Stumuli-responsive organogel based on poly(N-propargylamide)

Article abstract of DOI:10.1021/ma0300950

The first example for a novel stimuli-responsive organogel that senses external stimuli and undergoes very quick volume change in isotropic organic solvents was demonstrated. It was evidenced that the volume change was driven by the conformational change of the framework between the helical and disordered structures. In addition, it was observed that the volume change occured more rapidly when the solvent was changed from CHCl₃ to methanol and that this event could be monitored by the naked eye.

Full text of DOI:10.1021/ma0300950

Macromolecules 2003, 36, 6939-6941
6939
Notes
Sch em e 1
Stu m u li-Resp on sive Or ga n ogel Ba sed on
P oly(N-p r op a r gyla m id e)
Ryoji Nom u r a ,† Ka tsu h ir o Ya m a d a ,†
J u n ich i Ta bei,† Yosh ih ito Ta k a k u r a ,‡
Tosh ik a zu Ta k iga w 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, and
Department of Material Chemistry, Graduate School of
Engineering, Kyoto University, Kyoto 606-8501, J apan
Received February 7, 2003
In tr od u ction
The reversible volume change of gels upon external
stimuli was shown, for the first time, by Tanaka et al.1
After this discovery, much effort has been made on the
synthesis and application of the stimuli-responsive gels,
especially those of hydrogels, in view of their scientific
and technological importance.2 A wide variety of exter-
nal stimuli are available for the volume change of gels,
including temperature, pH, solvent composition, and so
on. Creation of organogel that undergoes quick volume
change in isotropic organic solvents would greatly
expand the potentiality of responsive gels.3 The helix-
coil transition is promising as a means to cause quick
volume change of gels by external stimuli.
Poly(N-propargylamides) can be prepared in a ster-
eoregular form (cis-transoidal) using Rh catalysts.4
They construct intramolecular hydrogen bonds between
the amide groups, which induce a helical conformation.
The most characteristic points of the poly(N-propargyl-
amide) helix is that, like R-helical polypeptides, the
transition from a disordered to a helical state involves
negatively large enthalpy and entropy changes.4b Thus,
changing the temperature and/or adding polar solvents
that can hydrogen-bond significantly affect the enthalpy
and entropy terms, which influence the equilibrium
between the helical and disordered states. Such a
conformational change should be accompanied by a
change in hydrodynamic volume, which would trigger
the volume change of the gels.6
p-toluenesulfonic acid monohydrate (Wako), [(nbd)RhCl]2 (Al-
drich), and triethylamine (Wako) were used without further
purification. Monomers 1a -c were prepared according to the
literature.4b
Syn th esis of Mon om er 2. A benzene solution (80 mL) of
adipic acid (2 g, 16.9 mmol), propargyl alcohol (3.3 g, 59.2
mmol), and p-toluenesulfonic acid monohydrate (1.6 g, 8.5
mmol) was refluxed for 2 h. The reaction mixture was washed
with saturated aqueous NaHCO3 and brine, dried over MgSO4,
and concentrated. Monomer 2 was isolated (2.6 g, 13.3 mmol,
79%) by flash column chromatography on silica gel (hexane/
AcOEt, 10/1, v/v).
P olym er iza tion P r oced u r es. A THF solution of the
monomers ([M]total ) 1.0 M) was added to a THF solution of
[(nbd)RhCl]2-Et3N ([monomer]/[cat]/[Et3N] ) 100/1/0.1) under
dry nitrogen, and the solution was kept at 30 °C for 1 h. The
resulting gel was stirred in methanol and CHCl3 for 5 h and
then dried under reduced pressure.
Mea su r em en ts. NMR spectra were recorded on a J EOL
EX-400 spectrometer. IR and UV-vis spectra were obtained
with Shimadzu FTIR-8100 and J ASCO V-500 spectrophotom-
eters. Optical rotations were measured with a J ASCO 600
spectropolarimeter. CD spectra were recorded on a J ASCO
V-530 spectropolarimeter.
Resu lts a n d Discu ssion
The gels were prepared with a combined catalyst,
[(nbd)RhCl]2 and Et3N, using N-propargyl-3-methyl-
butanamide (1a ) as a monomer (Scheme 1). Attempts
to prepare gels using bifunctional N-propargylamides
gave unsuccessful results. Specifically, the amide-based
cross-linkers prepared were poorly soluble in polymer-
ization solvents, and even the soluble ones inefficiently
provided insoluble gels. Thus, a bifunctional propargyl
ester, dipropargyl adipate (2), was employed as a cross-
linker, which led to good yields of insoluble gels.7 We
also prepared a gel based on poly(N-propargylpropan-
amide) [poly(1b)] for comparison since poly(1b) adopts
a disordered conformation at room temperature ir-
respective of the solvent.4b The IR spectra of the
resultant gels showed no absorption due to the C-C
triple-bond vibration that was observed for the mono-
mers. Thus, both of the triple bonds in the cross-linker
participate in the polymerization.
Here we show a new unique responsive organogel that
displays quick response to change in temperature or
solvent composition based on hydrogen bonding, which
is accompanied by volume change in organic isotropic
solvents. More specifically, the synthesis of poly(N-
propargylamides)-based copolymer gels poly(1) and their
stimuli-responsive properties are reported (Scheme 1).
Exp er im en ta l Section
Ma ter ia ls. The solvents were distilled by usual methods
prior to use. Propargyl alcohol (Wako), adipic acid (Wako),
† Department of Polymer Chemistry.
‡ Department of Material Chemistry.
* Corresponding author: Tel +81-75-753-5613; Fax +81-75-753-
5908; e-mail masuda@adv.polym.kyoto-u.ac.jp.
10.1021/ma0300950 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/06/2003

