Reversible fixation and release of carbon dioxide with a binary system consisting of polyethylene glycol and polystyrene-bearing cyclic amidine pendant group

Article abstract of DOI:10.1002/pola.27210

In this study, we investigated the CO₂-capture/release behavior of the polystyrene-bearing cyclic amidine pendant groups, which was synthesized via free radical polymerization of HCl salt of the corresponding styrene monomer followed by neutralization. For comparison, we also prepared the polystyrene bearing N-formyl-1,3-propanediamine pendant groups through the hydrolysis of the cyclic amidine group by treatment with an alkaline solution. First, we examined the CO₂-capture/release behaviors of the amidine and amine monomers in aqueous solution in terms of conductivity. The conductivity of a wet DMSO solution of the amidine monomer increased upon CO₂ bubbling at 25 C and reached a stationary value of about 11 mS/m, which indicated the formation of the bicarbonate salt. Conversely, the conductivity decreased to its original value upon N₂ bubbling at 50 C, reflecting the complete release of the trapped CO₂ molecules. Both solutions showed the changes in the conductivity with quick responses, and no appreciable difference was observed between them. We then investigated the CO₂-capture/release behaviors of the amidine and amine polymers, by taking advantage of the binary system with polyethylene glycol, and found that the binary system with the amidine polymer captured and released CO₂ more efficiently than that with the amine polymer.

Full text of DOI:10.1002/pola.27210

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Reversible Fixation and Release of Carbon Dioxide with a Binary System
Consisting of Polyethylene Glycol and Polystyrene-Bearing
Cyclic Amidine Pendant Group
Mina Sakuragi,* Naoto Aoyagi, Yoshio Furusho, Takeshi Endo
Molecular Engineering Institute, Kinki University, Iizuka, Fukuoka 820-8555, Japan
Correspondence to: T. Endo (E-mail: tendo@moleng.fuk.kindai.ac.jp)
Received 11 January 2014; accepted 12 April 2014; published online 2 May 2014
DOI: 10.1002/pola.27210
ABSTRACT: In this study, we investigated the CO2-capture/
release behavior of the polystyrene-bearing cyclic amidine
pendant groups, which was synthesized via free radical poly-
merization of HCl salt of the corresponding styrene monomer
followed by neutralization. For comparison, we also prepared
the polystyrene bearing N-formyl-1,3-propanediamine pendant
groups through the hydrolysis of the cyclic amidine group by
treatment with an alkaline solution. First, we examined the
CO2-capture/release behaviors of the amidine and amine
monomers in aqueous solution in terms of conductivity. The
conductivity of a wet DMSO solution of the amidine monomer
increased upon CO2 bubbling at 25 ꢀC and reached a stationary
value of about 11 mS/m, which indicated the formation of the
bicarbonate salt. Conversely, the conductivity decreased to its
original value upon N2 bubbling at 50 ꢀC, reflecting the com-
plete release of the trapped CO2 molecules. Both solutions
showed the changes in the conductivity with quick responses,
and no appreciable difference was observed between them.
