Synthesis of glycol diesters through the depolymerization of polyethylene glycols with carboxylic acids using a proton-exchanged montmorillonite catalyst

Article abstract of DOI:10.1016/j.tetlet.2018.01.056

A convenient and sustainable method for the synthesis of glycol diesters was developed through the depolymerization of polyethylene glycols (PEGs) with carboxylic acids using proton-exchanged montmorillonite as an efficient solid acid catalyst. Several functionalized glycol diesters were obtained in good yields from PEGs and readily available carboxylic acids. Upon reaction completion, the catalyst could be easily separated by filtration and reused with its activity remaining unchanged.

Full text of DOI:10.1016/j.tetlet.2018.01.056

Tetrahedron Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of glycol diesters through the depolymerization of
polyethylene glycols with carboxylic acids using a proton-exchanged
montmorillonite catalyst
⇑
Zen Maeno, Kaoru Midogochi, Takato Mitsudome, Tomoo Mizugaki, Koichiro Jitsukawa
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1–3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient and sustainable method for the synthesis of glycol diesters was developed through the
depolymerization of polyethylene glycols (PEGs) with carboxylic acids using proton-exchanged montmo-
rillonite as an efficient solid acid catalyst. Several functionalized glycol diesters were obtained in good
yields from PEGs and readily available carboxylic acids. Upon reaction completion, the catalyst could
be easily separated by filtration and reused with its activity remaining unchanged.
Ó 2018 Published by Elsevier Ltd.
Received 27 November 2017
Revised 13 January 2018
Accepted 19 January 2018
Available online xxxx
Keywords:
Glycol diesters
Depolymerization
Solid acid catalyst
C–O bond cleavage
Polyethylene glycols
Glycol diesters, which are highly useful as solvents and syn-
thetic intermediates of functionalized materials,1 are generally
synthesized through the acylation of glycols with acyl chlorides
or carboxylic anhydrides (Scheme 1a and b).2 However, these con-
ventional methods suffer from the formation of stoichiometric
amounts of HCl or carboxylic acids as by-products. In addition,
these acylating reagents are prepared through a stoichiometric
reaction of carboxylic acids with SOCl2 or dehydration reagents,
which results in the concomitant formation of hazardous halo-
genated or organic waste. Since these methods are not suitable
for sustainable green chemistry,3 a simple and atom-economical
method for synthesizing glycol diesters would be highly desirable.
In this context, the acid-catalyzed condensation of glycols and car-
boxylic acids has received significant attention,4 because car-
boxylic acids are readily available acylating reagents, and water
is formed as the sole by-product in this case (Scheme 1c). Recently,
our research group demonstrated the exploitation of unutilized
PEG waste5 as the oxyethylene units of glycol diesters through
the depolymerization of PEGs with carboxylic anhydrides using
proton-exchanged montmorillonite (H+-mont) as a layered solid
acid catalyst.6 It is envisioned that the use of carboxylic acids for
the depolymerization of PEGs would represent a more convenient
and sustainable method for the synthesis of glycol diesters
(Scheme 1d). In this study, the depolymerization of PEGs with car-
boxylic acids is investigated using solid acid catalysts, including
various cation-exchanged montmorillonite catalysts.7 The depoly-
merization of PEGs with carboxylic acids possessing various func-
tional groups is also examined.
Previously, we reported that H+-mont efficiently catalyzes the
depolymerization of PEGs with a mixture of acetic anhydride/
acetic acid (Ac2O/AcOH) at 100 °C to give ethylene glycol diacetate
(3a). However, the use of AcOH alone instead of Ac2O/AcOH results
in the formation of 3a in an extremely low yield.6 Accordingly, the
depolymerization of tetraethylene glycol (1a) with AcOH (2a) to
give 3a was carried out at 160 °C in the presence of various solid
acid catalysts, including cation-exchanged montmorillonites
(Mn+-mont, Mn+ denotes a metal species or proton).8 Of the various
Mn+-mont catalysts tested, H+-mont exhibited the highest catalytic
activity, affording 3a in 55% yield after 2 h (Table 1, entry 1). The
use of Al3+- and Ti4+-mont produced 3a in 22% and 10% yield,
respectively (Entries 4 and 5). On the other hand, other Mn+-monts,
such as Cu2+-, Ni2+-, La3+-, Zr4+-, and Ce3+-mont, did not promote
the depolymerization of 1a with 2a (Entries 6–10). Commercially
available solid acid catalysts, such as Amberlyst 36 and Nafion
NR50, produced 3a in 45% and 33% yield, respectively. Their turn-
over frequency (TOF) values were lower than that of H+-mont
(Entries 12 and 13).9 The solid acid catalysts SiO2-Al2O3, SO24-
/
ZrO2, and H+-mordenite showed extremely low activities (Entries
14–16). These results suggest that H+-mont acts as a highly active
⇑
Corresponding author.
E-mail address: jitkk@cheng.es.osaka-u.ac.jp (K. Jitsukawa).
https://doi.org/10.1016/j.tetlet.2018.01.056
0040-4039/Ó 2018 Published by Elsevier Ltd.
Please cite this article in press as: Maeno Z., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.01.056

