Effect of chiral substituents on the secondary structure of poly(N-alkynylamides)

Article abstract of DOI:10.1021/ma050313k

Optically active N-alkynylamides (S)-HC≡C(CH₂) _aNHCO(CH₂)_bCH(CH₃)CH ₂CH₃ (1: a = 1, b = 0; 2: a = 1, b = 1; 3: a = 1, b = 2; 4: a = 1, b = 3; 5: a = 2, b = 0; 6: a = 2, b = 1; 7: a = 3, b = 0), having several numbers of methylene spacers between the ethynyl group and chiral carbon or amide group, and (S)-HC≡C(CH₂)₂OCOCH ₂CH(CH₃)CH₂CH₃ (8) were polymerized with (nbd)Rh⁺[η⁶-C⁶H⁵B ^--(C₆H₅)₃] to afford the corresponding polymers with moderate molecular weights (M_n = 11 000-21 000) in 67-99% yields. The ¹H NMR spectra demonstrated that the resulting polymers had stereoregular structures (cis content = 79-100%). CD, UV-vis, and IR spectroscopic studies revealed that the position of the amide group and chiral center strongly affects the pitch and stability of helical structure of the polymers, and intramolecular hydrogen bonding is formed between the amide groups in CHCl₃. By comparison with the results of CD measurements and molecular orbital calculation, the sign of Cotton effect and the relationship between the screw sense of poly(N-propargylamides) was elucidated.

Full text of DOI:10.1021/ma050313k

5860
Macromolecules 2005, 38, 5860-5867
Articles
Effect of Chiral Substituents on the Secondary Structure of
Poly(N-alkynylamides)
Junichi Tabei, Masashi Shiotsuki, Fumio Sanda,* and Toshio Masuda*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura Campus, Kyoto 615-8510, Japan
Received February 14, 2005; Revised Manuscript Received May 5, 2005
ABSTRACT: Optically active N-alkynylamides (S)-HCtC(CH
2
)
a
NHCO(CH
2 b 3 2 3
) CH(CH )CH CH (1: a )
1
, b ) 0; 2: a ) 1, b ) 1; 3: a ) 1, b ) 2; 4: a ) 1, b ) 3; 5: a ) 2, b ) 0; 6: a ) 2, b ) 1; 7: a ) 3, b
)
0), having several numbers of methylene spacers between the ethynyl group and chiral carbon or amide
+
6
-
group, and (S)-HCtC(CH
2
)
2
OCOCH
2
CH(CH
3
)CH
2
CH
3
(8) were polymerized with (nbd)Rh [η -C
6 5
H B -
(
C
6
H
5
)
3
] to afford the corresponding polymers with moderate molecular weights (M
n
) 11 000-21 000) in
1
6
7-99% yields. The H NMR spectra demonstrated that the resulting polymers had stereoregular
structures (cis content ) 79-100%). CD, UV-vis, and IR spectroscopic studies revealed that the position
of the amide group and chiral center strongly affects the pitch and stability of helical structure of the
3
polymers, and intramolecular hydrogen bonding is formed between the amide groups in CHCl . By
comparison with the results of CD measurements and molecular orbital calculation, the sign of Cotton
effect and the relationship between the screw sense of poly(N-propargylamides) was elucidated.
8
Introduction
highly regulated as cis-transoid. When suitable pen-
dant groups are introduced in the side chain, the main
chain of the polymer cannot take planar structure.
Therefore, monosubstituted cis-polyacetylenes take heli-
cally twisted secondary structure, and they are classified
into dynamic helical polymers.9
Precise control of the higher order structure of
polymers is an issue of great importance. Synthesis of
optically active helical polymers with one-handed screw
sense attracts much attention.1 One of the goals of
helical polymer synthesis is to imitate the elegant and
sophisticated structures of naturally derived helices
We have recently reported that some chiral cis-poly-
(N-propargylamides) construct helical structure with
predominantly one-handed screw sense stabilized by
intramolecular hydrogen bonding as well as steric
repulsion between the side chains.10 It is expected that
the secondary structure of poly(N-propargylamides) is
influenced by changing the distance between the main
chain and chiral center (steric factor) and also the
distance between the main chain and amide group
2
3
such as double helix in DNA and R-helix in protein.
Helical polymers gather interest not only from the
viewpoint of synthesis and properties but also from
practical application because they exhibit useful func-
tions based on the secondary structure, including chiral
4
discrimination and catalytic activity for asymmetric
5
synthesis.
Helical polymers synthesized thus far are classified
(
hydrogen-bonding factor). In this article, we report the
into two types according to the nature of helix. One is
6
synthesis of optically active poly(N-alkynylamides) hav-
ing several numbers of methylene spacers between the
main chain and chiral carbon or amide group and their
secondary structures investigated by CD and UV-vis
spectroscopies (Scheme 1).
the polymers taking a static helical conformation due
to the rigid main chain, which originates from the bulky
pendant groups in the side chain. They can exist in
helical structure even in solution without undergoing
helix-helix or helix-coil transition. The other one is
7
the polymers forming dynamic helix, which results
Experimental Section
from stiff but nonrigid main chain. These polymers
undergo helix-helix and/or helix-random coil inter-
conversion due to the small energetic barriers for helix
reversal, and both right- and left-handed helices coexist
in the polymer main chain.
