Effects of steric repulsion on helical conformation of poly(N- propargylamides) with phenyl groups

Article abstract of DOI:10.1021/ma0492317

N-Propargylamides with one, two, and three phenyl groups at the α-position of the carboxyl group [HC=CCH₂NHCOR; 1, R = C(CH₃)₂C₆H₅; 2, R = CCH ₃(C₆H₅)₂; 3,

Full text of DOI:10.1021/ma0492317

7156
Macromolecules 2004, 37, 7156-7162
Effects of Steric Repulsion on Helical Conformation of
Poly(N-propargylamides) with Phenyl Groups
J ia n p in g Den g, J u n ich i Ta bei, Ma sa sh i Sh iotsu k i, F u m io Sa n d a , a n d
Tosh io Ma su d a *
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura Campus, Kyoto 615-8510, J apan
Received April 21, 2004; Revised Manuscript Received J uly 13, 2004
ABSTRACT: N-Propargylamides with one, two, and three phenyl groups at the R-position of the carboxyl
group [HC≡CCH2NHCOR; 1, R ) C(CH3)2C6H5; 2, R ) CCH3(C6H5)2; 3, R ) C(C6H5)3] were polymerized
with a rhodium catalyst, (nbd)Rh+B-(C6H5)4 (nbd ) 2,5-norbornadiene), to obtain the corresponding
polymers in 85-91% yields. Poly(1) possessed a moderate molecular weight (Mn ) 6300) and was
thoroughly soluble in chloroform and dichloromethane. On the other hand, poly(2) and poly(3) were not
totally soluble in the solvents. The Mn’s of chloroform-soluble parts were less than 3000. The secondary
structure of these three polymers in chloroform was examined by UV-vis spectroscopy with varying
temperature. It was found that only poly(1) could adopt helical conformation even at 60 °C. By the
copolymerization of either monomer 2 or 3 with HC≡CNH2CO(CH2)4H (4), the solubility of the polymers
was effectively improved, and the Mn’s were remarkably increased. When the content of unit 4 in poly-
(2-co-4)s was 25% and above, the copolymers could form helical conformation with different degrees,
among which poly(20.40-co-40.60) showed the largest helicity. When the content of unit 4 of poly(3-co-4)s
exceeded 95%, the copolymer took helical structure partly.
Sch em e 1
In tr od u ction
Biomacromolecules such as DNA and proteins form
the structural basis of life and often adopt helical
conformation.1 The molecular information contained by
nucleic acid bases and amino acids ranging from the
sequence and hydrophilicity to the chain helicity plays
significant roles in determining the high-order structure
of biomacromolecules. In addition, the conformational
transition of proteins is one of the key factors controlling
the activities of living organisms. Stimulated by this
elegant strategy of the nature, there has been a con-
tinuously growing interest in synthetic polymers that
are able to form ordered secondary structure in recent
years.2 Meanwhile, large progress in polymer chemistry
makes designing and synthesizing polymers possible,
which have the potential to take ordered secondary
structure. However, success has been achieved so
far only in a limited number of polymers including
polymethacrylates,2b,3 polyisocyanides,4 polyisocyan-
ates,5 polysilanes,6 polyaldehydes,7 polychlorals,8 and
polyacetylenes.9 Apart from these polymers, several
assembly systems10 can adopt a helical conformation.
Yashima et al.,9a-c Tang et al.,9d,e Aoki et al.,9f and
we11,12 found that polyacetylenes with appropriate sub-
stitutents can adopt helical conformation under certain
conditions. Specifically, poly(propiolic esters)11 form a
helix due to steric repulsion between the bulky pendent
groups as the predominant driving force. On the other
hand, poly(N-propargylamides)12 form a helix based on
hydrogen bonds intramolecularly formed between the
amide groups in side chains. Additionally, we have
recently proposed that steric repulsion should be an-
other key factor deciding the possibility for poly(N-
propargylamides) to adopt stable helical conformation.13a,b
In this article, we report the synthesis of novel poly(N-
propargylamides) bearing one, two, and three phenyl
groups at the R-position of the carbonyl group to clarify
the effect of bulkiness of the side chains on the higher
order structure of the main chains (Scheme 1).
