Aliphatic poly(urethane-amine)s synthesized by copolymerization of aziridines and supercritical carbon dioxide

Article abstract of DOI:10.1021/ma050549o

A reaction of aziridines and carbon dioxide (CO₂) proceeds under supercritical conditions to give random copolymers containing urethane and amine moieties. The ratio of urethane and amine linkages in the product can be controlled by reaction conditions including temperature and pressure as well as addition of N,N-dimethylacetamide as a cosolvent. An aqueous solution of the copolymer obtained from 2-methylaziridine and CO₂ undergoes a thermally induced phase transition at a lower critical solution temperature (LCST). The critical temperature is highly sensitive to changes in the primary structure of the products. The use of supercritical CO₂ (scCO ₂) allows us to construct a functional material through the effective chemical fixation of CO₂.

Full text of DOI:10.1021/ma050549o

Macromolecules 2005, 38, 6429-6434
6429
Aliphatic Poly(urethane-amine)s Synthesized by Copolymerization of
Aziridines and Supercritical Carbon Dioxide
Osamu Ihata and Yoshihito Kayaki*
PRESTO, Japan Science and Technology Agency, Tokyo Institute of Technology, 2-12-1, O-okayama,
Meguro-ku, Tokyo 152-8552, Japan
Takao Ikariya*
Graduate School of Science and Engineering and Frontier Collaborative Research Center,
Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Received March 15, 2005; Revised Manuscript Received May 12, 2005
2
ABSTRACT: A reaction of aziridines and carbon dioxide (CO ) proceeds under supercritical conditions
to give random copolymers containing urethane and amine moieties. The ratio of urethane and amine
linkages in the product can be controlled by reaction conditions including temperature and pressure as
well as addition of N,N-dimethylacetamide as a cosolvent. An aqueous solution of the copolymer obtained
from 2-methylaziridine and CO
temperature (LCST). The critical temperature is highly sensitive to changes in the primary structure of
the products. The use of supercritical CO (scCO ) allows us to construct a functional material through
the effective chemical fixation of CO
2
undergoes a thermally induced phase transition at a lower critical solution
2
2
2
.
Introduction
Scheme 1. Copolymerization of Aziridine and CO
2
There has been extensive interest in supercritical
fluids (SCFs) as alternatives to hazardous organic
1
solvents. The use of SCFs as reaction media potentially
leads to higher rates and/or higher product selectivity
based on their high gas miscibilities, high diffusivities
of solutes, and tunable solvent properties. In particular,
CO2 has been utilized as one of the most suitable media
for organic reactions because of its low cost, nontoxicity,
sponsive polymers such as poly(N-isopropylacrylamide)
PNIPAAm). This important class of materials has
attracted a great deal of interest for potential applica-
tions in drug delivery systems, microactuators, and
gene-transfection agents.11 In this paper, we describe
details of the copolymerization of aziridines and CO2
and the influence of the reaction conditions on the
structure of the products and the thermoresponsive
properties.
(
nonflammability, and moderate critical point (31.1 °C,
9
10
2
7
.38 MPa).
Utilization of CO2 as a reactant in synthetic processes
has been extensively investigated in the past decade.
However, few studies of copolymerization using CO2 as
a monomer under supercritical condition have been
3
carried out other than synthesis of aliphatic polycar-
bonates from epoxides and CO2 using metal catalysts.4
Since the early works were reported by Stevens in 1966
Experimental Section
5
and Inoue and Tsuruta in 1969, a significant improve-
Materials. 2-Methylaziridine was purchased from Tokyo
Kasei Co. and purified by distillation under reduced pressure;
other aziridines were synthesized by dehydration of the
ment in the activity and selectivity of the copolymeri-
zation has been achieved using well-defined molecular
1
2
6
corresponding amino alcohols with H
2
SO
4
.
These aziridines
catalysts. On the other hand, distorted three-membered
were stored over sodium hydroxide before use. Liquefied
carbon dioxide (99.999%) was purchased from Showa Tansan.
