Reversible cross-linking and de-cross-linking system of polystyrenes bearing the monohydrate structure of vicinal tricarbonyl group through water-alcohol exchange reactions at ambient conditions

Article abstract of DOI:10.1021/ma301139v

We describe in this paper reversible cross-linking and de-cross-linking system based on a polystyrene derivative bearing monohydrate structure of vicinal tricarbonyl groups with 1,6-hexanediol utilizing the direct water-alcohol exchange reactions on the v

Full text of DOI:10.1021/ma301139v

Article
pubs.acs.org/Macromolecules
Reversible Cross-Linking and De-Cross-Linking System of
Polystyrenes Bearing the Monohydrate Structure of Vicinal
Tricarbonyl Group through Water−Alcohol Exchange Reactions at
Ambient Conditions
Morio Yonekawa, Yoshio Furusho, and Takeshi Endo*
Molecular Engineering Institute, Kinki University, 11-6 Kayanomori, Iizuka, Fukuoka 820-8555, Japan
S
* Supporting Information
ABSTRACT: We describe in this paper reversible cross-
linking and de-cross-linking system based on a polystyrene
derivative bearing monohydrate structure of vicinal tricarbonyl
groups with 1,6-hexanediol utilizing the direct water−alcohol
exchange reactions on the vicinal tricarbonyl groups. By
employing diphenylpropanetrione as a unit model compound
for the polymer, we have demonstrated that the water−alcohol
exchange reactions could be carried out reversibly in both
directions by changing solvents. Notably, the water−alcohol
exchange reactions proceeded without any catalysts and under
mild conditions. For example, an equimolar mixture of the
hydrate of diphenylpropanetrione and benzyl alcohol in
chloroform (0.5 M) reached equilibrium after standing at ambient temperature within 48 h, where the content ratio of the
benzyl alcohol adduct increased up to 49%. The reaction rate and the position of the equilibrium were highly affected by the
concentrations of the substrates as well as the reaction temperature. By virtue of the above characteristic features of the water−
alcohol exchange reactions, the polystyrene derivative bearing monohydrate structure of vicinal tricarbonyl group (2.0 M) was
cross-linked with 1,6-hexanediol (0.2 equiv of OH group to the tricarbonyl unit) in acetone at ambient temperature for 5 days to
afford the networked polymer in almost quantitative yield. On the other hand, the networked polymer was treated with an excess
of water at ambient temperature for 3 days to afford the original linear polymer in high yield as a result of de-cross-linking
through the water−alcohol exchange reaction. The cross-linking and de-cross-linking behavior was also evidenced by SEC
analysis of the reaction mixture.
the contiguous three carbonyl groups, respectively, hence being
detectable by the naked eye.1,2 The vicinal tricarbonyl
compounds exhibit a similar behavior toward alcohols (Scheme
1b); the addition of alcohols readily proceeds at ambient
temperature to form the corresponding alcohol adducts
(hemiketal, DPPT-ROH, 3) without any catalyst, being
accompanied by disappearance of their distinctive color arising
from the vicinal tricarbonyl structures. Conversely, the vicinal
tricarbonyl compounds are obtained by heating under vacuum
to remove the alcohols.
