Boronic Ester Based Vitrimers with Enhanced Stability via Internal Boron-Nitrogen Coordination

Article abstract of DOI:10.1021/jacs.0c10244

Boron-containing polymers have many applications resulting from their prominent properties. Organoboron species with reversible B-O bonds have been successfully employed for the fabrication of various self-healing/healable and reprocessable polymers. However, the application of the polymers containing boronic ester or boroxine linkages is limited because of their instability to water. Herein, we report the hydrolytic stability and dynamic covalent chemistry of the nitrogen-coordinating cyclic boronic diester (NCB) linkages, and a new class of vitrimers based on NCB linkages is developed through the chemical reactions of reactive hydrogen with isocyanate. Thermodynamic and kinetic studies demonstrated that NCB linkages exhibit enhanced water and heat resistance, whereas the exchange reactions between NCB linkages can take place upon heating without any catalyst. The model compounds of NCBC-X1 and NCBC-X2 containing a urethane group and urea group, respectively, also showed higher hydrolytic stability compared to that of conventional boronic esters. Polyurethane vitrimers and poly(urea-urethane) vitrimers based on NCB linkages exhibited excellent solvent resistance and mechanical properties like general thermosets, which can be repaired, reprocessed, and recycled via the transesterification of NCB linkages upon heating. Especially, vitrimers based on NCB linkages presented improved stability to water and heat compared to those through conventional boronic esters because of the existence of N → B internal coordination. We anticipate that this work will provide a new strategy for designing the next generation of sustainable materials.

Full text of DOI:10.1021/jacs.0c10244

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Boronic Ester Based Vitrimers with Enhanced Stability via Internal
Boron−Nitrogen Coordination
Xiaoting Zhang,# Shujuan Wang,# Zikang Jiang, Yu Li, and Xinli Jing*
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ABSTRACT: Boron-containing polymers have many applications
resulting from their prominent properties. Organoboron species
with reversible B−O bonds have been successfully employed for
the fabrication of various self-healing/healable and reprocessable
polymers. However, the application of the polymers containing
boronic ester or boroxine linkages is limited because of their
instability to water. Herein, we report the hydrolytic stability and
dynamic covalent chemistry of the nitrogen-coordinating cyclic
boronic diester (NCB) linkages, and a new class of vitrimers based
on NCB linkages is developed through the chemical reactions of
reactive hydrogen with isocyanate. Thermodynamic and kinetic
studies demonstrated that NCB linkages exhibit enhanced water
and heat resistance, whereas the exchange reactions between NCB
linkages can take place upon heating without any catalyst. The model compounds of NCBC-X1 and NCBC-X2 containing a
urethane group and urea group, respectively, also showed higher hydrolytic stability compared to that of conventional boronic esters.
Polyurethane vitrimers and poly(urea-urethane) vitrimers based on NCB linkages exhibited excellent solvent resistance and
mechanical properties like general thermosets, which can be repaired, reprocessed, and recycled via the transesterification of NCB
linkages upon heating. Especially, vitrimers based on NCB linkages presented improved stability to water and heat compared to
those through conventional boronic esters because of the existence of N → B internal coordination. We anticipate that this work will
provide a new strategy for designing the next generation of sustainable materials.