6940 Notes
Macromolecules, Vol. 36, No. 18, 2003
F igu r e 2. (a) Temperature dependence of the relative volume
of the gel based on poly(1a ) in CHCl3. (b) Solvent-composition
dependence of the relative volume of the gels based on (O) poly-
(1a ) and (2) poly(1b) and (0) that of the relative swelling
degree of poly(1a )-based gel in mixed solvents of CHCl3 and
methanol.
F igu r e 1. (a) Temperature dependence of the relative inten-
sity of molar ellipticity (O) and absorbance (2) of poly(1a -co-
1c) at 400 nm in CHCl3. (b) Solvent-composition dependence
of the relative intensity of molar ellipticity (O) and absorbance
(2) of poly(1a -co-1c) at 400 nm in mixed solvents of CHCl3
and methanol.
deformation into a randomly coiled state. The confor-
mational change induced by the change of solvent
composition was also monitored in detail using the CD
and UV-vis spectra. Figure 1b plots the relative
intensity of the CD and UV bands at 400 nm for the
copolymer in mixed solvents of CHCl3 and methanol.
The magnitude of both UV and CD bands that originate
from the helical conformation steeply decreased upon
adding methanol, and no CD effects were observed when
the methanol content exceeded 5 vol %. This means that
the conformational change is completed when the
methanol content reaches 5 vol %.
The temperature dependence of the volume of a
cylindrical poly(1a )-based gel is illustrated in Figure 2a.
The cylindrical gel (1a /2 ) 97.5/2.5 in feed) with the
length and diameter of 7.1 and 2.2 mm, respectively,
started to shrink at approximately 35 °C, and the gel
volume leveled off at 50 °C. The temperature depen-
dence of the gel volume not strictly but roughly agreed
with that of the polymer conformation (Figure 1a).
Emphasis should be placed on the very rapid volume
change process of the gel; the temperature jump experi-
ment from 35.2 to 37.5 °C for this relatively large
cylindrical gel indicated that the relaxation time was
only 8.9 min. This is in contrast to the rather slow
volume change process of the conventional responsive
hydrogels based on poly(acrylic acids) and poly(acryla-
mides).
The volume change also occurred upon the change in
solvent composition. Figure 2b plots the relative volume
of the poly(1a )-based gel vs the solvent composition
between CHCl3 and methanol. The gel steeply shrank
with increasing methanol composition, meaning that the
continuous volume change took place. The shrinkage
stopped when the methanol content reached 5 vol %,
which is identical behavior to that observed in the
conformational change of poly(1a -co-1c) (Figure 1b). The
solvent-composition dependence of the swelling degree
also gave an identical curve as demonstrated in Figure
2b; the initial swelling degree (26.4) decreased with
The stimuli-induced conformational change of the
polymer backbone was first investigated in detail by
monitoring the CD and UV spectra of a copolymer of
1a with a small amount of 1c. The large persistence
length of the helical domain allows poly(1a ) to exist in
the helical conformation with an excess of one-handed
screw sense by incorporating a small amount of a chiral
segment.4b Thus, both poly(1a ) and poly(1a -co-1c) pos-
sess the helical conformation, but the former has
identical population of the right- and left-handed heli-
ces, while the screw sense of the latter copolymer is
biased as with the homopolymer from 1c. Therefore, the
copolymer displays intense Cotton effects in the absorp-
tion range of the main chain.7 Since the copolymer
prepared has only a very small amount of the chiral unit
(7 mol %), it is expected to show almost identical
behavior as with the homopolymer in terms of the
thermally and solvent-induced conformational changes.
Figure 1a shows the temperature dependence of the
relative magnitude of the Cotton effects at 400 nm for
poly(1a -co-1c) in CHCl3. The Cotton effect, centering
around 400 nm, gradually decreased in intensity on
heating, and the copolymer showed no CD bands at 56
°C.7 The lack of the CD signal at 56 °C is not due to the
equally populated right- and left-handed helices but is
due to the absence of the helical conformation, which
was confirmed by the UV-vis spectra.7 The helical
backbone of poly(N-propargylamides) absorbs light at
around 400 nm, while the disordered one displays
electric absorption at 320 nm.4b Thus, the UV-vis
spectrum is a good indicator for the conformation. With
increasing temperature, the absorption at 400 nm
gradually decreased in intensity and almost completely
disappeared at 56 °C (Figure 1a). A new band appeared
around 320 nm with increasing temperature.7 The plots
of the relative intensities of the CD and UV signals at
400 nm against temperature gave identical curves as
shown in Figure 1a. Therefore, lack of the CD band of
the copolymer originates from the thermally induced