Measurements. Melting points (mp) were measured with
a Yanaco micromelting point apparatus. Specific rotations [R]D
were measured by a JASCO DIP-1000 digital polarimeter. IR
spectra were obtained with a Shimadzu FTIR-8100 spectro-
photometer. NMR ( H: 400 MHz; C: 100 MHz) spectra were
recorded on a JEOL EX-400 spectrometer. Elemental analyses
were conducted at the Kyoto University Elemental Analysis
1
13
The polymerization of monosubstituted acetylenes
with Rh catalysts gives the corresponding polyacet-
ylenes, whose geometric structure of the main chain is
Center. Number-average-molecular weights (M
ular weight distributions (M /M ) of polymers were estimated
by
n
) and molec-
w
n
by GPC (Shodex KF-850L columns) eluted with CHCl
3
*
Corresponding authors: Tel +81-75-383-2589; Fax +81-75-
83-2590; e-mail sanda@adv.polym.kyoto-u.ac.jp, masuda@
adv.polym.kyoto-u.ac.jp.
3
polystyrene calibration. CD and UV-vis spectra were recorded
on a JASCO J-800 spectropolarimeter. Unless otherwise
1
0.1021/ma050313k CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/16/2005

Macromolecules, Vol. 38, No. 14, 2005
Poly(N-alkynylamides) 5861
Scheme 1
3308 (νH-Ct), 2930, 2138 (νCtC), 1671 (νCdO), 1509 (δN-H), 1219,
-
1 1
6
77 cm . H NMR (CDCl
H), 1.01-1.20 (CH CHCH
CHCH CH CH , m, 3H), 1.42-1.70 (CdOCH
H), 2.14 (CdOCH , m, 2H, J ) 7.60 Hz), 2.29 (CtCH, d, 1H,
, d, 2H, J ) 2.44 Hz), 5.88 (NH,
) δ: 11.32, 19.04 23.15, 29.16, 29.27,
4.17, 35. 91, 36.07, 71.49, 79.55, 173.03. Anal. Calcd for
19NO: C, 72.88; H, 10.56; N, 7.73. Found: C, 73.15; H,
0.49; N, 7.87. 5: yield 6%; mp 55-56 °C; [R] ) +10.4° (c )
.0955 g/dL in CHCl ). IR (KBr): 3306 (νH-Ct), 2934, 2173 (νCt
3
) δ: 0.64-1.00 (CHCH
CH CH , m, 2H), 1.20-1.42
CH , m,
3 2 3
CH CH , m,
6
2
3
2
3
(
CH
2
3
2
3
2
2
2
2
J ) 2.44 Hz), 4.02 (CtCCH
2
1
3
s, 1H). C NMR (CDCl
3
3
11
C H
1
0
D
3
-
1 1
C
), 1638 (νCdO), 1561 (δN-H), 1238, 662 cm . H NMR (CDCl
δ: 0.89 (CH CH , t, 3H, J ) 4.93 Hz), 1.05-1.52 (CHCH CH
CH , d, 3H, J ) 6.80 Hz), 1.41-1.63 (CHCH CH CH , m, 2H),
.94 (CtCH, d, 1H, J ) 2.80 Hz), 2.03-2.18 (CdOCH , m,
H), 2.25-2.43 (CtCCH , m, 2H), 3.34 (CH NH, t, 2H, J )
.49 Hz), 5.76 (NH, s, 1H). C NMR (CDCl ) δ: 11.58, 17.48,
9.51, 27.31, 37.73, 43.22, 69.93, 81.62, 176.57. Anal. Calcd
15NO: C, 70.55; H, 9.87; N, 9.14. Found: C, 70.25; H,
.72; N, 9.01. 6: yield 15%; colorless liquid; [R] ) +4.75° (c
0.198 g/dL in CHCl ). IR (CHCl ): 3308 (νH-Ct), 2964, 2137
CtC), 1664 (νCdO), 1518 (δN-H), 1215, 684 cm
CDCl ) δ: 0.76-0.92 (CHCH CH CH , m, 6H), 0.98-1.32
CHCH CH CH , m, 2H), 1.75-1.98 (CHCH CH CH , CtCH,
3
)
2
3
3
2
-
3
3
2
3
1
1
4
1
2
2
2
1
3
3
9
for C H
9
D
)
3
3
specified, UV-vis and CD spectra were measured at a polymer
concentration of 0.10 mM with respect to repeating unit in
-1
1
(
(
(
ν
. H NMR
3
3
2
3
3
CHCl at 20 °C.
3
2
3
3
2
3
Materials. Propargylamine (Aldrich), (S)-(+)-3-methyl-1-
pentanol (TCI), (S)-(+)-4-methyl-1-hexanol (TCI), (S)-(+)-5-
methyl-1-heptanol (TCI), 3-butyn-1-ol (Aldrich), 4-(4,6-dimeth-
oxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (Tokuya-
ma), and p-toluenesulfonic acid (Wako) were used as received.
