Control of helix sense by composition of chiral-achiral copolymers of N-propargylbenzamides

Article abstract of DOI:10.1002/chem.200401009

N-Propargylbenzamides 1-7 were polymerized with (nbd)Rh⁺[η ⁶-C₆H₅B^-(C₆H ₅)₃] to afford polymers with moderate molecular weights (M_n = 26000-51000) in good y

Full text of DOI:10.1002/chem.200401009

FULL PAPER
Control of Helix Sense by Composition of Chiral–Achiral Copolymers of N-
Propargylbenzamides
Junichi Tabei,[a] Masashi Shiotsuki,[a] Takahiro Sato,[b] Fumio Sanda,*[a] and
Toshio Masuda*[a]
Abstract: N-Propargylbenzamides 1–7
were polymerized with (nbd)Rh+[h6-
C6H5BÀ(C6H5)3] to afford polymers
with moderate molecular weights
(Mn = 26000–51000) in good yields.
The 1H NMR spectra demonstrated
that the polymers have fairly stereore-
gular structures (81–88% cis). The op-
tically active polymers, poly(1) and
poly(2), were proven by their intense
CD signals and large optical rotations
to adopt a stable helical conformation
with an excess of one-handed screw
sense when heated in CHCl3 or tolu-
ene. The sign of Cotton effect could be
controlled by varying the content in
the copolymers of either chiral bulky 1
and achiral nonbulky 3, or chiral non-
bulky 2 and achiral bulky 7. The small-
er the pendant group in the copolymer-
ization of achiral monomers with 1, the
more easily did the preferential helical
sense change with the copolymer com-
position. However, the copolymers of
chiral nonbulky 2 and achiral nonbulky
3 did not change the helical sense, irre-
spective of the composition. The free
energy differences between the plus
and minus helical states, as well as the
excess free energy of the helix reversal,
of those chiral–achiral random copoly-
mers were estimated by applying a
modified Ising model.
Keywords: Cotton effect · helical
structures · modified Ising model ·
polyacetylenes · polymers
Introduction
Green et al. have confirmed that a small proportion of
chiral isocyanate determines the helical sense of the copoly-
mers of chiral and achiral isocyanates (the “sergeants and
soldiers rule“).[5,6] Some polysilylenes[7] and polyacetylenes
also obey this rule,[8] where optical activities show positive
nonlinear relationships with the copolymer composition.
However, there are some random copolymers that change
their helical sense according to the chiral monomer content
(the soldiers may not obey their sergeants); A and B are ex-
amples.[9] The common characteristic of these copolymers is
the location of chiral centers on the pendant groups: that is,
The exclusively one-handed screw sense of biomacromole-
cules such as proteins and DNA strongly influences their
biological activities.[1] The study of helical polymers is im-
portant for an understanding of the self-organization of bio-
macromolecules such as a-helices of polypeptides and
double helices of nucleic acids, and also for production of
highly advanced materials with biomimetic functions. Syn-
thetic optically active polymers have received much atten-
tion[2] because their chiralities originating from their helical
conformations can be applied to functional materials exhib-
iting molecular recognition ability[3] and catalytic activity for
asymmetric synthesis.[4]
[a] J. Tabei, Dr. M. Shiotsuki, Prof. Dr. F. Sanda, Prof. Dr. T. Masuda
Department of Polymer Chemistry, Graduate School of Engineering
Kyoto University, Katsura Campus, Kyoto 615–8510 (Japan)
Fax : (+81)75-383-2590
E-mail: sanda@adv.polym.kyoto-u.ac.jp
masuda@adv.polym.kyoto-u.ac.jp
[b] Prof. Dr. T. Sato
Department of Macromolecular Science, Osaka University
1–1 Machikaneyama-cho, Toyonaka, Osaka 560-0043 (Japan)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2005, 11, 3591 – 3598
DOI: 10.1002/chem.200401009
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3591

of the resulting (1S)-(À)-(2-bornyloxycarbonyl)benzoic acid (7.97 g,
33.8 mmol) and propargylamine (3.41 mL, 33.8 mmol) at room tempera-
ture. The resulting solution was stirred at room temperature for 24 h.
After the white precipitate had been filtered off, the filtrate was concen-
trated. Ethyl acetate (ca. 100 mL) was added to the residue, and the so-
lution was washed with aqueous HCl (2m)and saturated aqueous
NaHCO3, dried over MgSO4, and concentrated. Monomer 1 was isolated
(1.62 g, 5.92 mmol, 12%) by flash column chromatography on silica gel
(hexane/ethyl acetate, 4:1 v/v). Monomers 2–7 were prepared in a similar
way.
a benzene ring is sandwiched between the main chain and
an asymmetric carbon atom.