on associative exchange reactions like “vitrimers” because of
their silica-like rheological behavior. Below the topology freezing
temperature (Tv), vitrimers behave like traditional thermosets
that exhibit excellent dimensional stability, chemical resistance,
and mechanical properties. At higher temperatures, vitrimers
can be repaired, reprocessed, and recycled via rapid exchange
reactions of dynamic covalent bonds, while maintaining a
relatively constant cross-linking density. In recent years, a variety
of associative covalent exchange chemistries have been
developed, such as transesterification, transalkylation, trans-
carbamoylation, siloxane equilibration, olefin metathesis, imine
exchanges, and so on.7−15
INTRODUCTION
■
Thermosetting polymers, which are cross-linked by irreversible
covalent bonds, are widely used for vast demanding applications
because of their superior mechanical properties, dimensional
stability, and chemical resistance. However, the cross-linked
architecture limits the polymer chain’s mobility, such that
thermosets cannot be reshaped, reprocessed, or recycled once
fully cured. Thus, thermosets are major contributors to
environmental pollution and responsible for a significant waste
of resources.1 Introducing dynamic covalent bonds into polymer
networks to prepare dynamic cross-linked polymers is an
efficient way of providing reversible characteristics to traditional
cross-linked polymers. Dynamic cross-linked polymers are able
to change their topological structures via the exchange reactions
of dynamic covalent bonds triggered by the external stimuli,
while still retaining the mechanical properties of traditional
cross-linked polymers. It not only brings malleability and
recyclability to cross-linked polymers but also endows many
excellent properties to traditional polymers, which has great
scientific significance for promoting the development of
sustainable materials.2−4
Boronic ester linkages formed by boronic acids and diols are a
typical class of dynamic covalent bonds;16,17 their dynamic
reversibility can be easily regulated by adjusting the pH value or
ethanol concentration of the medium.18−22 The unique
Received: October 1, 2020
Generally, dynamic cross-linked polymers can be classified
into two groups depending on their exchange mechanisms.5
Leibler’s group6 defined dynamic cross-linked polymers based
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properties of boronic esters have been employed to create the
porous covalent organic framework,23 supramolecular poly-
mers,24,25 and self-healing materials.26 Furthermore, boroxines
formed by the dehydration of boronic acid can also be used to
prepare dynamic covalent polymers.27−30 Despite the fact that
boronic ester and boroxine linkages have been successfully used
in the preparation of various self-healing/healable materials,
designing boronic ester based vitrimers with enhanced hydro-
lytic stability and superior mechanical properties remains
extremely challenging, as both boronic esters and boroxines
are very sensitive to water or alcohols.
Table 1. List of NCBCs
Organoboron chemistry studies have established that the
cyclic boronic diesters are more stable than the boronic acid
monoesters, especially for the cyclic boronic esters coordinated
by electron donors (e.g., nitrogen-donor compounds).31 As
mentioned above, we provide a new strategy for synthesizing
stable nitrogen-coordinating cyclic boronic diester compounds
(NCBCs) by introducing a N → B dative bond and then use
them to construct a cross-linked network. Differing from most
vitrimers, where dynamic covalent bonds were formed
simultaneously with final materials, we first synthesized small-
molecular model compounds containing dynamic covalent
bonds, and the cross-linked networks were constructed on the
basis of the dynamic linkages. This process is more conducive to
the precise control of vitrimer structures. Although boronic
esters with internal N → B interaction have been applied in
cross-coupling reactions,32 or employed as a curing agent for
epoxy resin,33 the thermodynamic and kinetic characteristics, as
well as the dynamic nature of nitrogen-coordinating cyclic
boronic diester (NCB) linkages, have not yet been systemati-
cally explored.
In this study, we first synthesized several NCBCs with
different structures and studied their hydrolytic and exchange
reactions, which demonstrated enhanced hydrolytic stability and
rapid transesterification of NCB linkages. Furthermore, the
model compounds of NCBC-X1 and NCBC-X2 were
synthesized by reacting hexyl isocyanate with monofunctional
NCBC-3 and NCBC-4 (Table 1), respectively, and their
hydrolysis stability was studied. The hydrolysis stability of
NCBC-X1 is comparable to that of NCBC, while NCBC-X2 is
slightly inferior to NCBC. On the basis of the model compound
results, polyurethane (PU) vitrimers and poly(urea-urethane)
(PUU) vitrimers were prepared by reacting a trifunctional
homopolymer of hexamethylene diisocyanate (THDI) with
NCBC-1 and NCBC-2, respectively. PU vitrimers and PUU
vitrimers not only inherit excellent properties of traditional
thermosets but also can be repaired, reprocessed, and recycled
through the exchange reactions of NCB linkages triggered by
heating. The design concept is shown in Figure 1. To our
knowledge, this is the first report on vitrimers with enhanced
water resistance and excellent recyclability based on NCB
linkages. We envision that this study will provide a more
extensive option for vitrimer design, which is also an important
means of achieving high-performance general purpose polymers.
continuously removed to ensure a complete reaction. When
NCBCs are synthesized, a small amount of water does not
matter because of the highly hydrophobic nature of the NCB
linkages with the intramolecular coordination of N → B. The
structures of NCBCs were confirmed by nuclear magnetic
resonance (1H NMR and 13C NMR), high-resolution mass
spectrometry (HRMS), Fourier transform infrared (FTIR)
spectra, and elemental analyses (Figures S1−S7 and Table S1).
