Conformational study of poly(N-propargylamides) having bulky pendant groups

Article abstract of DOI:10.1021/ma020320y

N-Propargylamides having chiral centers at the α-carbon of the amide groups, 1-3, were polymerized with (nbd)Rh⁺[η⁶-C₆H₅B^-(C ₆H₅)₃] to afford polymers with moderate molecular weights (M_n = 6000-32 000) in good yield. The ¹H NMR spectra demonstrated that the polymers have stereoregular structures (cis = 100%). The polymers were proven to take a helical conformation with an excess of one- handed screw sense in CHCl₃, which was supported by their intense CD effects and large optical rotations. It was confirmed that the helical structure was stabilized not only by the steric repulsion but also by the intramolecular hydrogen bonds between the pendant groups. CD spectroscopic study showed that the helical structure is more stable than that of the polymers without a branch at the α-position, which allowed the polymers to exist in the helical state in various solvents. The electronic absorption, CD effects, and optical rotations of the polymers closely correlated to the extent of the hydrogen bonding between the pendant amide groups.

Full text of DOI:10.1021/ma020320y

Macromolecules 2002, 35, 5405-5409
5405
Conformational Study of Poly(N-propargylamides) Having Bulky
Pendant Groups
J u n ich i Ta bei, Ryoji Nom u r a ,* a n d Tosh io Ma su d a *
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Kyoto 606-8501, J apan
Received March 1, 2002; Revised Manuscript Received May 7, 2002
ABSTRACT: N-Propargylamides having chiral centers at the R-carbon of the amide groups, 1-3, were
polymerized with (nbd)Rh+[η6-C6H5B-(C6H5)3] to afford polymers with moderate molecular weights (Mn
1
) 6000-32 000) in good yield. The H NMR spectra demonstrated that the polymers have stereoregular
structures (cis ) 100%). The polymers were proven to take a helical conformation with an excess of one-
handed screw sense in CHCl3, which was supported by their intense CD effects and large optical rotations.
It was confirmed that the helical structure was stabilized not only by the steric repulsion but also by the
intramolecular hydrogen bonds between the pendant groups. CD spectroscopic study showed that the
helical structure is more stable than that of the polymers without a branch at the R-position, which
allowed the polymers to exist in the helical state in various solvents. The electronic absorption, CD effects,
and optical rotations of the polymers closely correlated to the extent of the hydrogen bonding between
the pendant amide groups.
Sch em e 1
In tr od u ction
One-handed helical conformations of biopolymers
such as DNA1 and proteins2 are caused by homochirality
of their components (D-sugars and L-amino acids). The
exclusive one-handed screw sense of biopolymers is
related to their biological activities. The study of helical
polymers is important to understand the self-organiza-
tion process of biopolymers into helical structures, such
as R-helices of polypeptides and double helices of nucleic
acids, and also to produce highly advanced materials
having biomimetic functions. The study of helical poly-
mers has been conducted extensively, which dates back
to the discovery of isotactic polypropylene by Natta and
co-workers.3 Nowadays, synthetic, optically active poly-
mers, in which the chirality originates from the helical
conformation, are applied as functional materials based
on the molecular recognition ability4 and catalytic
activity for asymmetric synthesis.5
Appropriately substituted polyacetylenes can be heli-
cal polymers.6 Substituted polyacetylenes ideally take
four geometrical structures (trans-transoidal, trans-
cisoidal, cis-transoidal, and cis-cisoidal), and the stereo-
regular cis-polyacetylenes can form a well-ordered heli-
cal conformation. In a previous work, we reported that
N-propargylamides polymerize in the presence of Rh
catalyst to result in polymers having excellent cis
sterostructure (Scheme 1).7 We also found that the
polymers with a branched structure at the position â to
the carbonyl group, i.e., poly(5) and poly(7), adopt a
helical conformation in CHCl3.8 In general, the helical
conformation of substituted polyacetylenes is induced
and stabilized by the repulsion between the pendant
groups.6 On the contrary, the intramolecular hydrogen
bonds between the amide groups in the side chains
significantly contribute to stabilization of the helical
structure of poly(5) and poly(7).7 This is the driving force
for poly(N-propargylamides), which have small side
chains, to adopt the helical conformation. Therefore,
they are readily deformed to a disordered state by
external stimuli such as heating or adding polar sol-
vents.8 In the present work, the authors synthesized
poly(N-propargylamides), poly(1)-poly(3), having chiral
centers at the R-carbon of the amide group (Scheme 1).