Motivated by the characteristic features of vicinal tricarbonyl
compounds, we recently reported design and synthesis of a
polystyrene derivative bearing acyclic vicinal tricarbonyl
structures in the side chains (4) and detailed investigation of
its reversible addition−elimination behavior of water and
alcohols to the vicinal tricarbonyl polymer (Scheme 1c).13−16
The vicinal tricarbonyl pendants of 4 readily reacted with water
INTRODUCTION
■
Vicinal tricarbonyl compounds, such as alloxan, 1,2,3-
indanetrione, dehydroascorbic acid, and diphenylpropanetrione
(DPPT, 1), have by definition the three contiguous carbonyl
groups, and are characterized by a highly electrophilic central
carbonyl group, which is activated by the adjacent two
carbonyls and significantly reactive toward various nucleophiles,
such as water and alcohols (Scheme 1).1,2 Hence, vicinal
tricarbonyl compounds are usually obtained as their hydrated
forms (2,2-gem-diol, DPPT-H2O, 2) as a result of hydration
with moisture in air or solvents used during the isolation
processes (Scheme 1a). These hydrates can be dehydrated to
give pure free vicinal tricarbonyls (1) by heating under
vacuum,3−5 sublimation,5,6 distillation,7−10 crystallization,9
azeotropic removal of water,10 and utilization of dehydrating
agents such as molecular sieves 4A or P2O5.6,10,11 This
reversible hydration-dehydration nature of vicinal tricarbonyls
enables us to utilize them as recyclable dehydrating agents.12 Of
particular interest is that the reversible hydration process is
accompanied by disappearance and appearance of the
distinctive yellow color due to the collapse and recovery of
Received: June 4, 2012
Revised: July 21, 2012
Published: July 30, 2012
© 2012 American Chemical Society
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cross-linking reagent. However, the vicinal tricarbonyl polymer
system has some drawback toward materials application; the
de-cross-linking step utilizing the elimination of alcohols
requires heating at around 100 °C under reduced pressure. In
addition, the polymers bearing vicinal tricarbonyl pendants are
susceptible to moisture and need to be preserved under dry
conditions. Therefore, new processes or systems that can be
reversibly cross-linked and de-cross-linked under mild con-
ditions are desired to be developed.
Scheme 1. Reversible Addition of Water and Alcohols to
Vicinal Tricarbonyl Groups
Over the course of our study to develop new polymer
materials based on the vicinal tricarbonyl structures, we noted
that the two reversible reactions, the reversible addition of
water and that of alcohol, indicated that the apparently direct
exchange of water and alcohols on the vicinal tricarbonyls in situ
is possible by way of vicinal tricarbonyl intermediates (Figure
1a). On the basis of this equilibrium, we envisaged that use of
the gem-diol polymers would lead to a new system that can be
reversibly cross-linked and de-cross-linked system under mild
conditions through the direct water−alcohol exchange reaction
(Figure 1b). However, no detailed study for such direct water−
alcohol exchange reactions had been reported to date.
Therefore, we started from the systematic study of the
water−alcohol exchange reactions using a unit model
compound, and then we went into the gem-diol polymers.
Herein, we describe the reversible cross-linking and de-cross-
linking system of polystyrene bearing the gem-diol structure of
vicinal tricarbonyl group using an α,ω-alcohol exploiting the
direct water−alcohol exchange reaction along with the detailed
investigation using a unit model compound.
or alcohols to quantitatively yield the corresponding water or
alcohol adduct polymer 5. On the other hand, heating 5 under
vacuum afforded the original polymers with the vicinal
tricarbonyl pendants 4 without deteriorating the polymer
structure.
Polymer materials that can be reversibly cross-linked and de-
cross-linked through covalent bonds have recently attracted
much attention from the standpoint of chemical recycling as
well as self-healing materials.17−19 Taking advantage of the
reversible nature of the alcohol addition to vicinal tricarbonyl
groups, we developed a new reversible cross-linking system
from the vicinal tricarbonyl polymers 4 and an α,ω-diol as the
EXPERIMENTAL SECTION
■
Materials. Chloroform, chloroform-d (CDCl3) and dimethyl
sulfoxide-d6 (DMSO-d6) were distilled over molecular sieves 4A
(MS 4A). Acetone and acetone-d6 were distilled over CaCl2. Benzyl
alcohol and 1-hexanol were distilled over CaH2. 1,4-Dioxane was
distilled over sodium benzophenone ketyl. D2O (Merck KGaA,
Germany), n-Hexane (Wako Pure Chemical Industries, Japan) and
1,6-hexanediol (Wako) were purchased and used without further
purification. Diphenylpropanetrione monohydrate (2, DPPT−H2O),
Figure 1. Schematic illustrations of (a) water−alcohol exchange reaction on vicinal tricarbonyl groups and (b) reversible cross-linking and de-cross-
linking of polymers bearing vicinal tricarbonyl pendants using α,ω-diols through water−alcohol exchange reaction.