We have previously reported that N-propargylamides
polymerize in the presence of an Rh catalyst to afford poly-
mers with a cis stereostructure that adopt a helical confor-
mation stabilized by intramolecular hydrogen bonds be-
tween the amide groups in the side chains.[10] The helicity of
poly(N-propargylamide)s is an equilibrium conformation.
The secondary structure changes reversibly upon addition of
methanol, or with a rising temperature. In the present study,
we report the copolymerization of chiral and achiral N-
propargylbenzamides 1–7 that have chiral centers distant
from the acetylene moiety (Scheme 1), and demonstrate
that poly(1-co-3) and poly(2-co-7) show composition-driven
helical sense inversion.
Monomer 1: m.p. 45–468C; [a]D = +33.28 (c = 0.522 gdLÀ in CHCl3);
À
À
IR (KBr): n˜ = 3310 (=C H), 2959 (C H), 2124 (C=C), 1713 (C=O),
1651 (C=O), 1540 (dNÀH) cmÀ1 1H NMR (CDCl3): d
0.88–2.19 (m,
15H), 2.29 (d, 1H, J 2.44 Hz), 2.46 (m, 1H), 4.28 (d, 2H, J
;
=
=
=
2.44 Hz), 5.13 (m, 1H), 6.78 (s, 1H), 7.53 (t, 1H, J = 7.20 Hz), 8.04 (d,
1H, J = 7.20 Hz), 8.18 (d, 1H, J = 7.20 Hz), 8.43 ppm (s, 1H); 13C NMR
(CDCl3): d = 13.65, 18.92, 19.73, 27.41 28.08, 29.91, 36.83, 44.92, 47.94,
49.13, 72.07, 76.68, 79.18, 127.64, 128.84, 131.25, 131.57, 132.53, 133.98,
165.97, 166.11 ppm; elemental analysis
calcd (%) for C21H25NO3: C 74.31, H
7.42, N 4.13; found: C 74.10, H 7.40, N
4.08.
Monomer 2: viscous oil; [a]D
=
+18.28 (c = 0.113 gdLÀ1 in CHCl3);
À
IR (KBr): n˜
=
3294 (=C H), 2935
À
(C H), 2126 (C=C), 1723 (C=O), 1655
(C=O), 1542 (dNÀH) cmÀ1
;
1H NMR
0.85 (t, 3H,
6.80 Hz), 1.28 (m, 6H), 1.60 (m, 2H),
2.24 (d, 1H, J 2.44 Hz), 4.23 (d,
(CDCl3):
d
=
J =
=
2H, J = 2.44 Hz), 5.12 (m, 1H), 6.83
(s, 1H), 7.46 (t, 1H, J = 7.20 Hz), 8.00
(d, 1H, J = 7.20 Hz), 8.11 (d, 1H, J =
7.20 Hz), 8.39 ppm (s, 1H); 13C NMR
(CDCl3):
d = 13.94, 19.99, 22.46,
27.53, 29.78, 35.62, 71.84, 72.32, 79.26,
127.70, 128.66, 131.17, 131.52, 132.50,
133.91, 165.31, 166.19 ppm; elemental
analysis calcd (%) for C17H21NO3: C
71.06, H 7.37, N 4.87; found: C 71.22,
H 7.40, N 4.84.
Scheme 1. Rhodium-catalyzed copolymerization of chiral and achiral N-propargylbenzamides 1–7.
Experimental Section
Monomer 3: m.p. 77–788C; IR
À
À
(KBr):n˜ = 3279 (=C H), 2988 (C H), 2125 (C=C), 1721 (C=O), 1642
Materials: Solvents were distilled by the usual methods before use. Prop-
argylamine (Aldrich), isophthaloyl dichloride (Wako), pyridine (Wako),
ethanol (Wako), n-propanol (Wako), n-butanol (Wako), cyclohexanol
(Wako), 2-adamantanol (Wako), (S)-(À)-2-hexanol (Wako), (1S)-(À)-bor-
neol (Aldrich), and 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpho-
linium chloride (Tokuyama) were used without further purification.