We then investigated the CO2-capture/release behaviors of the
amidine and amine polymers, by taking advantage of the
binary system with polyethylene glycol, and found that the
binary system with the amidine polymer captured and released
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CO2 more efficiently than that with the amine polymer. 2014
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2014, 52, 2025–2031
KEYWORDS: amide; amidine; carbon dioxide; polyethers; poly-
ethylene glycol; polystyrene; renewable resources; structure
a pressure of 1 atm in solution as well as in the solid state,
and desorbed CO2 under N2 atmosphere at an elevated tem-
perature, suggesting that amidines could be better alternatives
for amines.20–23 Shortly thereafter, binary systems of amidines
and organic alcohols or water were reported to efficiently
capture CO2 as alkyl carbonates or bicarbonates and release it
under N2 or Ar atmosphere.24–27 Amidines or guanidines
were combined with hydroxy-functionalized ionic liquids,
reversibly capturing and releasing CO2 effectively.18,19 Based
on the binary systems, reversibly coagulatable and redispersi-
ble latexes were developed using polyacrylamide or polysty-
rene scaffolds having aliphatic amidine pendant groups and
were applied as switchable surfactants.28,29
INTRODUCTION The last decade has witnessed an ever-
increasing interest in CO2 fixation arising from global concern
about the rising atmospheric CO2 concentration as well as its
potential utility as a green, renewable C1 source owing to its
harmlessness and usefulness in organic transformations.1–5 CO2
capture with aqueous amine is a well-understood and widely
used technology, efficiently absorbing CO2 from exhaust gases
at low temperature via an exothermic reaction.6 However, the
release process of the trapped CO2 molecules requires heating,
thereby consuming a large amount of energy.2 Therefore,
reversible and efficient CO2-absorbing systems need to be
developed. A number of CO2 absorbents that can capture and
release CO2 with relatively good efficiency have been reported
to date, including metal–organic frameworks,7–9 amine-tethered
silica,10–12 amine-appended mesoporous silica,13–15 amine-
tethered porous polymer networks,16 and ionic liquids.17–19
Polyethylene glycols (PEGs) are known to have interesting
properties, such as good miscibility for common solids, ther-
mal stability, nonvolatility, innocuousness, and environmen-
tally benign characterization. In particular, PEGs have high
affinity toward CO2 and show some CO2-expansion effect
that increases gas/liquid diffusion rates.30 By taking
We reported a decade ago that polystyrene derivatives bear-
ing cyclic amidine pendant groups captured CO2 molecules at
Additional Supporting Information may be found in the online version of this article.
*Present Address: Mina Sakuragi, Department of Nanoscience, Faculty of Engineering, Sojo University, 4oˆ 22-1 Ikeda, Kumamoto
860-0082, Japan
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investigate it in detail for further development of the
polyamidine-based system. We investigated the CO2 fixation–
release behaviors of 1 and 2 in wet solution in terms of con-
ductivity. Moreover, we prepared binary systems PEG with
poly-1 or poly-2 of which the CO2-capture/release efficiencies
were estimated by thermogravimetric analysis (TGA)
technique.
EXPERIMENTAL
General
The NMR spectra were recorded on a JEOL JNM-ECS400 spec-
1
trometer operating at 400 MHz for H and 100 MHz for 13C.
Chemical shifts are reported in parts per million (d) based on
the residual proton peaks of the deuterated solvents. The
electron spray ionization mass spectra (ESI-MS) were
recorded using a JEOL JMS-T100CS spectrometer. The conduc-
tivities of the DMSO solutions of 1 and a mixture of 2A and
2B were measured using an electric conductivity meter
(MM60R with a conductivity sensor CT-57101C, TOADKK,
Japan) after the solutions were bubbled with N2 for 30 min at
50 ꢀC to obtain a stable conductivity. The TGA of the poly-
mers was performed with an EXSTAR TG/DTA 6200 instru-
ment (Seiko Instruments) with a flow of CO2 or N2 at a rate
of 200 mL/min. Gel permeation chromatography was carried
out with a JASCO LC-Net II high-performance liquid chroma-
tography with TOSOH TSKgel ALPHA-M and phosphate-
buffered saline as eluent. Molecular weights were evaluated
using PEG standards.
SCHEME 1 Synthesis of polystyrene derivatives bearing cyclic
amidine pendants (poly-1) and acyclic amine pendants (poly-2)
and their unit models (1, 2A, and 2B).
advantage of this property, it was demonstrated that addi-
tion of PEG significantly enhances the rates of CO2 absorp-
tion and desorption by ammonium salt/PEG mixtures.31
Combined mixture of PEG with amidine or guanidine super-
bases facilitated CO2 capture and activated CO2 molecules to
catalyze some CO2-fixing organic reactions.32 We recently
reported that a main chain-type polyamidine in which the
amidine groups are connected through hexamethylene link-
ers was miscible with PEG to form a homogeneous mixture,
which can reversibly capture and release CO2.33 A binary
system of PEG and polyethyleneimine also captured and
released CO2 although its efficiency was much less than that
of the polyamidine/PEG binary systems, suggesting that
amidine is a better functionality for CO2 absorbents than
amino group. However, the main chain-type polyamidines
are synthesized generally by polycondensation of a,x-dia-
mines and orthoformates, which limits the scope of further
design of polyamidines. On the other hand, the polystyrene
derivatives bearing amidine pendant groups can be pre-
pared by radical polymerization of the corresponding sty-
rene derivative.20,21 Therefore, the side chain-type
polyamidines have potential advantage in incorporating vari-
ous functionalities by molecular design and controlled radi-
cal polymerization techniques.