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diphenylpropanetrione−benzyl alcohol adduct (3a, DPPT-BnOH)
and polymer bearing monohydrate structure of vicinal tricarbonyl
moieties 5a were synthesized according to the literature.13 The
number-average molecular weight of 5a was estimated to be 1.5 × 104
with a polydispersity index (PDI) of 10.3 by size exclusion
chromatography (SEC) based on PSt standards. Polymer 5a with
number-average molecular weight of 3.8 × 103 with a PDI of 2.9 was
prepared from the polymer synthesized by radical polymerization of 4-
vinyldibenzoylmethane with 20 mol % of 2,2′-azobis(isobutyronitrile)
(AIBN) at 80 °C for 18 h.
Recovery of Polymer 5a by De-Cross-Linking of Networked
Polymer 7 through the Water−Alcohol Exchange Reaction
with Water. A pea-sized piece of networked polymer 7 (168 mg) was
immersed in 10 mL of water−acetone (1/9, v/v) and the mixture was
gently stirred at room temperature. The polymer swelled gradually and
then completely dissolved within 2 days. The resultant solution was
stirred for one more day, and then mixture was reprecipitated with a
large amount of water and filtered to obtain polymer 5a (149 mg,
0.527 mmol, 91%) as a pale yellow solid. 1H NMR (400 MHz,
acetone-d6, 298 K): see Supporting Information, Figure S5.
Measurements. 1H NMR spectra were measured on a JEOL
JNM-ECS 400 spectrometer at a resonance frequency of 400 MHz
with tetramethylsilane (TMS) as an internal standard. NMR chemical
shifts were reported in delta unit (δ). IR spectra were recorded on a
Thermo Scientific Nicolet iS10 spectrometer. UV−vis spectra were
measured with a JASCO V-570 spectrophotometer in a 1-cm quartz
cell. Number-average and weight-average molecular weights (Mn, Mw)
and polydispersity indices (Mw/Mn) of the polymers were estimated
by size exclusion chromatography (SEC) using tetrahydrofuran (THF)
as the eluent at a flow rate of 0.6 mL/min at 40 °C, performed on a
Tosoh chromatograph model HLC-8320 system equipped with Tosoh
TSKgel SuperHM-H styrogel columns (6.0 mm ϕ × 15 cm, 3 and 5
μm bead sizes), UV−vis detector (254 nm). The molecular weight
calibration curve was obtained with polystyrene standards. Thermog-
ravimetric analysis (TGA) was performed on a Seiko Instrument Inc.
TG-DTA 6200 with an aluminum pan under a 150 mL/min N2 flow at
a heating rate of 10 °C/min. The single crystal X-ray data for the
benzyl alcohol adduct (3a) was collected on a Bruker Smart Apex
CCD-based X-ray diffractometer with Mo−Kα radiation (λ = 0.71073
Å).20
RESULTS AND DISCUSSION
■
Reversible Water−Alcohol Exchange Reaction of
Monohydrate of Diphenylpropanetrione with Alcohols.
We chose the monohydrate of diphenylpropanetrione 2
(DPPT−H2O) as a unit model compound for polystyrene
bearing monohydrate structure of vicinal tricarbonyl group and
investigated its water−alcohol exchange reaction in detail.
Before investigating the model reaction, we prepared the benzyl
alcohol adduct of diphenypropanetrione 3a (DPPT−BnOH)
according to our previous report.15 Single crystals of 3a suitable
for X-ray analysis were grown from a solution in benzyl alcohol.
