Enantioselective pervaporation through membranes from poly(1,3- phenyleneethynylene)-based one-handed helical foldamer and unfoldamer

Article abstract of DOI:10.1246/cl.2011.384

We synthesized two kinds of poly(1,3-phenyleneethynylenes), one has (+)-menthoxycarbonyl groups at all repeating units and the other has (+)-menthoxycarbonyl groups and ndodecyloxy groups alternately. Self-supporting membranes could be obtained easily by

Full text of DOI:10.1246/cl.2011.384

doi:10.1246/cl.2011.384
384
Published on the web March 12, 2011
Enantioselective Pervaporation through Membranes from Poly(1,3-phenyleneethynylene)-based
One-handed Helical Foldamer and Unfoldamer
Makoto Inoue,1 Masahiro Teraguchi,*2,3 Toshiki Aoki,1,2,3,4 Takeshi Namikoshi,5 Edy Marwanta,3,4 and Takashi Kaneko3
1Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Niigata 950-2181
2Institute of Science and Technology, Niigata University, 2-8050 Ikarashi, Niigata 950-2181
3Center for Transdisciplinary Research, Niigata University, 2-8050 Ikarashi, Niigata 950-2181
4Venture Business Laboratory, Niigata University, 2-8050 Ikarashi, Niigata 950-2181
5Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507
(Received January 17, 2011; CL-110039; E-mail: teraguti@eng.niigata-u.ac.jp)
We synthesized two kinds of poly(1,3-phenyleneethynyl-
enes), one has (+)-menthoxycarbonyl groups at all repeating
O
O
[PdCl2(PPh3)2]
CuI, PPh3
units and the other has (+)-menthoxycarbonyl groups and n-
dodecyloxy groups alternately. Self-supporting membranes
could be obtained easily by solvent casting. Enantioselective
permeations of the polymers were investigated by pervaporation
of 2-butanol. Enantioselective permeabilities of the membrane
consisting of one-handed helical foldamer and unfoldamer were
clarified.
I
I
n
+
O
THF/ Et3N
O
R
R
O
R =
=
O
PMtMt
PMtODo
OC12H25
Syntheses of optically active helical polymers have attracted
particular interests of polymer scientists due to interesting
technological applications in optical resolution, chiral sensors,
chiroptics, microelectronics, chiral magnets, and so on.1
It is well known that some poly(1,3-phenyleneethynylene)s
can form helical foldamer conformation depending on their side
groups and/or solvent.2-6 These properties are very interesting
and many functions such as molecular recognition,7-10 magnet-
ism,11,12 and reactive sieve13 have been reported.
Scheme 1. Synthesis of chiral poly(1,3-phenyleneethynylenes).
Table 1. Polycondensation results by using [PdCl2(PPh3)2]-
PPh3-CuI (1:4:6)a
b
20 c
¹3 d
Yield Mw
/% /©104
½¡ꢀD
[ª]320 © 10
/deg¢cm2¢dmol
b
Code
Mw/Mn
¹1
/°
PMtMt
72
23.0
17.2
2.20
1.45
91.0
62.0
9.3
3.1
However, to the best of our knowledge, functions of
poly(1,3-phenyleneethynylenes) used as free-standing mem-
brane have been never reported.14 Needless to say, there are
no reports that functions of free-standing membranes consisting
of foldamer and unfoldamer were compared. Enantioselective
permeation by chiral main chains of polymers is a very
interesting subject in polymer science. Herein, we synthesized
two kinds of chiral poly(1,3-phenyleneethynylenes), one has
(+)-menthoxycarbonyl groups at all repeating units and the
other has (+)-menthoxycarbonyl groups and n-dodecyloxy
groups alternately in the repeating units (named PMtMt and
PMtODo, respectively) (Scheme 1). Then we prepared free-
standing membranes consisting of one-handed helical foldamer
and unfoldamer by casting and investigated the enantioselective
permeability of the foldamer and the unfoldamer membranes.
PMtMt was synthesized according to a previous report from
our laboratory.15 PMtODo was newly synthesized to improve
solubility and membrane forming ability.16 The polymerization
data for the polymers used in this paper are summarized in
Table 1. Both polymers were pale yellow solids and soluble in
chloroform, dichloromethane, carbon tetrachloride, THF, tol-
uene, and benzene and insoluble in hexane, diethyl ether,
methanol, and acetone.
PMtODo 50
aPolycondensations were carried out in THF/Et3N for 24 h at
b
r.t.; [M]0,total = 0.125 M, [Pd cat] = 0.025 M. Determined by
GPC correlating polystyrene standard; eluent: THF. cIn
¹1
CHCl3/benzene (volume ratio: 2:8); 0.10 g L
(2.5 © 10¹4 M) at 20 °C.
.
dIn toluene
other hand, no Cotton effects were observed in chloroform
solution because PMtODo exists in extended chain conforma-
tion (unfoldamer) in chloroform.
The CD profile of PMtODo in toluene was similar to that of
PMtMt,15 but the [ª] value of PMtODo in toluene was one third
of that of PMtMt in toluene (Table 1). It was attributed to the
chiral menthyl group, i.e., menthyl group content of PMtODo
was less than that of PMtMt and less contribution of
solvophobic effects by n-dodecyloxy groups as a driving force
for foldamer formation than that of menthoxycarbonyl groups.
To elucidate a one-handed helical conformation in the
membrane state, a thin membrane was prepared on a quartz disc
by spin-coating. A strong Cotton effect was observed in the CD
spectra of PMtODo membrane prepared from toluene solution,
which was similar to that of in solution. On the other hand, no
Cotton effect was observed in the CD spectra of PMtODo
membrane prepared from chloroform solution (Figure 1). These
results were the same in the case of PMtMt previously
reported.15 UV-vis spectra of foldamer and unfoldamer were
PMtODo in toluene displayed intense Cotton effects in
circular dichroism (CD) spectra in the range of 270-360 nm
which are attributable to the backbone. This indicates that
PMtODo in toluene forms one-handed helical foldamer. On the
Chem. Lett. 2011, 40, 384-386
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