The melting points of NCBCs were shown in Figure S8. The
strong coordination of N → B was confirmed by single-crystal X-
ray diffraction. At room temperature, the boron−nitrogen bond
length in NCBC-1 and NCBC-3 were ∼1.75 Å, while that for
NCBC-2 and NCBC-4 was ∼1.66 Å, which was between the
length of the boron−nitrogen covalent bond (e.g., in boron
nitride, 1.53 Å) and the N → B external coordination bond
(1.80−1.98 Å).34 The results indicate that NCBCs show higher
stability than the boronic esters with external coordination
structures. Data of single-crystal X-ray diffraction are shown in
Figures S9−S12 and Tables S2−S13.
The hydrolytic stability of NCBCs was investigated by taking
NCBC-1 and NCBC-2 as examples. Specifically, we dissolved
NCBC-1 in DMSO-d6 with different water concentrations, and
the hydrolysis stability of NCBC-1 was evaluated by 1H NMR
(Figure 2, Figure S13 and Table S14). NCBCs showed higher
hydrolysis stability than those of conventional boronic esters
because of the intramolecular coordination of N → B in the
NCB linkages. For example, when water was added to NCBC-1
or NCBC-2 in a molar ratio of 20:1 (approximately 20 equiv
compared to boronic ester groups), the hydrolysis percentages
of NCBC-1 and NCBC-2 were 15.8% and 18.0% after 5 min,
respectively. As reported by Sumerlin’s group,22 when water was
added to the five-member cyclic boronic esters in a 15:1 molar
ratio (approximately 15 equiv compared to boronic ester
groups), ∼97% of the boronic esters was hydrolyzed after 30 s.
We further studied the hydrolytic equilibrium of NCBC-1 and
RESULTS AND DISCUSSION
■
Synthesis of NCBCs and Evaluation of Their Hydrolytic
Stability. For investigation of the hydrolytic stability and
dynamic nature of the NCB linkages, seven model NCBCs were
designed and synthesized (Schemes S1−S7 in the Supporting
Information; Table 1). Conventional boronic esters synthesized
from boronic acids with diols or phenols are very susceptible to
water, so the water produced during the reaction must be
B
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Figure 1. Design concept: (A) Chemical route for the fabrication of PU vitrimers from the reaction of the active hydrogen in NCBC-1 with THDI. (B)
Rearrangement of PU networks via associative exchange reactions of NCB linkages. (C) Speculative exchange reactions of NCB linkages in PU
networks.
NCBC-2 at a fixed water concentration and obtained the
hydrolysis equilibrium constant (Keq = ([boronic acid] [do-
nor])/[NCBC] [H2O]2) of NCBCs. The Keq of NCBC-1 and
NCBC-2 at 25 °C are 4.78 × 10−3 and 3.67 × 10−3 L·mol−1,
respectively, which is significantly lower than that of the five-
member cyclic boronic ester reported in the reference.35
Furthermore, the model compounds containing urethane
group and urea group were synthesized by reacting NCBC-3 and
NCBC-4 with hexyl isocyanate (Schemes S8 and S9 and Figures
S14 and S15) and were named as NCBC-X1 and NCBC-X2,
1
respectively. Then their hydrolysis stability was studied by H
NMR (Figures S16 and S17). NCBC-X1 showed excellent
hydrolytic stability as NCBCs and the hydrolysis percentage was
Figure 2. Hydrolytic stability for NCBC-1. (A) Hydrolysis reaction of
NCBC-1. (B) 1H NMR spectra of the NCBC-1 in DMSO-d6 after the
addition of different amounts of water at 25 °C for 5 min. The molar
ratios of H2O versus NCBC-1 are 1:1, 2:1, 5:1, 10:1, and 20:1,
respectively.
30%, while that for NCBC-X2 was also completely hydrolyzed.
The reason is that once NCBCs containing an amine nitrogen
atom in NCBCs reacted with the isocyanate, the electron cloud
density on the nitrogen atom decreased as the lone pair of
electrons on nitrogen was engaged in the formation of the urea
C
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Figure 3. Dynamic exchange reactions between NCB linkages. (A) Transesterification reaction between NCBC-2 and NCBC-6. (B) 1H NMR spectra
for the methyl of the mixture of NCBC-2 (a1b1) and NCBC-6 (a2b3) at variable temperatures. (C) Kinetic data of the transesterification between
NCBC-2 (a1b1) and NCBC-6 (a2b3). The reaction kinetics were plotted as the concentration of the reactant NCBC-6 (a2b3) decreased, and the
concentration of the product NCBC-7 (a2b1) increased with the increase of temperature and time. The inset shows the Arrhenius plot of the logarithm
of the reaction rate constant (ln k) versus the reciprocal of the temperature (1/T); the Ea calculated from the Arrhenius plot was estimated to be 62.9
kJ/mol. The details of the exchange reaction kinetics are described in the Supporting Information.