The X-ray analysis revealed that the benzyl alcohol was added
to the central carbonyl group, and the central carbon atom
adopted the tetrahedral configuration (Figure 2).20 We
Water−Alcohol Exchange Reaction of DPPT−H2O, 2, with
Alcohols. A typical experimental procedure was as follows. 2 (64.1
mg, 0.250 mmol) was dissolved in 0.50 mL of CDCl3 under an argon
atmosphere. To the solution was added BnOH (26.2 μL, 0.250 mmol),
and the solution was transferred to an NMR tube. The reaction
1
progress was monitored by H NMR.
Water−Alcohol Exchange Reaction of DPPT−BnOH, 3a, with
Water. DPPT−BnOH (7.81 mg, 0.025 mmol) was dissolved in 0.50
mL of a mixture of D2O and acetone-d6 (1/9, v/v) under an argon
atmosphere, and the solution was transferred to an NMR tube. The
1
reaction progress was monitored by H NMR.
Water−Alcohol Exchange Reaction of Polystyrene Deriva-
tive 5a with Alcohols. A typical experimental procedure was as
follows. Polymer 5a (282 mg, 1.00 mmol) was dissolved in chloroform
(5.0 mL). To the solution was added 1-hexanol (374 μL, 3.00 mmol),
and the reaction mixture was stirred at ambient temperature under
argon atmosphere for 3 days. The resulting mixture was precipitated
with n-hexane and filtered to obtain polymer 6 (298 mg, 94%) as a
Figure 2. ORTEP structure of 3a with thermal ellipsoids at 50%
probability. Selected data: bond lengths = O1−C7 1.214 Å, O2−C15
1.208 Å, O3−C8 1.407 Å, O4−C8 1.394 Å; angles = O3−C8−O4
113.36°, C7−C8−C15 110.12°; dihedral angles = O1−C7−C8−O3
−133.36°, O2−C15−C8−O3 −129.29°.
1
pale yellow solid. H NMR (400 MHz, acetone-d6, 298 K): δ 8.17−
5.20 (br, aromatic-H, −OH), 3.70−3.30 (br, −OCH2−), 2.40−0.50
(br, aliphatic-H) ppm. IR (ATR) 3415 (O−H), 2929 (C−H), 1721
(central CO), 1682 (side CO) cm−1.
Recovery of Polymer 5a from Polymer 6 through Water−
Alcohol Exchange Reaction with Water. Polymer 6 (151 mg,
0.469 mmol) was dissolved in 10 mL of a mixture of water and acetone
(1/9, v/v), and the resultant solution was stirred at ambient
temperature for 3 days. The resulting mixture was precipitated with
a large amount of water and filtered to obtain polymer 5a (127 mg,
96%) as a pale yellow solid. 1H NMR (400 MHz, acetone-d6, 298 K):
see Supporting Information, Figure S4.
monitored the water−alcohol exchange reaction of 2 with
benzyl alcohol in CDCl3 by 1H NMR. Figure 3 shows the time
1
dependence of partial H NMR spectra in the aromatic region
of an equimolar mixture of 2 and benzyl alcohol ([2]0 =
[BnOH]0 = 0.5 M) after mixing at ambient temperature,
indicating that three species, i.e., 2, and diphenylpropanetrione
(DPPT, 1), and BnOH adduct (DPPT−BnOH, 3a) were
involved in the equilibrium. The existence of 1 was also
evidenced by the naked-eye observation that the reaction
mixture took on the distinctive yellow color due to the
contiguous three carbonyl groups.1,13−15 The ortho-proton
signals of the benzene rings of 1, 2, and 3a appeared as three
clearly separated ones at around 8 ppm, with which we
estimated the content ratios of the three species. The contents
of the three species were plotted against the reaction time as
Cross-Linking of Polystyrene Derivative 5a through the
Water−Alcohol Exchange Reaction with 1,6-Hexanediol. To a
solution of polymer 5a (282 mg, 1.00 mmol) in acetone (0.50 mL)
was added 1,6-hexanediol (11.8 mg, 0.100 mmol), and the reaction
mixture was stirred at ambient temperature under argon atmosphere
for 5 days. The resultant highly viscous mixture was filtered and
washed with acetone (5.0 mL). The insoluble part was dried in vacuo
at room temperature to afford the networked polymer 7 (292 mg,
99%) as a yellow solid. IR (ATR): 3392 (OH), 2932 (C−H), 1721
(central CO), 1674 (side CO) cm−1.