2
Z. Maeno et al. / Tetrahedron Letters xxx (2018) xxx–xxx
solid acid catalyst for the depolymerization 1a with 2a. Further-
more, when the reaction time was increased to 24 h, H+-mont gave
3a in 89% yield (Entry 2). After the reaction, H+-mont was separable
by filtration, and the recovered catalyst was reusable without a sig-
nificant loss in activity (Entry 3).10
The depolymerization of low-molecular-weight PEG (1b, Mn ꢀ
400, Mn denotes the number-average molecular weight) with var-
ious carboxylic acids was carried out using H+-mont (Table 2).
Butyric acid (2b) reacted with 1b to give ethylene glycol dibutyrate
3b in 81% yield (Entry 2). The reaction of 1b with a fatty acid, lauric
acid (2c), proceeded to afford ethylene glycol dilaurate (3c) in an
isolated 87% yield (Entry 3). Saturated carboxylic acids are suitable
for this reaction. However, the reaction of 1a with carboxylic acids
containing an olefin moiety, such as cinnamic acid and 2,3-
diphenylacrylic acid, did not give the desired product and yielded
unknown viscous products. Aromatic carboxylic acids, such as ben-
zoic acid (2d), p-methoxybenzoic acid (2e), and 1-naphthoic acid
(2f), were successfully used for the synthesis of the corresponding
monomer glycol diesters (3d, 3e, and 3f) from 1b (Entries 4–6).
Carboxylic acids containing a nitrogen atom, such as N-phthaloyl-
glycine (2g), were also used for the depolymerization of 1b, result-
ing in 1,2-bis(N-phthaloylglycyl)ethane (3g) in 54% yield (Entry 7).
The use of succinic acid (2h), which is a dicarboxylic acid, for the
depolymerization of 1b afforded the corresponding glycol diesters
containing two terminal carboxylic acid groups (3h) in 79% yield
(Entry 8). Although 3 h is a potentially suitable compound for
polyester synthesis,1e,1g poly(ethylene glycol succinate) was not
formed in this reaction. When formic acid (2i) was used as a
carboxylic acid source, glycol diformate (3i) was obtained in only
24% yield (Entry 9). The low yield of 3i might be due to the
Scheme 1. Conventional synthesis of glycol diesters from glycol with (a) acyl
chlorides, (b) acid anhydrides, and (c) carboxylic acids. (d) Synthesis of glycol
diesters from carboxylic acids and PEG waste as the glycol unit.
Table 1
Depolymerization of tetraethylene glycol with acetic acid using various acid catalysts.
b
Entry
Catalyst
Yield [%]a
TOF [hÀ1
]
1
2
3
4
5
6
7
8
H+-mont
55
89
90
22
10
4
<1
<1
<1
<1
<1
45
33
<1
<1
3
15.2
–
–
7.0
8.7
–
–
–
–
–
H+-mont (24 h)
H+-mont (24 h, 2nd reuse)
Al3+-mont
Ti4+-mont
instability of the desired product. From
a synthetic organic
Cu2+-mont
chemistry standpoint, depolymerization using chloroacetic acid is
of great interest, as shown in Scheme 2. The reaction of
chloroacetic acid (2j) with 1b gave 1,2-bis(chloroacetoxy)ethane
(3j) in 71% yield. Compound 3j is the starting compound for
Ni2+-mont
La3+-mont
9
Zr4+-mont
10
11
12
13
14
15
16
Ce3+-mont
Na+-mont
–
ethylene glycol bis(azidoacetate) 4, which is useful as a
plasticizer and cross-linker,1 h,1i as well as for the biodegradable
surfactant 5.1f Various functionalized glycol diesters were
synthesized through the depolymerization of PEGs with the
corresponding carboxylic acids using H+-mont as the catalyst.
The catalytic depolymerization of several PEGs with 2a was also
investigated using H+-mont. The reaction of low-molecular-weight
PEG (1b, Mn ꢀ 400) with 2a afforded 3a in 85% yield (Table 3, entry
1), while in the case of middle-molecular-weight PEGs (1c, Mn ꢀ
1000) the yield was 69% at 230 °C (Entry 2). The reaction of high-
Amberlyst 36
Nafion NR50
SiO2/Al2O3
2.0
10.0
–
–
–
SO24-/ZrO2
H+-mordenite
Reagents and conditions: catalyst (0.05 g), 1a (0.1165 g, 2.4 mmol per oxyethylene
subunit), AcOH (2a) (2.75 mL).
a
Determined by GC using an internal standard. Based on the total amount of the
oxyethylene unit of 1a.
b
TOF = (molar amount of 3a produced)/(mol H+ h).
Table 2
Depolymerization of polyethylene glycol (Mn ꢀ 400, 1b) with various carboxylic acids using H+-mont.
Entry
1
Carboxylic acid 2
2 [equiv.]
H+-mont [g]
Temp. [°C]
Time [h]
24
Product 3
Yield [%]
85a
10
0.4
160
3a
2
3
10
5
0.05
0.4
160
160
3
3b
3c
81a
87b
11
4
5
10
10
0.4
180
185
24
48
3d
3e
81c
74c
0.05
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Z. Maeno et al. / Tetrahedron Letters xxx (2018) xxx–xxx
