δ 6.81 (br, 1H, HCC), 3.92, 3.75,
3
3
2
.60, 3.43 (br, 2H, OCH ), 1.68 (br, 1H, CH(CH )), 1.40, 1.16 (br
H, CH CH ), 0.89 (br, 3H, CH(CH )), 0.88 (br, 3H, CH CH ).
2
3
13
C
2
3
3
2
3
NMR (ppm): δ 163.9, 135.3, 128.0, 69.5, 33.8, 26.2, 16.56, 16.43,
1
6
1.20, 11.10. Anal. Calcd for C H O : C, 68.54; H, 8.63. Found: C,
8 12 2
8.74; H, 8.72.
1
PR1: H NMR (CDCl ): δ 6.81 (br, 1H, HCC), 3.75, 3.60 (br,
3
2
0
1
H, OCH ), 1.67 (br, 1H, CH(CH )), 1.38, 1.15 (br 2H, CH CH ),
.88 (br, 3H, CH(CH )), 0.87 (br, 3H, CH CH ). C NMR (ppm): δ
2
3
2
3
13
3
2
3
63.9, 135.3, 128.0, 69.5, 33.8, 26.2, 16.5, 11.1. Anal. Calcd for
C H O : C, 68.54; H, 8.63. Found: C, 68.72; H, 8.71.
8
12
2
1
PS1: H NMR (CDCl ): δ 6.81 (br, 1H, HCC), 3.75, 3.60 (br,
3
2
0
1
H, OCH ), 1.67 (br, 1H, CH(CH )), 1.38, 1.15 (br 2H, CH CH ),
2
3
2
3
13
.88 (br, 3H, CH(CH )), 0.87 (br, 3H, CH CH ). C NMR (ppm): δ
3
2
3
63.9, 135.3, 128.0, 69.5, 33.8, 26.2, 16.5, 11.1. Anal. Calcd for
C H O : C, 68.54; H, 8.63. Found: C, 68.73; H, 8.75.
8
12
2
RESULTS AND DISCUSSION
Polymerization. The polymerizations of three monomers,
i.e., (rac)-, (R)-, and (S)-2-methyl-1-butyl propiolates, were
■
1
performed using the catalyst [Rh(nbd)Cl] at 40 °C for 4 h in
2
Figure 1. H NMR spectra of (a) Prac1 and (b) PR1 prepared with
MeOH to give the corresponding polymers, Prac1, PR1, and
PS1 as shown in Scheme 1c and Table 1. The polymers were
[Rh(nbd)Cl] in MeOH at 40 °C.
2
°
C to deduce the helical structures that result from the highly
Table 1. Synthesis of Prac1, PR1, and PS1 Using
stereoregular copolymerization induced by the Rh complex
catalyst (Figure 1 and Figure S2). The important feature is that
a
[
Rh(nbd)Cl] in MeOH for 4 h at 40 °C
2
ε
ε
b
c
Mn (×10−4
c
d
e
the ester methylene proton, i.e., O−CH H ′, of the side chain
polymer
yield (%)
)
M /M
cis (%)
[α]D
w
n
was observed not as a singlet but rather as a four-peak signal at
Prac1
PR1
PS1
49
43
43
3.2
2.1
2.5
5.5
4.9
5.5
91
96
95
1
3
.92, 3.75, 3.60, and 3.43 ppm (Figure 1a). In contrast, the H
+813
NMR spectra of PR1 and PS1 simply showed a two-peak signal
at 3.75 and 3.60 ppm (Figure 1b and Figure S2), unlike the case
of Prac1. The vinyl protons, H −C=, and all alkyl protons in
the side chain were completely identical (Figure 1a and Figure
S2).
−847
a
b
[
M]0 = 2.0 mol/L, [M] /[cat.] = 100. Insoluble fraction in
α
0
c
d
1
methanol. Estimated by GPC (PSt, CHCl ). Determined by H
NMR (CDCl ). c = 0.20 g/dL (CHCl ).