Figure 4. Dynamic nature and superior properties of PU vitrimers based on NCB linkages. (A) Stress−strain curves of PU vitrimers with different
stoichiometric ratios. (B)−(D) Storage modulus, tan δ curves, and stress−strain curves of PU1.1 after exposure to 40% RH for a certain time. (E) Stress
relaxation curves of PU1.1 at various temperatures. (F) Linear regression of the logarithm of relaxation time (ln τ) versus the reciprocal of temperature
(1/T) for PU1.1. (G) Visible light spectra of commercial silicate glass and PU1.1. (H) Stress−strain curves of virgin and recycled PU1.1. (I) Optical
microscope images of the scratched and repaired PU1.1 coatings at 170 °C for 40 min under a pressure of 10 kPa.
D
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bond, thus weakening or eliminating the coordination effect of
the N → B.
exhibited excellent mechanical properties. For example, the
tensile strengths of PU1.1 and PUU1.1 are 43.0 and 36.7 MPa,
respectively (Figure 4A and Figure S27), which are superior to
those of most reported PU materials with a dynamic covalent
network.36−40
Dynamic Exchange Reactions of NCB Linkages in
Model Compounds. Although the N → B coordination makes
NCBCs more stable to water, NCB linkages can undergo
exchange reactions upon heating without any catalyst. To
investigate the dynamic nature of NCB linkages, we first studied
the exchange reactions between NCBC and boronic acid (PBA)
qualitatively by liquid chromatography (LC). We mixed NCBC-
2 (a1b1) and PBA (b2) in a 1:1 stoichiometric ratio in anhydrous
dimethylformamide (DMF); after the mixture was incubated at
90 °C for 20 min, two new peaks of the target compounds
NCBC-4 (a1b2) and HPBA (b1) appeared (Figure S18). The
exchange reactions between NCBC-2 and PBA were further
studied to obtain the kinetic parameter using variable temper-
ature nuclear magnetic resonance (VT-NMR). Specifically, we
mixed NCBC-2 and PBA with a stoichiometric ratio of 1:1 in
Generally, the boronic ester linkages are easily hydrolyzed and
the corresponding materials exhibit poor water resistance.
Because of the N → B internal coordination of NCB linkages,
PU vitrimers showed significantly improved water resistance.
We confirmed this by exposing the bulk PU1.1 to the
environment with a constant temperature and humidity.
There was almost no decrease in the Tg and mechanical
properties for PU vitrimers, even after exposure to 40% RH at 30
°C for 4 weeks (Figure 4B−D; Figure S28). These results are in
agreement with those described in model NCBCs. This is a
breakthrough of the dynamic cross-linked polymers based on
the boronic ester linkages. Surprisingly, despite the fact that the
small molecule of NCBC-X2 is easy to hydrolyze, the Tg and
mechanical properties of PUU1.1 did not significantly reduce
after exposure to 40% RH at 30 °C for 4 weeks (Figure S29). We
believe that the cross-linking networks of the polymers can
prevent water molecules, thereby inhibiting hydrolysis of the
boronic esters.
Considering the potential applications for PU vitrimers and
PUU vitrimers based on NCB linkages, the hydrolytic stability of
the vitrimers was further investigated by immersing PU1.1 and
PUU1.1 samples in distilled water. After 7 days at room
temperature (25 3 °C), the overall appearance of the PU1.1
samples remained almost unchanged, while that for the PUU1.1
sample partially paled (Figure S30). The water uptake of PU1.1
and PUU1.1 was 3.4% and 8.9% (Tables S19 and S20),
respectively, under the same conditions.