Exp er im en ta l Section
Mea su r em en ts. Melting points (mp) were measured by a
Yanaco micro melting point apparatus. IR spectra were
1
recorded with a Shimadzu FTIR-8100 spectrophotometer. H
and 13C NMR spectra were recorded on a J EOL EX-400
spectrometer. Elemental analysis was carried out at the Kyoto
University Elemental Analysis Center. Molecular weights and
molecular weight distributions of the (co)polymers were de-
termined by GPC (Shodex KF-850 column) calibrated by using
polystyrene as standards and CHCl3 as an eluent. UV-vis
spectra were recorded on a J ASCO J -820 spectropolarimeter.
Ma ter ia ls. THF as polymerization solvent was distilled by
the usual method. Propargylamine (TCI), n-butyric acid (TCI),
2-phenylisobutyric acid (TCI), 2,2-diphenylpropionic acid (TCI),
triphenylacetic acid (TCI), thionyl chloride (Wako), and pyri-
dine (Wako) were used as received. The (nbd)Rh+B-(C6H5)4
catalyst was prepared as reported.14
Mon om er Syn th esis. Monomer 4 was synthesized accord-
ing to the method reported in a previous article.13a Monomers
1-3, which were new compounds, were synthesized according
to the method introduced earlier.12a,13c Now, taking the syn-
thesis of monomer 1 as an example, the synthetic procedures
* Corresponding author: Tel +81-75-383-2589; Fax +81-75-383-
2590; e-mail masuda@adv.polym.kyoto-u.ac.jp.
10.1021/ma0492317 CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/25/2004

Macromolecules, Vol. 37, No. 19, 2004
Poly(N-propargylamides) 7157
Ta ble 2. Solu bility of P oly(1)-P oly(3)a
Ta ble 1. P olym er iza tion of Mon om er s 1-3a
c
c
polymer
yieldb (%)
Mn
Mw/Mn
solvent
poly(1)
poly(2)
poly(3)
poly(1)
poly(2)
poly(3)
85
90
91
6300
2.85
CHCl3
CH2Cl2
THF
O
O
0
0
×
0
×
×
0
×
0
×
×
×
×
×
0
×
0
×
×
×
×
×
3000d
2600d
1.81d
2.41d
C6H5Cl
toluene
(CH2Cl)2
DMF
a
With (nbd)Rh+B-(C6H5)4 as catalyst in THF at 30 °C for 1 h.
[M]0 ) 0.60 M; [M]0/[Rh] ) 100. Hexane-insoluble part. c Mea-
sured by GPC (polystyrene as standards, CHCl3 as an eluent).d The
polymer contained CHCl3-insluble part. The data were measured
with CHCl3-soluble part after filtration.
b
DMSO
a
O ) soluble; 0 ) partly soluble; × ) insoluble; the solubility
of a polymer sample (2 mg) in a solvent (1 mL) was checked at
room temperature. THF ) tetrahydrofuran; DMF ) N,N-dimeth-
ylformamide; DMSO ) dimethyl sulfoxide.
are described in detail. 2-Phenylisobutyric acid (5.0 g, 30.0
mmol) was added to thionyl chloride (22.0 mL, 300.0 mmol),
and then the resulting solution was refluxed for 3 h. The
residual thionyl chloride was removed by rotary pervaporation,
ether (200 mL) was added to the residue, and then pyridine
(7.8 mL, 90.0 mmol) and propargylamine (6.6 mL, 90.0 mmol)
were added sequentially to the solution. The solution was
stirred at 0 °C for 2 h, and then white precipitate formed was
filtered off. The filtrate was washed with 2 M aqueous HCl
three times and then with saturated aqueous NaHCO3 to
neutralize the solution. Afterward, the solution was dried over
anhydrous MgSO4, filtered, and concentrated to give the target
monomer. The crude monomer was purified twice by flash
column chromatography on silica gel (hexane/AcOEt ) 2.5/1,
v/v). Monomers 2 and 3 were prepared in the same way using
the corresponding carboxylic acids. The analytical and spec-
troscopic data of monomers 1-3 were as follows:
Monomer 1: yield 61%, colorless crystal, mp 83-84 °C. IR
(KBr): 3294 (H-N), 2360 (H-C≡), 1653 (CdO), 1495, 704
(phenyl), 1529, 1279, 1184, 1008, 654, 540 cm-1 1H NMR
.