(nbd)Rh+[h6-C6H5BÀ(C6H5)3] was prepared as reported previously.[11]
(C=O), 1541 (dNÀH) cmÀ1
;
1H NMR (CDCl3): d
=
1.41 (t, 3H, J
=
6.80 Hz), 2.30 (d, 1H, J = 2.44 Hz), 4.27 (d, 2H, J = 2.44 Hz), 4.39 (q,
2H, J = 6.80 Hz), 6.49 (s, 1H), 7.53 (t, 1H, J = 7.20 Hz), 8.03 (d, 1H, J
7.20 Hz), 8.17 (d, 1H, J
(CDCl3): d 14.34, 29.91, 61.44, 72.09, 76.68, 79.19, 127.55, 128.87,
130.81, 131.73, 132.65, 133.93, 165.71, 166.04 ppm; elemental analysis
calcd (%) for C13H13NO3: C 67.52, H 5.67, N 6.06l; found: C 66.48, H
5.76, N 6.36.
=
=
7.20 Hz), 8.39 ppm (s, 1H); 13C NMR
=
Measurements: Melting points (m.p.) were measured with a Yanaco
micro-melting point apparatus. Elemental analyses were conducted at the
Kyoto University Elemental Analysis Center. NMR (1H: 400 MHz, 13C:
100 MHz) spectra were recorded on a JEOL EX-400 spectrometer. IR
spectra were obtained with a Shimadzu FTIR-8100 spectrophotometer.
Number-average molecular weights (Mn) and molecular weight distribu-
tions (Mw/Mn) of polymers were estimated by GPC (Shodex KF-850 L
columns: elution with CHCl3, calibration with polystyrene). CD spectra
were recorded on a JASCO J-820 spectropolarimeter.
À
À
Monomer 4: m.p. 48–498C; IR (KBr): n˜ = 3325 (=C H), 2941 (C H),
2122 (C=C), 1719 (C=O), 1651 (C=O), 1546 (dNÀH) cmÀ1 1H NMR
;
(CDCl3): d = 0.99 (t, 3H, J = 7.20 Hz), 1.76 (m, 2H), 2.26 (d, 1H, J =
2.44 Hz), 4.24 (m, 4H), 6.72 (s, 1H), 7.48 (t, 1H, J = 7.20 Hz), 8.01 (d,
1H, J = 7.20 Hz), 8.13 (d, 1H, J = 7.20 Hz), 8.38 ppm (s, 1H); 13C NMR
(CDCl3): d
= 10.45, 22.02, 29.82, 66.94, 71.92, 79.24, 127.65, 128.77,
130.77, 131.69, 132.53, 133.96, 165.77, 166.12 ppm; elemental analysis
calcd (%) for C14H15NO3: C 68.56, H 6.16, N 5.71; found: C 68.36, H
6.16, N 5.64.
Monomer synthesis: Synthesis of 1 is described as a typical procedure. A
mixture of (1S)-(À)-borneol (7.60 g, 49.3 mmol) and pyridine (8.70 mL,
98.7 mmol) was added slowly to a THF solution (150 mL) of phthaloyl
chloride (10.0 g, 49.3 mmol) at 08C. After the reaction mixture had been
refluxed for 6 h, water (20 mL) was added, and the resulting mixture was
further refluxed for 6 h, washed with aqueous HCl (2m) and then water,
and concentrated to give (1S)-(À)-(2-bornyloxycarbonyl)benzoic acid in
68% yield. 4-(4,6-Dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium
chloride[12] (4.60 mL, 33.8 mmol) was added to a THF solution (100 mL)
À
À
Monomer 5: m.p. 66–678C; IR (KBr): n˜ = 3282 (=C H), 2963 (C H),
2121 (C=C), 1711 (C=O), 1645 (C=O), 1537 (dNÀH) cmÀ1 1H NMR
;
(CDCl3): d = 0.98 (t, 3H, J = 6.80 Hz), 1.08–1.89 (m, 4H), 2.31 (d, 1H,
J = 2.44 Hz), 4.27 (d, 2H, J = 2.44 Hz), 4.35 (t, 2H, J = 6.32 Hz), 6.43
(s, 1H), 7.26 (t, 1H, J = 7.20 Hz), 8.03 (d, 1H, J = 7.20 Hz), 8.17 (d,
1H, J
19.24, 29.88, 30.71, 65.31, 72.05, 79.19, 127.59, 128.85, 130.86, 131.69,
= = 13.76,
7.20 Hz), 8.39 ppm (s, 1H); 13C NMR (CDCl3): d
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Chem. Eur. J. 2005, 11, 3591 – 3598

Helical Structures
FULL PAPER
132.60, 133.96, 165.78, 166.06 ppm; elemental analysis calcd (%) for
C15H17NO3: C 69.48, H 6.61, N 5.40; found: C 69.29, H 6.52, N 5.40.