385
almost the same unlike in solution.15 It is possible that in
membrane the ratio of oscillator strengths for absorption bands
of the unfoldamer were similar to that of the foldamer due to an
aggregation of the unfoldamer.5 Since both polymers had a
sufficiently high molecular weight, self-supporting membranes
can be fabricated by solvent-casting.17 PMtODo gave a more
flexible membrane compared with PMtMt.
4
0
toluene
chloroform
Figures 2 and 3 show the quantity (Q) of permeated (R)- and
(S)-2-butanol versus permeation time through foldamer and
unfoldamer membranes of PMtMt and PMtODo, respectively,
in pervaporation when racemic 2-butanol was supplied.18 Using
PMtMt unfoldamer membrane, (S)-isomer preferentially per-
meated, and the ee of permeate was 89.4% in good selectivity.17
On the other hand, using PMtMt foldamer membrane, perme-
ateselectivity was hardly observed.
-4
1
toluene
chloroform
300
0
250
350
400
In contrast with these results, using PMtODo membranes,
unfoldamer membrane scarcely showed enantioselective perme-
ability and foldamer membrane showed a good enantioselective
permeability, (R)-isomer preferentially permeated, and the ee of
permeate was 68.3%.
Wavelength/nm
Figure 1. CD and UV-vis spectra of PMtODo membrane at
25 °C.
a) Unfoldamer
b) Foldamer
10
10
Pc = 2.77 x 10-13 (m2 h−1
αS = 17.8 (89.4 %ee)
)
Pc = 2.33 x 10-12 (m2 h−1
αS = 1.07 (3.35 %ee)
)
8
6
8
6
4
4
2
2
0
0
0
10
20
30
0
20
40
60
80
100
Permeation time/h
Permeation time/h
Figure 2. Plots of quantity (Q) of permeated (R)-(+)-2-butanol ( ) and (S)-(¹)-2-butanol ( ) v.s. permeation time through PMtMt
membrane: a) prepared from chloroform solution (unfoldamer); b) prepared from toluene solution (foldamer).
a) Unfoldamer
12
b) Foldamer
14
Pc = 9.45 x 10-13 (m2 h−1
αR = 5.31 (68.3 %ee)
)
Pc = 6.61 x 10-12 (m2 h−1
αS = 1.06 (3.12 %ee)
)
12
10
8
10
8
6
6
4
4
2
0
2
0
0
10
20
30
50
Permeation time/h
0
100
150
200
Permeation time/h
Figure 3. Plots of quantity (Q) of permeated (R)-(+)-2-butanol ( ) and (S)-(¹)-2-butanol ( ) v.s. permeation time through PMtODo
membrane: a) prepared from chloroform solution (unfoldamer); b) prepared from toluene solution (foldamer).
Chem. Lett. 2011, 40, 384-386
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