Materials
Hydrochloride 1ꢁHCl was prepared from p-chloromethylstyr-
ene according to our previously reported method.20 The
polystyrene-bearing 6-membered cyclic amidinium hydro-
chloride (poly-1ꢁHCl) was synthesized by free radical poly-
merization of 1ꢁHCl in water according to a slightly modified
procedure of our previous article.20 Polyethylene glycol
(Mw 5 400, PEG400) was purchased from Wako Pure Chemi-
cal Industries (Osaka, Japan), and used as received. CO2 was
purchased from Fukuho Teisan (Fukuoka, Japan). Other
reagents and solvents were purchased from commercial sup-
pliers and used without further purification.
Methods
In this study, we synthesized the polystyrene-bearing cyclic
amidine pendant groups (poly-1) via free radical polymer-
ization of the hydrochloride of the corresponding styrene
monomer (1ꢁHCl) followed by neutralization, according to
our previous method (Scheme 1).20 For comparison, we also
prepared the polystyrene-bearing N-formyl-1,3-propanedi-
amine pendant groups (poly-2) through the hydrolysis of
the cyclic amidine group by treatment with an alkaline solu-
tion of which the structure was characterized by 1H, 13C,
and 2-D NMR, and IR along with the studies using its unit
models (2A and 2B) prepared by the hydrolysis of the cyclic
amidine group of 1ꢁHCl. Another reason to study the hydro-
lytic products of 1 and poly-1 is that we had been aware of
the possibility of hydrolysis of poly-1 by a small amount of
water during experimental manipulation. It was necessary to
Synthesis of 1-(4-Vinylbenzyl)-1,4,5,6-
tetrahydropyrimidine (1)
A 0.1-M solution of sodium methoxide in methanol (20 mL,
2 mmol) was added slowly to a 0.2-M solution of 1ꢁHCl in
methanol (10 mL, 2 mmol). The solvent was removed by
evaporation after being stirred at room temperature for 10
min under nitrogen. The residue was dissolved in chloroform
(10 mL) and the insoluble part was filtered off. The filtrate
was evaporated to dryness to afford 1 as a colorless oil in
98% yield after drying under vacuum.
1H NMR (400MHz, CD3OD): d 5 7.43 (d, J 5 8.24 Hz, 2H),
7.27 (s, 1H, disappeared after H/D exchange), 7.23 (d,
J 5 8.24 Hz), 6.73 (dd, J 5 11.0, 17.4 Hz, 1H), 5.78(d, J 5 17.4
Hz, 1H), 5.23 (d, J 5 11.0 Hz, 1H), 4.30 (s, 2H), 3.21 (t,
2026
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FIGURE 1 1H NMR (400 MHz, CD3OD) spectra of (a) 1, and poly-1 (b) right after and (c) 1 day after dissolving, (d) a mixture of 2A
and 2B, and (e) poly-2.
J 5 5.5 Hz, 2H), 3.07 (t, J 5 6.0 Hz, 2H), 1.80 (quint, J 5 5.73
Hz, 2H) ppm, 13C NMR (100 MHz, CD3OD): d 5 152.16 (split
into a triplet after H/D exchange), 138.64, 138.00, 137.64,
128.88, 127.60, 114.29, 57.28, 44.09, 42.89, and 21.82 ppm.
1H NMR (400 MHz, CD3OD): d 5 7.26 (brs, 1H, disappeared
after H/D exchange), 7.00 (brs, 2H), 6.52 (brs, 2H), 4.24
(brs, 2H), 3.17 (brs, 2H), 3.01 (brs, 2H), 1.72 (brs, 2H), 1.20–
2.10 (brs, 3H) ppm, 13C NMR (100 MHz, CD3OD): d 5 152.10
(s, before H/D exchange, and highly broadened after H/D
exchange), 145.91, 136.02, 129.09, 128.68, 57.31, 45.89,
44.12, 42.82, 41.68, and 21.83 ppm.