We show that the stability of the helical conformation
is largely improved and that poly(1)-poly(3) can take
the helical structure in various solvents including polar
ones. The authors also demonstrate that the CD spectra,
UV-vis spectra, and optical rotation of the polymers
closely correlate to the extent of the hydrogen bonding.
Exp er im en ta l Section
Ma ter ia ls. The solvents were distilled by usual methods
prior to use. Propargylamine (Aldrich), (S)-(-)-2-methyl-1-
butanol (Tokyo Kasei), 2-methylbutyric acid (Wako), isobutyl
chloroformate (Wako), 4-methylmorpholine (Wako), (1S)-(-)-
camphanic acid (Aldrich), and (R)-(+)-tetrahydro-2-furoic acid
(Aldrich) were used without further purification. (nbd)Rh+[η6-
C6H5B-(C6H5)3]9,10 and pyridinium dichromate (PDC)11 were
prepared as reported. Monomers 4, 5, and 7 were prepared
according to the literature.8
Mon om er Syn th esis. Synthesis of 1 is described as a
typical procedure. PDC (150 g, 0.39 mol) was added to a DMF
* Corresponding author: Tel +81-75-752-5613; Fax +81-75-756-
5908; e-mail masuda@adv.polym.kyoto-u.ac.jp.
10.1021/ma020320y CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/04/2002

5406 Tabei et al.
Macromolecules, Vol. 35, No. 14, 2002
Ta ble 1. P olym er iza tion of 1, 2, 3, a n d 6
solution (300 mL) of (S)-2-methyl-1-butanol (10 g, 0.11 mol),
and the reaction mixture was stirred for 2 days at room
temperature. The reaction mixture was poured into 1700 mL
of water and extracted with diethyl ether. The organic layer
was washed with HCl(aq), dried over MgSO4, and concentrated
to give (S)-2-methylbutyric acid in 57% yield. Isobutyl chloro-
formate (5.90 mL, 45.7 mmol) was added to a THF solution
(100 mL) of the resulting (S)-2-methylbutyric acid (4.66 g, 45.7
mmol) and 4-methylmorpholine (5.13 mL, 45.7 mmol) at 0 °C.
After 15 min, propargylamine (3.14 mL, 45.7 mmol) was added
to the solution. The solution was stirred at room temperature
for 1 h. After the white precipitate was filtered off, the filtrate
was concentrated. Ethyl acetate (ca. 100 mL) was added to
the residue, and the solution was washed with HCl(aq) and
saturated aqueous NaHCO3, dried over MgSO4, and concen-
trated. Monomer 1 was isolated (4.11 g, 29.6 mmol, 37%) by
flash column chromatography on silica gel (hexane/AcOEt, 1/1,
v/v). Monomers 1 (racemate), 2, and 3 were prepared in a
similar way from the corresponding carboxylic acids. The
spectral data were as follows.
d
d
monomer
yield (%)
Mn
Mw/Mn
[R]D (deg)e
+1610
1
82a
63a
76b
68c
75c
19 000
22 000
6 000
32 000
9 200
1.93
1.99
1.26
1.77
21.2
1f
2
+842
-973
+50.1
3
6
a
b
n-Hexane-insoluble part. Et2O-insoluble part. c Methanol-
d
insoluble part. Estimated by GPC (CHCl3, PSt standards). e c )
0.082-0.050 (g/dL) in CHCl3. f ee ) 0%.
1.00-1.17 (OCdOCC(CH3)2), 1.58-2.20 (CCH2CH2C), 2.58-
3.22 (CCH2CH2C), 3.80-4.42 (CHdCCH2), 5.88-6.22 (NH),
7.52-7.81 (CHdC).
Mea su r em en ts. Molecular weights and molecular weight
distributions of polymers were estimated by GPC (Shodex KF-
850L columns) calibrated by using standard polystyrenes in
chloroform solution. NMR spectra were recorded on a J EOL
EX-400 spectrometer. IR spectra were obtained with a Shi-
madzu FTIR-8100 spectrophotometer. UV-vis spectra were
recorded on a J ASCO V-500 spectrophotometer. Optical rota-
tion was measured with a J ASCO 600 spectropolarimeter. CD
spectra were recorded on a J ASCO V-530 spectropolarimeter.