(CDCl3, 400 MHz, 20 °C): δ 1.55 [s, 6H, -CPh(CH3)2], 2.10 (s,
1H, CH≡C), 3.93 (s, 2H, CH≡CCH2), 5.29 (broad s, 1H, NH),
7.15-7.20 (m, 5H, Ph-H).13C NMR (CDCl3, 100 MHz, 20 °C):
δ 26.97 (×2), 29.44, 46.90, 71.28, 79.51, 126.33 (×2), 127.05,
128.69 (×2), 144.58, 176.98. Anal. Calcd for C13H15NO: C,
77.58; H, 7.51; N, 6.96. Found: C, 77.68; H, 7.65; N, 6.93.
Monomer 2: yield 55%, white solid, mp 90-91 °C. IR (KBr):
3242 (H-N), 2112 (H-C≡), 1660 (CdO), 1493, 702 (phenyl),
1269, 1029, 922, 609, 555 cm-1 1H NMR (CDCl3, 400 MHz,
.
20 °C): δ 2.03 (s, 3H, CPh2CH3), 2.17 (s, 1H, CH≡C), 4.03 (s,
2H, CH≡CCH2), 5.63 (broad s, 1H, NH), 7.30-7.50 (m, 10H,
Ph-H).13C NMR (CDCl3, 100 MHz, 20 °C): δ 26.95, 29.64,
56.76, 71.47, 79.31, 127.02 (×2), 128.01 (×4), 128.50 (×4),
144.54 (×2), 174.74. Anal. Calcd for C18H17NO: C, 82.10; H,
6.51; N, 5.32. Found: C, 82.03; H, 6.57; N, 5.27. Monomer 3:
yield 57%, white solid, mp 130-131 °C. IR (KBr): 3285 (H-
N), 2380 (H-C≡), 1668 (CdO), 1491, 704 (phenyl), 1449, 1226,
1
1035, 748, 648, 561 cm-1. H NMR (CDCl3, 400 MHz, 20 °C):
δ 2.14 (s, 1H, CH≡C), 4.03 (s, 2H, CH≡CCH2), 5.93 (broad s,
1H, NH), 7.20-7.35 (m, 15H, Ph-H).13C NMR (CDCl3, 100
MHz, 20 °C): δ 29.80, 67.41, 71.56, 79.18, 127.05 (×3), 127.89
(×6), 130.41 (×6), 142.95 (×3), 173.04. Anal. Calcd for
C23H19NO: C, 84.89; H, 5.89; N, 4.30. Found: C, 84.77; H, 6.09;
N, 4.26.
P olym er iza tion a n d Cop olym er iza tion . (Co)polymeriza-
tions were carried out with (nbd)Rh+B-(C6H5)4 as a catalyst
in dry THF at 30 °C for 1 h under the following conditions:
[monomer]0 ) 0.60 M, [monomer]0/[catalyst] ) 100. After
polymerization, the reaction mixture was poured into a large
amount of hexane and then (co)polymer precipitated out. Then,
the (co)polymer was filtered and dried under reduced pressure.
In the case of copolymerization, the total monomer concentra-
tion was kept 0.60 M, and the other conditions were the same
as those for the homopolymerization.
F igu r e 1. UV-vis spectra of poly(1)-poly(3) measured in
CHCl3 (c ) 0.10 mM). The data of poly(2) and poly(3) were
approximate because they were partly (30%) insoluble in
CHCl3.
yield (85%). On the other hand, poly(2) and poly(3)
contained chloroform-insoluble parts, and so only the
molecular weights of chloroform-soluble parts (ca. 70%)
could be determined as 2600-3000. It is considered that
the chloroform-insoluble parts were polymers with
higher molecular weights. Poly(1) completely dissolved
in chloroform and dichloromethane and partly dissolved
in THF, chlorobenzene, and 1,2-dichloroethane; how-
ever, poly(2) and poly(3) were only partly soluble in
Molecu la r Mech a n ics Ca lcu la tion s. All the calculations
were carried out using MMFF94 force field using Spartan’04
Windows version 1.01.