3
e
3
3
a
produced in a moderate yield of 43−49%, a number-averaged
Table 2. Copolymerization Using R1 and S1 Monomers
4
molecular weight (M ) of (2.1−3.2) × 10 , and a molecular
n
b
c
d
R1:S1
(mol %)
yield
Mn
cis
weight dispersity (M /M ) of 4.9−5.5. The relative amounts of
(×10−4)
c
e
w
n
run
(%)
M /Mn
w
(%)
[α]D
cis isomers (cis %) were determined to be 91−96% by
1
2
3
4
5
6
7
8
9
10:90
20:80
30:70
40:60
50:50
60:40
70:30
80:20
90:10
43
42
41
43
41
44
41
44
43
2.1
2.4
2.5
2.7
2.6
2.4
2.5
2.7
2.4
8.5
5.1
9.5
7.1
6.6
9.6
5.5
5.1
91
92
92
94
93
93
94
95
91
−847
−779
−763
−636
0
α
comparing the relative intensities, i.e., H −CC/CH of the
3
main-chain vinyl proton at 6.80 ppm and the methyl proton at
0
.87 ppm in the side chain of Prac1, PR1, and PS1 (Figure 1
1
1−14
and Figure S2).
The optical rotations [α] of PR1 and
D
PS1 were determined to be +813° and −847°, respectively,
using chloroform at room temperature (Table 1). Copolymers,
Pcos were also prepared by changing the monomer ratio (mol
+655
+781
+793
+806
%
) of R1 to S1 using the [Rh(nbd)Cl] catalyst at 40 °C for 4 h
2
8.1
in MeOH, as shown in Scheme 1d and Table 2. The
a
b
[
M]0 = 2.0 mol/L, [M] /[cat.] = 100. Insoluble fraction in
methanol. Estimated by GPC (PSt, CHCl ). Determined by H
0
copolymers were also produced in a moderate yield of 41−
c
d
1
3
4
1
4%, a number-averaged molecular weight (M ) of (2.1−2.7) ×
e
n
NMR (CDCl ). c = 0.20 g/dL (CHCl ).
4
3
3
0 , and a molecular weight dispersity (M /M ) of 5.1−9.6,.
w
n
The amount of cis configuration was determined to be 91−95%
using the above-mentioned method. The [α] of copolymers is
Dependence of the Four-Peak Signal on Monomer-
Feed Ratio As Detected by H NMR. The copolymers Pcos
D
1
presented in Table 2. The resulting Prac1, PR1, PS1, and Pco
were soluble in CHCl and toluene but insoluble in CH Cl ,
were prepared by mixing the chiral monomers R1 and S1 in the
3
2
2
C H Cl , Et O, n-hexane, dimethyl sulfoxide, N,N-dimethylfor-
presence of the [Rh(nbd)Cl] −MeOH catalyst at 40 °C. The
2
2
4
2
2
mamide, MeOH, and EtOH.
question is why the above four-peak signal is observed in the
1
1
H NMR Spectra of Prac1, PR1, and PS1. The H NMR
case of Prac1. Figure 2a shows the monomer-feed dependence
1
spectra of Prac1, PR1, and PS1 were measured in CDCl at 30
of H NMR spectra line shape for Pco (Scheme 1d). The
3
C
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intensities of the outer peaks indicate that a and d, at 3.92 and
3
.43 ppm, respectively, decreased with increasing monomer
ratio of either R1 or S1 within the copolymer. Inversely, the
intensities of the inner two b and c peaks at 3.75 and 3.60 ppm
gradually increased. However, the chemical shifts and the line
α
shapes of the other alkyl side chain, and the vinyl protons, H −
C= remained unchanged. The shapes of the four peaks of Pco
agreed with that of Prac1 when the monomer ratio of R1 and
S1 was 50:50. A two-peak signal at 3.75 and 3.60 ppm observed
in PR1 or PS1 was also observed for the blend polymer, Pble
(
Figure 2b), obtained when mixing PR1 with PS1 at a molar
ratio of 50:50 in CDCl , although with this method the four-
3
peak signal never appeared. This suggests that the main chains
of copolymers Prac1 and Pco are composed of a chiral
monomer sequence different from that of chiral homopolymers
as follows: neither a random nor a block monomer sequence,
but rather a strictly alternating monomer sequence has been
formed. Furthermore, the intensity changes in the four-line
signal are attributed to the ratio of sequences in Pcos, i.e.,
sequences composed of only one monomer (identical to PR1
or PS1) or an alternating monomer sequence (identical to
Prac1).