Table 1. Polymerization of 1–7.[a]
[c]
[c]
Monomer
yield[b][%]
Mn
Mw/Mn
Cis[d] [%]
À
À
Monomer 6: m.p. 81–828C; IR (KBr): n˜ = 3297 (=C H), 2967 (C H),
1
2
3
4
5
6
7
76
91
50
73
86
62
43
51000
26000
28000
31000
27000
45000
26000
2.05
1.92
2.90
1.76
2.25
2.10
3.45
81
85
83
88
87
88
86
2127 (C=C), 1720 (C=O), 1639 (C=O), 1535 (dNÀH) cmÀ1 1H NMR
;
(CDCl3): d = 1.21–2.15 (m, 10H), 2.29 (d, 1H, J = 2.44 Hz), 4.27 (d,
2H, J = 2.44 Hz), 5.03 (m, 1H), 6.69 (s, 1H), 7.52 (t, 1H, J = 7.20 Hz),
8.03 (d, 1H, J = 7.20 Hz), 8.17 (d, 1H, J = 7.20 Hz), 8.41 ppm (s, 1H);
13C NMR (CDCl3): d = 23.71, 25.35, 29.83, 31.59, 71.93, 73.74, 79.24,
127.63, 128.72, 131.29, 131.53, 132.57, 133.89, 165.09, 166.17 ppm; elemen-
tal analysis calcd (%) for C17H19NO3: C 71.56, H 6.71, N 4.91; found: C
71.38, H 6.67, N 4.94.
[a] Polymerized with (nbd)Rh+[h6-C6H5BÀ(C6H5)3] in CHCl3 at 308C for
24 h. [M]0 10 mm. [b] Methanol-insoluble part.
1.0m and [Rh+]
=
=
À
À
Monomer 7: m.p. 125–1268C; IR (KBr): n˜ = 3236 (=C H), 2949 (C H),
2123 (C=C), 1709 (C=O), 1643 (C=O), 1551 (dNÀH) cmÀ1 1H NMR
;
[c] Estimated by GPC (CHCl3, polystyrene standards). [d] Determined
1
by H NMR spectroscopy.
(CDCl3): d = 1.62–2.13 (m, 14H), 2.30 (d, 1H, J = 2.44 Hz), 4.28 (d,
2H, J = 2.44 Hz), 5.19 (s, 1H), 6.62 (s, 1H), 7.27 (t, 1H, J = 7.20 Hz),
8.03 (d, 1H, J = 7.20 Hz), 8.21 (d, 1H, J = 7.20 Hz), 8.44 ppm (s, 1H);
13C NMR (CDCl3): d = 26.94, 27.21, 29.86, 31.96, 31.99, 36.29, 37.29,
71.98, 76.67, 79.22, 127.70, 128.78, 131.47, 131.49, 132.54, 133.97, 164.96,
166.15 ppm; elemental analysis calcd (%) for C21H23NO3: C 74.75, H
6.88, N 4.15; found: C 74.66, H 6.90, N 3.95.
ate molecular weights (Mn = 26000–51000) were obtained
1
in good yields. The H NMR spectra of the resulting poly-
mers, poly(1)–poly(7), showed the olefinic proton in the
main chain around d = 6 ppm. By comparison of the inte-
grated intensity of the olefinic and other protons, the esti-
mated cis structure content of these polymers was 81–88%.
Polymerization procedures:
A CHCl3 solution of the monomers
([M]total = 2m) was added to a CHCl3 solution of (nbd)Rh+[h6-C6H5BÀ-
(C6H5)3] ([monomer]/[cat] = 100:1) under dry nitrogen. The solution was
kept at 308C for 24 h, then poured into a large volume of methanol to
precipitate polymers. The resulting polymers were filtered from the su-
pernatant and dried under reduced pressure.
Secondary conformation of homopolymers: We have previ-
ously demonstrated that the electronic absorption of the
main chain chromophore of poly(N-propargylamide)s de-
pends strongly on the secondary structure.[10b] When the
polymers exist in a helical structure, an absorption peak cen-
tered at approximately 390 nm is observed. In contrast, ran-
domly coiled poly(N-propargylamide)s exhibit an absorption
maximum (lmax) at 320 nm. To examine the secondary struc-
ture, the UV/Vis spectra of poly(1)–poly(7) were measured
in CHCl3 and in toluene: they all showed UV/Vis absorption
around 380 nm (Figure 1). Thus, it can be concluded that
poly(1)–poly(7) have helical conformations in these solvents.
We next examined the stability of the helical conformations
of poly(1) and poly(2). They both displayed an intense
Cotton effect in CHCl3 and in toluene. When the measuring
temperature was raised from 0 to 558C, the intensity of the
Cotton effects of poly(1) and poly(2) decreased only slightly
(Figure 2). The helical structures of poly(1) and poly(2) are
therefore fairly stable to heating in CHCl3 or toluene, and
these homopolymers do not exhibit temperature-driven
helix inversion.
