Dynamically stable helices of poly(N-propargylamides) with bulky aliphatic groups

Article abstract of DOI:10.1021/ma049321b

N-Propargylamides with bulky pendent groups [HC≡CCH₂NHCOR, 9: R = CH₂C(CH₃)₃, 10: R=C(CH ₃)₃ 11: R = C(CH₃)₂CH ₂CH₂CH₃,12: R = CH(CH₂CH ₃)₂, 13: R = CH(CH₂CH₂CH ₃)₂] were polymerized with a rhodium catalyst, (nbd)Rh⁺B^-(C₈H₅)₄ (nbd = 2,5-norbornadiene), to obtain the polymers in 80-92% yields. Poly(11) and poly(12) possessed moderate molecular weights (M_n ≥ 10 000) and were totally soluble in a few solvents including chloroform. On the other hand, the M_n values of poly(9), poly(10), and poly(13) were no more than 5000, and these polymers were not completely soluble in any solvents. The conformational transition behavior of these polymers was examined by temperature-variable UV-vis spectroscopy in chloroform solution, which revealed that poly(10) - poly(13) could form dynamically stable helical conformation even at 60°C. By copolymerizations of monomers 9 and 10 with monomer 4, HC≡CCH₂NHCO(CH₂)₄H, the solubility of the polymers was effectively improved and almost all the copolymers totally dissolved in chloroform, while the molecular weights of the copolymers increased up to 18 600-45 000. Moreover, the helix contents of poly(4_0.63-co- 9_0.37) and poly(4_0.40-co-10_0.60) were the highest among the two series of (co)polymers, respectively. It is concluded that the copolymerization of 9 and 10 with 4 effectively decreased the steric repulsion between the crowded side chains, which probably allowed the copolymers to take helical conformation efficiently.

Full text of DOI:10.1021/ma049321b

Macromolecules 2004, 37, 5149-5154
5149
Dynamically Stable Helices of Poly(N-propargylamides) with Bulky
Aliphatic 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, Kyoto 615-8510, J apan
Received April 7, 2004; Revised Manuscript Received May 11, 2004
ABSTRACT: N-Propargylamides with bulky pendent groups [HCtCCH2NHCOR, 9: R ) CH2C(CH3)3,
10: R ) C(CH3)3, 11: R ) C(CH3)2CH2CH2CH3, 12: R ) CH(CH2CH3)2, 13: R ) CH(CH2CH2CH3)2] were
polymerized with a rhodium catalyst, (nbd)Rh+B-(C6H5)4 (nbd ) 2,5-norbornadiene), to obtain the polymers
in 80-92% yields. Poly(11) and poly(12) possessed moderate molecular weights (Mn g 10 000) and were
totally soluble in a few solvents including chloroform. On the other hand, the Mn values of poly(9), poly-
(10), and poly(13) were no more than 5000, and these polymers were not completely soluble in any solvents.
The conformational transition behavior of these polymers was examined by temperature-variable UV-
vis spectroscopy in chloroform solution, which revealed that poly(10)-poly(13) could form dynamically
stable helical conformation even at 60 °C. By copolymerizations of monomers 9 and 10 with monomer 4,
HCtCCH2NHCO(CH2)4H, the solubility of the polymers was effectively improved and almost all the
copolymers totally dissolved in chloroform, while the molecular weights of the copolymers increased up
to 18 600-45 000. Moreover, the helix contents of poly(40.63-co-90.37) and poly(40.40-co-100.60) were the highest
among the two series of (co)polymers, respectively. It is concluded that the copolymerization of 9 and 10
with 4 effectively decreased the steric repulsion between the crowded side chains, which probably allowed
the copolymers to take helical conformation efficiently.
In tr od u ction
can take helical conformation.3 In our preceding study,4
some poly(N-propargylamides) with pendent groups of
various lengths [poly(1)-poly(8) in Scheme 1] were
synthesized, and their secondary structures were in-
vestigated by UV-vis spectroscopy. Poly(N-propargyl-
amides) with pendent groups of medium lengths [(poly-
(5) and poly(6)] could take relatively stable helices at
room temperature.4 In contrast, the other polymers
hardly took helical conformation at room temperature.