Temperature Dependence of the Line Shapes of
ε
OCH Protons. The intensity of the two peaks observed at
2
3
.75 and 3.60 ppm in PR1 or PS1 was not of equal intensity;
instead, the intensity ratio reversibly changed to a fairly sharp
and asymmetrical doublet with an intensity ratio of
approximately 2:1 when their xylene-d10 solutions were heated
to 130 °C (Figure 3a). The two-peak signal showed a large
separation of approximately 111−66 Hz, too large for a geminal
ε
coupling between ester methylene protons, CH (Figure
2
6
9−71
3
a).
Therefore, this peak intensity must be interpreted by
ε
assuming restricted rotation around the ester bond, O−CH ,
through a three-site jump rotation generating three dynamic
1
2
Figure 2. (a) Monomer-feed dependence of H NMR spectra of PR1,
1
PS1, and Pco and (b) H NMR spectra of the blend polymer (Pble)
helical conformations called rotamers A, B, and C (Figures 4
observed at 30 °C in CDCl . Pble was obtained by mixing PR1 with
3
49−51
and Figure S3).
Thereby, a so-called M-helix is assumed;
PS1 at a molar ratio of 50:50 in CDCl3.
in the case of a P-helix, the three rotamers are depicted in
Figure S3. The ratio of a relatively contracted cis−cisoid helix
1
ε
2
Figure 3. Temperature-dependent H NMR spectra due to OCH protons observed at 30, 80, 100, and 130 °C and their simulated spectra
expanded in the range 4.5−3.5 ppm.
D
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Table 3. Observed and Simulated Kinetic Dynamical Data
1
for Restricted Rotation in the H NMR Spectra of Prac1,
a
PR1, and PS1
splitting (Hz)
jump rate (Hz)
‡
polymer
30 °C
130 °C
30 °C
130 °C
ΔG (kcal/mol)
b
Prac1
PR1
PS1
161, 163
111
−
10
20
20
150
280
280
19.1
19.4
19.4
70
113
66
a
Calculated by gNMR; peaks corresponding rotamer A, B, and C were
b
calculated as equal populations. Coalesced temperature.
determine which peak combination for the four-peak signal
belonged to the chemically exchanging protons (Figures 1a, 2a,
and 3b). The relevant proton pairs were successfully detected
as cross-peaks with a positive sign (green), when the diagonal
peak was depicted as a positive sign (Figure 5). Thus, the four-
Figure 4. (a) Newman projection of rotamers A, B, and C. (b) An
accordion-like helical oscillation (HELIOS) of PR1 with M-helix
composed of rotamers A, B, and C.
sequence due to rotamer A (red color) and slightly stretched
cis−cisoid sequences comprising rotamers B and C (blue color)
is approximately 1:2. This is similar to the case of poly(n-heptyl
50
propiolate), PnHepP. These rotamers undergo the so-called
accordion-like helix oscillation (HELIOS) along the main chain
1
1
axis within the time scale of the H NMR experiment, i.e., 10 −
6
72
1
0 Hz. The ester four-peak signal (intensity ratio, 2:2:1:1) of
Prac1 and Pco also showed a reversible, temperature-depend-
ent change in line shape and intensity, where the four-peak
signal ultimately becomes an extremely broad singlet (Figure
Figure 5. The 920 MHz ROESY NMR spectrum of Prac1 expanded in
the range of 4.2−3.2 ppm (40 °C in CDCl3).