Free Radical Polymerization of 1ꢁHCl (Synthesis of Poly-1ꢁHCl)
1ꢁHCl (1.050 g, 4.4 mmol), 2,20-azobis (2-amidinopropane)
dihydrochloride (V-50, 36 mg, 0.133 mmol), and H2O (10
mL) were fed into round flask. Then, the mixture was cooled,
degassed, and heated at 75 ꢀC for 20 h. The product was
precipitated from tetrahydrofuran and collected by filtration.
The product was dissolved in a small amount of methanol,
and precipitated from diethyl ether several times and col-
lected by filtration, and then in a vacuum oven to dry for 24
h. The yellow powder was obtained with the yield of 99%.
SEC-analysis (eluent, H2O) of the polymer was carried out to
estimate the number-average molecular weight (Mn) and
the molecular weight distribution (Mw/Mn) (Mn 5 83,000,
Synthesis of a Mixture of N-(3-((4-vinylbenzyl)amino)propyl)
formamide (2A) and N-(3-aminopropyl)-N-(4-vinylbenzyl)
formamide (2B)
An aqueous solution of potassium hydroxide (3 wt %, 4.0
mL) was added to a 0.2-M solution of 1ꢁHCl in water (5 mL,
1 mmol). The water was removed by evaporation after being
stirred at room temperature for 10 min. The residue was
dissolved in chloroform (10 mL) and the insoluble part was
removed by suction filtration. The filtrate was evaporated to
dryness to give a mixture of 2A and 2B as a colorless oil in
55% yield (2A:2B 5 59:41).
Mw/Mn 5 1.99). IR (ATR), 1675 cm21
.
1H NMR (400 MHz, CD3OD): d 5 8.53 (d, J 5 27.9 Hz, 1H),
7.21 (m, 2H), 6.58 (m, 2H), 4.71 (brs, 2H), 3.11–3.60 (m,
4H), 1.98 (brs, 2H), 0.89–2.55 (m, 3H), 13C NMR (100 MHz,
CD3OD): d 5 153.77, 147.06, 132.75, 129.83, 129.30, 59.18,
45.67, 44.47, 41.83, 38.51, and 19.73 ppm.
For the characterization, these two isomers were separated
by preparative TLC (SiO2, MeOH/CHCl3 5 2/1 v/v, Rf values:
0.5 (2A); 0.13 (2B)). 2A: 1H NMR (400 MHz, CD3OD, two
conformers arising from the E/Z isomerism of the formamide
group were present (E:Z 5 7.5:92.5).
Synthesis of Poly-1
Chemical shift values belonging to the major Z-conformer are
underlined: d 5 8.0217.97 (s1s, 0.925H10.075H), 7.40 (m,
J 5 8.0 Hz, 2H), 7.30 (d, J 5 8.2 Hz, 2H), 6.72(dd, J 5 11.0,
17.9 Hz, 1H), 5.76 (d, J 5 20 Hz, 1H), 5.2 (d, J 5 9.4 Hz, 1H),
3.73013.729 (s1s, 2H), 3.27 (t, J 5 6.9 Hz, 2H), 2.62 (t,
J 5 8.4 Hz, 2H), 1.7411.73 (quin1quin, J 5 7.1 Hz, 2H) ppm,
13C NMR (100 MHz, CD3OD): d 5 163.84, 140.27, 137.99,
137.86, 129.73, 127.26, 113.74, 54.16, 47.11, 36.86, and
30.11 ppm. ESI-MS (MeOH, positive): Calcd for
A 0.1-M solution of sodium methoxide in methanol (5 mL,
0.5 mmol) was added slowly to a 0.2-M solution of poly-
1ꢁHCl (2.5 mL, 0.5 mmol) in methanol. After being stirred at
room temperature for 10 min under nitrogen, the mixture
was poured into dry diethyl ether (50 mL). The resulting
precipitate was collected by suction filtration under nitrogen,
and then dried in a vacuum oven for 24 h to afford poly-1 in
87% yield as a white powder. IR (ATR), 1626 cm21
.
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[C13H16D2N2NaO 2A1Na]1: m/z 5 243; Found: m/z 5 243
(The two NH protons were replaced with deuterium as the
solution of 2A in CD3OD used for the NMR measurements
was subjected to ESI-MS analysis). 2B: 1H NMR (400 MHz,
CD3OD, two conformers arising from the E/Z isomerism of
the formamide group were present (E:Z 5 57.5:42.5).
