Poly(hydroxyurethane): Catalytic applicability for the cyclic carbonate synthesis from epoxides and CO2

Article abstract of DOI:10.1039/d0cc04463f

We have developed a synthetic methodology using poly(hydroxyurethane) as an organocatalyst for the chemical fixation of CO2 into epoxides, leading to the formation of five-membered cyclic carbonates with remarkably high selectivity and yields. The catalyzed reaction was applicable to various epoxides.

Full text of DOI:10.1039/d0cc04463f

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Poly(hydroxyurethane): catalytic applicability
for the cyclic carbonate synthesis from epoxides
Cite this:DOI: 10.1039/d0cc04463f
and CO2†
Suguru Motokucho *a and Hiroshi Morikawa
Received 28th June 2020,
Accepted 4th August 2020
b
DOI: 10.1039/d0cc04463f
rsc.li/chemcomm
We have developed a synthetic methodology using poly(hydroxy-
urethane) as an organocatalyst for the chemical fixation of CO2 into
epoxides, leading to the formation of five-membered cyclic carbo-
nates with remarkably high selectivity and yields. The catalyzed
reaction was applicable to various epoxides.
Polyurethanes are industrially manufactured by the reaction
between polyfunctional isocyanates and polyols. Isocyanates pre-
pared from toxic phosgene are extremely harmful to both humans
and the environment. Therefore, poly(hydroxyurethane)s (PHUs)
can replace polyurethanes because their synthetic procedure is
environmentally benign. PHUs (also called non-isocyanate
polyurethanes) are easily produced via the aminolysis reactions
of five-membered cyclic carbonates (5CCs) without formation
of by-products (Fig. 1).1,2 In addition, PHUs have been studied
as potential coating materials, adhesives, and resins for three-
dimensional printing applications, owing to their unique
properties.1–6 5CCs, the key reagent for PHU synthesis, are
produced by the cycloaddition reaction of epoxides and carbon
dioxide (CO2) (Fig. 1);7–10 they are also used as electrolytes in
lithium ion batteries.11 The efficient transformation of CO2,
which is an industrial waste, into value-added reagents is a
critical research objective for sustainable chemistry and envir-
onmental preservation. CO2 is a renewable, inexhaustible,
cheap, and nontoxic resource, which allows synthetic flow
of PHUs through 5CC products with high atom economy.
As of today, various metal-based and organic catalysts have
been developed for 5CC synthesis.7–10 Among them, organoca-
talysts are preferred because of their low cost, low toxicity,
and versatile molecular design that allows the control of the
Fig. 1 PHU synthetic flow from epoxide through 5CC.
activity.12–14 Recently, we have found that traditional polyur-
ethane can be used as an organocatalyst for manufacturing
5CCs from epoxides and CO2,15 which stimulated the develop-
ment of various polyurethane structures. In this study, we
examine the applicability of PHU as a catalyst in the synthesis
of 5CCs (Scheme 1).
Fig. 1 illustrates a simple but innovative concept of this
study. 5CC derivatives are precursors for PHU production;
therefore, the use of PHU as a catalyst for 5CC synthesis can
help to build a material cycle involving 5CC and PHU without
any synthetic catalysts. In other words, the polymer catalyzes
the formation of the monomer serving as its own precursor,
which is reported for the first time here. Notably, this reaction
is performed under halogen-free and metal-free conditions,
which are required for the industrial manufacturing of 5CCs.16
We selected phenyl glycidyl ether (PGE) as the epoxide
substrate. The mixture of PGE and PHU reacted under 5.2 MPa
a Chemistry and Material Engineering Program, Nagasaki University, 1-14,
Bunkyo-Machi, Nagasaki-shi 852-8521, Japan. E-mail: motoku@nagasaki-u.ac.jp
b Department of Applied Chemistry, Kanagawa Institute of Technology,
1030, Shimo-ogino, Atsugi, Kanagawa 243-0292, Japan
† Electronic supplementary information (ESI) available: Reagents; synthesis and
characterization; 1H-NMR spectra, FT-IR spectra, GPC profiles; weights of recov-
ered PHU; plausible reaction mechanism, characteristic 5CC data (PDF). See DOI:
10.1039/d0cc04463f
Scheme 1 PHU-catalyzed PGE-5CC synthesis through the reaction of
PGE with CO2.