Although the breaking strength and Young’s modulus of PU1.1
and PUU1.1 decreased slightly compared to those of the original
samples after immersion in water for a period of time (Figure
S31 and Table S21), one can obviously observe that PU1.1’s
water resistance was superior to that of PUU1.1 because of the
difference in the stability of NCB linkages embedded in the
polymer networks. In fact, because the large number of urethane
groups and/or urea groups existed in the PU1.1 and PUU1.1, the
higher water absorption and plasticization of which had a
significant impact on the mechanical properties. Even then, the
water resistance of PU1.1 and PUU1.1 is still superior to that of
dynamic cross-linked polymers based on conventional boronic
esters and boroxine linkages.11,28 When the NCB linkages are
introduced into the relatively hydrophobic polystyrene (PS)
skeleton (Scheme S11), the water resistance of the NCB
linkages can be fully utilized. After immersion in water for 24 h
(25 3 °C), the breaking strength and Young’s modulus of
NCB based PS vitrimer can still maintain 90% of the original
sample (Figure S32 and Table S21). This is consistent with the
results reported by Guan et al.26 Furthermore, PU vitrimers can
maintain the structural integrity, even after immersion in boiling
water. PU1.1 can maintain the structural integrity after being
boiled in water at 80 °C for 8 h (Figure S33a). After drying (100
°C, 12 h), the appearance of PU1.1 was restored to the original
sample, and the mechanical properties only slightly decreased
(Figure S33b). For example, the breaking strengths of PU1.1
before and after in immersion in boiling water and drying were
54.47 and 49.34 MPa, respectively (Table S22).
1
DMSO-d6, and the H NMR spectra of the mixture were
collected every 30 s at certain temperatures until the reaction
reached an equilibrium state. According to the Arrhenius
equation, the activation energy (Ea) of the reaction between
NCBC-2 and PBA was calculated to be 43.6 kJ/mol (Figures
S19 and S20). With use of the same method, the Ea for the
reaction of NCBC-2 (a1b2) with NCBC-6 (a2b3) was calculated
to be 62.9 kJ/mol (Figure 3, Figure S21); this value was higher
than that of the five-membered cyclic boronic ester (15.9 kJ/
mol).12 Thus, because of the stable N → B internal coordination,
the exchange rate of the NCB linkages is lower than that of the
common cyclic boronic esters.
Synthesis of PU Vitrimers and PUU Vitrimers. After
demonstrating the dynamic reversibility of NCB linkages based
on small molecules, we set out to construct polymer networks
based on NCB linkages. Classical polyurethanes (PUs) or
poly(urea-urethane)s (PUUs) were usually synthesized from
polyisocyanates with polyols or polyamines. Herein, PU
vitrimers and PUU vitrimers based on NCB linkages can be
readily assembled from difunctional NCBCs (such as NCBC-1
and NCBC-2) and trifunctional isocyanate (THDI) under mild
conditions without any catalyst; the general schemes of PU
vitrimers and PUU vitrimers are depicted in Figure 1A and
Scheme S10, respectively.
Considering the deviation between theoretical and actual
values resulting from the purity of the reagents, PU vitrimers and
PUU vitrimers with different stoichiometric ratios were
synthesized. The cross-linked structures of PU vitrimers and
PUU vitrimers were confirmed by FTIR (Figure S22) and
swelling tests. PU vitrimers and PUU vitrimers showed good
chemical resistance and are insoluble but swollen in DMF and
other polar solvents (Figure S23 and Tables S15−S17),
indicating that the integrity of the cross-linked structure was
maintained even when heated. PU vitrimers and PUU vitrimers
with different stoichiometric ratios exhibited different thermal
properties (Figures S24−S26 and Table S18). PU1.1 and PUU1.1
showed the highest glass transition temperature (Tg, 87 and 78
°C, respectively) and thermal decomposition temperature (T5%,
215 and 228 °C, respectively) among samples with different
stoichiometric ratios, indicating that a relatively complete
reaction occurred in the corresponding reactants ratio.
Mechanical Properties and Water Resistance of PU
Vitrimers and PUU Vitrimers. PU vitrimers and PUU
vitrimers showed good processability as the pulverized resin
can be reprocessed by means of compression molding at high
temperatures. As expected, PU vitrimers and PUU vitrimers
Stress Relaxation Behavior of PU Vitrimers and PUU
Vitrimers. The viscoelasticity properties of PU vitrimers and
PUU vitrimers were studied by stress relaxation tests. Full stress
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relaxation was observed at high temperatures despite the high
cross-linking structure of PU vitrimers and PUU vitrimers
(Figure 4E, Figure 5, and Figure S34), which confirmed the
were significantly lower than that of PU1.1; this trend was also
applied to the PUU vitrimers.