Typical Procedure for Copolymerization. Caution: since
high gas pressures are involved, safety precautions must be
taken at all stages of studies. The reaction was carried out in
a 50 mL stainless steel autoclave. To the autoclave filled with
argon gas, 2-methylaziridine (8.8 mmol) was introduced with
cyclic amines (secondary aziridines) were reported to
react with CO2 to give polymeric products consisting of
urethane and amine units even in the absence of a metal
7
catalyst (Scheme 1). The chain growth reaction, involv-
ing a successive ring-opening reaction of the aziridines
to form polyamines in preference to the copolymeriza-
tion with CO2, led to products with insufficient content
of the urethane linkage. We have recently reported that
the copolymerization of aziridines and CO2 under su-
percritical conditions efficiently proceeded to give the
corresponding poly(urethane-amine), which exhibited
a thermally induced reversible transition property in
the aqueous solution around the lower critical solution
2
a syringe. Then CO was charged through a cooling apparatus
with an HPLC pump. After stirring at 100 °C for 24 h, the
reaction was stopped by cooling the autoclave in a dry ice-
methanol bath. The autoclave was slowly cooled in the ice bath,
2
followed by the slow release of CO . The polymeric product
was purified by reprecipitation from methanol/ether to remove
low-molecular-weight products, and finally, the product was
dried in vacuo.
8
temperature (LCST), as observed in typical thermore-
Measurements. All NMR experiments were performed on
a JEOL Lambda 300 spectrometer. The experimental proce-
dures for high-pressure NMR studies have previously been
*
Corresponding authors: e-mail ykayaki@o.cc.titech.ac.jp,
tikariya@apc.titech.ac.jp; Fax +81-3-5734-2637; Tel +81-3-
734-2881.
1
3
described in detail. C, H, and N elemental analyses were
performed on a Perkin-Elmer 2400 analyzer. Urethane con-
5
1
0.1021/ma050549o CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/21/2005

6430 Ihata et al.
Macromolecules, Vol. 38, No. 15, 2005
Table 1. Copolymerization of Aziridines and CO2a
temp,
°C
CO2,
MPa
yield,
%
urethane
content, %
aziridine
2
2
2
-methylaziridine
-ethylaziridine
-ethylaziridine
100
100
100
100
140
22
10
22
10
22
35
10
18
6
62
54
62
73
52
cyclohexeneimine
2
,2-dimethylaziridine
8
a
Conditions: 50 mL autoclave, aziridine ) 8.8 mmol, 24 h.
Table 2. Pressure Effect on the Copolymerization of 1
and CO2a
urethane
CO2, MPa
3
yield, %
content, %
LCST, °C
Mw
3
5
5
4
14
18
22
35
32
47
53
62
85
64
41
34
7.0 × 10
1.5 × 10
1.4 × 10
2.7 × 10
10
16
22
1
Figure 1. H NMR spectra of the crude product prepared from
(22 MPa) with various reaction times in D O as
a
Conditions: 50 mL autoclave, 1 ) 8.8 mmol, 100 °C, 24 h.
1 and scCO
2
2
the NMR solvent: (a) 1.0 h, (b) 2.0 h, (c) 4.0 h, (d) 12 h, and
(e) 24 h (b, 1; 2, 4-methyl-2-oxazolidinone; 9, 5-methyl-2-
oxazolidinone).
Table 3. Temperature Effect on the Copolymerization of
1
and CO2a
temp, °C
yield, %
urethane content, %
LCST, °C
product was purified by reprecipitation from a methanol
solution of the crude product into diethyl ether to give
a white powder in 35% yield. Size exclusion chroma-
tography coupled with multiangle laser light scattering
(SEC-MALLS) analysis indicated that the isolated
product was indeed a high molecular weight material
6
8
0
0
0
8
35
18
11
58
62
52
47
67
34
24
28
100
120
140
a
Conditions: 50 mL autoclave, 1 ) 8.8 mmol, CO2 ) 22 MPa,
4 h.