2 2
m, 3H), 2.03-2.18 (CdOCH , m, 2H), 2.25-2.43 (CtCCH , m,
2H), 3.34 (CH NH, t, 2H, J ) 4.49 Hz), 5.92 (NH, s, 1H). 13
2
C
NMR (CDCl ) δ: 11.31, 19.08, 19.45, 29.37, 32.30, 37.88, 44.09,
3
69.85, 81.59, 172.71. Anal. Calcd for C H NO: C, 71.81; H,
1
0
17
10.25; N, 8.37. Found: C, 71.63; H, 10.21; N, 8.59. 7: yield
+
6
-
11
12
(
1
6 5 6 5 3
nbd)Rh [η -C H B (C H ) ], pyridinium dichromate (PDC),
25%; colorless liquid; [R] ) +8.42° (c ) 0.240 g/dL in CHCl ).
D
3
1
3
-amino-3-butyne, 1-amino-4-pentyne, (S)-2-methylbutanoic
IR (CHCl ): 3308 (ν
H-Ct CtC CdO
), 2936, 2127 (ν ), 1663 (ν ), 1518
3
1
0c
(δN-H), 1230, 679 cm-1. H NMR (CDCl ) δ: 0.81 (CHCH -
1
acid, and monomer 1
literature. CHCl
to use.
were prepared according to the
3
3
3
used for polymerization was distilled prior
2 3 3 2 3
CH CH , t, 3H, J ) 4.80), 1.04 (CHCH CH CH , d, 3H, J )
6.80), 1.34-1.62 (CHCH CH CH , m, 2H), 1.62-1.71 (Ct
3
2
3
Synthesis of Monomers 2-8. Monomer 2 was prepared
CCH
OCH, m, 1H), 2.25-2.43 (CtCCH
3.28 (CH NH, t, 2H, J ) 4.27 Hz), 6.01 (NH, s, 1H). C NMR
(CDCl ) δ: 11.83, 16.00, 17.44, 27.21, 28.13, 38.36, 42.98, 68.93,
2
CH
2
, m, 2H), 1.90-1.98 (CtCH, m, 1H), 2.01-2.12 (Cd
as follows: PDC (30 g, 78 mmol) was added to a DMF solution
2
, dt, 2H, J ) 2.40, 7.20 Hz),
1
3
(
60 mL) of (S)-(+)-3-methyl-1-pentanol (1.6 g, 16 mmol), and
2
the reaction mixture was stirred for 24 h at room temperature.
The reaction mixture was poured into 500 mL of water and
extracted with diethyl ether. The organic layer was washed
3
83.43, 176.51. Anal. Calcd for C10H17NO: C, 71.81; H, 10.25;
N, 8.37. Found: C, 71.92; H, 10.05; N, 8.44.
with HCl(aq), dried over MgSO
4
, and concentrated to give (S)-
Monomer 8 was prepared as follows: a benzene solution
(100 mL) of p-toluenesulfonic acid (0.16 g, 0.91 mmol), (S)-3-
methylpentanoic (1.0 g, 9.1 mmol), and 3-butyn-1-ol (0.60 g,
9.1 mmol) was heated with refluxing for 5 h. The resulting
3
-methylpentanoic acid in 57% yield. 4-(4,6-Dimethoxy-1,3,5-
1
4
triazin-2-yl)-4-methylmorpholinium chloride (4.60 g, 9.1
mmol) and then propargylamine (0.92 mL, 9.1 mmol) were
added to a THF solution (100 mL) of the obtained (S)-3-
methylpentanoic acid (1.0 g, 9.1 mmol) at room temperature.
The resulting solution was stirred at room temperature for
4 h. After the white precipitate formed was filtered off, the
filtrate was concentrated by rotary evaporation. Ethyl acetate
ca. 100 mL) was added to the residue, and the resulting
solution was washed with 2 M aqueous HCl and saturated
solution was washed with saturated aqueous NaHCO
over MgSO , and concentrated. Monomer 8 was isolated (1.15
g, 6.8 mmol, 43%) by distillation under reduced pressure. 8:
yield 43%; bp 67-68 °C/5 mmHg; [R] ) +6.78° (c ) 0.401
g/dL in CHCl ). IR (KBr): 3292 (νH-Ct), 2926, 2138 (νCtC), 1636
(νCdO), 1541 (δN-H), 1232, 675 cm . H NMR (CDCl
0.92 (CHCH CH CH , m, 6H), 1.16-1.35 (CHCH CH
2H), 1.85-1.98 (CHCH CH CH , CtCH, m, 3H), 2.08-2.33
(CdOCH , m, 2H), 2.42-2.58 (CtCCH , m, 2H), 4.16 (CH
NH, t, 2H, J ) 4.49 Hz). C NMR (CDCl ) δ: 11.27, 19.01,
19.25, 29.29, 31.90, 41.31, 61.83, 69.78, 80.11, 173.02. Anal.