In addition to Tg, Tv is another key characteristic for vitrimers,
as it corresponds to the transition from the viscoelastic solid to
the viscoelastic liquid resulting from the exchange reactions
within the polymeric network. Conventionally, Tv for vitrimers is
considered the temperature at which the viscosity reaches 1012
Pa·s;42−46 accordingly, Tv values for PU1.1 and PUU1.1 were
calculated to be 110 and 107 °C (details are shown in the
Supporting Information). As the temperature increased to Tv,
the exchange reactions of NCB linkages became fast. Once the
concentration of the exchangeable dynamic covalent bonds
reached a certain threshold, the rearrangement of cross-linked
networks became possible and the macroscopic flows occurred
in vitrimers; ultimately, the vitrimers can be reprocessed and
recycled.
Reprocessing and Recycling of PU Vitrimers and PUU
Vitrimers. It is known that vitrimers can be reprocessed,
repaired, and recycled at high temperatures, while still
maintaining a relatively constant cross-linking density thanks
to the associative exchange reactions. As expected, solid-phase
reprocessing and liquid-phase recycling of PU vitrimers and
PUU vitrimers can be realized via the exchange reactions of
NCB linkages upon heating without any catalyst, and the
multiple reprocessed and recycled vitrimers showed mechanical
properties similar to the virgin materials.
Figure 5. (A)−(C) Stress relaxation curves at various temperatures for
PUU vitrimers. (D) Linear regression of the logarithm of relaxation
time ln(τ) versus reciprocal of temperature (1/T) for PUU vitrimers.
The reprocessability of PU vitrimers and PUU vitrimers was
investigated by compression molding. The specific reprocessing
procedure is shown in Figure 6A. The reprocessed bulks were
uniformly transparent with the transmittance above 80% in the
visible wavelength range (Figure 4G). The structure of PU
vitrimers and PUU vitrimers showed no obvious change over
five reprocessing cycles (Figure S38). The Tg’s of reprocessed
PU1.1 were almost equal to that of the virgin sample (Figure
S39). Furthermore, there was no evident decrease in tensile
strength, Young’s modulus, or elongation at break after being
reprocessed five times (Figure 4H and Figures S40 and S41).
Therefore, because of the exchange reactions among the NCB
linkages, the network rearrangement and bond reshuffling
occurred, enabling the PU networks and PUU networks to be
reshaped and reprocessed in a solid state at high temperatures.
However, this solid-phase recycling method is somewhat
limited, especially for the resin−matrix composites, as damages
on the reinforced fibers will be unavoidably produced.
In this work, we proposed a more useful recovery method as
presented in Figure 6B. Taking PU1.1 as an example, in essence,
the chemical equilibrium of the cross-linked network can be
completely destroyed by the exchange reactions induced by
adding excessive TEA or HPBA; then PU vitrimers can be fully
recycled to their oligomers. After the addition of 3 equiv of TEA,
the NCB linkages in the PU network were broken as a result of
the exchange reactions upon heating. When the number of cross-
links decreased to the percolation threshold, the PU vitrimer
could be fully dissolved and returned to the liquid state. After the
addition of 3 equiv of THDI and 3 equiv of HPBA, the resin
could be recurred through the reaction between active hydrogen
atoms and isocyanate groups. The structure and mechanical
properties of the recycled PU1.1 and PUU1.1 were considerable to
those of the virgin samples (Figures S42 and S43). This method
is highly significant for the complete recovery of the matrix resin
and the expensive carbon fiber from resin−matrix composites
without sacrificing their commercial use and value.
malleability of PU vitrimers and PUU vitrimers. The relaxation
time (τ) rapidly decreased with increasing temperatures for PU
vitrimers and PUU vitrimers. Because of the associative
exchange mechanism of the NCB linkages, the relationship
between τ and temperatures for PU vitrimers and PUU vitrimers
can be well described by the Arrhenius equation (Figure 4F,
Figure S34b,d, and Figure 5d). Ea for PU vitrimers and PUU
vitrimers can be obtained according to the linear correlation of
ln(τ) with 1/T. Because of the high stability of NCB linkages,
the Ea values for PU1.1 and PUU1.1 were 138.4 and 130.0 kJ/mol,
respectively, which were much higher than those of vitrimers
based on conventional boronic esters (76.7 kJ/mol) and
boroxine linkages (79.5 kJ/mol).30,41 It can be seen that
vitrimers based on NCB linkages show not only good hydrolytic
stability but also good heat resistance (such as higher Ea) as
compared to other dynamic cross-linked polymers based on
conventional boronic esters, which greatly expands the
application of this kind of material. To further prove the
integrity of the cross-linked structure of NCB-based vitrimers at
high temperatures, we used a dynamic thermomechanical
analyzer and rheometer to conduct frequency sweep experi-
ments and temperature sweep experiments for PU1.1 and
PUU1.1, respectively. The results showed that the PU1.1 and
PUU1.1 at different temperatures did not exhibit a significant
decline in storage modulus, indicating that the cross-linking
density of the vitrimers remained unchanged at different
temperatures (Figures S35−S37).