386
Quite interestingly, whether foldamer or unfoldamer was
10, 4283.
a good membrane showing higher selectivity in permeation
depended on whether PMtMt or PMtODo was used. That is,
when PMtMt membrane was used, the unfoldamer was better.
On the other hand, when PMtODo membrane was used, the
foldamer was better. Furthermore, PMtMt unfoldamer mem-
brane and PMtODo foldamer membrane showed opposite
selectivity. These results indicate a one-handed helical con-
formation of poly(1,3-phenyleneethynylenes) affected strongly
permselectivity. It is possible that the main chain of poly(1,3-
phenyleneethynylene) with helical sense inducted by (+)-men-
thoxycarbonyl groups in side groups interacted preferentially
with (S)-2-butanol. In contrast, (+)-menthoxycarbonyl groups in
side chains themselves maybe interacted preferentially with (R)-
isomer. Therefore, it is assumed that when the PMtMt foldamer
membrane was used, both effects unfortunately canceled each
other and when the PMtODo foldamer membrane was used,
the effect of one-handed helical main chain surpassed that of
(+)-menthoxycarbonyl groups due to the lower contents of
chiral substituents than PMtMt. Anyway, a detailed investiga-
tion of enantioselective permeation of various chiral poly(1,3-
phenyleneethynylenes) is now in progress.
In summry, two kinds of poly(1,3-phenyleneethynylenes)
having (+)-menthoxycarobonyl groups in all repeating units
(PMtMt) and having (+)-menthoxycarobonyl groups and n-
dodecyloxy groups alternately in the repeating units (PMtODo)
could be obtained. Fabrication of self-supporting membranes
consisting of different conformations, i.e., foldamer (one-handed
helical conformation) membrane and unfoldamer membrane was
achieved by casting from toluene and chloroform, respectively.
It was found that chiral poly(1,3-phenyleneethynylenes) were
novel promising materials for optical resolution membranes and
the optical resolution ability of chiral poly(1,3-phenyleneethyn-
ylenes) was quite different depending on polymer conformation
such as unfoldamer and foldamer.
7
8
9
R. B. Prince, S. A. Barnes, J. S. Moore, J. Am. Chem. Soc.
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2001, 42, 351.
11 Y. Miura, Y. Ushitani, Macromolecules 1993, 26, 7079.
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Polyhedron 2007, 26, 1825.
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14 Recently, gas permeation using poly(1,4-phenyleneethynyl-
ene)s were reported: T. Sakaguchi, D. Nojiri, T. Hashimoto,
Polym. Bull. 2008, 60, 271.
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E. Marwanta, T. Kaneko, Synth. Met. 2009, 159, 854.
16 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
17 Preparation of a self-supporting membrane: A solution of a
polymer in toluene or chloroform (ca. 2-5 © 10¹2 M based
on the monomer unit) was cast on a poly(tetrafluoroethylene)
sheet, and the solvent was evaporated for 24 h at room
temperature. The resulting solid membrane was detached
from the sheet and dried in vacuo for a day. Thickness (L):
30-60 ¯m, area (A): 8.55 cm2.
18 Enantioselective pervaporation: The feed side of a stainless
steel cell was filled with racemic 2-butanol. The permeate
side was connected to a cold trap in dry ice/methanol and
evacuated at 0.2 mmHg. Pervaporation was carried out for
30-150 h (t) at 25 °C. The permeate trapped was weighed (Q,
g) and the normalized quantity (Qc, g m m¹2) was calculated
from Qc = QL/A. The permeation rate (P, g m m¹2 h¹1) was
estimated from the slope of the permeation time (t, h) plot.
The permeability coefficient (Pc, m2 h¹1) was calculated by
dividing P by the difference in the concentration between
the feed and permeate sides. Enantioselectivity in the
permeation, i.e. optical purity of the permeate (%ee), was
determined by the equation
This work was partially supported by a Grant-in-Aid for
Science Research in
a Priority Area “Super-Hierarchical
Structures” (Nos. 18039011, 19022009, and 19022010) from
the MEXT, Japan, and for Science Research (B) (Nos. 19350054
and 20310052) from JSPS, and by Mukai Science and
Technology Foundation.
%ee = 100 © (AR ¹ AS)/(AR + AS)
The peak areas of (R)- and (S)-isomer (AR and AS) in the
permeate were measured by HPLC with an optical resolution
column (CHIRALCELL OD-H manufactured by Daicel
Chemical Co.) after carbamoylation of 2-butanol permeate.
Carbamoylation of 2-butanol: 2-Butanol permeate was
diluted with ether (2 mL). To the solution containing 2-
butanol (10 mg, 0.14 mmol) were added pyridine (0.2 mL,
2.5 mmol) and then phenylisocyanate (0.2 mL, 1.8 mmol).
The solution was stirred for 3 min at 50 °C, and the solvent
was removed. The resulting product was dissolved in
hexane, and the solution was used for HPLC measure-
ment.
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Chem. Lett. 2011, 40, 384-386
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/