Resu lts a n d Discu ssion
P olym er Syn th esis. Table 1 shows that monomer 1
underwent polymerization smoothly to give the polymer
with a moderate molecular weight (Mn ) 6300) in a high

7158 Deng et al.
Macromolecules, Vol. 37, No. 19, 2004
F igu r e 2. Relationship between the energy and dihedral
angle at the single bond of the main chain (ψ) of the 10-mer
of 1. The calculation was carried out by molecular mechanics
using the MMFF94 force field.
chloroform and THF, while insoluble in other solvents
(Table 2). Precipitates of polymers were observed during
the polymerization of 2 and 3, especially monomer 3.
1
In addition, the three polymers exhibited very broad H
NMR signals around 6 ppm in chloroform-d at 50 °C,
which are assigned to the olefinic protons in the main
chain. This can be explained in terms of the bulky
phenyl groups, which make the polymer main chains
rather stiff and reduce the mobility of the polymer main
chains. In fact, the corresponding 1H NMR signal of
poly(4) was much sharper, whose side chains are steri-
cally less demanding than those of poly(1)-poly(3).
Secon d a r y Str u ctu r e of P oly(1)-P oly(3). Our
previous research has revealed that poly(N-propargyl-
amides) with appropriate substitutents can adopt helical
conformation; moreover, both intramolecularly formed
hydrogen bonds and steric repulsion between pendent
groups are significant factors affecting the stability of
the helix.13 In the current study, the secondary structure
of poly(1)-poly(3) was examined using the soluble parts
in chloroform by recording UV-vis spectra at different
temperatures, which has been proved to be a simple but
effective methodology to investigate the secondary
structure of poly(N-propargylamides).12,13 In the UV-
vis spectra of poly(1) (Figure 1a), only one absorption
peak was observed at about 400 nm, while no absorption
peak appeared at around 320 nm. According to our
earlier studies on the secondary structure of poly(N-
propargylamides),12,13 the absorption peak at 390 nm
demonstrates the formation of a helix, while the peak
at 320 nm corresponds to random coil conformation. It
is therefore concluded that poly(1) takes a helical
conformation in chloroform even at 60 °C, which indi-
cates that the helix is fairly stable at high temperature
compared to those in poly(N-propargyl-n-alkylamides).13
On the other hand, only one absorption peak appeared
around 320 nm in the UV-vis spectra of poly(2) and
poly(3) (Figure 1b). It is concluded that they cannot
adopt helical conformation at the temperature range of
-40 to 60 °C. In the sequence from poly(1) to poly(3),
steric repulsion between the pendent groups becomes
larger, and hence it is assumed that the steric repulsion
is appropriate for poly(1) to adopt a helix but too large
F igu r e 3. Top and side views of the geometry of 10-mer of 1
optimized by MMFF94. The dihedral angle at the single bond
of the main chain (ψ) was fixed at 120°.
in poly(2) and poly(3), which probably hinders the
formation of hydrogen bonds and in turn exerts negative
effects on the stability of the helical conformation.
Con for m a tion a l An a lysis by Molecu la r Mech a n -
ics Ca lcu la tion . Molecular mechanics calculations
were carried out to gain information about the reason
for the difference of higher order structures of poly(1)-
poly(3), using the MMFF94 force field that can take into
account the stabilization effect by hydrogen bonding.
Figure 2 depicts the relationship between the dihedral
angle at the single bond of the main chain (ψ) and the
energy of the 10-mer of 1, whose both end groups are
terminated with hydrogen atoms. The dihedral angle
at the double bond of the main chain was fixed at 0°,
and ψ was varied from 170° to 90° with 10° decrement.
The side chain conformation of the initial geometry was
arranged to form hydrogen bonding between the amide
groups at nth and (n + 2)th units, where the distance
between N-H hydrogen and OdC oxygen atoms of the

Macromolecules, Vol. 37, No. 19, 2004
Poly(N-propargylamides) 7159
F igu r e 4. Effects of added solvents on UV-vis spectra of poly(1)/CHCl3 at 20 °C (c ) 0.10 mM). The total volume of the sample
solution was 10 mL.
eight pairs was set at 1.9 Å. Otherwise, all the other
geometries were optimized. It was confirmed that the
10-mer of 1 with the dihedral angle of 120°-130° is the
most stable among the conformers with ψ ranging from
90° to 170°. The average N-H- - -OdC distance be-
came 1.98 Å after geometry optimization (ψ ) 120°),
which was almost the same as that of the initial
geometry.
possibly to form hydrogen bonds (displayed with green
dotted lines); i.e., the distance between the hydrogen
and oxygen atoms is ranging from 1.6 to 2.1 Å, and the
angle of N-H- - -O is larger than 120°. It clearly
indicates that the hydrogen bonds between the amide
groups at nth and (n + 2)th units contribute to the
formation of helical structure. On the other hand, the
corresponding 10-mers of 2 could form only five hydrogen-
bonding pairs due to the repulsion between the bulky
pendent groups, and that of 3 could form no pair among
the eight pairs of N-H- - -OdC possibly to form
hydrogen bonds. These results support well the as-
sumption that too bulky pendent groups make the
formation of hydrogen bonding disadvantageous.