Macromolecules, Vol. 36, No. 18, 2003
Notes 6941
increasing methanol content and became constant (14-
17) when the methanol content exceeded 5 vol %. In
contrast, the volume of the gel based on poly(1b) (1b/2
) 97.5/2.5 in feed) remained almost unchanged under
the same conditions as illustrated in Figure 2b. Because
poly(1b) exists in a randomly coiled state even in CHCl3
at room temperature,4b it is concluded that the helix-
coil transition results in the volume change. Further,
it was confirmed that this transition is completely
reversible. The solvent-induced volume change also very
quickly proceeded. Specifically, the relaxation time of
volume expansion upon switching the solvent from
methanol to CHCl3 was only 43 min for a cylindrical
gel with the length and diameter at the swollen state
of 7.1 and 2.2 mm, respectively.
isotropic organic solvents would allow us to expand the
utility of stimuli-responsive polymer networks as chemi-
cal valves, gentle actuators, and so forth.
Su p p or tin g In for m a tion Ava ila ble: Results for the
preparation of the gel and the variable temperature CD and
UV spectra of poly(1a -co-1c) in CHCl3. This material is
available free of charge via the Internet at http://pubs.acs.org.
Refer en ces a n d Notes
(1) Tanaka, T. Phys. Rev. Lett. 1978, 40, 820-823.
(2) Dusˇek, K. Responsive Gels: Volume Transition I and II;
Springer: Berlin, 1993.
(3) Few examples have been reported for stimuli-responsive
organogels that undergo volume change in liquid crystal
solvents. See: (a) Sarkar, M. D.; Urayama, K.; Kawamura,
T.; Kohjiya, S. Liq. Cryst. 2000, 27, 795-800. (b) Urayama,
K.; Luo, Z.; Kawamura, T.; Kohjiya, S. Chem. Phys. Lett.
1998, 287, 342-346.
Con clu sion
We have demonstrated the first example for a novel
stimuli-responsive organogel that senses external stimuli
and undergoes very quick volume change in isotropic
organic solvents. It has been evidenced that the volume
change is driven by the conformational change of the
framework between the helical and disordered struc-
tures. We also observed that the volume change oc-
curred more rapidly when the solvent was changed from
CHCl3 to methanol and that this event could be moni-
tored by the naked eye. Because this process took place
quite rapidly, we could not obtain quantitative data for
the relaxation time. We believe that such very quick
response of the poly(N-propargylamide)-based gel in
(4) (a) Nomura, R.; Tabei, J .; Masuda, T. J . Am. Chem. Soc.
2001, 123, 8430-8431. (b) Nomura, R.; Tabei, J .; Masuda,
T. Macromolecules 2002, 35, 2955-2961.
(5) Nomura, R.; Yamada, K.; Masuda, T. Chem. Commun. 2002,
478-479.
(6) Responsive hydrogels that contain helical proteins at the
cross-linking points have been synthesized, in which the
reversible conformational change of the cross-linker yields
continuous volume change. See: (a) Wang, C.; Stewart, R.
J .; Kopecˇek, J . Nature (London) 1999, 397, 417-420. (b)
Chen, L.; Kopecˇek, J .; Stewart, R. J . Bioconjugate Chem.
2000, 11, 734-740.
(7) See the Supporting Information.
MA0300950