À
À
Poly(1): IR (KBr): n˜ = 3319 (N H), 2955 (C H), 1721 (C=O), 1642 (C=
O), 1541 (dNÀH) cmÀ1 1H NMR (CDCl3): d = 0.78–0.99 (CH2CH3), 0.99–
;
1.18 (CHCH3), 1.18–1.51 (CH2CH3), 1.51–1.80 (CH2CH3), 2.08–2.37
(CHCH3), 3.61–4.50 (CH=CCH2), 5.92–6.38 (NH), 7.98–8.45 ppm (CH=
C).
À
À
Poly(2): IR (KBr): n˜ = 3319 (N H), 2932 (C H), 1723 (C=O), 1646 (C=
O), 1546 (dNÀH) cmÀ1 1H NMR (CDCl3):
;
d
=
0.78–0.99 (CH-
(CH2CH2)2CH2), 0.99–1.18 (CHCH3), 1.18–1.51 (CH2CH3), 1.51–1.80
(CH2CH3), 2.08–2.37 (CHCH3), 3.61–4.50 (CH=CCH2), 5.92–6.38 (NH),
7.98–8.45 ppm (CH=C).
À
À
Poly(3): IR (KBr): n˜ = 3325 (N H), 2980 (C H), 1721 (C=O), 1655 (C=
O), 1542 (dNÀH) cmÀ1
;
1H NMR (CDCl3): d = 0.98–1.41 (CH2CH3), 3.82–
À
4.94 (CH2CH3, CH=CCH2), 5.92–6.48 (C=CH), 7.19–8.72 (Ar H), 8.72–
9.40 ppm (NH).
À
À
Poly(4): IR (KBr): n˜ = 3325 (N H), 2968 (C H), 1728 (C=O), 1651 (C=
O), 1542 (dNÀH) cmÀ1 1H NMR (CDCl3): d = 0.61–1.10 (CH2CH3), 1.32–
;
1.94 (CH2CH3), 3.58–5.14 (CH2CH2CH3, CH=CCH2), 5.82–6.63 (C=CH),
7.19–8.79 (Ar H), 8.79–9.45 ppm (NH).
À
À
À
Poly(5): IR (KBr): n˜ = 3325 (N H), 2949 0(C H), 1728 (C=O), 1651
(C=O), 1542 (dNÀH) cmÀ1 1H NMR (CDCl3): d = 0.66–1.08 (CH2CH3),
;
1.08–144 (CH2CH3), 1.44–1.79 (CH2CH2CH3), 3.42–5.12 (OCH2CH2,
CH=CCH2), 5.92–6.63 (C=CH), 7.19–8.79 (Ar H), 8.79–9.44 ppm (NH).
À
À
À
Poly(6): IR (KBr): n˜ = 3325 (N H), 2937 (C H), 1723 (C=O), 1654 (C=
O) cmÀ1 1540 (dNÀH) cmÀ1 1H NMR (CDCl3):
,
;
d =
0.52–2.16 (CH-
(CH2CH2)2CH2), 3.48–5.10 (OCHCH2, CH=CCH2), 5.61–6.72 (C=CH),
Control of helical sense by copolymer composition: In gen-
eral, helical chiral–achiral random copolymers take the
same helical sense as the homopolymers of the chiral mono-
mers, and the Cotton effect of the copolymers is larger than
that predicted from the composition of the chiral monomer
unit. This chiral amplification behavior is called the “ser-
geants and soldiers rule“ as described in the Introduc-
tion.[6–8] However, it is reported that the preferential helical
sense of polysilylenesA and polyisocyanates B, which have
asymmetric centers apart from their main chain, is changed
by varying the composition.[9] There are two factors that de-
termine the helical senses of most chiral–achiral copolymers.
One is the direct interaction between the main chain and
the chiral centers of the pendant groups; for example, alkyl
À
7.17–8.78 (Ar H), 8.78–9.50 ppm (NH).
À
À
Poly(7): IR (KBr): n˜ = 33343 (N H), 2907 (C H), 1723 (C=O), 1654
(C=O), 1540 (dNÀH) cmÀ1 1H NMR (CDCl3): d = 0.97–2.18 (CH(CH)2-
;
(CH2)4(CH)2CH2), 3.45–5.32 (OCH, CH=CCH2), 5.96–6.65 (C=CH),
À
7.17–8.84 (Ar H), 8.84–9.46 ppm (NH).