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34%, and 28%, respectively (Figure 4B). In contrast, the rate of
the exchange reaction became much higher at 50 °C than that
at ambient temperature and the equilibrium state was achieved
within half a day. The position of the equilibrium state was also
shifted to a certain extent toward the free tricarbonyl 1; the
contents of 1, 2, and 3a at equilibrium were 49%, 15%, and
36%, respectively at 50 °C (Figure 4C).
Next, we investigated the effect of solvent on the water−
alcohol exchange reaction (Figure 5). We chose 1-hexanol
1
Figure 3. H NMR spectra of the reaction mixture of DPPT-H2O 2
Figure 5. Changes in the content ratios of 3a and 3b upon mixing with
BnOH in CDCl3 (a), and with 1-hexanol in CDCl3 (b), in DMSO-d6
(c), and in acetone-d6 (d) at ambient temperature. [2]0 = [1-hexanol]0
= 0.5 M.
with equimolar benzyl alcohol in CDCl3 at ambient temperature after
(a) 10 min, (b) 1 h, (c) 12 h, and (d) 48 h (400 MHz, 298 K). [2]0 =
[BnOH]0 = 0.5 M.
shown in Figure 4A. At an early stage of the reaction (10 min,
Figure 3a), the major part of the mixture was 2, and a small
amount of 1 with a trace amount of 3a were detected after 1 h
(Figure 3b). The content of 1 reached maximum a few hours
later, and the content of 3a gradually increased with time, while
1 and 2 decreased monotonously (Figure 3c). The mixture
reached equilibrium within 48 h, at which the content ratios of
1, 2, and 3a were 18%, 33%, and 49%, respectively (Figure 3d).
The initial concentration of the starting materials and the
reaction temperature markedly affected the reaction rate as well
as the position of the equilibrium of the water−alcohol
exchange reaction. When the initial concentrations of 2 and
BnOH were set to 0.1 M, the reaction became significantly slow
and 4 days were required for reaching equilibrium at ambient
temperature, where the content ratios of 1, 2, and 3a were 38%,
instead of benzyl alcohol, because the BnOH adduct showed
poor solubility and insufficient peak separation of the 1H NMR
spectra in acetone and DMSO. In CDCl3, the exchange
reaction with 1-hexanol exhibited a similar behavior to that with
benzyl alcohol and the mixture reached equilibrium within 48 h,
at which the content ratios of 1, 2, and 3b were estimated to be
1
11%, 21%, and 68% by using the integrals of the H NMR
spectrum. The content of the 1-hexanol adduct 3b at
equilibrium was much higher than that of 3a in CDCl3
probably because of the less steric hindrance of the hydroxyl
group in 1-hexanol than that of benzyl alcohol. In contrast, the
exchange reaction rate slowed very much in DMSO-d6 and it
took ca. 120 h until equilibrium was attained. The content of 3b
was as low as 23% at equilibrium, which was much lower than
that in CDCl3. This is consistent with our previously reported
Figure 4. Changes in the content ratio of 1, 2, and 3a upon mixing DPPT-H2O 2 and BnOH in CDCl3: (A) [2]0 = [BnOH]0 = 0.5 M, room
temperature, (B) [2]0 = [BnOH]0 = 0.1 M, room temperature, and (C) [2]0 = [BnOH]0 = 0.5 M, 50 °C.