Low stereoregularity may be responsible in polymers
with long side chains [poly(7) and poly(8)], whereas the
bulkiness of pendent group does not seem enough for
the formation of helices in polymers with short alkyl
chains [poly(2)-poly(4)].4 The stereoregularity and higher
order structure of poly(1) could not be determined
because of its insolubility.
In recent years, synthetic helical polymers have
drawn growing attention, and a number of significant
achievements have been made in this field.1 Synthetic
helical polymers remind us the stable helical conforma-
tion of naturally occurring biomacromolecules, such as
proteins and DNA. The helical senses of proteins are
generally predetermined by the chiral information
contained in the sequence of amino acids. The helices
of proteins are usually stabilized by intramolecular
hydrogen bonds. Generally speaking, it is unfavorable
for biomacromolecules to take stable helical conforma-
tion from the viewpoint of entropy; nevertheless, the
enthalpy of the formation of hydrogen bonds compen-
sates it. Stimulated by this strategy of nature, polymer
chemists have tentatively designed and synthesized
polymers which can form intramolecular hydrogen
bonding with the hope to obtain polymers which are able
to form stable helices.1 Unfortunately, it still remains
difficult that nonconvalent interactions such as hydro-
gen bonds govern the secondary structure of synthetic
polymers. Indeed, success has been made only in a few
special oligomers and polymers.1
We have designed and synthesized two kinds of
polyacetylenes, poly(propiolic esters)2 and poly(N-pro-
pargylamides),3 which can form dynamically stable
helical conformation under certain conditions. The driv-
ing forces with which they form helices are different
from each other. Steric repulsion contributes to the
formation of helices in the former polymers,2 while in
the latter, the intramolecular hydrogen bonding be-
tween the neighboring amide groups in the side chain
plays a predominant role so that the latter polymers
Nevertheless, all the helices formed in poly(2)-poly-
(8) were more or less unstable at high temperature.
More specifically, all these polymers took random coil
conformation instead of helix upon raising temperature
up to 27 °C and above. Improvement of the stability of
helical structure in these polymers is not only interest-
ing from the viewpoint of synthesis of novel helical
polymers but also important from the viewpoint of
application of the helical polymers to chiral separating
materials. The present study deals with the polymeri-
zation of five N-propargylamides with bulky pendent
groups, 9-13 (Scheme 1), and the elucidation of the
conformational transition behavior of the formed poly-
mers by UV-vis spectroscopy. We also examine the
copolymerizations of either monomer 9 or 10 with
monomer 4, on one hand, to keep the helix-forming
ability of poly(9) and poly(10) and, on the other hand,
to improve the solubility of poly(9) and poly(10) and
increase the molecular weights.
* Corresponding author: Tel +81-75-383-2589; Fax +81-75-383-
2590; e-mail masuda@adv.polym.kyoto-u.ac.jp.
10.1021/ma049321b CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/15/2004

5150 Deng et al.
Macromolecules, Vol. 37, No. 14, 2004
Ta ble 1. P olym er iza tion of Mon om er s 9-13a
Sch em e 1
c
c
monomer
yieldb (%)
Mn
Mw/Mn
9
10
11
12
13
92d
80d
86
4000
3100
10000
19000
2300
1.06
2.57
1.64
3.14
3.24
89
90d
a
With (nbd)Rh+B-(C6H5)4 catalyst in THF at 30 °C for 1 h; [M]0
) 1.0 M; [M]0/[Rh] ) 100. Precipitated in hexane. c Measured by
GPC (polystyrenes as standards; chloroform as eluent). Partly
b
d
soluble in THF (65%) and CHCl3 (70%).