Results and Discussion
Polymer synthesis: Polymerization of monosubstituted ace-
tylenes, including N-propargylamides, with Rh catalysts
gives polymers with high stereoregularity (cis).[10,13] Thus,
polymerization of 1–7 was conducted with (nbd)Rh+[h6-
C6H5BÀ(C6H5)3] in CHCl3 (Table 1). Polymers with moder-
Chem. Eur. J. 2005, 11, 3591 – 3598
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F. Sanda, T. Masuda et al.
groups attached to the chiral centers of a polyisocyanate
side chain may interact chirally with the main chain carbon-
yl groups.[9d] The other is the interaction or packing of the
neighboring side chains. The helical senses of the polysily-
lenes A and polyisocyanates B, whose asymmetric centers
are apart from their main chains, depend only on the inter-
action between neighboring pendant groups. If the interac-
tions of chiral–chiral units and of chiral–achiral units stabi-
lize the opposite helical state, the preferred polymer helical
sense may change with the chiral monomer content.
In the present study, we synthesized copolymers of chiral
and achiral N-propargylbenzamides 1–7, whose chiral cen-
ters are known from those previous reports to be apart from
the main chain. First, chiral bulky monomer 1 was copoly-
merized with achiral monomers 3–6. Copolymers with mod-
erate molecular weights were obtained, and the copolymer
compositions were practically identical to the feed ratios.[14]
As the CD and UV/Vis spectra of poly(1-co-3)s with various
compositions (Figure 3) all showed the absorption of the
main chain at 380 nm, we conclude that poly(1-co-3)s form
helices regardless of the copolymer composition. Interesting-
ly, there was clear variation with the copolymer composition
in the CD spectra. Poly(1-co-3)s containing 48% or more of
chiral monomer unit 1 (1/3 = 88:12, 73:27, and 48:52) dis-
played negative Cottons effect just like poly(1), and their
magnitudes decreased with decreasing composition of 1.
However, poly(1-co-3)s containing 38% or less of unit 1
(1/3 = 38:62 and 15:85) displayed positive Cotton effects.
These results indicate that the helical sense of poly(1-co-3)
Figure 1. UV/Vis spectra of a) poly(1) and poly(2), measured in CHCl3
and in toluene, and b) poly(3)–poly(7), measured in CHCl3 at 208C (c =
0.074–0.19 mm).
Figure 2. Variable-temperature CD spectra of poly(1), measured in a)
CHCl3 and b) toluene; and of poly(2), measured in c) CHCl3 and d) tolu-
ene (c = 0.084–0.19 mm).
Figure 3. CD and UV/Vis spectra of poly(1-co-3), measured in CHCl3 at
208C (c = 0.11–0.15 mm).
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Chem. Eur. J. 2005, 11, 3591 – 3598

Helical Structures
FULL PAPER
has been reversed by the copolymer compositions. Similar
tendencies were observed in poly(1-co-4) and poly(1-co-5)
(Figure 4a,b) but the tendency became ambiguous as the
bulkiness of the achiral monomer unit increased (Figure 5a).
When achiral monomer 6 was used for the copolymerization
with 1, the helical sense was not altered by changing the
composition (Figure 4c). The small chiral nonbulky mono-
mer 2 was then copolymerized with achiral nonbulky 3, and
with achiral bulky 7. Copolymers with moderate molecular
weights were obtained, and the copolymer compositions
were identical with the feed ratios. All the poly(2-co-3)s
showed positive Cotton effects and a nonlinear relationship
between the molar ellipticities at 380 nm and chiral contents
(Figure 4d and Figure 5b). Poly(2-co-7) copolymers with
51% or more of chiral monomer units (2/7 = 88:12, 71:29,
and 51:49) displayed positive Cotton effects, while those
with 33% or less of chiral units (2/7 = 33:67 and 15:85) dis-
played negative Cotton effects. When a helical polymer car-
rying chiral side chains takes right- and left-handed screw
senses, the two conformations are diastereomers. This
should be why poly(2-co-3) and poly(2-co-7) showed a hyp-
sochromic shift of the CD and absorption bands with in-
creasing concentrations of the achiral comonomer. Chiral–
achiral copolymers of N-propargylalkylamides whose chiral
centers are close to the main chain show Cotton effects of
the same sign as the chiral homopolymer, even if the chiral
and achiral monomers are quite different in size from each
other.[15] From these data, we conclude that, in order to con-
trol of variability of the screw sense of chiral–achiral
poly(N-propargylamide)s, the chiral centers should be locat-
ed apart from the main chain, and furthermore the chiral
and achiral monomers should differ in bulkiness; that is, the
monomer pairs should be either a bulky chiral plus a non-
bulky achiral monomer or a nonbulky chiral plus a bulky
achiral monomer.