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result that the addition of alcohol to 1 is slowed in polar
solvents,15 thus indicating that the rate-determining step for the
water−alcohol exchange reaction is most likely to be the
addition of the alcohol. The reaction rate was even lowered in
acetone-d6 than in DMSO-d6, and it required more than 240 h
to achieve equilibrium, although the content of 3b was higher
than that in DMSO-d6 at the final equilibrium state.
gem-diol and hemiketal units (Figure S2, Supporting
Information). A weak but distinctive absorption peak at 455
nm observed in the UV/vis spectrum also the existence of the
vicinal tricarbonyl unit in the polymer 6 (Figure S3, Supporting
Information).1,13−15 The hemiketal structure in 6 was
confirmed by the appearance of a broad signal between 3.3
and 3.7 ppm in the 1H NMR spectra that corresponds to that of
alkoxymethylene protons of the hemiketal side chain (Figure
S4, Supporting Information). The contents of gem-diol,
tricarbonyl, and hemiketal structures were estimated by using
We demonstrated the back reaction directly from 3a to 2
exploiting the water−alcohol exchange reaction in a similar way
to that for the hydration of 1 according to our previous
report.13 The isolated 3a was dissolved in a mixture of D2O and
acetone-d6 (1/9, v/v) at ambient temperature, and the
1
the integrals of the H NMR spectra. When equimolar 1-
hexanol was used, the contents of the three units were 39%,
27%, and 34% for gem-diol, vicinal tricarbonyl, and hemiketal
units, respectively. The content of the hemiketal unit (34%)
halved when compared to that of the equilibrium solution of an
equimolar mixture of 2 and 1-hexanol in CDCl3 (68%, vide
infra). This could be attributed to the lowered total
concentration as well as the steric hindrance of the polymer
backbones. The content of hemiketal unit in 6 increased with
an increase in the feed ratio of 1-hexanol, and it reached 62%
when 10 equiv of 1-hexanol was used. The thermogravimetric
1
exchange reaction was monitored by H NMR (Scheme 2).
Scheme 2. Water−Alcohol Exchange Reaction of DPPT−
BnOH 3a with D2O
1
analysis (TGA) profile of 6 was consistent with the H NMR
results (Figure 7). Only a negligible weight loss was observed
up to 220 °C for the tricarbonyl polymer 4 (dehydrated form of
5a) upon being heated from 30 °C at a rate of 10 °C/min
under nitrogen flow. In contrast, the gem-diol polymer 5a
showed a weight loss of 6 wt % up to 200 °C, which was in
good agreement with the weight of water that can be released
potentially from 5a. The TGA profile of 6 (obtained with 3 eq
of 1-hexanol) had two-step weight loss at around 100 and 200
°C, which probably correspond to the releases of water and 1-
hexanol, respectively. The weight loss up to 250 °C coincides
with the combined weight of water and 1-hexanol incorporated
After 3 days, an almost quantitative water−alcohol exchange
was achieved with generation of 2′ (DPPT−D2O) in 97% yield
1
confirmed by H NMR (Figure 6A). Similarly to the forward
reaction, the backward reaction was accelerated by heating at 50
°C, so that 2′ was generated in 97% yield (1H NMR) within 24
h (Figure 6B).
Reversible Water−Alcohol Exchange Reaction of
Polystyrene Derivative 5a with 1-Hexanol. On the basis
of the results of the model reactions, we investigated the
reversible water−alcohol exchange reaction of the polystyrene
bearing monohydrate structure of vicinal tricarbonyl group 5a
with 1-hexanol (Table 1). The polymer with gem-diol structure
5a (Mn = 1.5 × 104, PDI = 10.3) was prepared according to our
previous report.13 1-Hexanol was added to a solution of 5a in
CHCl3 ([5a]0 = 0.2 M) and the reaction mixture was stirred at
ambient temperature and took on a yellow color, which
suggested the existence of vicinal tricarbonyl units. After 3 days,
the resulting mixture was precipitated with n-hexane to obtain
1-hexanol adduct of the polymer 6 in 91−96% yield. The IR
spectrum showed a weak but distinctive absorption peak at
1721 cm−1 assignable to the three contiguous carbonyl groups,
indicating that 6 contained vicinal tricarbonyl unit along with
1
in 6 that was estimated by the H NMR analysis.