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CO2 pressure in the absence of any organic solvents (Scheme 1).
The molar ratio of PHU urethane moieties to PGE was varied
from 0.01 to 0.40 (the most frequently used value was 0.20); these
numbers are equivalent to catalyst amounts of 1–40 mol%. PHU
is soluble in PGE at temperatures higher than 90 1C under
atmospheric pressure; therefore, the reaction was expected to
proceed homogeneously under these conditions.
After a specified time, the reaction mixture was analyzed by
performing 1H nuclear magnetic resonance (1H-NMR) measure-
ments of the CDCl3 solution, estimating both the conversion of
PGE as the consumption and its selectivity from PGE to PGE-
5CC17 in the reaction mixture (Fig. S1 and eqn (S2), (S3), ESI†).
Finally, the reaction mixture was separated into diethyl ether-
soluble components and insoluble solids. The former con-
tained a mixture of PGE and PGE-5CC and was purified to
isolate pure PGE-5CC. In contrast, the insoluble solid consisted
of recovered PHU.
Table 1 lists the various parameters of the reactions, such as
conversion, selectivity, and isolated yield of PGE-5CC,18 per-
formed under different conditions. Notably, the PGE-5CC
selectivity in each reaction (entries 1–11 in Table 1) was
extremely high (499%) without forming any by-products,
which was consistent with the similar values of conversion
and isolated yield. At 90 1C, increasing the molar ratio from
0.01 to 0.40 increased both the conversion and yield. Surpris-
ingly, 96% conversion and 93% isolated yield were achieved at
the ratio of 0.40 (entry 7) despite a relatively mild temperature
of 90 1C, confirming the high catalytic activity of PHU. Reac-
tions at temperatures above 90 1C were also performed at a
fixed catalyst ratio of 0.20. At 120 1C, the carbonation reaction
achieved the highest yield of 97% after 16 h (entry 10). Further
increase in the temperature to 150 1C resulted in 82% yield
(entry 11) after only 4 h of reaction.
Fig. 2 1H-NMR spectra of the pristine PHU and the PHU recovered by
filtration (in DMSO-d6).
(FT-IR) spectroscopy, and gel permeation chromatography (GPC);
the obtained data were compared with the parameters of the
1
pristine PHU. No changes in the H-NMR spectra were observed
for the samples treated at 90 and 120 1C. In contrast, new peaks at
5.1–4.4 ppm appeared after the treatment at 150 1C, which were
assigned to the protons (p, q, r) of 5CC moieties (Fig. 2 and Fig. S2,
ESI†). In this case, the yield of 5CC moieties produced from
hydroxyurethane linkage (Scheme 2), as estimated from the
intensity ratio of signals q, k, and l, was 24%. This observation
was in good agreement with the results of FT-IR and GPC
measurements. The obtained FT-IR spectra (Fig. S3, ESI†) show
a characteristic peak at 1806 cmÀ1, which is assigned to the
carbonyl groups of 5CC moieties in the sample treated at 150 1C.
In the corresponding GPC profiles (Fig. S4, ESI†), the number-
and weight-average molecular weights noticeably decreased for
the PHU sample treated at 150 1C (Table S2, ESI†), but not for the
samples treated at 90 and 120 1C. The observed temperature
dependency presumably originated from the thermal decomposi-
tion of PHU urethane moieties leading to 5CC formation, as was
reported previously (Scheme 2).19,20 These results indicate that
temperatures below 120 1C are favorable for the recovery of PHU
without degradation.
In contrast to the isolated PGE-5CC, high yields of recovered
PHU solids were obtained from the reactions conducted over
the temperature range 90–150 1C (Table S1, ESI†), indicating
that PHU could be easily separated from the reaction mixture.