The stable geometries of the 10-mers of 2 and 3 were
also searched in a manner similar to that of 1. The 10-
mers of 2 and 3 also exhibited the energy minima when
ψ was 120°-130°. It should be noted that the average
interatomic distance between the N-H hydrogen and
OdC oxygen atoms of the 10-mers of 2 and 3 became
as long as 2.12 and 2.72 Å after geometry optimization,
respectively. This result indicates that intramolecular
hydrogen bonds tend to break, especially in the 10-mer
of 3. Large steric repulsion between the side chains
should be responsible for the fission of hydrogen bonds.
It is therefore likely that the bulkiness of the pendent
groups of poly(2) and poly(3) prevents the effective
formation of hydrogen bonds for helix formation, and
they take a random structure instead.
E ffect s of Solven t s on t h e H elices F or m ed in
P oly(1). To further elucidate the stability of the helical
structure of poly(1), several solvents were added to the
chloroform solution, and UV-vis spectra were recorded
as presented in Figure 4. When DMF was added to the
solution, the UV-vis absorption at 390 nm weakened
gradually with the increase of DMF, and a new peak
appeared simultaneously at about 350 nm. It is consid-
ered that DMF destroyed the intramolecular hydrogen
bonds between the amide groups in the side chains to a
certain degree because DMF can form hydrogen bonds
with the amide groups, resulting in the deformation of
helical structure. However, a higher order structure
should remain because the position of the UV-vis
absorption is not at 320 nm (random coil) but at 350
nm, presumably based on another helical structure with
Figure 3 illustrates the 10-mer of 1 after the geometry
was optimized, in which ψ was constrained at 120°.
Although the present polymers consist of equivalent
amount of right- and left-handed helices, here we
assume that poly(1) takes right-handed helical struc-
ture, wherein all the eight pairs of N-H at nth and
OdC at (n + 2)th units are located at the positions

7160 Deng et al.
Macromolecules, Vol. 37, No. 19, 2004
F igu r e 5. Effects of copolymer composition on the UV-vis spectra of poly(2-co-4)s measured in CHCl3 (c ) 0.10 mM) at -45 °C.
Ta ble 3. Cop olym er iza tion of Mon om er s 2 a n d 4a
from helix to random coil, which should stem from the
c
c
loss of intramolecular hydrogen bonds by added metha-
nol. It can be assumed that methanol, a protic solvent,
effectively destroys the intramolecular hydrogen bonds
between the amide groups of the side chains compared
to DMF and THF, aprotic solvents, but the concrete
reason is not clear. Addition of toluene was also exam-
ined to find that it hardly affected the helical conforma-
tion of the polymer (Figure 4d). On the basis of these
results, we can conclude that, even though poly(1)
carries bulky pendent groups, the hydrogen bonds are
affected especially by polar solvents, resulting in the
deformation of helical structure.
4 in monomer feed (mol %)
yieldb (wt %)
Mn
Mw/Mn
0
10
90
97
98
97
100
99
99
100
98
100
100
80
3000
4400
4700
7400
6200
6010
9100
9000
5200
5200
6200
9500
1.81
1.10
1.07
2.03
2.67
2.31
1.68
1.26
3.11
5.09
3.98
2.32
20
25
30
40
50
60
70
80
90
100d
With (nbd)Rh+B-(C6H5)4 as catalyst in THF at 30 °C for 1 h.
a
[M]total ) 0.60 M; [M]0/[Rh] ) 100. Hexane-insoluble part. c Mea-
b
Syn th esis a n d Secon d a r y Str u ctu r e of P oly(2-
co-4)s. Table 3 shows that the copolymerization of
monomers 2 and 4 readily proceeded to give the copoly-
mers in high yields (97-100%). All the copolymers
possessed moderate molecular weights (4400-9100),
and they were totally soluble in chloroform and THF.
sured by GPC (polystyrene as standards, CHCl3 as an eluent).