4
2
(Mw ) 2.7 × 10 ). The urethane content in the product
can be estimated to be 62% by elemental analysis.
In a similar manner, 2-ethylaziridine, cyclohexene-
imine, and 2,2-dimethylaziridine (2) reacted with scCO2
to give copolymerization products. The results of the
copolymers obtained at 100 or 140 °C are summarized
in Table 1. The reaction of cyclohexeneimine under a
pressure of 10 MPa provided a polymeric product with
relatively high urethane content (73% urethane link-
ages).
Notably, the CO2 pressure was found to affect the
outcome of the copolymerization. The representative
results in the copolymerization of 1 are summarized in
Table 2. An increase in the reaction pressure caused the
enhancement of CO2 incorporation into the products
derived from 1 or 2-ethylaziridine, the urethane content
of the product reaching up to 62%. The reaction with
subcritical CO2 (3.0 MPa) provided unsatisfactory re-
sults in terms of the isolated yields, the urethane
content, and the Mw.
The reaction temperature also affected the outcome
of the copolymerization as shown in Table 3. When the
reaction was conducted at 60 °C for 24 h, the aziridine
was mostly consumed, but no solid products were
obtained after the reprecipitation. Copolymer with high
molecular weight was isolated in 8% yield after the
reaction at 80 °C. The maximum yield and urethane
content were obtained from the reaction at 100 °C.
Phase Behavior for the Copolymerization under
Supercritical Conditions. The outcome of the copo-
lymerization was found to be significantly dependent
upon reaction conditions, especially the CO2 pressure,
suggesting that the phase behavior of the copolymeri-
zation in scCO2 might be varied by the change in the
CO2 pressure or the reaction temperature. In fact, as
depicted in Figure 2, a high-pressure NMR spectrum
of the reaction mixture in scCO2 (40 °C, 8.0 MPa)
showed sharp signals due to the monomer 1, where 1
dissolved in the CO2 medium to form a homogeneous
tents in Tables 1-3 were calculated from the elemental
analyses of the copolymerization products containing urethane
and amine moieties. GC-MS analyses were carried out using
a Shimadzu QP-5000 spectrometer. Infrared spectra of KBr
pellet samples were recorded with a JASCO FTIR-610 spec-
trometer.
The molar mass distributions of the products were deter-
mined by SEC using a dual-detector system consisting of a
MALLS device (DAWN-EOS Model, Wyatt Technology) and a
refractive index detector set in the direction of the flow. All
samples were filtered through Millipore Millex-LG (0.20 µm)
before the analyses. SEC-MALLS measurements were carried
out at 35.0 °C using Shodex OHpak SB-804HQ and SB-
06MHQ columns at the polymer concentration of 10 mg/mL.
Eluent was an aqueous solution of 0.2 M NaNO /0.5 M CH
COOH at a flow rate of 0.5 mL/min. The LCST for the product
was measured by monitoring the transparency at 500 nm of a
.0 wt % of aqueous solution at a rate of heating of 1 °C/min
with a temperature-variable UV-vis spectrometer Shimadzu
UV-1600PC. The LCST was defined as the onset point in the
absorbance curve of the polymer solution during the heating
process.
8
3
3
-
2
Results and Discussion
Copolymerization of Aziridines and CO2. A reac-
tion of 2-methylaziridine (1) with CO2 at 22 MPa and
1
00 °C in a 50 mL stainless steel autoclave proceeded
smoothly without any additives to give viscous crude
1
products. As shown in Figure 1, the H NMR spectra of
the reaction products revealed the formation of the
desired polyurethanes in addition to the lower molecular
weight cyclic urethanes, 4-methyl-2-oxazolidinone and
5
-methyl-2-oxazolidinone, which are possibly formed by
1
4
ring expansion of the monomer or by degradation
1
5
(
backbiting) of the product polyurethane. Formation
of the five-membered cyclic urethanes was also con-
firmed by GC-MS analyses of the crude products.