Monomer 10: yield 35%, colorless crystal, mp 58-60 °C. IR
(KBr): 3290 (H-N), 2928, 2360 (H-Ct), 1641 (CdO), 1550,
1
1460, 1261, 1211, 1010, 655, 626, 559 cm-1. H NMR (CDCl3,
400 MHz, 20 °C): δ 1.18-1.58 [-C(CH3)3], 2.20-2.25 (CHt
C), 4.00 -4.05 (CHtCCH2), 5.70-5.80 (NH).13C NMR (CDCl3,
400 MHz, 20 °C): δ 20.79, 27.47, 29.46, 56.24, 71.61, 74.50,
185.56. Anal. Calcd for C8H13NO: C, 69.03; H, 9.41; N, 10.06.
Found: C, 68.82; H, 9.63; N, 9.94. Monomer 11: yield 25%,
colorless liquid, bp 129-131 °C (760 mmHg). IR (KBr): 3292
(H-N), 2370 (H-Ct), 1641 (CdO), 1545, 1425, 1277, 1118,
696, 667, 632, 569 cm-1. 1H NMR (CDCl3, 400 MHz, 20 °C): δ
0.81-0.89 (CH2CH2CH3), 1.12-1.30 [C(CH3)2, CH2CH3), 1.40-
1.50 (CH2CH2CH3), 2.15-2.20 (CHtC), 3.95-4.05(CHtCCH2),
5.80 -5.85 (NH).13C NMR (CDCl3, 400 MHz, 20 °C): δ 14.54,
17.99, 25.24, 29.29, 42.05, 43.59, 71.41, 79.83, 177.47. Anal.
Calcd for C10H17NO: C, 71.81; H, 10.25; N, 8.37. Found: C,
71.87; H, 10.50; N, 8.14. Monomer 13: yield 45%, colorless
crystal, mp 47-49 °C. IR (KBr): 3295 (H-N), 2932 (H-Ct),
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. The molecular weights
and molecular weight distributions of (co)polymers were
determined by GPC (Shodex KF-850 column) calibrated by
using polystyrene as standards and THF or chloroform as an
eluent. IR spectra were recorded with a Shimadzu FTIR-8100
spectrophotometer. Elemental analysis was carried out at the
Kyoto University Elemental Analysis Center. UV-vis spectra
1
were recorded on a J ASCO J -820 spectropolarimeter. H and
13C NMR spectra were recorded on a J EOL EX-400 spectrom-
eter.
Ma ter ia ls. THF as polymerization solvent was distilled by
the usual method. Propargylamine (TCI), tert-butylacetic acid
(TCI), pivalic acid (TCI), 2,2-dimethyl-n-valeric acid (TCI),
2-ethyl-n-butyric acid (TCI), 2-propyl-n-valeric acid (TCI),
thionyl chloride (Wako), pyridine (Wako), isobutyl chlorofor-
mate (Wako), and 4-methylmorpholine (Wako) were used as
received without further purification. The (nbd)Rh+B-(C6H5)4
catalyst was prepared as reported.5
1632 (CdO), 1541, 1232, 1122, 675, 553, 490 cm-1 1H NMR
.
(CDCl3, 400 MHz, 20 °C): δ 0.89-1.05 (CH2CH2CH3), 1.60-
1.65 (CH2CH2CH3), (2.05-2.21 (CHtC, CH2C2H5), 4.03-4.10
(CHtCCH2), 5.55-5.68 (NH).13C NMR (CDCl3, 400 MHz, 20
°C): δ 14.12, 19.02, 20.80, 29.01, 35.19, 47.52, 71.51, 72.37,
186.52. Anal. Calcd for C11H19NO: C, 72.88; H, 10.56; N, 7.73.
Found: C, 72.62; H, 10.76; N, 7.69.