1: mp 30-31 °C; [R]D ) +13.0° (c ) 1.84 g/dL in CHCl3).
IR (in CHCl3): 3455, 2971, 1673, 1505, 1217, 791 cm-1 1H
.
NMR (CDCl3): δ 0.91 (t, 3H, J ) 7.32 Hz), 1.14 (d, 3H, J )
6.84 Hz), 1.45 (m, 1H), 1.68 (m, 1H), 2.16 (m, 1H), 2.23 (t, 1H,
J ) 2.44 Hz), 4.06 (dd, 2H, J ) 1.46, 2.44 Hz), 5.91 (s, 1H).
13C NMR (CDCl3): δ 11.79, 17.22, 27.24, 28.99, 42.81, 71.33,
79.73, 176.21. Anal. Calcd for C8H13NO: C, 69.06; H, 9.35; N,
10.1. Found: C, 68.83; H, 9.15; N, 9.42.
1 (racemate): yield 65%; mp 30-31 °C. IR (in CHCl3): 3455,
3013, 1670, 1507, 1215, 781 cm-1. 1H NMR (CDCl3): δ 0.88 (t,
3H, J ) 7.32 Hz), 1.12 (d, 3H, J ) 6.84 Hz), 1.42 (m, 1H), 1.63
(m, 1H), 2.13 (m, 1H), 2.20 (d, 1H, J ) 2.44 Hz), 4.03 (dd, 2H,
J ) 1.46, 2.44 Hz), 5.94 (s, 1H). 13C NMR (CDCl3): δ 11.02,
17.24, 27.24, 28.99, 42.86, 71.44, 79.71, 176.10. Anal. Calcd
for C8H13NO: C, 69.06; H, 9.35; N, 10.1. Found: C, 68.78; H,
9.41; N, 10.0.
Resu lts a n d Discu ssion
P olym er Syn th esis. The polymerization of N-pro-
pargylamides with Rh catalyst gives polymers with high
stereoregularity (cis).7 Thus, polymerization of 1-3 was
conducted with (nbd)Rh+[η6-C6H5B-(C6H5)3] in THF.
Monomer 6 was also polymerized in a similar way for
comparison because the ester groups give similar steric
effects to amide groups but cannot give rise to hydrogen
bonding. The results of the polymerization are listed in
Table 1. Polymers with moderate molecular weights (Mn
) 6000-32 000) were obtained in good yields. All of the
polymers displayed unimodal GPC chromatograms,
which means the presence of a single propagation
2: yield 57%; mp 57-58 °C; [R]D ) +33.4° (c ) 1.90 g/dL in
CHCl3). IR (in CHCl3): 3422, 3310, 3013, 1673, 1514, 1219,
1073, 791 cm-1 1H NMR (CDCl3): δ 1.87 (m, 2H), 2.04 (m,
.
1H), 2.22 (m, 1H), 2.22 (s, 1H), 3.86 (t, 1H, J ) 7.81 Hz), 3.93
(t, 1H, J ) 6.83 Hz), 4.03 (m, 2H), 4.34 (t, 1H, J ) 2.44 Hz),
6.86 (s, 1H). 13C NMR (CDCl3): δ 25.01, 25.46, 28.59, 30.01,
69.43, 71.48, 78.28, 172.96. Anal. Calcd for C8H10NO2: C,
62.75; H, 7.19; N, 9.15. Found: C, 62.57; H, 7.25; N, 9.02.
3: yield 78%; mp 102-103 °C; [R]D ) -21.2° (c ) 1.92 g/dL
in CHCl3). IR (in CHCl3): 3436, 3013, 1786, 1682, 1526, 1397,
1221, 1063 cm-1. 1H NMR (CDCl3): δ 0.86 (s, 3H), 1.08 (s, 6H),
1.66 (t, 1H, J ) 8.78 Hz), 1.90 (m, 2H), 2.24 (d, 1H, J ) 2.45
Hz), 2.50 (p, 1H, J ) 11.7 Hz), 6.27 (s, 1H). 13C NMR (CDCl3):
δ 9.59, 16.39, 16.60, 28.72, 28.92, 30.15, 54.03, 55.18, 71.92,
78.70, 92.21, 166.70, 177.99. Anal. Calcd for C13H17NO3: C,
66.38; H, 7.23; N, 5.96. Found: C, 66.25; H, 7.27; N, 5.90.