Notably, monomer 1 was totally recovered after being
heated to 100 °C in the absence of CO2. The polymeric

Macromolecules, Vol. 38, No. 15, 2005
Copolymerization of Aziridines and Supercritical CO
2
6431
Figure 2. 1H NMR spectra of 1 in scCO
(40 °C, 8.0 MPa):
2
(
a) 0, (b) 1.5, and (c) 17 h.
single phase. These signals became weaker as the
reaction progressed, implying that CO2-insoluble sub-
stances might be precipitated from the scCO2 phase
during the reaction.
Visual inspection of the reaction phase behavior using
a 50 mL high-pressure reactor equipped with sapphire
windows revealed that the homogeneous phase contain-
ing 1 (8.8 mmol) was observed at the initial stage of the
reaction at 100 °C and 22 MPa of CO2, and the
subsequent phase separation by precipitation of the
polymeric products from CO2 phase appeared after 2 h
of reaction, as shown in Figure 3a.
Figure 3. Photographs of the internal of the reaction auto-
2
clave after 2 h of polymerization with 1 in scCO (22 MPa) (a)
To overcome the phase separation due to the lower
solubility of the product, we examined the effect of a
cosolvent, N,N-dimethylacetamide (DMAc), which may
have a marked affinity to both the hydrophilic polymers
without cosolvent and (b) with DMAc N,N-dimethylacetamide
(1 mL).
copolymeric products by increasing the reaction pres-
sure resulted in a marked increase in the signals due
to urethane units. These NMR data are consistent with
the results of elemental analysis listed in Table 2.
In contrast to the spectra of the polymers obtained
from 1, the products from 2,2-dimethylaziridine (2) gave
1
6
and the CO2 medium. An addition of DMAc to the
reaction mixture could cause a marked enhancement
of the solubility of the products into CO2 to provide a
homogeneous reaction phase, leading to facilitating
chain elongation by introducing CO2 molecules. In fact,
the copolymerization of 1 in scCO2 containing DMAc (1.0
mL) under appropriate conditions gave the polymeric
product with a urethane content of up to 74%. These
results strongly imply that the supercritical homoge-
neous phase is a crucial factor for attaining well-
controlled copolymerization with CO2.
1
simple NMR spectra, as shown in Figure 5a. The H
NMR spectrum of the copolymer prepared from 2 at 100
°C exhibited signals due to the methylene protons
adjacent to the urethane oxygen and the amine nitrogen
at δ ) 3.8-4.2 and 2.3-2.6 ppm, respectively. No strong
signals assigned to the methylene group bound to the
nitrogen atom in the urethane linkage were observed
at around 3 ppm, suggesting that the monomer 2 reacts
Spectral Analysis of Copolymer. Figure 4 il-
1
lustrates the H NMR spectra of the copolymeric
1
products from the monomer 1 and CO2 at 3.0-22 MPa.
The signals due to methyl protons in the products are
observed at 0.7-1.2 ppm in each spectrum. The signals
assigned to both methylene and methine protons bound
to heteroatoms of the amine and urethane linkages
should appear in the region from 2.0 to 4.3 ppm. The
comparable integral values calculated for the signals at
regioselectively. As shown in Figure 5b,c, the H NMR
spectra markedly changed with the reaction tempera-
ture. In particular, the signals due to the methylene
group bound to amine moiety were weakened, and the
signals at δ ) 3.0-3.6 ppm assignable to the methylene
protons adjacent to urethane nitrogen appeared with
increasing reaction temperature.