Mon om er Syn th esis. Monomers 4 and 10 were synthe-
sized according to the method reported in the preceding
article.4a Monomers 9 and 11-13 were synthesized according
to the method introduced earlier.3a Monomers, 10, 11, and 13
were new compounds. Now, taking the synthesis of monomers
10 and 11 as examples, the main synthetic procedures of the
two different methods are described. Monomer 10 (Scheme S1
in the Supporting Information): pivalic acid (5.6 mL, 49.0
mmol), isobutyl chloroformate (6.4 mL, 49.0 mmol), and
4-methylmorpholine (5.4 mL, 49.0 mmol) were added to THF
(200 mL) sequentially. The solution was stirred at room
temperature for 10 min, and then propargylamine (3.4 mL,
49.0 mmol) was added to the solution. After 1 h, white
precipitate formed was filtered off, and the filtrate was
collected, to which ethyl acetate (ca. 50 mL) was added to
extract the desired product. The combined solution was washed
with 2 N HCl three times and then washed with saturated
aqueous NaHCO3 to neutralize the solution. Then, the solution
was dried over anhydrous MgSO4, filtered, and concentrated
to give the target monomer. The crude monomer was further
purified by flash column chromatography on silica gel (hexane/
AcOEt ) 2/1, v/v). Monomer 11 (Scheme S2 in the Supporting
Information): 2,2-dimethyl-n-valeric acid (5.0 mL, 35.0 mmol)
was added to thionyl chloride (1.3 mL, 17.5 mmol), and then
the solution was refluxed. After 1 h, ethyl ether (200 mL),
pyridine (2.8 mL, 35.0 mmol), and propargylamine (2.4 mL,
35.0 mmol) were added sequentially to the solution. The
solution was stirred at 0 °C for 2 h, and then the white
precipitate formed was filtered off. The filtrate was washed
with 2 N 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 further
purified twice by flash column chromatography on silica gel
(hexane/AcOEt ) 3/1, v/v). Monomers 9, 12, and 13 were
prepared similarly to monomer 11 from the corresponding
carboxylic acids. The data of monomers 10, 11, and 13 were
as follows:
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 ) 1.0 M, [catalyst] ) 10 mM. After polymerization,
the reaction mixture was poured into a large amount of hexane
to precipitate the formed (co)polymer. Then, the (co)polymer
was filtered off, washed with hexane, and dried under reduced
pressure. In the case of copolymerization, the total monomer
concentration was kept 1.0 M, while other conditions were the
same as for homopolymerization.
Resu lts a n d Discu ssion
Syn t h esis a n d H elica l Con for m a t ion of H o-
m op olym er s. Table 1 shows that monomers 11 and 12
underwent polymerization smoothly to give polymers
having moderate molecular weights (Mn g 10 000) in
good yields (ca. 90%). However, polymerizations of other
three monomers 9, 10, and 13 just provided oligomers
under the same conditions, whose molecular weights
were lower than 5000. The reason should be probably
the low solubility of these oligomers in THF. Poly(12)
dissolved in chloroform, THF, and toluene smoothly,
while poly(11) dissolved totally in chloroform but only
partly in THF and toluene (Table S1 in the Supporting
Information). The solubility of other three oligomers was
not satisfactory; i.e., they only partly dissolved in
chloroform, CH2Cl2, and THF and hardly dissolved in
1,2-dichloroethane, chlorobenzene, o-dichlorobenzene,
toluene, DMF, DMSO, and methanol. The 1H NMR
spectra of the polymers/oligomers recorded in CDCl3 at
50 °C exhibited broad olefinic proton signals, which
prevented us from determining the cis contents of the
main chains.

Macromolecules, Vol. 37, No. 14, 2004
Poly(N-propargylamides) 5151
F igu r e 2. UV-vis spectra of poly(10)-poly(13) (c ) 0.10 mM
measured in CHCl3 at 60 °C). The data of poly(10) and poly-
(13) were approximately determined because they were partly
(30%) insoluble in CHCl3.
maximum. This means that the helix-to-coil transition
was completed at 58 °C and that the polymer predomi-
nantly took a random coil conformation above this
temperature. Figure 1b shows the change of UV-vis
spectrum with decreasing temperature. When the tem-
perature was lowered, the coil-to-helix transition began
at 58 °C and the transition process lasted down to 40
°C. It is concluded from Figure 1 that poly(9) can form
helices at about 40-60 °C higher temperatures than
poly(5) and poly(6) which have been studied in the
preceding articles.4
Figure 2 presents the UV-vis spectra of poly(10)-
poly(13) measured at 60 °C. It should be further
described that all the UV-vis spectra in Figure 2 hardly
changed below 60 °C compared to those at 60 °C.
According to the UV-vis spectra in Figure 2, all these
polymers/oligomers can form helices even at 60 °C,
which is evidenced by the strong absorption peak at 390
nm and the absence of the peak at 320 nm. By compar-
ing these polymers/oligomers with those studied earlier,4
it can be concluded that the steric repulsion between
the bulky pendent groups also make a large contribution
to the formation of helices in addition to the hydrogen
bonding intramolecularly formed between the amide
groups.