We also examined the recovery of the gem-diol polymer 5a
directly from the hemiketal polymer 6 in the same manner as
that for the unit model system (Scheme 3). Being stirred at
ambient temperature, a mixture of 6 in water-acetone (1/9, v/
v) became homogeneous and colorless. After 3 days, the
mixture was reprecipitated with water to afford 5a in 96% yield.
1
The H NMR spectrum of the obtained polymer 5a agreed
completely with that of the original gem-diol polymer 5a
(Figure S4, Supporting Information).
Reversible Cross-Linking and De-Cross-Linking of
Polystyrene Derivative 5a through the Water−Alcohol
Figure 6. Change in the content ratio of DPPT (1), DPPT−D2O (2′) and DPPT−BnOH (3a) upon mixing 3a in D2O/acetone-d6 (1/9, v/v) at (A)
ambient temperature and (B) at 50 °C. [3a]0 = 0.05 M.
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Table 1. Water−Alcohol Exchange Reaction of 5a with 1-Hexanol and Compositions of Resulting Polymers
b
content ratio /%
a
c
entry
hexanol/equiv
gem-diol
tricarbonyl
hemiketal
yield /%
1
2
3
1
3
39
38
24
27
16
34
46
62
91
94
96
10
14
a
b
c
1
Versus gem-diol moieties of 5a. Determined by integral ratios on H NMR spectra. Isolated yield.
part was dried in vacuo at room temperature to afford the
networked polymer 7 in 99% yield as a yellow solid. In the IR
spectrum of the obtained networked polymer, a peak due to
methylene moiety of 1,6-hexanediol and a weak peak
originating from the vicinal tricarbonyl structure were observed
at 2932 cm−1 and 1721 cm−1, respectively (Figure S2,
Supporting Information).1,13−15 The TGA profile of 7 showed
a weight loss of 9 wt % up to ca. 250 °C, which agreed well with
the combined weight of water and 1,6-hexanediol covalently
attached to the polymer side chain (Figure 7). Figure 8 shows
the photographs for the polymer mixture before (a) and after
(b) the cross-linking. While the solution of 5a in acetone
exhibited high fluidity as solution, a transparent self-standing
gel was formed after the cross-linking with 1,6-hexanediol. The
scanning electron microscopy (SEM) images of the fracture
surface of the networked polymer are shown in Figure S6
(Supporting Information). The morphology of the networked
polymer 7 showed smooth and homogeneous surface, and no
phase-separated morphology was observed in the micrometric
scale.
Figure 7. TG profiles of 4, 5a, 6, and 7 (heating rate 10 °C/min,
under N2 flow).
Scheme 3. Recovery of Polymer 5a
We demonstrated the de-cross-linking of the obtained
networked polymer 7 through water−alcohol exchange
reaction. A pea-sized piece of 7 was immersed in water-acetone
(1/9, v/v) and the mixture was gently stirred at ambient
temperature. The polymer swelled gradually and then
completely dissolved within 2 days. The resultant solution
was stirred for one more day and became colorless. After
reprecipitation with water, the water-insoluble part was dried in
vacuo at ambient temperature to afford the gem-diol polymer 5a
1
in 91% yield, of which the H NMR spectrum was completely
Exchange Reaction with 1,6-Hexanediol. We investigated
the reversible cross-linking−de-cross-linking of the gem-diol
polymer 5a through the water−alcohol exchange reaction by
using 1,6-hexanediol as a bifunctional cross-linking reagent
(Figure 8). We used acetone for the solvent, because it is a
good solvent for 5a so that we could achieve a high
concentration of 5a, which is advantageous for the cross-
linking reaction. A small amount of 1,6-hexanediol (0.2 equiv of
OH group relative to the gem-diol moiety of 5a) was added to a
concentrated solution of 5a (Mn = 1.5 × 104, PDI = 10.3) in
acetone (2.0 M). After 1 day at ambient temperature, the
magnetic stirring had been stopped because of significantly
increased viscosity of the mixture. The mixture displayed an
orange color, which indicated the existence of vicinal
tricarbonyl units. The highly viscous mixture was allowed to
stand for 4 more days and washed with acetone. The insoluble
identical to that of the original polymer 5a (Figure S5,
Supporting Information).