0
.7-1.2 and 2.0-4.3 ppm are in agreement with the
structure of the polymeric products, as shown in Scheme
. Formation of urethane units by incorporation of CO2
Figure 6 displays the IR spectra of the products
synthesized from 1 and scCO2 at 100 °C. The urethane
moiety in the polymer provided sharp peaks at 1710 and
1
-
1
molecules into the polymer can be confirmed with the
signals at 3.6-4.3 ppm attributable to protons on the
carbon adjacent to the urethane’s oxygen. As shown in
Figure 4, an increase in the urethane content of the
1540 cm for carbonyl groups, in addition to the peaks
-
1
at 1250 and 1080 cm
due to urethane C-O-C
symmetric and asymmetric bending vibrations. Under
higher CO2 pressure, the intensities of these peaks

6432 Ihata et al.
Macromolecules, Vol. 38, No. 15, 2005
Figure 6. IR spectra of the polymeric products prepared from
1
2
and CO at (a) 3.0, (b) 12, and (c) 22 MPa.
1
Figure 4. H NMR spectra of the polymeric products prepared
2
from 1 and CO at (a) 3.0, (b) 8.0, (c) 12, (d) 16, and (e) 22
MPa.
Figure 7. IR spectra of the polymeric products prepared from
in scCO (22 MPa) at (a) 80, (b) 100, (c) 120, and (d) 140 °C.
1
2
Figure 5. 1H NMR spectra of the products prepared from 2
in scCO (22 MPa) at (a) 140, (b) 120, and (c) 100 °C.
2
should exist in an equilibrium with the corresponding
ammonium carbamate (b) under copolymerization con-
ditions (eq 1).18 It seems likely that nucleophilic attack
of the carbamate anion on an aziridinium cation is
favored to a neutral aziridine molecule. Since any
transformation of the monomer 1 was not observed
under otherwise identical conditions to the copolymer-
ization except for the absence of CO , the CO -assisted
increased relatively with increasing urethane content
in the product polymer.
The temperature-dependent change in the IR spectra
of polymers was observed, as shown in Figure 7. At
higher reaction temperatures, a new peak at 1670 cm-1
due to the carbonyl groups in the polymers was observed
2
2
-
1
in addition to the band at 1710 cm . Furthermore, a
reversible formation of the aziridinium cation might be
an essential step for the ring-opening reaction. A
plausible mechanism of the copolymerization of aziri-
2
-
1
peak at 1540 cm related to hydrogen bonding at the
urethane’s nitrogen decreased in absorbance. These
changes in the IR spectra are consistent with the change
dine and CO is outlined in Scheme 2.
1
in the H NMR spectra of the product by reaction
temperature as shown in Figure 5b,c, indicating that
an increase in the reaction temperature caused the side
reaction of the secondary amine units in the polymer
main chain, leading to formation of the N,N-disubsti-
tuted urethane structure as will be discussed later.17
Mechanism of Copolymerization. As many pri-
mary and secondary amines have been known to revers-
ibly generate the corresponding carbamic acid deriva-
tives by uptaking of CO2, the monomer cyclic amine (a)
The nucleophilic attack of the aziridine molecule or
the carbamate anion on the carbon atom of the aziri-

Macromolecules, Vol. 38, No. 15, 2005
Copolymerization of Aziridines and Supercritical CO
2
6433
Scheme 2. Plausible Copolymerization Routes of
Scheme 5. Formation of the Branched Structure in
the Copolymerization of Aziridines and CO
Aziridines and CO
2
2
bound to a urethane oxygen atom (mode B). These
methylene proton signals might be distinguishable by
1
the chemical shifts in the H NMR spectrum. As shown
1
in Figure 5a, the H NMR spectrum of the reaction
products of 2 and CO2 shows the typical signals for the
methylene group bound to the urethane’s oxygen at δ
Scheme 3. Possible Mechanisms for the Formation of
Cyclic Urethanes
)
3.8-4.3 ppm, indicating that the ring-opening takes
place via the attack of the carbamate oxygen on the less
hindered carbon and the methylene carbon-nitrogen
scission (mode B). In a similar way, the formation of
amine moieties in either mode C or D provides products
with a methylene group in the R-position to the amino
1
group. The H NMR spectrum in Figure 5a shows the
signals due to these methylene protons at around 2.5
ppm. Notably, when the reaction temperature was
increased up 140 °C, IR and NMR spectra of the reaction
products dramatically changed, as discussed in the
previous section (see Figures 5b,c and 7). These can be
explained as a result of the side reaction caused by the
secondary amine in the polymer main chain to give
branched polymers containing the N,N-disubstituted
urethane structure, as shown in Scheme 5.