3
b). Therefore, this change of Prac1 must also be caused by
restricted rotation around the racemic ester bond, as in the case
of the chiral homopolymers. This difference in temperature also
indicates that these polymers are thermally stable, as compared
to Ps2OcP and PnHepP.
must be attributed to magnetically different environments, i.e.,
the existence of two types of O−CH moieties due to
restricted rotation occurring in Prac1. The Gibbs free energy
peak signal was identified as a combination of the (a and c)
peaks, and (b and d) peaks, respectively. The ROESY spectrum
of Prac1 also corroborated the assignment, since the
exchangeable proton and the NOE proton have opposite
signs with respect to the diagonal sign (Figures 5 and 6). The
(a, d) protons and (b, c) protons were spatially close, within
approximately 5 Å (Figure S4). Accordingly, in the helical
main-chain of the cis−cisoid structure, the chirality of n + 6 and
n + 3 monomer units was the same as and opposite to that of
the n unit, respectively (Figure 6 and Figure S5). Therefore, the
polymer with a one-handed helical sense main chain comprises
an alternating sequence of R1 and S1 monomers (Figure 7).
The four-peak signal, thus, points toward two diastereotopic
relationships between the two helical senses (P and M) and the
two chirality within the alkyl group ((R) and (S)). Moreover
each peak is split into two peaks by the chemical exchange.
49−51
The observed four-peak signal
ε
2
‡
ε
(
ΔG ) of activation for rotation about the O−CH bond was
2
calculated from the observed temperature and the value of the
6
9−71,73,74
1
peak splitting (Table. 3).
simulations of the two- and four-line signals were successfully
accomplished using the gNMR program to prove the
chemical exchange assuming a three-site jump model without
helix inversion (Figure 3). Furthermore, the kinetic data
H NMR spectral
67
ε
indicated that (i) the rotation of the O−CH bond is
2
restricted, (ii) the side chain of Prac1 rotates much faster than
that of the homochiral polymer, PR1 or PS1, and (iii) the
flexibility of Prac1 is higher than that of PR1 or PS1.
1
3
13
C NMR Spectra of Prac1, PR1, and PS1. The C NMR
spectrum of Prac1 was measured (Figure 8a) to elucidate the
Determination of Proton Pairs Exchanging Their
Magnetics Sites by 2D ROESY NMR Spectrum. The 2D
ROESY spectrum of Prac1 was measured to explicitly
monomer sequence of the resulting polymer chain. The two
η
methyl signals due to the branched C ′H and the terminal
3
CH were split into asymmetric doublets whose peaks (16.8−
3
E
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increased from 0 to 60 °C. These spectral changes again point
ε
ζ
to restricted rotation around the C −*C bond in the alkyl side
chain, although such restricted rotation was not observed for
PR1 or PS1. It should be noted that in the case of poly(s-2-
octyl propiolate), Ps2OctP, such restricted rotation around the
49
ester bond was clearly detected. The restricted rotation
models are depicted using a Newman projection as three
conformers, C , C , and C(iii), of the R1 or S1 moiety (Figure
(
i)
(ii)
8
c and Figure S7). Among the three conformers, C(ii) has fairly
large steric hindrance compared with C(i) or C(iii). Therefore,
the lifetime of the C(ii) conformer among the three conformers
1
3
is shorter than that of C(i) and C(iii) in the time scale of the C
NMR experiment. The relative weight among the three
η
conformers due to branched C ′H is approximately
3
C :C :C = 1:0:1 at 60 °C (Figure 8b). On the other
hand, the intensity ratio of the peak at 11.28 and 11.14 ppm
(
i) (ii) (iii)
due to the terminal CH becomes approximately 2:1, indicating
3
that the ratio of such conformers is C(i) + C(iii):C(ii) = 2:1. This
means that the intramolecular steric hindrance between the
alkyl side chain in the racemic and chiral polymers is
significantly different. This is understandable, as Prac1 is
composed of different chiral units, i.e., R1 and S1, while the
chiral polymers have a completely stereoregular microsequence
(
Figure 6 and Figure S5).