Chemical shift values belonging to the major conformer are
underlined: d 5 8.3418.23 (s1s, 0.57H10.43H), 7.4517.40
(d1d, J 5 7.8, 8.2 Hz, 1.14H10.86H), 7.2617.24 (d1d,
J 5 7.3, 7.8 Hz, 1.14H10.86H), 6.7516.71 (dd1dd, J 5 7.1,
10.1 Hz and J 5 7.32, 11.0 Hz, 0.43H10.57H), 5.8015.76
(d1d, J 5 10.5, 10.1 Hz, 0.57H10.43H), 5.2515.22 (d1d,
J 5 11.0, 10.0 Hz, 0.57H10.43H), 4.5514.49 (s1s, 0.43H1
0.57H), 3.28 (t1t, J 5 3.64 Hz, 1.14H10.86H), 2.60 (t1t,
J 5 6.88 Hz, 1.14H10.86H), 1.7011.63 (quint 1 quint, J 5 7.08,
6.86 Hz, 1.14H10.86H) ppm, 13C NMR (100 MHz, MeOD):
d 5 165.53 1 165.34, 138.95 1 138.45, 137.64 1 137.54, 137.45 1
137.21, 129.50 1 129.24, 127.74 1 127.68, 114.55 1 114.21,
52.00, 46.04 1 45.98, 40.31 1 39.45, 32.23, 30.77 ppm. ESI-MS
(MeOH, positive): Calcd for [C13H16D2N2NaO 2B1Na]1: m/
z5 243; Found: m/z5 243 (The two NH protons were
replaced with deuterium as the solution of 2B in CD3OD used
for the NMR measurements was subjected to ESI-MS analysis).
FIGURE 2 1H NMR spectra of (a) 2A, (b) 2B, and (c) a mixture
of 2A and 2B.
1
free cyclic amidine group (1) of which the H NMR spectrum
is shown in Figure 1(a). In CD3OD, the methine proton under-
went H/D coupling so that its 1H signal at 7.27 ppm disap-
peared and its 13C singlet at 152.16 ppm was split into a
triplet as a result of the 13C-D coupling (Supporting Informa-
tion Fig. S1). As the free radical polymerization of the styrene
with the neutral amidine group (1) did not proceed at all, we
synthesized poly-1ꢁHCl by free radical polymerization of the
hydrochloride monomer (1ꢁHCl) according to a slightly modi-
fied procedure of our previously published study.20 The poly-
styrene with amidinium hydrochloride pendant groups (poly-
1ꢁHCl) was neutralized with an equimolar amount of NaOMe
to afford the polystyrene with free cyclic amidine groups
(poly-1) in 87% yield. The structure of poly-1 was character-
ized by 1-D and 2-D 1H NMR spectra (Fig. 1 and Supporting
Information Figs. S2–S4). The 1H NMR spectrum in CD3OD
was in good agreement of the polymer structure; the vinyl
proton signals disappeared and the signals of the methine
and methylene proton of the polymer backbone were newly
appeared [Fig. 1(a,b)]. The methine proton of the cyclic ami-
dine moiety underwent H/D exchange reaction with CD3OD,
Synthesis of Poly-2
An aqueous solution of potassium hydroxide (3 wt %, 2.0
mL) was added to a 0.25-M solution of poly-1ꢁHCl (2 mL, 0.5
mmol) in water. The resulting precipitate was collected by
suction filtration and dried under vacuum to give poly-2 in
66.4% yield as a white solid. IR (ATR), 1651 cm21
.
1H NMR (400 MHz, CD3OD). The chemical shifts belonging to
the primary amine unit (corresponding to 2B) are double-
underlined: d 5 8.2418.0517.95 (br s1s1s, 0.475H1
0.495H10.03H), 7.02 (brs, 2H), 6.50 (brs, 2H), 4.4113.65
(brs1brs, 0.95H11.05H), 3.26 (brs, 2H), 1.71 (brs, 5H) ppm.
13C NMR (100 MHz, CD3OD). The chemical shifts belonging
to the primary amine unit (corresponding to 2B) are double
underlined: d 5 165.371165.211163.81, 138.40, 135.35,
129.40, 128.94, 54.29151.79, 48.36145.81, 44.12, 41.49,
40.08139.48136.92, and 32.11130.77130.21 ppm.