Scheme 4. Ring-Opening Reactions of 2
Thermoresponsive Properties. The product co-
polymers were soluble in aqueous solutions but under-
went a phase transition in response to thermal stimulus.
Figure 8 shows the UV-vis light transmittance mea-
surements (λ ) 500 nm) of 1 wt % solutions of the
copolymers of 1 determined at temperatures ranging
from 20 to 90 °C. The transmittance of the solution of
product prepared at 3.0 MPa of CO2 decreased gradually
above 80 °C, while the products obtained under super-
critical conditions showed quick and reversible phase
transition behaviors around the LCST. The LCST of
copolymer was strongly affected by the polymerization
conditions. As seen from Table 2, an increased in the
pressure of CO2 from 3.0 to 22 MPa caused a significant
drop of the LCST of the product from 85 to 34 °C. When
the reaction temperature was increased from 80 to 120
dinium cation provides ring-opening intermediates A or
B, respectively, which will repeatedly react to form the
polymeric products, as shown in Scheme 2. The molar
ratio of A and B intermediates might be determined by
the concentration of the carbamate anion in the reaction
system. The shift to ammonium carbamate in the
equilibrium in eq 1 under higher pressure CO219 would
result in the preferential formation of the urethane
linkages in the copolymerization of aziridines. In fact,
the urethane content of the product from 1 was up to
62% under supercritical conditions, whereas the product
obtained at 3.0 MPa of CO2 had only 33% of urethane
linkages. The formation of byproducts, such as cyclic
urethanes, might be explained either by a rearrange-
ment of the twitter ionic intermediate X or by backbiting
1
5
from the intermediate B, as shown in Scheme 3.
Further information on the copolymerization mech-
anism was obtained with separate experiments. When
the substituted aziridine 2 is used as a monomer, there
are two possible ring-opening modes in terms of the
regioselectivity in the formation of the urethane, as
illustrated in Scheme 4. If the carbon-nitrogen bond
cleavage in the aziridinium cation occurred on the
sterically hindered side (mode A), the resulting urethane
nitrogen would be attached to the methylene group,
while the ring-opening at the less hindered carbon atom
should give the products bearing the methylene units
Figure 8. Light transmittances of 2.0 wt % aqueous solutions
of the copolymeric products from 1 at (a) 100 °C, CO
2
) 3.0
MPa, (b) 100 °C, CO ) 10 MPa, (c) 100 °C, CO ) 22 MPa,
2
2
and (d) 120 °C, CO
2
) 22 MPa.

6434 Ihata et al.
Macromolecules, Vol. 38, No. 15, 2005
°
C, the LCST was decreased from 67 to 24 °C, as listed
941. (d) Wells, S. L.; DeSimone, J. Angew. Chem., Int. Ed.
2001, 40, 518-527. (e) Chemical Synthesis Using Supercriti-
in Table 3.
cal Fluids; Jessop, P. G., Leitner, W., Eds.; Wiley-VCH:
Weinheim, 1999.
2) Green Chemistry Using Liquid and Supercritical Carbon
Dioxide; DeSimone, J. M., Tumas, W., Eds.; Oxford University
Press: New York, 2003.