3
, dried
4
2
D
3
-1 1
(
3
) δ: 0.76-
CH , m,
3
2
3
3
2
3
aqueous NaHCO
3
, dried over MgSO
4
, and concentrated. Mono-
3
2
3
mer 2 was isolated (1.3 g, 8.5 mmol, 53%) by flash column
2
2
2
-
1
3
chromatography on silica gel eluted with hexane/ethyl acetate
3
)
1/1 (v/v). Monomers 3-7 were prepared in a similar way. 2:
yield 53%; mp 57-58 °C; [R]
CHCl ). IR (CHCl ): 3308 (νH-Ct), 2964, 2117 (νCtC), 1666 (νCd
), 1522 (δN-H), 1246, 716 cm . H NMR (CDCl
.09 (CH CHCH CH CH , m, 6H), 1.09-1.58 (CH CHCH
CH , m, 2H), 1.52-1.73 (CH CHCH CH CH , m, 2H), 2.08-
.19 (CH CHCH CH CH , CtCH, m, 2H), 4.05 (CtCCH , d,
H, J ) 2.44 Hz), 5.87 (NH, s, 1H). C NMR (CDCl ) δ: 10.83,
8.67, 28.57, 28.91, 31.83, 43.29, 70.89, 79.19, 171.82. Anal.
15NO: C, 70.55; H, 9.87; N, 9.14. Found: C,
0.42; H, 9.93; N, 9.04. 3: yield 86%; mp 49-50 °C; [R]
4.95° (c ) 0.129 g/dL in CHCl ). IR (CHCl ): 3308 (νH-Ct),
964, 2138 (νCtC), 1671 (νCdO), 1507 (δN-H), 1217, 679 cm
D
) +5.22° (c ) 0.146 g/dL in
Calcd for C10
9.33.
16 2
H O : C, 71.39; H, 9.59. Found: C, 71.52; H,
3
3
-
1
1
O
3
) δ: 0.82-
CH
Polymerization Procedure. A CHCl
3
solution of a mono-
solution of (nbd)-
6 5 6 5 3 0
Rh [η -C H B (C H ) ] ([M] /[Rh ] ) 100) under dry nitrogen,
1
2
3
2
3
2
3
2
-
mer ([M] ) 1.0 M) was added to a CHCl
0
3
+
6
-
+
3
2
3
2
3
2
2
1
2
3
2
3
2
and the solution was kept at 30 °C for 24 h. The reaction
mixture was poured into a large amount of hexane or methanol
to precipitate a polymer. The resulting polymer was separated
by filtration and dried under reduced pressure.
1
3
3
9
Calcd for C H
1
5
10c
7
D
)
Spectroscopic Data of Polymers. Poly(1), Poly(2): IR
1 1
-
+
3
3
(CHCl
(CDCl
CH ), 1.79-2.38 (CH(CH
5.64-6.48 (CHdC), 7.42-8.48 (NH). Poly(3): IR (CHCl
3
3
): 3288, 2963, 1634, 1541, 1223, 743 cm . H NMR
) δ: 0.65-1.22 (CH(CH CH ), 1.22-1.79 (CH(CH
CH ), 3.24-4.48 (CHdCCH
) 3293,
) δ: 0.99-
), 3.58-4.44 (CHd
2
), 5.88-6.22 (CHdC), 7.58-7.91 (NH). Poly(4): IR
-
1
2
.
3
2
3
)
2
2
-
2
),
1
H NMR (CDCl
.51 (CH CHCH
m, 1H), 2.01-2.35 (CdOCH
3
) δ: 0.72-0.96 (CHCH
CH CH , m, 4H), 1.54-1.73 (CHCH
, CtCH, m, 3H), 4.02 (CtCCH
3
CH
2
CH
3
, m, 6H), 1.02-
3
)
2
2
3
)
2
1
2
3
2
3
3
CH
2
CH
3
,
,
-
1 1
2
2
2963, 1653, 1541, 1215, 664 cm . H NMR (CDCl
1.41 (CH C(CH ), 1.41-1.75 (CH C(CH
CCH
(CHCl
(CDCl
3.58-4.44 (CHdCCH
3
1
3
d, 2H, J ) 2.44 Hz), 5.84 (NH, s, 1H). C NMR (CDCl
3
) δ:
1.27, 18.84, 29.11, 29.15, 32.18, 34.01, 34.15, 71.42, 79.64,
72.98. Anal. Calcd for C10 17NO: C, 71.81; H, 10.25; N, 8.37.
2
)
3 3
2
3 3
)
1
1
-
1
1
H
3
): 3306, 2955, 1655, 1546, 1236, 758 cm . H NMR
) δ: 0.99-1.41 (CH C(CH ), 1.41-1.75 (CH C(CH ),
), 5.88-6.22 (CHdC), 7.58-7.91 (NH).