Monomer Feed Ratio Dependence of the Two Methyl
1
3
13
Peaks Detected by C NMR. The C NMR spectral line
shape of Pco gradually approached that of Prac1 when the ratio
of R1 to S1 was increased or decreased in the copolymer
(Figure 9a). In contrast, the 13C NMR spectrum of the blend
polymer, Pble, completely agreed with that of the homochiral
polymers PR1 or PS1 (Figure 9a). The peak intensities at lower
Figure 6. Side view of Prac1 with M-helix showing spatial relations of
ε
−
OCH − protons between the nearest-neighbor side chains, i.e., (a
2
and c) proton pair and (b and d) proton pair between the nth unit and
n ± 3)th unit, together with the (n + 6)th unit, in the helical main
(
a
c
b
d
chain. The protons (H and H ) and (H and H ) are located on the
R1 unit and S1 unit sides, respectively.
η
magnetic field due to C ′H and CH increased with increasing
3
3
monomer ratio of either R1 or S1 within the copolymer.
Furthermore, the chemical shift of the increased peaks agreed
with those of the chiral and blend polymers. This spectral
change suggest that Pco is composed of only two sequences,
i.e., homochiral and alternating sequences, and never includes a
random sequence.
1
6.2 and 16.56−16.43, 11.4−11.0 and 11.20−11.10 ppm) are
depicted in Figure 8b. However, in the case of PR1 or PS1, the
η
peaks due to C ′H at 16.6 ppm, the terminal CH at 11.2 ppm
3
3
(Figure S6), and the remaining signals of Prac1, PR1, and PS1
were not split (Figure 8 and Figure S6).
Temperature Dependence of the Line Shapes of Two
Methyl Carbons. The two split peaks due to the two methyl
carbons of Prac1 also showed a typical temperature-dependent
Calculation of Main Chain Dihedral Angles, Pitch
Width, and Strain Energies. Dihedral angles between the
two cis double bonds CC−CC in the helical main chain,
75
shape (Figure 8b); the splitting width of each doublet
the helical pitch width, and the strain energies of those helices
η
decreased from 15.6 to 9.9 Hz in the C ′H and from 18.0 to
were calculated using the MMFF94 force field program for a
20-mer as a model of PR1, PS1, and Prac1 (Figure 10 and
3
7
.4 Hz in the terminal CH when the temperature was
3
Figure 7. Perspective views of enantiomers and diastereomers with P- and M-helices in Prac1.
F
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Figure 9. (a) Monomer feed dependence of 13C NMR spectra due to
η
13
branched C ′H and terminal CH moieties in Pco and (b) C NMR
3
3
spectrum of Pble observed at 40 °C in CDCl . Pble was obtained by
mixing PR1 with PS1 at a molar ratio of 50:50 in CDCl3.
3
Figure 8. (a) 13C NMR spectra of Prac1 observed at 30 °C. (b)
1
3
η
Temperature-dependent C NMR spectra of branch C ′H and
3
terminal CH in Prac1. (c) The three-rotamer model depicted using a
3
Newman projection for the side chain of PR1.
6
6
Table 4). The relevant helical sense polymers are depicted
together with the relation between the diastereomer and the
enantiomer in Figures 6 and 7 as well as Figure S5. The strain
energy of PR1 or PS1 is smaller than that of Prac1, indicating
that the conformation of Prac1 is energetically unfavorable
compared with that of PR1 or PS1. These calculations also
corroborate that each helical sense of PR1 or PS1 corresponds
to that of the M-helix or P-helix, respectively, because the strain
energies of each helical sense are larger than those of opposite
senses, ΔE = ca. 1.4 kJ/mol at the dihedral angles, 70° and
2
90°. Furthermore, this indicates that the helical oscillated
molecular chains of (PR1 or PS1) and Prac1 have small pitch
widths (3.6 and 4.0 Å). The top and side views of M- and/or P-
helix models in PR1, PS1, and Prac1 are presented in Figure
S8.
Figure 10. Calculated dihedral angles of C−C and CC bonds,
helical pitches, and strain energies of PR1, PS1, and Prac1 using
MMFF94.