Analysis of experimental data
by means of statistical mechani-
cal theory: In the case of
chiral–achiral copolymers obey-
ing the “sergeants and soldiers”
principle, the optical activities
of the copolymers change nonli-
nearly but monotonically with
the composition, which can be
explained
quantitatively
in
terms of a random field Ising
model.[16] However, when co-
polymers change their helical
sense according to the chiral
monomer content, such behav-
ior cannot be explained by the
conventional Ising model. Re-
cently, Sato and co-workers
have modified the Ising model
by considering the chiral dis-
crimination to be dependent on
the pair type of interacting
pendant groups.[9d] There are
three different types of pairs
along the copolymer chain, the
chiral–chiral (CC), chiral–achi-
ral (CA), and achiral–achiral
(AA) pairs, and the CC and
CA pairs can contribute to the
chiral discrimination in differ-
ent manners.
The chiral discrimination in
our copolymer model can be
characterized in terms of two
free energy differences per
monomer unit between the
right-handed (P) and left-
handed (M) helical states:
Figure 4. CD spectra of poly(1-co-4), poly(1-co-5), poly(1-co-6), poly(2-co-3), and poly(2-co-7) (a)–e), respec-
tively), measured in CHCl3 at 208C (c = 0.072–0.14 mm).
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F. Sanda, T. Masuda et al.
seem to reach as high as 5.6ꢁ104 degcm2 dmolÀ1 at low tem-
peratures, so that we took this value for [q]P for poly(2).
We have calculated 2fpÀ1 and then [q] choosing different
values of DGh,CC, DGh,CA, and DGr. Figure 6 shows favorably
fitting results for the copolymers shown in Figure 5; the free
Figure 5. Molar ellipticities at 380 nm, measured in CHCl3 at 208C,
versus chiral content of a) poly(1-co-3), poly(1-co-4), and poly(1-co-5);
and b) poly(2-co-3) and poly(2-co-7).
2DGh,CC for the CC pair and 2DGh,CA for the CA pair. (The
conventional Ising model contains only one chiral-discrimi-
nation free energy difference.) In addition, the difficulty of
helix reversal for a copolymer chain is taken into account by
the excess free energy DGr of the helix reversal (per mono-
mer unit). The fraction fP of monomer units taking the P
state, or the enantiomeric excess 2fpÀ1 for the modified
Ising model, has been calculated as follows.[16b–d] By comput-
er generation of a large number of sequences of chiral–achi-
ral random copolymers with a given degree of polymeri-
zation N and mole fraction x of the chiral unit, we have cal-
culated fP for each sequence by the matrix method and aver-
aged it over all the sequences generated.
To convert from the theoretical value of 2fpÀ1 into the el-
lipticity [q], we need the maximum ellipticity [q]P for the
intact P helix. [q] for poly(1) is almost saturated, but that of
poly(2) seems to increase gradually with decreasing temper-
ature (Figure 2). It has been reported that some helical poly-
mers undergo inversion of helical sense upon a change in
temperature.[17] Temperature dependencies of [q] for homo-
polymers of N-propargylbenzamides bearing similar chiral
groups are also reported in reference [17g]. A saturated [q]
similar to that of poly(1) has been observed[17g] for poly(5)
with a similar chiral group. Thus we chose a [q]P value of 3ꢁ
104 degcm2 dmolÀ1 for poly(1). On the other hand, absolute
values of [q] for poly(4) and poly(6) in reference [18g],
which have similar chiral groups to poly(2) in this study,
Figure 6. Chiral contents versus [q]380 of a) poly(1-co-3) and b) poly(2-co-
&
7), measured in CHCl3 at 208C. : observed data; solid curves: theoreti-
cal values calculated with the parameters from Table 2.
energy parameters used are listed in Table 2. The value of
DGr (15.5 kJmolÀ1) for all copolymers is close to those used
in the CD analysis for chiral–achiral random copolymers of
phenylacetylene derivatives with the same theory.[18] Fur-
thermore, the same value of DGr also explains consistently
the thermo-driven inversion of the helical sense of poly(N-
propargylbenzamide)s in reference [17g], Therefore, the
DGr value may be an intrinsic property of the polyacetylene
Table 2. Free energy parameters of the copolymers.