(Co)p olym er iza tion P r oced u r es. A THF solution of the
monomers ([M]total ) 2 M) was added to a THF solution of
(nbd)Rh+[η6-C6H5B-(C6H5)3] ([monomer]/[cat] ) 100) under dry
nitrogen, and the solution was kept at 30 °C for 1 h. The
reaction solution was poured into a large amount of hexane,
methanol, or diethyl ether to precipitate polymers. The result-
ing polymers were dried under reduced pressure.
1
species. The H NMR spectra of the resulting polymers,
poly(1)-poly(3) and poly(6), showed the olefinic proton
in the main chain around 6 ppm. By comparison of the
integrated intensity of the other protons, the content of
the cis structure was estimated to be 100% for poly(1)
1
and poly(2) and 92% for poly(6). The H NMR spectra
of poly(3) showed very broad signals for the protons of
the main chain, which inhibited accurate estimation of
the cis content. The peak broadening results because
the bulky pendant groups decrease the mobility of the
main chain. Therefore, a few drops of CD3OD were
added to the CDCl3 solution of poly(3) (ca. 50 µL), and
1
the measurement of H NMR was conducted at 60 °C.
The signal of the main-chain proton became sharp under
these conditions, and the cis content of poly(3) was
proven to be 100%.
Secon d a r y Con for m a tion . We previously demon-
strated that the copolymerization of 4 with 5 shows no
chiral amplification phenomenon.8 Specifically, the opti-
cal rotation ([R]D) of the copolymers of 4 with 5 is
smaller than that calculated linearly from the feed ratio
of 4 to 5, when the feed content of 5 is less than 40 mol
%. In contrast, a positive nonlinear relationship between
the optical rotation and the feed content of the chiral
monomer is observed in the copolymerization of 5 with
7. This means that monomer 7, which has a branch at
the â-position, gives a polymer with long persistence
length of the helical domain. From these results, we
concluded that poly(N-propargylamides) having a branch
at the R-carbon of the amide group do not take helical
conformation, at least, at ambient temperature. How-
ever, as shown in Figure 1, poly(1)-poly(3), which have
Poly(1) (ee ) 100%). IR (in CHCl3): 3305, 2934, 2342, 1636,
1541, 1460, 1215 cm-1 1H NMR (CDCl3): δ 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 (CHdCCH2),
5.92-6.38 (NH), 7.98-8.45 (CHdC).
Poly(1) (ee ) 0%). IR (in CHCl3): 3852, 3305, 2969, 1636,
1539, 1210, 787 cm-1. 1H NMR (CDCl3): δ 0.65-0.97 (CH2CH3),
0.97-1.20 (CHCH3), 1.20-1.51 (CH2CH3), 1.51-1.78 (CH2-
CH3), 1.98-2.43 (CHCH3), 3.58-4.42 (CHdCCH2), 5.85-6.34
(NH), 7.62-8.43 (CHdC).
Poly(2). IR (in CHCl3): 3569, 2988, 2346, 1655, 1522, 1213,
1
1078 cm-1. H NMR (CDCl3): δ 1.75-1.90 (OCH2CH2), 1.90-
2.08 (OCHCH2), 2.08-2.31 (OCHCH2), 3.52-4.08 (OCH2CH2),
4.08-4.41 (OCHCH2), 5.72-6.08 (NH), 7.42-7.78 (CHdC).
Poly(3). IR (in CHCl3): 3357, 2973, 2361, 1781, 1661, 1530,
1267 cm-1 1H NMR (CDCl3): δ 0.78-1.00 (OCdOCCH3),
.

Macromolecules, Vol. 35, No. 14, 2002
Poly(N-propargylamides) 5407
F igu r e 1. CD spectra of poly(1), poly(2), poly(3), and poly(6)
in CHCl3 at room temperature.