Found: C, 70.93; H, 10.12; N, 8.30. 4: yield 61%; colorless
liquid; [R] ) +2.37° (c ) 0.228 g/dL in CHCl ). IR (CHCl ):
3
2
)
3 3
2
3 3
)
D
3
3
2

5862 Tabei et al.
Macromolecules, Vol. 38, No. 14, 2005
Table 1. Polymerization of 1-8a
Table 2. Specific Rotations of Poly(1)-Poly(8)
[R]Da (deg)
b
Mnd
Mw/Mnd
e
monomer
yield (%)
cis content (%)
1
2
3
4
5
6
7
8
99
98
67
73
79
85
98
16 000
21 000
13 000
11 000
f
14 000
11 000
13 000
2.55
1.99
1.71
1.85
f
1.53
3.46
2.03
100
79
100
96
f
82
90
96
polymer
in toluene
in CHCl3
in THF
poly(1)
poly(2)
poly(3)
poly(4)
poly(5)
poly(6)
poly(7)
poly(8)
b
+1610.00
+876.0
+320.00
-13.4
b
+46.70
-82.80
-4.31
b
+36.50
-124.00
-6.11
b
b
+199.00
+12.50
-15.00
+268.00
+1.75
+251.00
+9.94
-17.20
c
92
a
+
-
Polymerized with (nbd)Rh [(C6H5)B (C6H5)3] in CHCl3 at 30
-6.46
+
b
°
C for 24 h. [M]0 ) 1.0 M, [Rh ] ) 10 mM. Hexane-insoluble part.
a
Measured by polarimetry at room temperature, c ) 0.051-
c
d
Methanol-insoluble part. Estimated by GPC (eluent CHCl3, PSt
b
0
.083 g/dL. Not determined.
calibration). e Determined by H NMR measurement. Not deter-
1
f
mined due to the insolubility of the polymer.
Poly(5): IR (KBr): 3306, 2964, 1650, 1551, 1235 cm-1. 1H NMR
(
CDCl
3 2 3 3 2 3 3
) δ: 0.99-1.41 (CH C(CH ) ), 1.41-1.75 (CH C(CH ) ),
3.58-4.44 (CHdCCH
2
), 5.88-6.22 (CHdC), 7.58-7.91 (NH).
-
1
Poly(6): IR (CHCl
3
): 3285, 2932, 1628, 1543, 1225, 765 cm .
) δ: 0.99-1.41 (CH
1
H NMR (CDCl
CH C(CH ), 3.58-4.44 (CHdCCH
.58-7.91 (NH). Poly(7): IR (CHCl ): 3301, 2934, 1636, 1541,
) δ: 0.99-1.41 (CH C(CH ),
), 3.58-4.44 (CHdCCH ), 5.88-6.22
): 3306, 2955,
) δ: 0.69-1.01
CH CH ), 1.89-
), 3.38-3.84 (CHdCCH ), 3.78-4.34
), 5.73-6.22 (CHdC).
3
2
C(CH
3 3
) ), 1.41-1.75
), 5.88-6.22 (CHdC),
(
2
)
3 3
2
7
1
1
3
-
1 1
235, 756 cm . H NMR (CDCl
.41-1.75 (CH C(CH
CHdC), 7.58-7.91 (NH). Poly(8): IR (CHCl
3
2
3 3
)
2
3
)
3
2
(
3
655, 1546, 1236, 758 cm-1. H NMR (CDCl
CH CHCH CH CH ), 1.67-1.89 (CH CHCH
.31 (CH CHCH CH CH
1
1
3
(
2
3
2
3
2
3
2
3
1
2
3
2
3
2
(
OCH
2
Molecular Mechanics and Semiempirical Molecular
Orbital Calculations. All the calculations were carried out
with Wavefunction, Inc. Spartan ’04 Windows version 1.01.
Results and Discussion
Polymerization. The polymerization of N-propargyl-
amides with an Rh catalyst gives polymers with high
10
stereoregularity (cis). Thus, the polymerization of 1-8
+
6
-
was carried out with (nbd)Rh [η -C6H5B (C6H5)3] as a
catalyst in CHCl3. The results of the polymerization are
listed in Table 1. Polymers with moderate molecular
weights (Mn ) 11 000-21 000) were obtained in 67-
9
9% yields. Poly(5) began to precipitate in 10 min after
initiating the polymerization; the polymer was insoluble
in common organic solvents such as toluene, THF, and
CHCl3. Poly(2)-poly(4) and poly(6)-poly(8) dissolved in
toluene, THF, and CHCl3, while poly(1) did only in
methanol and CHCl3. Poly(1)-poly(4) and poly(6)-poly-
(
8) displayed unimodal GPC chromatograms, which
means that the polymerization proceeded through a
1
single propagating species. The H NMR spectra of the
Figure 1. CD and UV-vis spectra of poly(1) measured in
CHCl at various temperatures (c ) 0.101 mM).
resulting polymers, poly(1)-poly(4) and poly(6)-poly-
3
(8), showed a signal assignable to the cis-olefinic proton
Figure 1 depicts the CD and UV-vis spectra of poly-
1) measured in CHCl3 at various temperatures. Poly-
1) exhibited an intense Cotton effect and a UV-
in the main chain around 6 ppm. The cis contents of
the polymers were estimated to be 79-100% by com-
parison of the cis-olefinic proton signal with other proton
signals.