[a]
[c]
[c]
Copolymer
Mw
N[b]
DGh,CC
DGh,CA
DGr[c]
Poly(1-co-3)
Poly(1-co-4)
Poly(1-co-5)
Poly(2-co-3)
Poly(2-co-7)
81000–122000
47000–144000
93000–110000
16000–51000
24000–50000
385
320
380
140
160
À18.8
À18.8
À18.8
16.7
À9.7
À7.3
À2.4
À9.7
À4.9
15500
15500
15500
15500
15500
16.7
[a] Estimated by GPC (CHCl3, polystyrene standards). [b] Averaged
(true) degree of polymerization estimated from Mw (in the second
column) and the relationship between molecular weights determined by
GPC and light scattering for a poly(N-propargylamide) (ref. [11d]). [c] In
units of JmolÀ1
.
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Chem. Eur. J. 2005, 11, 3591 – 3598

Helical Structures
FULL PAPER
backbone. This value is close to or slightly larger than DGr
for polyisocyanates and poly(dialkylsilylene)s.[9d,19]
Conclusion
In general, DGh,CC and DGh,CA depend on how far the
chiral center is removed from the main chain. For the pres-
ent copolymers, the chiral center is so distant from the poly-
acetylene backbone that the DGh,CC and DGh,CA values in
Table 2 are considerably smaller than those for chiral–achi-
ral random copolymers investigated so far.[9d,18] Strong CD
induced by such small values of DGh,CC and DGh,CA demon-
strates the sensitivity of the polyacetylene backbone to
chiral perturbation.
N-Propargylbenzamides 1–7 were polymerized with an Rh
catalyst to afford stereoregular cis polymers with moderate
molecular weights in good yields. The optically active poly-
mers poly(1) and poly(2) were proven to take a helical
structure with an excess of one-handed screw sense, which
was stable to heating in CHCl3 or toluene. External stimuli
did not drive helical inversion in these homopolymers.
Meanwhile, it was confirmed that the helical sense of the co-
polymers changed with changing copolymer composition
when the sizes of the chiral and achiral monomer units were
quite different from each other, as for poly(1-co-3) and
poly(2-co-7). The free energy differences between the P and
M helical states, as well as the excess free energy DGr of
helix reversal, for several copolymers of the chiral unit 1 or
2 were estimated by applying the modified Ising model.
While the value of DGr is almost the same in all polyacety-
lene derivatives, the signs of DGh,CC and DGh,CA seem to
depend on the bulkiness of the chiral and achiral units.
The sign of DGh,CA is the opposite of that of DGh,CC for all
copolymers except poly(2-co-3). The opposite sign indicates
that the helical sense induced by the CA interaction is the
opposite of that induced by the CC interaction, and thus
that the helical sense can be controlled by copolymerization,
which tunes the populations of CC and CA interactions.
When chiral–achiral copolymers of N-propargylalkyla-
mides have a chiral center near the amide group, it may be
expected that interactions between the asymmetric carbon
atoms and amide groups in the nearest-neighbor side chain
strongly affect the determination of the preferential helical
sense. Because the absolute configuration of chiral centers
does not change with the copolymer compositions, the inter-
action of the chiral centers with the amide groups in the
nearest-neighbor chiral and achiral side chains should
induce the same helical sense, and prevent composition-
driven inversion of the helical sense.[15] On the other hand,
because the asymmetric centers of poly(1-co-3), poly(1-co-
4), poly(1-co-5), and poly(2-co-7) are apart from their main
chains, it is possible that each chiral center interacts with
various portions of neighboring side chains, including the
chiral portion if the neighboring side chain is chiral. In such
a case, the chiral interactions of the CC and CA side-chain
pairs may not necessarily induce the same helical sense.
As shown in Table 2, dissimilarity in the bulkiness of
chiral and achiral units seems to be an important factor in
the causes of the opposite signs of DGh,CC and DGh,CA for N-
propargylbenzamide copolymers. For copolymers of the
bulky chiral unit 1, the absolute value of DGh,CA decreases
with the increasing bulkiness of the achiral unit. This may
be due to the screening of the chiral interaction by the achi-
ral interaction between the CA pairs, which becomes stron-
ger with increasing bulkiness of the achiral unit. We have no
clear interpretation for the opposite signs of DGh,CA for
poly(2-co-3) and poly(2-co-7). Values of DGh,CC and DGh,CA
are usually so small (see Table 2) that it is difficult to predict
the signs from the chemical structures of the chiral and achi-
ral units by using force-field calculations. For polysilylene
derivatives such as A, the difference in bulkiness of the
chiral and achiral units is not a necessary condition for the
helical-sense inversion driven by copolymer composition.[9d]
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Received: October 5, 2004
Published online: April 5, 2005
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Chem. Eur. J. 2005, 11, 3591 – 3598