Finally, we attempted SEC analysis of the reaction mixture
prior to the gel point, in order to visualize the cross-linking−de-
cross-linking behavior of 5a.21 To this end, we prepared
polymer 5a with a lower molecular weight (Mn = 3.8 × 103,
PDI = 2.9) and allowed to react with 1,6-hexanediol in the
same manner. The SEC analysis of the reaction mixture showed
an increase in molecular weight as well as PDI after 24 h (Mw =
4.5 × 103, PDI = 5.0) and after 48 h (Mn = 4.6 × 103, PDI =
8.5), indicating the formation of network polymer via cross-
linking (Figure 9a−c). Upon treatment of the resulting partially
networked polymer with excess amount of water at ambient
temperature for 3 days, the SEC profile of the polymer (Mn =
3.4 × 103, PDI = 2.9) showed good accordance with that of the
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Figure 8. Photographs of before (a) and after (b) cross-linking of the polystyrene derivative 5a through water−alcohol exchange reaction on its
vicinal tricarbonyl moieties.
alcohol exchange reaction. Thus, we believe that the cross-
linking and de-cross-linking system based on the water−alcohol
exchange reactions on vicinal tricarbonyl compounds is
promising for development of new chemical recycling and
self-healing materials because of its reversible nature as well as
the feasibility under mild conditions.
ASSOCIATED CONTENT
* Supporting Information
Detailed X-ray crystal data of 3a, IR, UV−vis, and H NMR
spectra of the polymers, and SEM images of 7. This material is
■
S
1
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tendo@moleng.fuk.kindai.ac.jp.
■
Figure 9. SEC profiles of polymer 5a (a) as-prepared, (b) after cross-
linking for 48 h, (c) after cross-linking for 96 h, and (d) after de-cross-
The authors declare no competing financial interest.
linking.
Notes
ACKNOWLEDGMENTS
■
original polymer 5a as-prepared (Figure 9d), indicating that the
de-cross-linking was completed.
This work was supported by Grant-in-Aid for Scientific
Research (B) (21350068) from the Japan Society for the
Promotion of Science (JSPS), Japan and JSR Corporation. The
authors are grateful to Dr. Kazuhide Morino for the fruitful
discussion on the preparation of the vicinal tricarbonyl
compounds. The authors also thank to Dr. Yoshihisa Sei for
the X-ray single crystal analysis of the compound 3a.
CONCLUSION
■
We have thus demonstrated here that the direct water−alcohol
exchange reactions on the vicinal tricarbonyl compounds can be
carried out reversibly in both directions by changing solvents,
exploiting the reversible nature of the addition of alcohols to
vicinal tricarbonyl compounds as well as that of the elimination
of alcohols from the alcohol adducts. It should be noted here
that the water−alcohol exchange reactions proceed without any
catalysts and under mild conditions such as ambient temper-
ature. By virtue of the above characteristic features of the
water−alcohol exchange reactions on vicinal tricarbonyl
compounds, the polystyrene derivative bearing monohydrate
structure of vicinal tricarbonyl group can be networked with
1,6-hexanediol at ambient temperature through the water−
alcohol exchange reaction to afford the networked polymer in
almost quantitative yield. On the other hand, the treatment of
the networked polymer with water affords the original polymer
in high yield as a result of de-cross-linking through the water−
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