(
(
absorption peak at 390 nm based on the conjugated
10c
main chain. By raising the temperature, the CD and
Effect of Alkylene Spacer between Chiral Cen-
ter and Amide Group. The position of chiral centers
affects the helical structure of polymers such as poly-
UV-absorption peak gradually decreased, and a peak
appeared around 320 nm in the UV-vis spectrum,
which was obvious at 55 °C. We have previously
demonstrated that the electronic absorption of the main-
chain chromophore of poly(N-propargylamides) strongly
depends on the conformation.1 When poly(N-pro-
pargylamides) exist in a helical conformation, they show
a UV absorption peak centered at 390 nm. In contrast,
randomly coiled poly(N-propargylamides) show an ab-
sorption maximum at 320 nm. Consequently, it is
considered that poly(1) underwent helix-coil transition
at higher temperature, and this is the reason why the
1
6
17
(
isocyanides),
poly(thiophenes),
and poly(acet-
9
a,18
ylenes).
Thus, [R]D, CD, and UV-vis spectra of poly-
0b
(
1)-poly(4) were measured in order to examine the
effect of alkylene spacers on the helical structure. As
summarized in Table 2, the absolute values of [R]D in
CHCl3 tended to decrease with increasing spacer length
between the amide group and chiral center. Poly(3)
exhibited [R]D with different sign in CHCl3 from that
in toluene and THF.

Macromolecules, Vol. 38, No. 14, 2005
Poly(N-alkynylamides) 5863
Figure 2. (A) CD and UV-vis spectra of poly(2) measured in CHCl
3
at various temperatures. (B) CD and UV-vis spectra of
poly(2) measured in THF and toluene at 20 °C (c ) 0.188-0.339 mM).
intensity of the Cotton effect decreased upon heating.
Poly(2) also exhibited a Cotton effect with positive sign
similarly to poly(1) (Figure 2A). In general, the sign of
Cotton effect of polymers represents the handedness of
position of chiral center of poly(4) is so far from the main
chain that it cannot take one-handed helical structure.
Poly(N-propargylamides) possibly form intramolecu-
lar hydrogen bonding between the amide groups at
either the nth and (n+2)th or the nth and (n+3)th units.
From the fact that CD and UV absorption around 390
nm is unusually red-shifted among polyacetylenes, along
with the conformational analysis by the molecular
orbital method, we have concluded that poly(N-propar-
gylamides) regularly form hydrogen bonding between
1
9
helicity. Both of the stereogenic units in monomers 1
and 2 possess (S)-configuration, and it is likely that the
screw senses of poly(1) and poly(2) are the same, which
is also supported by the sign of [R]D. This result
contrasts to the observation of odd-even rules in the
chiroptical properties for several chiral polymers such
1
6
17
20
as poly(isocyanides) and poly(thiophenes). For ex-
ample, the helix sense of poly(thiophenes) having iden-
tical stereogenic units alternates upon sequential varia-
tion of the spacer length. No such effect was observed
in the present system and poly(propiolic esters) excep-
the amide groups at the nth and (n+2)th units. Poly-
(1) and poly(2) presumably take this helical structure
in CHCl3. On the other hand, it is suggested that poly-
(3) exhibiting a Cotton effect around 300 nm form
another type of helical structure, i.e., the one ac-
companied by intramolecular hydrogen bonding be-
tween the amide groups at the nth and (n+3)th units
in a manner similar to poly(6),21 which will be described
below. It seems that the coplanarity of the polymer
backbone is enhanced by eliminating the alkylene
spacer, and the degree of twist of the polymer backbone
increases by introducing the alkylene spacer because
of enlargement of steric repulsion.22 In other words,
when the chiral center is positioned away from main
chain, the helical pitch becomes short, and the polymer
exists in helical structure with ill-controlled screw sense
because the efficiency of chirality transmittance from
the side chain to main chain is decreased.
18
tionally. The helical structure of poly(2) was thermally
less stable than that of poly(1), and the helix-coil
transition was clearly confirmed as shown in Figure 2A.
Poly(2) slightly showed a Cotton effect in THF and
2
toluene ([θ]max ) ca. +2000 deg cm /dmol) (Figure 2B).
This result well agrees with the report that poly(N-
propargylamides) without R-branched structure at the
amide carbonyl group can exist in a helical conformation
10b
only in CHCl3 and CH2Cl2. It is considered that poly-
3) also formed one-handed helical structure because
(
poly(3) showed a strong Cotton effect at 300 nm, the
UV absorption region of the main chain (Figure 3). In
THF and toluene, poly(3) showed a strong Cotton effect
with different shape and sign from those measured in
CHCl3. As shown in Figure 4, poly(4) did not display a
Cotton effect in these solvents. It is suggested that the
Effect of the Position of Amide Group. The helical
structure of poly(N-propargylamides) is stabilized by
intramolecular hydrogen bonding between the amide

5864 Tabei et al.
Macromolecules, Vol. 38, No. 14, 2005
Figure 3. CD spectra of poly(3) measured in (a) CHCl
of poly(3) measured in CHCl
3
, (b) THF, and (c) toluene at various temperatures and (d) UV-vis spectra
3
, THF, and toluene at 20 °C (c ) 0.162-0.177 mM).