We propose that the full range of the vitrimers’ stress
relaxation behaviors are primarily affected by temperature and a
small number of impurities, such as water, boronic acid, or other
hydroxyl-containing compounds. Compared to PU1.1, there may
be some residual hydroxyl groups in the PU1.0 and PU0.9
networks, which are conducive to the exchange reactions of
the NCB linkages. The stress relaxation results support our
hypothesis. As shown in Table S23, the Ea’s of PU1.0 and PU0.9
F
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boronic esters. Both PU vitrimers and PUU vitrimers exhibited
excellent solvent resistance and mechanical properties like
general thermosets, solid-phase reprocessing of PU vitrimers
and PUU vitrimers can be achieved via the transesterification of
NCB linkages upon heating. Besides, reversible conversion
between the oligomer and polymers can also be realized by the
exchange reactions among NCB linkages; the multiple
reprocessed and recycled vitrimers showed mechanical proper-
ties similar to the virgin samples. Moreover, NCB-based
vitrimers showed enhanced water and heat resistance as a result
of the highly hydrophobic structure of the NCB linkages, in
which there was almost no decrease in Tg or mechanical
properties when exposed to 40% relative humidity at 30 °C for 4
weeks. This study provides a novel chemical strategy and
concept for designing a range of robust and stable vitrimers with
the potential for commercial industrial applications.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c10244.
■
sı
X-ray crystallographic data of NCBC-1 (CIF)
X-ray crystallographic data of NCBC-2 (CIF)
X-ray crystallographic data of NCBC-3 (CIF)
X-ray crystallographic data of NCBC-4 (CIF)
All experimental details including synthesis and character-
izations, model reaction, polymerization, sample prepara-
tion, and mechanical studies (PDF)
Figure 6. Reprocessing and recycling processes of PU vitrimers. (A)
Reprocessing of PU1.1 powders (left) by compression molding to
reform the bulk material (right, after the fifth cycle) at 150 °C and 1
MPa. (B) Recycling process of PU vitrimers. (i) PU bulks were ground
into powders. (ii) Powders were dissolved in DMF and treated with 3
equiv of TEA at 130 °C for 2 h. (iii) Reformation of PU network by
adding 3 equiv of THDI and 3 equiv of HPBA incubated at 90 °C for 2
h. (iv) Recycled square-shaped PU sample via hot-pressing of the
reformed PU after pulverization and vacuum-drying treatment.
AUTHOR INFORMATION
Corresponding Author
■
Xinli Jing − School of Chemistry, Xi’an Jiaotong University,
Xi’an, Shaanxi 710049, P. R. China; MOE Key Laboratory for
Nonequilibrium Synthesis and Modulation of Condensed
Matter, Xi’an, Shaanxi 710049, P. R. China; Xi’an Key
Laboratory of Sustainable Energy Material Chemistry, Xi’an,
Shaanxi 710049, P. R. China; orcid.org/0000-0002-
0056-4864; Email: xljing@mail.xjtu.edu.cn
PU vitrimers and PUU vitrimers can be used to prepare
remendable adhesives and coatings through the NCB exchange-
induced network rearrangement. As the raw material NCBC-1
for PU vitrimers showed good solubility, PU adhesive and
coating were prepared and their repairable properties were
studied. As shown in Figure S44, the PU adhesive can be easily
repaired by heating it to 170 °C under 0.5 MPa, as this condition
will activate the exchange reactions of NCB linkages. The
repaired adhesive’s lap-shear strength showed no significant
decrease compared to that of the pristine sample (Figure S44e).
A scratch on the PU1.1 coating can be repaired by heating it at
170 °C for 20 min under a pressure of 10 kPa because of the
reformation of covalent bonds across the interfaces. The
obtained PU1.1 coating showed good adhesion (Grade 0) and
flexibility (1 mm). Optical microscopy images of PU coating are
described in Figure 4I.