These solids were analyzed by 1H-NMR, Fourier-transform infrared
The described PHU-catalyzed reaction was performed for
various substituted epoxides, resulting in the formation of differ-
ent 5CCs (5CC-1 to 5CC-10) with relatively high yield (Fig. 3). For
optically active 5CC-9 and 5CC-10, the stereochemical configu-
ration was maintained during the entire reaction (Scheme S1,
ESI†). In addition to monofunctional 5CCs, bifunctional 5CC-11
Table 1 Effects of the reaction conditionsa on the synthesis of PGE-5CC
Temp./ Catalyst Time/ Conversionc,d
/
Selectivityc,e
%
/
Isolated
yield f/%
Entry 1C
ratiob
h
%
1
2
3
4
5
6
7
8
90
90
90
90
90
90
90
120
120
120
150
0.01
0.05
0.10
0.20
0.20
0.20
0.40
0.20
0.20
0.20
0.20
16
16
16
4
11
16
16
4
Trace
18
31
8
28
50
96
31
95
499
499
499
499
499
499
499
499
499
499
499
2
14
26
6
26
46
93
27
91
97
82
9
10
11
11
16
4
Quant.
83
a
b
Under 5.2 MPa CO2. Molar ratio of [urethane moiety of PHU]0/
[PGE]0. Determined from 1H-NMR analysis of the reaction mixture.
c
d
e
f
Scheme 2 Thermal decomposition reaction of the hydroxyurethane
Consumption of PGE. From PGE to PGE-5CC. After the purification
linkage in the main chain.
via SiO2 column chromatography.
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NH proton of the urethane moiety and the oxygen atom of the
epoxide, which influenced the electronic density distribution of
the urethane moiety. This consideration of formation of hydro-
gen bond is supported by similar 1H-NMR analyses in previous
studies.10,23 In these studies, the epoxide was reported to be
activated by the hydrogen bonds for the synthesis of 5CCs.
From the results of Scheme S1 (ESI†) and the aforemen-
tioned 1H-NMR analyses (Fig. S7 and S8, ESI†), a plausible
mechanism of the PHU-catalyzed 5CC formation reaction is
illustrated in Scheme 4. First, the epoxide is activated by the
hydrogen bond through the NH proton of urethane moiety. A
cyclic compound (1) is then formed owing to the reaction
between the epoxide and the carbonyl group of urethane
moiety, followed by a ring-opening reaction resulting in the
formation of zwitterionic species involving alkoxide ion (2).
Subsequently, it reacts with CO2 to form corresponding carbo-
nate anion (3). Finally, (3) undergoes a ring-closure reaction
causing the release of urethane moiety and the formation of the
desired 5CC. Jiang et al. have reported synthesis of 5CC from
epoxide and CO2, catalyzed by N,N-dimethylformamide (DMF)
in the absence of nucleophiles such as chloride ion.24 In that
study, DMF reacts with epoxide to form cyclic species, followed
by a ring-opening and subsequent ring-closure reactions. The
reaction mechanism shown in Scheme 4 is in good agreement
with the DMF-catalyzed reaction mechanism.
Fig. 3 Syntheses of 5CCs from various epoxides. Reaction conditions:
120 1C, 5.2 MPa CO2, 16 h, 5.0 mmol epoxide, [PHU urethane groups]0/
[epoxide]0 = 0.20. a Isolated yield. b Conversion of epoxy moieties in the
reaction.
and 5CC-12 were produced from the corresponding diepoxides.
Bisphenol A-based 5CC-12 is a well-known starting compound for
PHU synthesis,1,2,21,22 and it allows the implementation of the
cycle outlined in Fig. 1. The polymerization of 5CC-12 obtained in
this study was conducted with 1,6-hexamethylenediamine for the
preparation of PHU.21 The reaction successfully produced PHU as
a white solid with 87% yield, which was confirmed after obtaining
1H-NMR spectrum and GPC profile similar to that of the pristine
PHU (Fig. S5 and S6, ESI†). These results indicate achievement of
the concept shown in Fig. 1.
Finally, to examine the effect of the PHU structure on this
reaction, TMS-PHU was selected. As a preliminary experiment,
the reaction of PGE with CO2 was performed using TMS-PHU18
instead of PHU (Scheme 1) under the conditions specified in
entry 6 of Table 1. After the reaction, TMS-PHU solid was
successfully recovered by re-precipitation into diethyl ether.