Syn th esis a n d Helica l Con for m a tion of P oly(4-
co-9). The polymers/oligomers studied in the present
research are interesting since they can form helices at
relatively high temperature. However, the low solubility
of poly(9), poly(10), and poly(13) is a drawback, resulting
in difficult analysis of their higher order structure by
UV-vis spectroscopy. Aiming at detailed investigation
of the secondary structure of these polymers, copoly-
merization was attempted to improve the solubility of
the polymers.
F igu r e 1. Temperature dependence of UV-vis spectra of poly-
(9) (c ) 0.10 mM measured in CHCl3).
As we had previously reported,3 poly(N-propargyl-
amides) with appropriate pendent groups could take
helical conformation owing to hydrogen bonding in-
tramolecularly formed between the amide groups in the
side chains. Using the part soluble in chloroform, the
secondary structure of the polymers/oligomers synthe-
sized in the current study was investigated by recording
UV-vis spectra at different temperatures, which had
been proved to be a simple and effective method to study
the secondary structure of poly(N-propargylamides).3,4
Applying this UV-vis methodology, we can conclude
that poly(9) exists predominantly in helical conforma-
tion below 36 °C because only one strong absorption
peak was observed at about 390 nm in the UV-vis
spectrum (Figure 1a). When the temperature was raised
to 38 °C, the conformational transition from helix to
random coil began to take place. With further increasing
temperature, the absorption intensity of the peak at 390
nm continued to weaken, while the absorption intensity
at 320 nm became gradually stronger, which reflects the
helix-to-coil transition of the polymer. When the tem-
perature reached 58 °C, the absorption peak at 390 nm
completely disappeared and the peak at 320 nm took a
Copolymerization of monomers 4 and 9 was carried
out, whose results are given in Table 2. These monomers
smoothly copolymerized to provide copolymers with
moderate Mn’s in high yields. The compositions of the
resulting copolymers were determined from their 1H
NMR spectra by calculating the relative peak intensity
of protons in -CdC-CH2- (4 ppm) to all the protons
in the alkyl groups (0.8-2.5 ppm). The compositions of

5152 Deng et al.
Macromolecules, Vol. 37, No. 14, 2004
Ta ble 2. Cop olym er iza tion of Mon om er s 4 a n d 9a
monomer feed yieldb
4 in copolymerd
cisd
(%)
c
c
(4/9, mol/mol)
(%)
Mn
Mw/Mn
(mol %)
0/100
20/80
40/60
60/40
80/20
100/0
92e
93e
90
89
87
4000
7600
16000
18600
9200
1.06
8.41
1.99
1.38
2.11
2.32
0
21
42
63
79
n.d.f
100
100
99
100
98
80
9500
100
a
With (nbd)Rh+B-(C6H5)4 catalyst in THF at 30 °C for 1 h; [M]0
) 1.0 M; [M]0/[Rh] ) 100. Precipitated in hexane. c Measured by
GPC (polystyrenes as standards; chloroform as eluent except for
b
d
the monomer feed 4/9 ) 100/0, where THF was used). Deter-
mined by 1H NMR in CDCl3 at 50 °C. e Partly soluble in THF and
CHCl3. f Cis content could not be determined because the olefinic
proton signal was too broad.
the copolymers were almost identical to those of the
corresponding monomer feeds. The solubility of the
copolymer containing 21% unit 4 was not effectively
improved compared to the homopolymer of 9; i.e., it
dissolved only partly in common organic solvents. In
contrast, when the content of monomer 4 was 42% and
above, the solubility of the copolymers was satisfactory.
The copolymers had high stereoregularity; namely, their
cis contents were almost quantitative.