Controlling Factors for Determining the Ther-
moresponsive Properties of the Copolymers. Since
the thermally induced phase transition behavior in the
aqueous solution is attributed to the dehydration and
hydrophobic aggregation of the polymers, a suitable
balance of the both hydrophilic and hydrophobic moi-
eties in the polymer structures should determine the
thermoresponsive properties. It has been reported that
control of the LCST of PNIPAAm copolymers can be
attained by employing hydrophilic and hydrophobic
(
(
3) (a) Sargent, D. E. US Patent US 2,462,680, 1949; (b) Buckley,
G. D.; Ray, N. H. US Patent US 2,550,767, 1951. (c)
Darensbourg, D. J.; Stafford, N. W.; Katsurao, T. J. Mol.
Catal. 1995, 104, L1-L4. (d) Super, M.; Berluche, E.; Cos-
tello, C.; Beckman, E. Macromolecules 1997, 30, 368-372.
20
(
e) Mang, S.; Cooper, A. I.; Colclough, M. E.; Chauhan, N.;
Holmes, A. B. Macromolecules 2000, 33, 303-308.
4) (a) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.;
Billodeaux, D. R. Acc. Chem. Res. 2004, 37, 836-844. (b)
Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004,
(
21
monomers. The influence of CO2 pressure on the LCST
of the copolymeric product from 1 as shown in Figure 8
can be simply explained by the fact that the unit ratio
of the hydrophilic amine moiety in the polymer chain
to the relatively hydrophobic urethane moiety was
precisely controlled by the reaction pressure and tem-
perature. The increase in the urethane content in the
copolymers obtained under higher pressure could have
led to a lowering of the LCST with a decrease in the
hydrophilicity of the polymers (Table 2). However, on
the other hand, the reaction temperature proved to be
the dominant factor in determining the LCST rather
than the urethane content in the copolymer, especially
for the products obtained at 120 and 140 °C as discussed
above (Table 3). The formation of branched structures
might cause a loss in the NH protons in the polymer
43, 6618-6639.
(
(
5) (a) Stevens, H. C. US Patent US 3,248,415, 1966. (b) Inoue,
S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Polym. Lett. Ed.
1969, 7, 287-292.
6) (a) Darensbourg, D. J.; Holtcamp, M. W.; Struck, G. E.;
Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Robertson, J. B.;
Draper, J. D.; Reibenspies, J. H. J. Am. Chem. Soc. 1999,
121, 107-116. (b) Cheng, M.; Lobkovsky, E. B.; Coates, G.
W. J. Am. Chem. Soc. 1998, 120, 11018-11019. (c) Darens-
bourg, D. J.; Yarbrough, J. C. J. Am. Chem. Soc. 2002, 124,
6335-6342. (d) Qin, Z.; Thomas, C. M.; Lee, S.; Coates, G.
W. Angew. Chem., Int. Ed. 2003, 42, 5484-5487.
(
(
7) Soga, K.; Chiang, W. Y.; Ikeda, S. J. Polym. Sci., Polym.
Chem. Ed. 1974, 12, 121-131.
8) (a) Ihata, O.; Kayaki, Y.; Ikariya, T. Angew. Chem., Int. Ed.
2
004, 43, 717-719. (b) Ihata, O.; Kayaki, Y.; Ikariya, T.
Chem. Commun. 2005, 2268-2270. (c) Ihata, O.; Kayaki, Y.;
Ikariya, T. Kobunshi Ronbunshu 2005, 62, 196-199.
chains, leading to lowering of the hydrophilicity of the
whole copolymer22 and a drop in the LCST, even if the
(9) (a) Okano, T.; Bae, Y. H.; Jacobs, H.; Kim, S. W. J. Controlled
Release 1990, 11, 255-265. (b) Bromberg, L. E.; Ron, E. S.
Adv. Drug Deliv. Rev. 1998, 31, 197-221. (c) Chilkoti, A.;
Dreher, M. R.; Meyer, D. E.; Raucher, D. Adv. Drug Deliv.
Rev. 2002, 54, 613-630.
ratio of urethane and amine linkages were not altered.