1
and its H signal almost disappeared within 1 day [Fig. 1(c)].
The 13C NMR spectrum was also in good agreement with the
structure, and the singlet owing to the methine carbon highly
broadened after 1 day as a result of 13C-D coupling after the
H/D exchange reaction with CD3OD (Supporting Information
Fig. S2). The IR spectrum of poly-1 showed a strong peak
attributable to the C@N stretching at a lower wavenumber of
1626 cm21 than that of poly-1ꢁHCl (1675 cm21). The molecu-
lar weight of poly-1ꢁHCl was determined to be Mn 5 83,000
(Mw/Mn 5 1.99) by SEC analysis (H2O, PEG standards).
RESULTS AND DISCUSSION
Synthesis and Characterization of Polystyrene
Derivatives having 6-Membered Cyclic Amidine or N-
formyl-1,3-propanediamine Pendant Groups
The hydrochloride of the styrene derivative bearing hydro-
chloride of 6-membered cyclic amidine group (1ꢁHCl) was
prepared from p-chloromethylstyrene according to our previ-
ously published method.20 Although we reported previously
that we neutralized successfully the cyclic amidinium hydro-
chloride moiety of 1ꢁHCl by using potassium hydroxide,20 we
could not reproduce the result and the most of the cyclic ami-
dine moiety was hydrolyzed as described below (Scheme
1).34 In this study, the neutralization was achieved by using
an equimolar amount of NaOMe to afford the styrene with a
As mentioned above, the cyclic amidine was highly suscepti-
ble to hydrolysis in alkaline solution. We treated 1ꢁHCl with
an excess of KOH in water to completely hydrolyze the ami-
dine moiety, and obtained a mixture of two N-formylated
propanediamines, secondary amine 2A, and primary amine
2B [Fig. 1(d)]. The molar ratio of 2A and 2B was calculated
1
to be 59:41 based on the integrals of the H NMR spectrum.
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the signals at 8.1 and 3.2 ppm, indicating the Z-conformation.
The signal at 8.2 ppm showed a crosspeak with 4.37 ppm,
which indicated the E-conformation. The abundance ratio of
the two conformers was evaluated to be E:Z 5 57:43. The bias
in conformation was quite small when compared to 2A, as
reported for analogous N-formylated ethylenediamines.35
We prepared poly-2 from poly-1ꢁHCl by treatment with an
excess of KOH in water by a similar procedure to that for the
2A/2B mixture from 1ꢁHCl. The IR spectrum showed a strong
absorption at 1651 cm21, which was attributed to the C@O
stretching of amide functionalities. The 1H NMR of poly-2
showed its signals at chemical shifts similar to those for 2A
and 2B except for disappearance of the vinyl proton signals
as well as appearance of the signals of the newly formed
polymer backbone (Fig. 1(e) and Supporting Information Figs.
S7 and S8). The 13C NMR spectrum was also consistent with
the structure of poly-2 (Supporting Information Fig. S9). The
molar ratio of the secondary amine unit (corresponding to
2A) and the primary amine unit (corresponding to 2B) was
estimated to be 47.5:52.5 based on the relative integral values
of the benzylic proton signals, which was comparable to that
for the 2A/2B mixture obtained by the hydrolysis of 1ꢁHCl
although the bias in the regioselectivity was inverted.
FIGURE 3 Partial NOESY spectrum of 2B (mixing time 5 0.5 s).