The secondary structure of these (co)polymers was
investigated by UV-vis spectroscopy. The UV-vis
absorption at 390 nm was the strongest in poly(40.63
-
co-90.37) among the copolymers of 4 and 9. The UV-vis
spectrum of poly(40.63-co-90.37) is presented in Figure 3
for the sake of comparison with those of poly(9) (Figure
1) and poly(4).4 To discuss the conformational transition
behavior of polymers, several temperatures are defined
as follows: T1 and T2 are the temperatures at which
the coil-to-helix transition begins and ends, and T3 and
T4 are the temperatures at which the helix-to-coil
transition begins and ends, respectively. The helix
content at T1 and T4 is zero, whereas the helix content
at T2 and T3 is not necessarily 100% but is a maximum
for each (co)polymer under the given conditions. T1 and
T2 of these (co)polymers are given in Figure 4a and T3
and T4 in Figure 4b. Figure 4a indicates that poly(4)
begins to form helical conformation at -13 °C and that
this coil-to-helix transition process lasts down to -40
°C. When temperature was raised again, the polymer
reversibly underwent helix-to-coil transition (Figure 4b).
This transition process ranged from -37 °C (T3) to -11
°C (T4).4a Poly(9) started to undergo a helix-to-coil
transition at 36 °C, which ended at 58 °C. It showed
coil-to-helix transition in the temperature range from
60 to 40 °C (Figure 1). In the copolymers composed of
monomers 4 and 9, all of T1-T4 were located between
those of the corresponding homopolymers. Take poly-
(40.63-co-90.37) for example. T1-T4 were 20, -14 (Figure
3b), -12, and 24 °C (Figure 3a), respectively.
F igu r e 3. Temperature dependence of UV-vis spectra of poly-
(40.63-co-90.37) (c ) 0.10 mM measured in CHCl3).
easily compared to both of the homopolymers, poly(4)
and poly(9). It is likely that the bulkiness of the pendent
group of poly(4) is not enough to induce a helix ef-
ficiently. On the other hand, it is assumed for poly(9)
as follows: the pendent groups may be too bulky, and
hence the large steric repulsion between the pendent
groups possibly will have an adverse effect on the
formation of hydrogen bonding that generates a stable
helix. It seems that an exquisite balance for the forma-
tion of helical structure is satisfactorily achieved in the
copolymer poly(40.63-co-90.37) to provide the helical struc-
ture more readily than both poly(4) and poly(9) do.
Thermodynamic parameters, i.e., ∆H (change in en-
thalpy), ∆S (change in entropy), and ∆G (change in free
energy), for the coil-to-helix transition of these (co)-
polymers were determined by the method described
earlier,4 and the data are deployed in Figure 6. Both
∆H and ∆S became larger in negative sign with in-
creasing unit 4 in the copolymers, and they took
maximum values at 63% of unit 4 and 37% of 9. This
finding agrees well with the results observed in Figure
5, where poly(40.63-co-90.37) showed a maximum UV-
vis absorption. Thus, it is concluded that the formation
of the helix is accompanied by large decreases in ∆H
Among these (co)polymers, poly(40.63-co-90.37) showed
the largest UV-vis absorption at 390 nm (Figure 3).
Hence, the UV-vis absorption intensities of other (co)-
polymers relative to that of this copolymer [poly(40.63
-
co-90.37)] were calculated by the method employed in the
preceding articles,4 whose result is depicted in Figure
5. The relative intensities of poly(4) and poly(9) were
52% and 63%, respectively. With increase of the content
of unit 4 in the copolymers, the relative intensity of
ꢀ390 nm of the copolymers gradually increased, and then
it drastically decreased when the content of unit 4 in
the copolymer exceeded 63%. These results imply that
the copolymers could form helical conformation more

Macromolecules, Vol. 37, No. 14, 2004
Poly(N-propargylamides) 5153
F igu r e 6. Plots of ∆H, ∆S, and ∆G (at -45 °C) vs composition
of poly(4-co-9)s.