UV and CD Spectra of Contracted and Stretched
Helices. The UV−vis spectra for PR1, PS1, and Pco exhibit
G
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Table 4. Helical Senses, Dihedral Angles of C−C and CC
Bonds, Helical Pitches, and Strain Energies of PR1, PS1, and
Prac1 Using MMFF94
monomer ratio. Furthermore, when CD intensity at 317 nm
was plotted as a function of the feed ratio of the chiral
monomer (Figure 12b), there was no linear relation between
a
b
helical
sense
dihedral angle
(deg)
pitch
(Å)
strain energy
polymer
(kJ/(mol unit))
PR1
M-helix
P-helix
M-helix
P-helix
M-helix
P-helix
290
70
3.6
3.6
3.6
3.6
4.0
4.0
59.4
60.8
60.7
59.4
65.5
PS1
290
70
Prac1
280
80
65.4
a
b
Calculated by molecular mechanics method MMFF94. Optimized
structure of P-helix or M-helix.
two absorption maxima λmax at 275 and 317 nm (Figure 11b)
that are unaffected when the chiral monomer ratio R1:S1 is
Figure 12. (a) Relationship between optical rotation and the feed ratio
of R1. (b) CD spectral intensity dependence of R1 monomer in the
copolymer Pco as observed at 317 nm in CHCl at 30 °C.
3
optical rotation and the monomer feed ratio (Figures 11 and
1
2a). This suggests that the helix sense is biased by either the
M- or P-helix, corresponding to the R1 or S1 monomers,
respectively.
Polymerization Mechanism of Chiral R1 or S1 and
Racemic rac1 Monomers. The monomeric rhodium complex
as an important catalytic species is generated when methanol,
with its electron lone pair, is used as the solvent (Scheme
11−14
1
).
On the basis of the results shown above, we propose a
polymerization mechanism to explain the observed alternating
copolymer sequences. A unique copolymerization with
alternating monomer sequence occurs when introducing five
R1 or S1 monomers (Scheme 2). In the initiation step, the
chiral monomer R1 or S1 coordinates to the monomeric Rh
complex with the methanol molecule to generate the chiral Rh
propagation chemical species. The resulting chiral Rh complex
species selects R1 or S1 monomer to newly generate an
important chiral Rh complex (CHIRHEX) catalyst. The
resulting CHIRHEX catalyst coordinated with R1 selects the
S1 monomer, and inversely, the chiral Rh complex coordinated
with S1 monomer selects the R1 monomer. Thus, the chiral Rh
complex consecutively induces copolymerization with alternat-
ing incorporation of monomers to create the polymer Prac1. In
the case of Pco, the highly alternating copolymer under
incorporation of the R1 or S1 monomer is produced until the
less abundant monomer is completely consumed, at which
point, highly stereoregular homopolymerization proceeds by
incorporating the remaining monomers of R1 or S1 (Scheme
2b). Then the helical sense of Pco is determined by the
remaining excess chiral monomer sequence, although R1 and
S1 monomers coordinate to the achiral monomeric Rh complex
at the initial step of the copolymerization (Scheme 2 and Figure
13). Furthermore, the proposed mechanism rationalizes why
the optical rotation depends on the monomer ratio (Figure
12a,b) and the helical sense excess also depend on the feed
monomer ratio, e.g., R1 monomer (Figure 13). Therefore, the
Figure 11. (a) Dependence of CD spectra of PR1, PS1, and Pco on
chiral monomer feed (CHCl at 30 °C). (b) Dependence of UV−vis
3
spectra of PR1, PS1, and Pco on monomer feed (CHCl at 30 °C).
3
changed from 100:0 to 0:100. This indicates that PR1, PS1, and
Pco are typical helical polymers having an accordion-like helix
oscillation (HELIOS), as in the case for shrunk (= contracted)
50
helices, such as PnHepP. For comparison with the chiral
polymers, Pco and PnHepP, the two absorptions at 275 nm
and 317 ppm can be attributed to the contracted cis−cisoid helix
(
rotamer A) and stretched cis−cisoid helices (= rotamers B and/
or C) of Pco and (PR1 or PS1), respectively (Figure 4 and
Figure S3).