With (nbd)Rh+B-(C6H5)4 as catalyst in THF at 30 °C for 1 h.
d
[M] ) 1.0 M; [M]0/[Rh] ) 100.
a different pitch (Figure 4a). Addition of THF gave
similar results to the case of DMF as shown in Figure
4b. It is also possible to assume that the UV-vis
absorption peak at 350 nm is caused by aggregation of
the polymers. However, the UV-vis spectra of the
polymer solution were measured again after filtration
to find little difference from those before filtration. Thus,
this idea is denied.
When methanol was added to the chloroform solution
of poly(1), the UV absorption at 390 nm gradually
weakened and then totally disappeared; in the mean-
time, absorption at 320 nm gradually increased (Figure
4c), which is different from the cases of DMF and THF.
This means that conformational transition took place
1
The H NMR signals assigned to the olefinic protons in
the main chain became sharp gradually along with
increasing the content of unit 4; however, the signals
were still quite broad compared to that of poly(4). Hence,
the steric repulsion caused by the pendent groups in
monomer 2 may be still large even in the copolymers,
and the flexibility of the polymer chain is low.
The UV-vis spectra of poly(2-co-4)s were measured
in chloroform. In Figure 5, the spectra of the copolymers
containing 20 and below, 25, 60, and 90% of unit 4 are
presented as examples. When the content of unit 4 was
20% or smaller, only one absorption peak appeared at

Macromolecules, Vol. 37, No. 19, 2004
Poly(N-propargylamides) 7161
F igu r e 6. Temperature dependence of UV-vis spectra of poly-
(2-co-4)s measured in CHCl3 (c ) 0.10 mM). T1 and T2 are the
temperatures at which the coil-to-helix transition begins and
ends, respectively.
F igu r e 7. Effects of copolymer composition on the relative
intensity of ꢀ at 390 nm of poly(2-co-4)s determined by UV-
vis spectroscopy measured in CHCl3 (c ) 0.10 mM) at -45 °C.
Ta ble 4. Cop olym er iza tion of Mon om er s 3 a n d 4a
4 in monomer feed
c
c
(mol %)
yieldb (wt %)
Mn
Mw/Mn
0d
20d
40
91
99
2600
9300
2.41
2.27
1.27
2.42
2.73
2.86
2.51
2.27
97
22500
11000
11700
10000
9400
60
98
100
97
98
80
80
90
95
100e
9500
a
With (nbd)Rh+B-(C6H5)4 as catalyst in THF at 30 °C for 1 h.
[M]total ) 0.60 M; [M]0/[Rh] ) 100. Hexane-insoluble part. c Mea-
b
sured by GPC (polystyrene as standards, CHCl3 as an eluent).
Partly dissolved in THF and CHCl3. e With (nbd)Rh+B-(C6H5)4
d
as catalyst in THF at 30 °C for 1 h. [M] ) 1.0 M; [M]0/[Rh] ) 100.
around 320 nm above -40 °C, exemplified by the UV-
vis spectra in Figure 5a. This indicates that the copoly-
mers bearing low content of unit 4 exists mainly in
random coil at the temperature, judging from our pre-
vious studies.13 When the content of unit 4 was 25%,
the absorption peak at 320 nm gradually weakened as
the temperature was lowered from 30 to 1 °C, and in
the meantime, an absorption peak appeared at about
390 nm and became stronger as shown in Figure 5b.
This phenomenon became more obvious along with the
increase of unit 4 content in the copolymers and demon-
strated that more copolymer main chains adopted hel-
ical conformation under the examined temperatures.
When the content of unit 4 reached 60%, the absorption
at 390 nm became the strongest (Figure 5c). The absorp-
tion at 390 nm progressively weakened again with
further increasing the content of 4 in the copolymers
(Figure 5d).
F igu r e 8. Effects of copolymer composition on the UV-vis
spectra of poly(3-co-4)s measured in CHCl3 (c ) 0.10 mM).