Ta ble 3. Cop olym er iza tion of Mon om er s 4 a n d 10a
monomer feed
(4/10, mol/mol)
yieldb (%)
Mn
Mw/Mn
cisd (%)
c
c
0/100
20/80
30/70
40/60
50/50
60/40
80/20
100/0
80e
100e
98
3 100
20 000
33 000
45 000
19 400
15 000
11 250
9 500
2.57
1.37
1.93
1.88
2.89
3.19
4.03
2.32
n.d.f
n.d.f
n.d.f
n.d.f
100
100
100
98
99
100
100
96
80
a
With (nbd)Rh+B-(C6H5)4 catalyst in THF at 30 °C for 1 h; [M]0
) 1.0 M; [M]0/[Rh] ) 100. Precipitated in hexane. c Measured by
GPC (polystyrenes as standards; THF as eluent except for the
b
F igu r e 4. Temperature dependence of conformational transi-
tion of poly(4-co-9)s (c ) 0.10 mM measured in CHCl3). T1 and
T2 are the temperatures at which the coil-to-helix transition
begins and ends, respectively; T3 and T4 are the temperatures
at which the helix-to-coil transition begins and ends, respec-
tively.
d
monomer feed 4/10 ) 100/0, where THF was used). Determined
by 1H NMR in CDCl3 at 50 °C. e Partly soluble in THF and CHCl3.
f Cis content could not be determined because the olefinic proton
signal was too broad.
F igu r e 7. Effect of copolymer composition on the relative
intensity of ꢀ at 390 nm of poly(4-co-10)s determined by UV-
vis spectroscopy measured in CHCl3 (c ) 0.10 mM) at -45 °C.
In the parentheses, the temperatures at which the (co)-
polymers began to take the helix-to-coil transition are shown.
F igu r e 5. Effect of copolymer composition on the relative
intensity of ꢀ at 390 nm of poly(4-co-9)s determined by UV-
vis spectroscopy measured in CHCl3 (c ) 0.10 mM) at -45 °C.
and ∆S, which seems to be reasonable from the view-
lower than 50%, the cis contents could not be deter-
mined because the signals assigned to the olefinic
protons were very broad.
point of thermodynamics. In addition, poly(40.63-co-90.37
)
gave the largest ∆G with negative sign, providing the
evidence for reasoning that poly(40.63-co-90.37) formed the
most stable helix among the assessed (co)polymers.
The secondary structure of the copolymers was ex-
amined by UV-vis spectroscopy measured in chloro-
form. The (co)polymers hardly underwent a helix-to-coil
transition below 60 °C, indicating that the helices of
these (co)polymers are dynamically stable at this tem-
perature range. The relative intensities of UV-vis
absorption at 390 nm of the (co)polymers at -45 °C are
plotted against the compositions of the (co)polymers
using the value of poly(40.40-co-100.60) as a standard
(Figure 7). In the figure, the temperatures at which the
(co)polymers start undergoing a helix-to-coil conforma-
Syn th esis a n d Helica l Con for m a tion of P oly(4-
co-10). Table 3 shows that the copolymerizations of
monomers 4 and 10 also proceed smoothly to afford the
corresponding copolymers with moderate Mn’s in high
yields. When the content of unit 4 in the copolymers
was 30% and above, the copolymers totally dissolved in
chloroform. When the content of unit 4 in the copolymer
exceeded 50%, the cis contents of the copolymers were
quantitative. However, when the content of unit 4 was

5154 Deng et al.
Macromolecules, Vol. 37, No. 14, 2004
low solubility of the products. Poly(10)-poly(13) could
form helices even at 60 °C. By copolymerization of either
9 or 10 with 4 which contains linear alkyl group, the
solubility of the copolymers could be improved ef-
fectively. In addition, the composition and temperature
dependence of the conformational transition behavior
of these copolymers were clearly observed. Poly(40.63
-
co-90.37) and poly(40.40-co-100.60) exhibited the highest
helix contents among poly(4-co-9)s and poly(4-co-10)s,
respectively. Cooperational effects of hydrogen bonding
and steric repulsion were observed for the process of
helix formation.
Su p p or tin g In for m a tion Ava ila ble: Solubility of poly-
(9)-poly(13) and synthetic schemes of monomers 10 and 11.
This material is available free of charge via the Internet at
http://pubs.acs.org.
F igu r e 8. Effect of copolymer composition on the ratio of the
ꢀ390 nm values at 60 and -45 °C of poly(4-co-10)s determined
by UV-vis spectroscopy measured in CHCl3 (c ) 0.10 mM).
tional transition upon raising temperature are shown
in parentheses. We can observe similar results to those
in Figure 5. The temperature dependence of the helix-
to-coil conformational transition of these (co)polymers
was similar to that in Figure 4b.