The CD spectra of Pcos and the chiral polymers PR1 and
PS1 were also measured to determine the quantity of the
predominant helical sense polymer by changing the feed ratio
of the chiral monomer (Figure 11a). A Cotton effect at 325 nm
in the CD spectra was observed, although the sign due to the
helix of the main chain changed according to the chiral
H
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Scheme 2. (a) Proposed Polymerization Mechanisms of rac1
To Generate the Alternative Copolymer; (b) Proposed
Formation Mechanism of Pco Using a Monomer Ratio of
R1:S1 = 8:2
four-peak signal, and PR1 or PS1 exhibit a two-peak signal in
1
their H NMR spectra. Thus, these results are reasonably
55−59
explained even if the so-called dynamic helix inversion
not accepted at present.
is
CONCLUSIONS
■
Monomers of DL-2-methyl-1-butyl propiolate (rac1), (R)-2-
methyl-1-butyl propiolate (R1), and (S)-2-methyl-1-butyl
propiolate (S1) were stereoregularly polymerized using the
catalyst [Rh(nbd)Cl] at 40 °C in methanol to yield the
2
corresponding racemic Prac1 and chiral (PR1 and PS1)
1
polymers. Temperature-dependent H NMR spectra indicated
ε
restricted rotation around the ester O−CH bonds of the alkyl
2
side chains. However, this phenomenon does not support the
55−59
helix inversion model.
The asymmetric doublet peaks of
ε
the H proton (observed at 30−130 °C), with relative
intensities of approximately 2:1, suggested three rotamers A,
B, and C. The intensity ratio is in accord with a contracted cis−
cisoid helix (A) and two relatively stretched cis−cisoid helices (B
and C). The interesting four-peak signal observed in Prac1 was
interpreted in terms of a perfectly alternating copolymerization,
i.e., RSRSRS, which was further corroborated by the 2D ROESY
1
NMR spectrum. The H NMR spectra and their simulation
suggest that the alternating copolymer possesses a unique
accordion-like helix oscillation (HELIOS) mode. Furthermore,
the stable helical senses resulting from PR1 and PS1 were
calculated by the MMFF94 program in their energetically
favorable conformations. The two UV-absorption maxima
observed at 275 and 317 nm also point to the formation of
three helices. Based on these results, a polymerization
mechanism was proposed to explain the formation of strictly
stereoregular and alternating monomer sequences. The smallest
helical pitch width, 3.6 Å, calculated in the two chiral polymers,
is close to that of graphite, a typical conductive and layered
76−78
material.
The present study is the starting point for further
research into, for example, the fabrication of the smallest single
helical polymer device, e.g., a sensitive magnetic nanosensor
generating magnetic flux from the molecular axis to create an
eternally oscillating nano molecular engine. These results are
forthcoming.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b02508.
Figures S1−S8 (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*
*
E-mail tabata@mmm.muroran-it.ac.jp (M.T.).
E-mail mawatari@mmm.muroran-it.ac.jp (Y.M.).
Figure 13. Helix sense excess (%) estimated using the feed ratio of R1
monomer based on the Scheme 2.
ORCID
̈
Klaus Mullen: 0000-0001-6630-8786
Masayoshi Tabata: 0000-0001-6650-9991
Notes
copolymer possess both alternating and chiral homopolymer
sequences. This feature can also explain why Prac1 exhibits a
The authors declare no competing financial interest.
I
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(
phenylacetylene) Prepared with a [Rh(nbd)Cl] -Amine Catalyst. J.
ACKNOWLEDGMENTS
2
■
Polym. Sci., Part A: Polym. Chem. 2014, 52, 752−759.
Thanks are extended to the Instrument Center at the Institute
for Molecular Science for collecting the 920 MHz 2D NMR
NOESY and ROESY) and CD spectra measurements.
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DOI: 10.1021/acs.macromol.6b02508
Macromolecules XXXX, XXX, XXX−XXX