3
Table 2 (continued)
Entry
6
Carboxylic acid 2
2 [equiv.]
H+-mont [g]
0.4
Temp. [°C]
Time [h]
48
Product 3
Yield [%]
65c
15
180
3f
7
4
0.4
190
24
3g
54c
8
9
4
0.05
0.4
185
160
20
24
3h
3i
75d
24d
30
HCOOH
2i
Reagents and conditions: H+-mont, 1b (0.1106 g, 2.4 mmol per oxyethylene subunit).
a
Determined by GC using an internal standard. Based on the total amount of the oxyethylene unit of 1b.
Isolated yield. Based on the total amount of the oxyethylene unit of 1b.
Determined by HPLC using an internal standard. Based on the total amount of the oxyethylene unit of 1b.
Determined by NMR using an internal standard. Based on the total amount of the oxyethylene unit of 1b.
b
c
d
suppressed the reaction. These results suggest that the Brønsted
acid site within the interlayer space of H+-mont is the active spe-
cies during the catalysis process.12 The high activity of H+-mont
can be ascribed to the ready accessibility of the interlayer space
to the PEGs, as revealed by XRD measurements in a previous
study.6 The Hammett plot for the depolymerization of tetraethy-
lene glycol, 1a, with four types of p-substituted benzoic acids exhi-
bits a negative slope (
of the cation intermediate. Accordingly, the depolymerization of
this PEG with carboxylic acids probably occurs in the interlayer
q
= À0.83) (Fig. 1), indicating the formation
Scheme 2. Synthesis of ethylene glycol di(chloroacetate) 3j through depolymer-
ization of PEG 1b with chloroacetic acid 2j for the production of useful glycol diester
derivatives 4 and 5.
molecular-weight PEGs, 1d (Mn ꢀ 8000) and 1e (Mn ꢀ 2,000,000),
with 2a also afforded 3a in moderate yields at the same tempera-
ture (Entries 3 and 4). Commercially available surfactants, such as
Tween 20 (1f), PEG-10 monolaurate (1g), and Triton X-100 (1h),
were efficiently degraded to form 3a in yields of 76%, 80%, and
69%, respectively (Entries 5–7). Since this depolymerization of
1b–h with 2a gave 1,4-dioxane as a by-product,11 the yield of 3a
was relatively low in comparison to that in the case of the reaction
system using a carboxylic acid anhydride.6
Na+-mont did not promote the depolymerization of 1a with 2a
(Table 1, entry 11). The addition of 2,6-lutidine during the
H+-mont-catalyzed depolymerization of PEGs with 2a completely
Fig. 1. Relationship between the activity and Hammett parameter of p-substituted
benzoic acids in the depolymerization. Reaction conditions are shown in the ESI.
Table 3
Depolymerization of various PEGs with 2a to give 3a using H+-mont.
Entry
Polyether 1
Temp. [°C]
Time [h]
Yield [%]a
1
2
3
4
5
6
7
PEG (Mn ꢀ 400) (1b)
PEG (Mn ꢀ 1000) (1c)
PEG (Mn ꢀ 8000) (1d)
PEG (Mn ꢀ 2,000,000) (1e)
Tween 20 (1f)
160
230
230
230
160
160
230
24
2
2
85
69
53
47
76
80
69
2
24
24
10
PEG-10 monolaurate (1g)
Triton X-100 (1h)
Reagents and conditions: H+-mont (0.4 g), polyether compound (2.4 mmol per oxyethylene subunit), AcOH 2a (2.75 mL).
a
Determined by GC using an internal standard. Based on the total amount of the oxyethylene unit of polyether compound.
Please cite this article in press as: Maeno Z., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.01.056

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Z. Maeno et al. / Tetrahedron Letters xxx (2018) xxx–xxx
of H+-mont through cleavage of the C–O bond due to attack by the
acyl cations generated from the carboxylic acids at the Brønsted
acid sites.6
In conclusion, the efficient synthesis of glycol diesters through
the depolymerization of PEGs with carboxylic acids was achieved
using H+-mont as a solid acid catalyst. The proposed reaction can
potentially be used as an effective method for the use of PEG waste
because both the carboxylic acids and the heterogeneous catalyst
used are easy-to-handle materials and their use will contribute
to the effective utilization of resources.
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the Japan Society for the Promotion of Science and the Ogasawara
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A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.tetlet.2018.01.056.
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Please cite this article in press as: Maeno Z., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.01.056