To understand the results in Figure 5 more clearly,
two temperatures are designated as follows: T1 and T2
are the temperatures at which the random coil-to-helix
transition begins and ends, respectively. Here, it is
necessary to point out that the helix content at T1 is
zero, whereas the helix content at T2 is not necessarily
100% but is a maximum for each (co)polymer under the
given conditions.13 The T1 and T2 of these (co)polymers
are given in Figure 6. The copolymers previously studied
gave maximum T1 and T2 at a certain composition,13b,c
while in the current study, the copolymers with unit 4
smaller than 60% showed almost the same T1 and T2
irrespective of the composition (T1 was about 30 °C and
T2 1 °C). On the other hand, both T1 and T2 drastically
decreased when the content of unit 4 exceeded 70%. It

7162 Deng et al.
Macromolecules, Vol. 37, No. 19, 2004
is proposed that the large steric repulsion between the
crowded pendent groups in unit 2 should be responsible
for these results.
other hand, poly(3-co-4)s could not form stable helical
conformation under the same conditions. Accordingly,
it is concluded that the pendent groups with appropriate
bulkiness are favorable for the polymers to adopt stable
helical conformation, and too bulky pendent groups
exert negative effects on the formation of a stable helix.
Poly(20.40-co-40.60) showed the strongest UV-vis ab-
sorption at 390 nm among the copolymers. Therefore,
relative intensities of ꢀ at 390 nm of the other copoly-
mers to that of poly(20.40-co-40.60) were determined
taking this copolymer as a standard based on the earlier
calculation methods,13b,c and the results are presented
in Figure 7. The relative intensities of ꢀ at 390 nm in
the copolymers linearly increased with increasing the
content of unit 4 up to 60%, and then they gradually
decreased again as the unit 4 content was raised
further. It is suggested from Figure 7 that small steric
repulsion between the pendent groups leads to unstable
helix in poly(4), whereas too bulky pendent groups may
prevent the formation of hydrogen bonds and thus place
negative influence on the formation of stable helices
[poly(2)]. Poly(20.40-co-40.60) achieves well the balance
between the two types of pendent groups, and therefore
this copolymer forms a stable helix.
Syn th esis a n d Secon d a r y Str u ctu r e of P oly(3-
co-4)s. The copolymerization of monomers 3 and 4 also
underwent satisfactorily to provide the copolymers with
moderate Mn’s (9300-22 500) in high yields (g 97%).
The copolymers showed good solubility in chloroform
except for poly(30.80-co-40.20). However, the cis content
could not be determined by the 1H NMR spectra
measured in chloroform-d due to broadness of olefinic
proton signals.The UV-vis spectra of poly(3-co-4)s were
also measured in chloroform. The copolymers did not
show absorption at 390 nm even though the content of
unit 4 was as large as 80% (Figure 8a). When the
content of unit 4 was 90% or higher (Figure 8b,c), poly-
(3-co-4)s demonstrated some absorption at 390 nm with
lowering temperature. However, the absorption at around
320 nm increased only slightly, even though the absorp-
tion at 390 nm increased and leveled off with decreasing
temperature to -40 °C. These results are quite different
from those in poly(2-co-4)s (Figure 5), which should be
due to the difference of steric repulsion between the
pendent groups in poly(2) and poly(3). Specifically, poly-
(2) possesses large steric repulsion, but the steric
repulsion can be decreased appropriately and effectively
by the copolymerization with monomer 4. On the other
hand, poly(3) possesses so large pendent groups that the
steric repulsion between the pendent groups still re-
stricts the formation of effective hydrogen bonds even
in the copolymers with monomer 4, which is indispen-
sable for the polymers to adopt a stable helix.
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Con clu sion s
Novel N-propargylamides 1-3 with one, two, or three
phenyl groups at the R-position of the carbonyl group
were synthesized. Among the three monomers, mono-
mer 1 with the smallest substitutent smoothly polym-
erized to provide poly(1) with moderate molecular
weight and good solubility in chloroform. However,
monomers 2 and 3 with larger substitutents gave poly-
mers with low solubility. Poly(1) adopted a relatively
stable helix even at high temperatures up to 60 °C,
while poly(2) and poly(3) did not take a helical confor-
mation under the examined conditions. By the copo-
lymerization with N-propargylpentanamide 4, poly(2-
co-4)s could adopt helical conformation when the content
of unit 4 exceeded 25%; poly(20.40-co-40.60) showed the
highest helix content among the copolymers. On the
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