Figure 4. CD and UV-vis spectra of poly(4) measured in CHCl
3
, THF, and toluene at 20 °C (c ) 0.179-0.210 mM).
groups in the side chain as described.10a Thus, it is likely
that the distance from the main chain to amide group
also affects the secondary structure. We measured the
that poly(6) takes one-handed helical structure in these
solvents. The shape of the Cotton effect of poly(6) was
very similar to that of poly(3), and it was thermally more
stable than that of poly(3). On the other hand, poly(7)
exhibited a small Cotton effect around 300 nm, whose
shape was identical to those of poly(3) and poly(6). From
these results, we can say that the position of amide
group does not affect the pitch of the helix but only
affects the stability. If poly(5) exhibits a Cotton effect
around 390 nm, this assumption would be supported
much more. Unfortunately, the secondary structure of
poly(5) could not be elucidated because of the poor
solubility. Anyhow, we can say that the ethylene spacer
[
R]D, CD, and UV-vis spectra of poly(3), poly(6), and
poly(7) in order to examine the effect. These polymers
have the same number of atoms between the main chain
and chiral center and different number of methylenes
(1-3) between the main chain and amide group. Con-
sequently, we can simply evaluate the effect of position
of the amide group. As shown in Figure 5 and Table 2,
poly(6) exhibited an intense Cotton effect at 300 nm,
UV absorption region of the main chain, and large [R]D
in CHCl3, THF, and toluene. Therefore, it is concluded

Macromolecules, Vol. 38, No. 14, 2005
Poly(N-alkynylamides) 5865
Figure 5. CD spectra of poly(6) measured in (a) CHCl
3
, (b) THF, and (c) toluene at various temperatures and (d) UV-vis spectra
3
of poly(6) measured in CHCl , THF, and toluene at 20 °C (c ) 0.269-0.359 mM).
polymers appeared around 1636 and 1541 cm- under
the same conditions, respectively. The wavenumbers of
these IR bands were independent of the concentrations
between 50 and 0.5 mM. These data lead to a reasonable
conclusion that hydrogen bonding is formed between the
pendant amide groups of the polymers intramolecularly.
All the monomers and polymers displayed the same IR
spectroscopic behavior irrespective of the fact that the
polymers take helical structure or not. We could not
observe the correlation between the secondary structure
and IR spectra of these polymers. It is therefore as-
sumed that intramolecular hydrogen bonding exists in
all the polymers, but the efficiency for stabilization of
the helical structure and the effect on the selectivity
between left- and right-handed screw senses are differ-
ent according to the spacer length and position of chiral
center.
1
between the main chain and amide group is preferable
to induce a stable helix.
Confirmation of Hydrogen Bonding. Poly(8), an
ester counterpart of poly(6), was synthesized and the
chiroptical properties were examined in order to inves-
tigate the nature and extent of hydrogen bonding of the
amide group of poly(6). Poly(8) having no hydrogen-
bonding donor showed a very weak CD signal and poor
optical rotation in solution (Figure 7 and Table 2). It is
hence considered that the main chain is too flexible to
adopt an ordered helical conformation and that it
essentially exists in a randomly coiled state. Thus, this
result supports the assumption that the helical struc-
ture of poly(6) is stabilized by hydrogen bonding be-
tween the amide groups in a fashion similar to poly(N-
10a
propargylamides). Asdescribedabove,theCDspectroscopic
patterns and [R]D of poly(N-alkynylamides), poly(1)-
poly(4), poly(6), and poly(7), were strongly influenced
by the position of amide group and chiral center. The
way of intramolecular hydrogen bonding may affect the
chiropotical properties. The IR spectra of the monomers
and corresponding polymers were measured in CHCl3
Helical Structure of Poly(1). Although we have
already reported the helical pitch of poly(N-pro-
pargylamides), we have not reported the screw sense
of the helix yet.20 In the present study, we attempted
the computational calculation of poly(1) to gain knowl-
edge on the secondary structure of poly(1) having the
simplest structure among chiral poly(N-propargyl-
amides). We first constructed 10-mers of N-pro-
pargylacetamide terminated with hydrogen as the ini-
tial models, wherein the amide moieties of the nth and
(n+2)th units form hydrogen boding. The dihedral angle
at the double bonds in the main chain was fixed at 0°,
and that at the single bonds was varied from 90° to 180°
by the step of 10°. Otherwise, all the geometries were
optimized by MMFF94.23 The computationally esti-
mated dihedral angle of the most stable 10-mer was
(c ) 50 mM) (Table 3). The following results evidenced
the presence of intramolecular hydrogen bonds formed
between the pendant amide groups in these polymers,
which plays an important role for helix formation. The
amide I and II absorption peaks of the monomers were
-
1
observed at 1662-1673 and 1501-1518 cm , respec-
tively. These peaks should be derived from the amide
groups free from hydrogen bonding because the measur-
ing concentration is low enough to prevent them from
forming intermolecular hydrogen bonding. In contrast,
the amide I and II absorption peaks of the corresponding

5866 Tabei et al.
Macromolecules, Vol. 38, No. 14, 2005
Figure 6. CD spectra of poly(7) measured in (a) CHCl
of poly(7) measured in CHCl
3
, (b) THF, and (c) toluene at various temperatures and (d) UV-vis spectra
3
, THF, and toluene at 20 °C (c ) 0.192-0.199 mM).