As it was difficult to assign all the signals of the 1H NMR
spectrum owing to its complexity, we separated the two iso-
mers by preparative TLC with SiO2 stationary phase (eluent:
MeOH/CHCl3 5 2/1 v/v). Both isomers were characterized
1
by H, 13C, and 2-D NMR as well as ESI-MS (Fig. 2 and Sup-
CO2 Capture and Release by Styrenes having a 6-
Membered Cyclic Amidine or an N-formyl-1,3-
1
porting Information Figs. S5–S7). The H NMR spectra of the
two isomers, 2A and 2B, showed the absence of NH signals
as a result of the H/D exchange reaction with CD3OD. The
CHO proton of 2A appeared as two singlets at 8.02 and 7.97
ppm with an integral ratio of 92.5:7.5, respectively, arising
from the existence of the two conformers owing to the E/Z
isomerism of the formamide moiety [Fig. 2(a)]. The major
isomer was assigned to be the Z-conformer according to Pet-
ride’s report on analogous mono-formylated ethylenedi-
amines.35 On the other hand, the isomer 2B exhibited two
singlets for its CHO proton at 8.34 and 8.23 ppm, which are
comparable in integral intensity [Fig. 2(b)]. The assignment
of the signals to the two isomers was achieved based on the
measurements of NOESY of 2B (Fig. 3 and Supporting Infor-
mation Fig. S6). A distinct crosspeak was observed between
propanediamine Moiety
The CO2 capture and release behaviors of 1 and 2A/2B in
aqueous solution were investigated in terms of conductivity
(Fig. 4). Upon bubbling CO2 gas through a solution of the
cyclic amidine monomer 1 in wet DMSO containing 1% of
water (10 mM) at 25 ꢀC, its conductivity increased rapidly
and reached a saturated value within approximately 3 min
[Fig. 4(a)]. The maximum value of about 11 mS/m was com-
parable to those for analogous amidines,24,29 indicating the
formation of the bicarbonate of 1 (1HꢁHCO3). When N2 was
bubbled through the DMSO solution saturated with CO2 at
50 ꢀC, its conductivity decreased to almost the initial value
within approximately 4 min, meaning all the captured CO2
was released. The CO2 capture and release cycle could be
FIGURE 4 Changes in the conductivity of (a) 1 and (b) a mixture of 2A and 2B in DMSO containing 1 vol % of H2O (10 mM) with
bubbling CO2 at 25 ꢀC and N2 at 50 ꢀC in an alternate manner.
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formation of the alkylcarbonate salt.32,33 The molar ratio of
captured CO2 to the amidine unit was calculated based on
the weight increase of the binary mixture, reaching 45%
after 240 min under the conditions. Then the mixture was
exposed to a N2 flow at 45 ꢀC, showing a decrease in the
trapped CO2 down to 12% after 240 min. Similarly, the
binary homogeneous mixture prepared from poly-2 and
ꢀ
PEG400 was subjected to a CO2 flow at 25 C. After 240 min,
the trapped CO2 reached 29%, which was 16% less than
that of poly-1. The mixture was then exposed to a N2 flow at
45 ꢀC. However, the binary system released CO2 after 240
min to a lesser extent when compared with the binary sys-
tem of poly-1. Although both the binary systems could be
repeated at least three times, the mixture with poly-1 cap-
tured and released CO2 more efficiently than that with poly-
2 as it is clearly shown in Figure 5. However, the TG profile of
the binary mixture of poly-1 became closer to that of poly-2
with each cycle. Then we examined the binary samples by IR
spectroscopy (Supporting Information Fig. S11). The freshly
prepared binary sample of poly-1 showed a strong absorption
at 1632 cm21 assignable to C@N stretching of the amidine
group, whereas the binary sample of poly-2 showed an absorp-
tion peak at 1659 cm21, which was assigned to C@O stretching
of the N-formyl-1,3-propannediamine group. On the other
hand, the binary sample of poly-1 that had been stored in air
at ambient temperature for 2 weeks showed an absorption
peak at 1650 cm21, which was in between those of poly-1 and
poly-2, indicating that the amidine group was partially hydro-
lyzed to yield the amino group after the sample was stored in
air at ambient temperature for a long period of time. Therefore,
the deterioration in the CO2 capture and release of the binary
system of poly-1 could be attributed to the partial hydrolysis
of the amidine group by a small amount of moisture contained
in CO2 or N2 gas during the TG measurements.
FIGURE 5 TG profiles of homogeneous mixtures of (a) poly-1
and PEG400 (black) and (b) poly-2 and PEG400 (gray) with flow-
ing CO2 at 25 ꢀC and N2 at 45 ꢀC in an alternate manner (200
mL/min).
repeated three times with quick responses. However, the
maximum conductivity value showed a slight decrease at
every step, which could be attributed to hydrolysis of the
cyclic amidine moiety in the wet DMSO solution. On the
other hand, CO2 gas was bubbled through a solution of a
mixture of 2A and 2B in wet DMSO containing 1% of water
(10 mM) at 25 ꢀC, its conductivity increased promptly, and
reached a saturated value within approximately 7.5 min [Fig.