1H-NMR and GPC analyses revealed that the recovered TMS-
PHU did not decompose (e.g., via desilylation) during the
To elucidate the interaction between PGE and urethane moiety
of PHU at the initial stage of the reaction, 1H-NMR spectra of PHU
in the presence and absence of PGE were measured in DMSO-d6 at
25 1C (Fig. S7, ESI†). Addition of 2 equivalent (molar ratio) of PGE
to the urethane moiety resulted in the shift of chemical shift
values of the N–H and O–H protons of PHU from 5.230 and 4.869
to 5.264 and 4.912 ppm, respectively in comparison to the 1H-
NMR spectra of PHU. We also measured 1H-NMR spectra of PGE
and a modified PHU, i.e., a hydroxy group-protecting combination
of PHU with trimethylsilyl ether (denoted as TMS-PHU,
Scheme 3). The chemical shift value of the N–H proton of TMS-
PHU shifted from 4.725 to 4.738 ppm in CDCl3 owing to the
presence of PGE (Fig. S8, ESI†). The change in the chemical shift
values indicates the formation of a hydrogen bond between the
Scheme 4 Plausible reaction mechanism for the urethane-catalyzed
5CC synthesis through the reaction of epoxide with CO2.
Scheme 3 Chemical structure of TMS-PHU.
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´
˙
3 M. Tryznowski, A. Swiderska, T. Gołofit and Z. Zołek-Tryznowska,
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reaction, indicating that its original structure was well pre-
served (Fig. S9, S10 and Table S3, ESI†). After performing the
evaluation and purification procedures that were similar to
those conducted using PHU, the conversion of PGE and iso-
lated yield of PGE-5CC in the TMS-PHU catalyzed reaction were
52% and 49%, respectively. These values are very close to those
obtained using PHU catalyst (entry 6), which indicates that the
urethane group of TMS-PHU exhibited sufficient catalytic activ-
ity for the cyclic carbonate synthesis. This result suggests that
the catalytic activity of the urethane moiety in the original PHU
is much higher than that of its hydroxy functional group
(Scheme 1). However, PHU and TMS-PHU are inherently differ-
ent in terms of hydrophobicity and main chain mobility, which
presumably affect their catalytic activities. Therefore, a further
study on the structural dependency of PHU on the reaction
behavior is currently ongoing.
4 H. Matuskizono and T. Endo, J. Am. Chem. Soc., 2018, 140, 884.
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carbonates from epoxides and CO2, ed. X.-F. Wu and M. Beller,
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18 The supplied PHU and TMS-PHU were further purified by the
precipitation into methanol and extraction with CHCl3 and water,
respectively. In the reactions employing the purified PHU and TMS-
PHU as catalysts, PGE-5CC was obtained in similar yields to those
using the supplied pristine PHU and TMS-PHU.
19 S. Hu, X. Chen and J. M. Torkelson, ACS Sustainable Chem. Eng.,
2019, 7, 10025.
In summary, PHU effectively catalyzed the reactions of PGE and
other epoxides with CO2 to produce various 5CCs with very high
yields. This is the first demonstration of the chemical transforma-
tion of CO2 by PHU, which may assist in conducting further studies
related to petroleum- and bio-based polyurethane1,2,7–10-catalyzed
reactions and their practical applications.
The authors thank Dainichiseika Color & Chemicals Mfg.
Co., Ltd. for providing PHU and TMS-PHU samples.
Conflicts of interest
20 X. Chen, L. Li, K. Jin and J. M. Torkelson, Polym. Chem., 2017,
8, 6349.
There are no conflicts of interest to declare.
21 B. Ochiai, S. Inoue and T. Endo, J. Polym. Sci., Part A: Polym. Chem.,
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Chem. Eng., 2020, 8, 1651.
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S. Zhang, Chin. J. Chem., 2018, 36, 293.
24 J.-L. Jiang and R. Hua, Synth. Commun., 2006, 36, 3141.
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