Conclusions
(
(
10) Harmon, M. E.; Tang, M.; Frank, C. W. Polymer 2003, 44,
We have demonstrated that copolymerization of aziri-
dines with CO2 proceeded smoothly without any cata-
lysts under scCO2 to give the polymeric products having
a thermoresponsive functionality covering a wide tem-
perature range. Since their primary structures and
physical properties have proven to be strongly influ-
enced by external polymerization conditions, the ther-
moresponsive properties can be simply tuned by reaction
conditions, in particular the CO2 pressure. Current
efforts involve the exploration of other properties, the
analysis of this copolymerization mechanism, and the
use of other aziridine families as monomers.
4547-4556.
11) (a) Yokoyama, M. Drug Discov. Today 2002, 7, 426-432. (b)
Twaites, B. R.; Alarc o´ n, C. D. L. H.; Cunliffe, D.; Lavigne,
M.; Pennadam, S.; Smith, J. R.; G o´ recki, D. C.; Alexander,
C. J. Controlled Release 2004, 97, 551-566.
(
(
12) Cairns, T. L. J. Am. Chem. Soc. 1941, 63, 871-872.
13) Kayaki, Y.; Suzuki, T.; Noguchi, Y.; Sakurai, S.; Imanari, M.;
Ikariya, T. Chem. Lett. 2002, 424-425.
(14) Soga, K.; Hosoda, S.; Nakamura, H.; Ikeda, S. J. Chem. Soc.,
Chem. Commun. 1976, 617.
(
(
(
(
15) Neffgen, S.; Keul, H.; H o¨ cker, H. Macromol. Rapid Commun.
1996, 17, 373-382.
16) Tsukahara, T.; Kayaki, Y.; Ikariya, T.; Ikeda, Y. Angew.
Chem., Int. Ed. 2004, 43, 3719-3722.
17) Kweon, J.-O.; Lee, Y.-K.; Noh, S.-T. J. Polym. Sci., Part A:
Polym. Chem. 2001, 39, 4129-4138.
Acknowledgment. This work was financially sup-
ported by a Grant-in-Aid from the Ministry of Educa-
tion, Science, Sports, and Culture of Japan (No.
18) (a) Fischer, H.; Gyllenhaal, O.; Vessman, J.; Albert, K. Anal.
Chem. 2003, 75, 622-626. (b) Carretti, E.; Dei, L.; Baglioni,
P.; Weiss, R. G. J. Am. Chem. Soc. 2003, 125, 5121-5129.
19) (a) Penny, D. E.; Ritter, T. J. J. Chem. Soc., Faraday Trans.
1 1983, 79, 2103-2109. (b) McGhee, W.; Riley, D.; Christ, K.;
Pan, Y.; Parnas, B. J. Org. Chem. 1995, 60, 2820-2830. (c)
Aresta, M.; Quaranta, E. Tetrahedron 1992, 48, 1515-1530.
20) (a) Heskins, M.; Guillet, J. E. J. Macromol. Sci., Chem. Ed.
1
2
4078209). A part of this study was supported by the
1st Century COE Program. The authors thank Dr. Y.
(
Sato and Professor A. Maruyama of Kyushu University
for performing temperature-controlled UV-vis spec-
troscopy.
(
1
968, A2, 1441-1445. (b) Bae, Y. H.; Okano, T.; Kim, S. W.
J. Polym. Sci., Polym. Phys. Ed. 1990, 28, 923-936.
(21) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules
993, 26, 2496-2500.
22) van Vliet, R. E.; Hoefsloot, H. C. J.; Iedema, P. D. Polymer
003, 44, 1757-1763.
References and Notes
1
(
1) (a) Jessop, P. G.; Ikariya, T.; Noyori, R. Science 1995, 269,
(
1
065-1069. (b) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem.
2
Rev. 1995, 95, 259-272. (c) Oakes, R. S.; Clifford, A. A.;
Rayner, C. M. J. Chem. Soc., Perkin Trans. 1 2001, 917-
MA050549O