4(b)]. The maximum value of about 3 mS/m was much
smaller than that of 1 in wet DMSO, reflecting that the reac-
tion of 2A or 2B with CO2 formed an electronically neutral
carbamic acid instead of the bicarbonate salt formation from
1 and CO2. Upon purging with N2 gas at 50 ꢀC, the conduc-
tivity of the wet DMSO solution was reduced quickly to the
original value, indicating a complete release of CO2. The cycle
could also be repeated three times without decrease in the
maximum conductivity value. Thus, the mixture of the
amines 2A and 2B in aqueous solution captured and
released CO2 as efficiently as that of the cyclic amidine 1, as
it is the case of wet DMSO solutions of a linear polyamidine
and polyethyleneimine.33
CONCLUSIONS
In this study, we carried out the comparison of the CO2 cap-
turing and releasing abilities of the cyclic amidines and their
hydrolyzed products, N-formyl-1,3-propanedimaines in solu-
tion as well as in the solid state. We synthesized the polysty-
rene having 6-membered cyclic amidine pendant groups by
free radical polymerization of the corresponding styrene
monomer as the HCl salt. In the course of the polymer syn-
thesis, we found that treatment of the polymer with an
excess of KOH resulted in hydrolysis of the cyclic amidine
unit to form N-formyl-1,3-propanedimaine moiety. The struc-
ture of the resulting polymer with N-formyl-1,3-propanedi-
amine pendant groups was characterized by 1H, 13C, 2-D
NMR, and IR along with the studies using its unit models
prepared from the styrene monomer with the cyclic amidine.
Using the styrene monomer having the cyclic amidine group
and its hydrolyzed N-formyl-1,3-propanediamines, we inves-
tigated their CO2-capturing and -releasing abilities in solu-
tion in terms of conductivity, and found that both efficiently
captured and released CO2 with a quick response, and no
appreciable difference was observed between them. We then
examined the CO2-capturing and -releasing behaviors of the
CO2 Capture and Release by Polystyrenes Bearing 6-
Membered Cyclic Amidine or N-formyl-1,3-
propanediamine Pendant Groups
Finally, we compared the CO2 capturing and releasing abil-
ities of poly-1 and poly-2 by TGA (Fig. 5 and Supporting
Information Fig. S10).36 Mixing poly-1 with PEG with an
average molecular weight of 400 (PEG400) gave a colorless,
highly viscous oil of which the sample weight was monitored
by TG apparatus under CO2 or N2 atmosphere. When
exposed to a CO2 flow at 25 ꢀC, the mixture of poly-1 and
PEG400 showed an increase in the weight as a result of the
2030
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ARTICLE
15 S. Araki, Y. Kiyohara, S. Tanaka, Y. Miyake, J. Colloid Inter-
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amidine polymer and the amine polymer in the solid state
by taking advantage of the binary system with PEG, and
found that the binary system with the amidine polymer cap-
tured and released CO2 more efficiently than the binary sys-
tem with the amine polymer, in contrast with the results on
the solution study. Amidines are known to be versatile syn-
thons for supramolecular chemistry, owing to their high
affinity toward oxo acids, such as carboxylic acids and sul-
fonic acids, as well as their coordinating ability toward tran-
sition metals.37–41 We believe that further investigation of
the binary system studied here could lead to more sophisti-
cated CO2 absorbing systems with highly ordered nanostruc-
ture by utilizing amidine units for both CO2 capture and
supramolecular synthons with acidic groups or transition
metals. Such studies are ongoing in our group.
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24 D. J. Heldebrant, P. G. Jessop, C. A. Thomas, C. A. Eckert,
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This study was financially supported by JSR Corporation.
Grant-in-Aids for Scientific Research (C) from the Japan Society
for the Promotion of Science (JSPS), Tatematsu Foundation, and
Asahi Glass Foundation are also acknowledged. The authors
thank Takeshi Tanaka of Evaluation Center of Materials Proper-
ties and Function, Institute for Materials Chemistry and Engi-
neering, Kyushu University, for the ESI-MS measurements.
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