F igu r e 2. Plot of the optical rotation [R]D vs enantiomeric
excess (%) for R/S copolymerization of 1. The [R]D was
measured in CHCl3 at room temperature.
R-branched structures, exhibited intense CD effects in
the UV absorption region of the main chain and large
[R]D. Therefore, poly(N-propargylamides) having chiral
centers at the R-carbon also exist in the helical confor-
mation if the side chain is bulkier than the isopropyl
groups. These results may convince one that the helical
structure of these polymers is stabilized only by the
steric repulsion between the side chains. However, a
poly(propargyl ester) having bulky, R-branched chiral
groups, poly(6), showed weak CD effects (Figure 1) and
small [R]D (Table 1), which means that poly(6) cannot
adopt the helical structure. Thus, not only the steric
repulsion between the side groups but also intra-
molecular hydrogen bonding contributes to stabilization
of the helical conformation.
Sta bility of th e Helica l Con for m a tion . First of all,
we examined the stability of the helical conformation,
i.e., the persistence length of the helical structure, by
means of the R/S copolymerization of monomer 1. The
results of the copolymerization are listed in Table 1 in
the Supporting Information. In CHCl3, a positive non-
linear relationship was observed between the enantio-
meric excess of the monomer and the optical rotation
of the copolymers (Figure 2). Approximately 20% enan-
tiomeric excess was enough to obtain copolymers with
a similar chiroptical property to that of the homopoly-
mer from (+)-1. Therefore, the persistence length of the
R-branched poly(N-propargylamides) is relatively long,
similar to that of the â-branched counterpart.8
F igu r e 3. Variable temperature CD spectra of (a) poly(1), (b)
poly(2), and (c) poly(3) in chloroform.
temperature, poly(2) showed intense CD effects even at
55 °C.12 These results are in contrast to the thermal
instability of sterically less hindered poly(N-propargyl-
amides). For example, poly(7) exists in a randomly coiled
conformation above 50 °C.8 Since the Cotton effects of
poly(5) negligibly changed with increasing tempera-
ture,7 a long alkyl chain is indispensable to improve
thermal stability of the helical conformation for the
â-branched polymer.
Helix In d u ction in P ola r Solven ts. A copolymer
of 5 with 7 (5/7 ) 1/9 in feed) exhibits intense CD and
large [R]D in CHCl3. However, in other solvents such
as methanol, DMF, THF, and toluene, this polymer does
not show CD signals as shown in Figure 4d.8 Therefore,
poly(N-propargylamides) having a branch at the â-car-
bon of the amide group construct the helical structure
only in CHCl3. This is because the helical structure -
of poly(N-propargylamides) is stabilized by the intra-
molecular hydrogen bonds between the pendant amide
groups. The hydrogen bonds are readily broken by
polar solvents, which forces the main chain to take a
randomly coiled conformation. In contrast, poly(1),
poly(2), and poly(3) displayed large molar ellipticity [θ]
and showed large [R]D in various solvents (Figure 4,
Table 2). For example, poly(1) displayed intense CD
effects not only in CHCl3 but also in DMF (Figure 4a).
The CD spectrum of poly(1) in CHCl3 is not completely
mirror-imaged to that in DMF. However, the differences
in the sign of the Cotton effect and the optical rotation
suggest that the screw sense of poly(1) was opposite in
DMF and in CHCl3. Interestingly, a weak but clear
Cotton effect ([θ] ) 9000 deg cm2/dmol) was detected
even in methanol. As with poly(1), poly(2) takes the
The thermal stability of the helical conformation
was next studied, and we found that introduction of
R-branched bulky pendant groups improves the thermal
stability of the helical conformation of poly(N-propargyl-
amides). For example, when the measuring temperature
was raised from 20 to 55 °C, the intensity of the CD
effects of poly(1) and poly(3) only slightly decreased
(Figure 3). Although the intensity of the Cotton effects
of poly(2) decreased to some extent with increasing

5408 Tabei et al.
Macromolecules, Vol. 35, No. 14, 2002
F igu r e 5. UV-vis spectra of poly(1)-poly(4) in CHCl3 at room
temperature.