Figure 7. CD and UV-vis spectra of poly(8) measured in CHCl
30°, which was identical to the previous report.20 We
3
, THF, and toluene at 20 °C (c ) 0.301-0.387 mM).
1
Table 3. IR Spectroscopic Data of 1-4, 6, 7,
Poly(1)-Poly(4), Poly(6), and Poly(7)a
next substituted the methyl groups of the 10-mers into
chiral 2-butyl groups [(S)-configuration], and the geom-
etries were optimized by the semiempirical molecular
orbital method using the AM1 Hamiltonian to deter-
mine the screw sense of poly(1). Figure 8 shows the
dependence of the potential energy of the 10-mers of 1
on the CdC-CdC torsional angle. The potential curve
exhibited two energy minima at the torsional angles of
amide I
amide II
amide I
amide II
-1
-1
-1
-1
compd
(cm
)
(cm
)
compd
(cm
)
(cm )
1
2
3
4
6
7
1673
1667
1671
1671
1665
1662
1501
1512
1507
1509
1518
1518
poly(1)
poly(2)
poly(3)
poly(4)
poly(6)
poly(7)
1636
1634
1638
1639
1628
1636
1541
1541
1541
1539
1543
1541
1
30° and 230°. The global energy minimum at 130°
means that the right-handed helical structure is pre-
ferred to the left-handed one by 0.323 kcal/mol per
repeating unit. Therefore, poly(1) seems to possess the
right-handed helical structure predominantly. By com-
a Measured in CHCl (c ) 50 mM).
3
parison with the results from the CD measurements,
we assume that poly(N-propargylamides) with right-

Macromolecules, Vol. 38, No. 14, 2005
Poly(N-alkynylamides) 5867
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Liq. Chromatogr. 1986, 9, 369-384.
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(
8) (a) Masuda, T.; Sanda, F. In Handbook of Metathesis; Grubbs,
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(
handed helical structure display a plus Cotton effect
around 390 nm. Accordingly, a minus Cotton effect at
the main-chain absorption band implies an excess of
left-handed helix.
1
Macromolecules 1974, 7, 174-179. (c) Nakako, H.; Nomura,
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Conclusion. Stereoregular poly(N-alkynylamides),
poly(1)-poly(7), having several alkylene spacers be-
tween the amide group and the chiral center or the main
chain were synthesized by the polymerization of the
6345-6359. (f) Li, B. S.; Cheuk, K. K. L.; Salhi, F.; Lam, J.
W. Y.; Cha, J. A. K.; Xiao, X.; Bai, C.; Tang, B. Z. Nano Lett.
2001, 323-328.
+
6
-
corresponding monomers using (nbd)Rh [η -C6H5B -
C6H5)3] as a catalyst. CD spectroscopic study implied
(10) (a) Nomura, R.; Tabei, J.; Masuda, T. J. Am. Chem. Soc. 2001,
1
23, 8430-8431. (b) Nomura, R.; Tabei, J.; Masuda, T.
(
Macromolecules 2002, 35, 2955-2961. (c) Tabei, J.; Nomura,
R.; Masuda, T. Macromolecules 2002, 35, 5405-5409.
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that the secondary structure of the polymers is influ-
enced by the position of the chiral center and amide
group. Namely, the position of the chiral center mainly
affects the helical pitch, which becomes short when the
chiral center is positioned away from the main chain.
The stability of the helical structure was also influenced
by the position of amide group. It was confirmed that
the helical structure of the poly(N-alkynylamides) is
stabilized by intramolecular hydrogen bonding between
the amide groups in the side chains from the IR
spectroscopic analysis, along with the comparison of the
CD spectra with that of poly(8). Semiempirical molec-
ular orbital calculations indicated that poly(1) takes
right-handed helical structure preferably to left-handed
one. It is concluded that poly(N-propargylamides) with
right-handed helical structure displays a plus Cotton
effect around 390 nm by comparison with the results of
the CD measurements.
(
(
12) Corey, E. J.; Schmidt, G. Tetrahedron Lett. 1979, 5, 399-
402.
(
13) Qiao, C.; Jeon, H. B,; Sayre, L. M. J. Am. Chem. Soc. 2003,
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1
3
(
15) The C NMR peaks of the polymers were very broad and
gave little information, and so we omitted the data.
(16) Amabilino, D. B.; Ramos, E.; Serrano, J. L.; Sierra, T.;
Veciana, J. J. Am. Chem. Soc. 1998, 120, 9126-9134.
(
(
17) Lermo, E. R.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.;
Meijer, E. W. Chem. Commun. 1999, 791-792.
18) Nakako, H.; Nomura, R.; Masuda, T. Macromolecules 2000,
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(
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