Refer en ces a n d Notes
(1) For the synthetic helical polymers, see: (a) Yashima, E.;
Maeda, K.; Nishimura, T. Chem.sEur. J . 2004, 10, 42-51.
(b) Nakano, T.; Okamoto, Y. Chem. Rev. 2001, 101, 4013-
4038. (c) Green, M. M.; Park, J .-W.; Sato, T.; Teramoto, A.;
Lifson, S.; Selinger, R. L. B.; Selinger, J . V. Angew. Chem.,
Int. Ed. 1999, 38, 3138-3154. (d) Rowan, A. E.; Nolte, R. J .
M. Angew. Chem., Int. Ed. 1998, 37, 63-68. (e) Pu, L. Acta
Polym. 1997, 48, 116-141. (f) Okamoto, Y.; Nakano, T. Chem.
Rev. 1994, 94, 349-372. (g) Nolte, R. J . M. Chem. Soc. Rev.
1994, 11-19.
(2) (a) Nakako, H.; Nomura, R.; Tabata, M.; Masuda, T. Macro-
molecules 2001, 34, 1496-1502. (b) Nomura, R.; Fukushima,
Y.; Nakako, H.; Masuda, T. J . Am. Chem. Soc. 2000, 37,
8830-8831. (c) Nakako, H.; Mayahara, Y.; Nomura, R.;
Tabata, M.; Masuda, T. Macromolecules 2000, 33, 3978-
3982. (d) Nakako, H.; Nomura, R.; Tabata, M.; Masuda, T.
Macromolecules 1999, 32, 2861-2864.
(3) (a) Tabei, J .; Nomura, R.; Sanda, F.; Masuda, T. Macromol-
ecules 2003, 36, 8603-8608. (b) Nomura, R.; Nishiura, S.;
Tabei, J .; Sanda, F.; Masuda, T. Macromolecules 2003, 36,
5076-5080. (c) Nomura, R.; Tabei, J .; Nishiura, S.; Masuda,
T. Macromolecules 2003, 36, 561-564. (d) Tabei, J .; Nomura,
R.; Masuda, T. Macromolecules 2002, 35, 5405-5409. (e)
Nomura, R.; Tabei, J .; Masuda, T. Macromolecules 2002, 35,
2955-2961. (f) Nomura, R.; Tabei, J .; Masuda, T. J . Am.
Chem. Soc. 2001, 123, 8430-8431.
(4) (a) Deng, J .; Tabei, J .; Shiotsuki, M.; Sanda, F.; Masuda, T.
Macromolecules 2004, 37, 1891-1896. (b) Deng, J .; Tabei, J .;
Shiotsuki, M.; Sanda, F.; Masuda, T. Macromol. Chem. Phys.
2004, 205, 1103-1107.
As the temperature was raised, the helix content of
these (co)polymers gradually decreased. The relative
ꢀ390 nm of these (co)polymers at 60 °C to the correspond-
ing ꢀ390 nm at -45 °C is illustrated in Figure 8. Poly(10)
kept the same ꢀ390 nm at 60 °C as that at -45 °C. The
ratio of ꢀ at 60 and -45 °C gradually decreased as the
unit of 4 increased. When the content of unit 4 exceeded
50%, the ratio of ꢀ at 60 and -45 °C abruptly decreased.
The relative ꢀ390 nm of poly(40.60-co-100.40) was no more
than about 10%. Poly(40.80-co-100.20) and poly(4) existed
in random coil conformation at 60 °C. On the basis of
these results, it can be concluded that the copolymeri-
zation not only improved the solubility of the copolymers
effectively and increased the molecular weight largely
but also adjusted the stability and content of helix, when
the copolymer composition was tuned.
Con clu sion s
N-Propargylamides with bulky pendent groups (9-
13) were synthesized. Monomers 11 and 12 satisfacto-
rily underwent polymerization with Rh catalyst to give
polymers with moderate molecular weights and good
solubility in chloroform. On the other hand, monomers
9, 10, and 13 provided only oligomers under the given
polymerization conditions, presumably because of the
(5) Schrock, R. R.; Osborn, J . A. Inorg. Chem. 1970, 10,
2339-2343.
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