Ta ble 3. IR Sp ectr a Da ta of Mon om er s 1-4 a n d
P oly(1)-P oly(4)a
compound
amide I (cm-1
)
amide II (cm-1
)
1
2
3
4
1673
1673
1682
1673
1636
1655
1661
1639
1505
1514
1526
1509
1541
1522
1530
1542
F igu r e 4. CD spectra of (a) poly(1), (b) poly(2), (c) poly(3),
and (d) poly(5-co-7) (5/7 ) 1/9 in feed) in various solvents at
room temperature.
poly(1)
poly(2)
poly(3)
poly(4)
Ta ble 2. Op tica l Rota tion of P oly(1), P oly(2), P oly(3), a n d
P oly(5-co-7) (5/7 ) 1/9 in F eed ) in Va r iou s Solven ts
a
In CHCl3 (c ) 48 mM).
[R]D (deg)a
located at 390, 340, and 350 nm, respectively, and their
[θ]max were +38 000, +22 000, and -24 000 deg cm2/
dmol. As with the CD effects, the absolute value of [R]D
decreased in the order of poly(1), poly(3), and poly(2)
(+1610, -973, and +842°, respectively). A similar
behavior was observed when the polarity of the solvent
was changed. As shown in Figure 4b and Table 2, the
magnitude of the CD effects and [R]D increased with
decreasing polarity of the solvents, and simultaneously
the absorption maximum red-shifted. In a similar way,
heating of poly(2) in CHCl3 not only decreased the
intensity of the CD effect but also blue-shifted the CD
band. Such a blue shift was also observed for poly(3),
although the degree of the shift was smaller due to the
very small variation of the CD intensity on heating.
The largely red-shifted absorption of the helical
polymers such as poly(1), poly(5), and poly(7) is appar-
ently extraordinary. For example, a potential energy
calculation of polypropyne suggested that its λmax is
located at 285 nm.13 Even a polyacetylene prepared from
1-hexyne, an aliphatic 1-alkyne with a W catalyst,
exhibits the λmax around 350 nm,14 although the degree
of the main-chain conjugation is larger for W-based
trans-rich polymers than that for Rh- or Fe-based cis
polymers. Helical cis-poly(1-alkynes) prepared with an
Fe catalyst give λmax around 300-320 nm.6b Thus, the
abnormal red shift of helical poly(N-propargylamides)
cannot be explained without taking account of the
influence of the hydrogen-bonded amide groups. To
investigate the nature and extent of hydrogen bonding
of the amide groups, the IR spectra of monomers 1-4
and poly(1)-poly(4) (c ) 48 mM) were measured in
CHCl3 (Table 3). Because the amide II bands of mono-
mers 2 (1514 cm-1) and 3 (1526 cm-1) were shifted to
polymer
poly(1)
poly(2)
poly(3)
toluene CHCl3 THF
DMF methanol H2O
+1610
-755
+400
+179
+355
+842 +663
-973
+219
-970
poly(5-co-7) -13.5 -2060 -15.0 -19.4
-7.27
a
c ) 0.0966-0.0504 g/dL.
helical structure in polar solvents such as methanol,
THF, and DMF (Figure 4b), which is evidenced by the
intense CD effects in these solvents. It is interesting
that poly(2) displayed moderate molar ellipticity (7000
deg cm2/dmol) and optical rotation (+219°) even in
water, which indicates that the polymer takes a helical
conformation in water. Because poly(3) is soluble in only
CHCl3 and toluene, we could not measure its CD spectra
in polar solvents. However, the magnitude of the CD
effects of poly(3) in CHCl3 was almost the same as that
in toluene (Figure 4c), which is a sharp contrast to the
copolymer of 5 with 7 (Figure 4d). The enhanced
stability of the helical conformation in polar solvents is
due to the bulky substituents in the side chain. The
bulky pendant groups can shield the hydrogen bonds
from the solvents, which consequentially stabilizes the
secondary conformation.
E ffect s of H yd r ogen Bon d on t h e Ch ir op t ica l
P r op er ties. We previously demonstrated that the
electronic absorption of the main-chain chromophore
of poly(N-propargylamides) strongly depends on the
conformation.8 When polymers, such as poly(5) and
poly(7), exist in the helical conformation, an absorption
centered at ca. 400 nm is observed. In contrast, ran-
domly coiled polymers such as poly(4) show an absorp-
tion maximum (λmax) at 320 nm.8 In the case of polymers
with R-branched pendant groups, structural change of
the side chain drastically changed the electronic absorp-
tion of the main chain. Figure 5 shows the UV/vis
spectra of poly(1)-poly(4) in CHCl3. The main chain of
poly(4) takes a random conformation as reported.8 The
more red-shifted polymer tended to show more intense
CD effects and a larger absolute value of [R]D in CHCl3.
Specifically, the λmax of poly(1), poly(2), and poly(3) were
the high-frequency region compared with 1 (1505 cm-1
)
and 4 (1509 cm-1), the N-H moieties in monomers 2
and 3 were hydrogen-bonded. The carbonyl frequencies
of the amide groups in these monomers are, however,
located in the non-hydrogen-bonded region, and the
N-propargylamides we have investigated so far do not
hydrogen bond intermolecularly at this concentration.

Macromolecules, Vol. 35, No. 14, 2002
Poly(N-propargylamides) 5409
Sch em e 2
than that of the polymers having chiral centers at the
â-position. While the â-branched polymers can exist in
helical conformation only in CHCl3 and CH2Cl2, the
present polymers can maintain the helix in a variety
of solvents including polar solvents. The optical and
chiroptical properties of the polymers depend on the
hydrogen bonds that are intramolecularly located in the
pendant groups. As the intramolecular hydrogen bond-
ing is disturbed by steric repulsion between the side
chains, polar solvents, and/or cyclic hydrogen bonding
in every repeating unit, the absorption maximum of the
polymer is blue-shifted, which is accompanied by a
decrease in intensity of CD effects and the absolute
value of [R]D.
Therefore, this hydrogen bond is a five-membered cyclic
and is constructed intramolecularly as shown in Scheme
2. With all of the polymers, the amide I bands are
observed in the region of the hydrogen-bonded amide
frequency, which indicates that the amide groups are
intramolecularly hydrogen-bonded between the pendant
groups. There was no difference in amide I frequency
between poly(1) and poly(4). However, the amide I
frequencies of poly(2) and poly(3) were shifted by 19 and
25 cm-1 to high wavenumber, respectively. This means
that the hydrogen bonds between the pendant groups
are weak for poly(2) and poly(3) compared with those
for poly(1) and poly(4). This is probably because the
cyclic hydrogen bonding hinders the hydrogen bonds
between the pendant groups. The bulky side chains in
poly(2) and poly(3) may also weaken the hydrogen bond
between the pendant groups. Poly(2) and poly(3), which
have bulky substituents, displayed blue-shifted absorp-
tions compared with poly(1) as demonstrated in Figure
5.
The following conclusions are provided from these
results. Even if the pendant amide groups form hydro-
gen bonds, poly(N-propargylamides) exist in a randomly
coiled state, when the side chain is not sterically
demanding. The randomly coiled polymers absorb UV
light around 320 nm, which is in good agreement with
a general feature of polymers from monosubstituted
aliphatic acetylenes. However, when appropriate steric
repulsion arises between the pendant groups, the
polymer is folded to the helical conformation, which
is promoted by the formation of well-arranged intra-
molecular hydrogen bonds. The arranged hydrogen
bonds significantly change the electronic state of the
main chain, red-shifting the main-chain absorption.
When the intramolecular hydrogen bonds between the
pendant amide groups are weakened by very bulky side
chains, polar solvents, and/or the cyclic hydrogen bonds,
the effects of the hydrogen bonds on the main-chain
absorption are reduced, resulting in blue shift. The blue
shift of poly(2) with increasing solvent polarity is
explained by the idea that polar solvents disturb the
hydrogen bonds between the pendant groups. In a
similar way, the blue shift observed in the variable
temperature CD spectra of poly(2) and ploy(3) originates
from thermally induced cleavage of the hydrogen bonds.
Con clu sion . Poly(N-propargylamides) having chiral
centers at the R-carbon of the amide groups have proven
to form the helical structure in solution. The helical
conformation of the polymers is thermally more stable
Ack n ow led gm en t. The authors are grateful to
Professor Yoshihiko Ito and Professor Shunsaku
Kimura for permission to use CD spectropolarimeters.
Su p p or tin g In for m a tion Ava ila ble: The results of the
R/S copolymerization of 1. This material is available free of
charge via the Internet at http://pubs.acs.org.
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