Authors
Xiaoting Zhang − School of Chemistry, Xi’an Jiaotong
University, Xi’an, Shaanxi 710049, P. R. China; orcid.org/
0000-0001-7330-2251
Shujuan Wang − School of Chemistry, Xi’an Jiaotong
University, Xi’an, Shaanxi 710049, P. R. China; orcid.org/
0000-0001-7004-1551
Zikang Jiang − School of Chemistry, Xi’an Jiaotong University,
Xi’an, Shaanxi 710049, P. R. China
Yu Li − School of Chemistry, Xi’an Jiaotong University, Xi’an,
Shaanxi 710049, P. R. China; orcid.org/0000-0002-
7066-6110
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c10244
CONCLUSION
■
In summary, based on the principle that the nitrogen-donor
compounds can “lock” the vacant orbital of the boron atom to
stabilize the boronic ester linkages, we studied the chemistry of
NCB linkages and fabricated the PU vitrimers and PUU
vitrimers with enhanced stability based on NCB linkages.
NCBCs showed high hydrolytic stability but still undergo
transesterification upon heating without any catalyst. The
NCBC-X1 containing urethane group also showed excellent
hydrolytic stability like NCBCs, even higher than conventional
Author Contributions
#X.T.Z. and S.J.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work is supported by the National Natural Science
Foundation of China (Nos. 51703179, 11732012), National
■
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Article
(19) Piest, M.; Zhang, X.; Trinidad, J.; Engbersen, J. F. J. pH-
Responsive, Dynamically Restructuring Hydrogels Formed by
Reversible Crosslinking of PVA with Phenylboronic Acid Functional-
ised PPO-PEO-PPO Spacers (Jeffamines®). Soft Matter 2011, 7 (23),
11111−11118.
(20) He, L.; Fullenkamp, D. E.; Rivera, J. G.; Messersmith, P. B. pH
Responsive Self-healing Hydrogels Formed by Boronate−Catechol
Complexation. Chem. Commun. 2011, 47 (26), 7497−7499.
(21) Meng, H.; Zheng, J.; Wen, X.; Cai, Z.; Zhang, J.; Chen, T. pH-
and Sugar-Induced Shape Memory Hydrogel Based on Reversible
Phenylboronic Acid-Diol Ester Bonds. Macromol. Rapid Commun.
2015, 36 (6), 533−537.
(22) Cash, J. J.; Kubo, T.; Bapat, A. P.; Sumerlin, B. S. Room-
Temperature Self-healing Polymers Based on Dynamic-Covalent
Boronic Esters. Macromolecules 2015, 48 (7), 2098−2106.
(23) Cote, A. P.; El-Kaderi, H. M.; Furukawa, H.; Hunt, J. R.; Yaghi, O.
M. Reticular Synthesis of Microporous and Mesoporous 2D Covalent
Organic Frameworks. J. Am. Chem. Soc. 2007, 129 (43), 12914−12915.
(24) Danjo, H.; Hirata, K.; Yoshigai, S.; Azumaya, I.; Yamaguchi, K.
Back to Back Twin Bowls of D3-Symmetric Tris(spiroborate)s for
Supramolecular Chain Structures. J. Am. Chem. Soc. 2009, 131 (5),
1638−1639.
(25) Bapat, A. P.; Roy, D.; Ray, J. G.; Savin, D. A.; Sumerlin, B. S.
Dynamic-Covalent Macromolecular Stars with Boronic Ester Linkages.
J. Am. Chem. Soc. 2011, 133 (49), 19832−19838.
(26) Cromwell, O. R.; Chung, J.; Guan, Z. Malleable and Self-Healing
Covalent Polymer Networks through Tunable Dynamic Boronic Ester
Bonds. J. Am. Chem. Soc. 2015, 137 (20), 6492−6495.
(27) Ono, K.; Johmoto, K.; Yasuda, N.; Uekusa, H.; Fujii, S.; Kiguchi,
M.; Iwasawa, N. Self-Assembly of Nanometer-Sized Boroxine Cages
from Diboronic Acids. J. Am. Chem. Soc. 2015, 137 (22), 7015−7018.
(28) Lai, J. C.; Mei, J. F.; Jia, X. Y.; Li, C. H.; You, X. Z.; Bao, Z. A Stiff
and Healable Polymer Based on Dynamic-Covalent Boroxine Bonds.
Adv. Mater. 2016, 28 (37), 8277−8282.
(29) Bao, C.; Jiang, Y. J.; Zhang, H.; Lu, X.; Sun, J. Room-
Temperature Self-Healing and Recyclable Tough Polymer Composites
Using Nitrogen-Coordinated Boroxines. Adv. Funct. Mater. 2018, 28,
1800560.
Key R&D Program of China (2019YFA0706801), and
Fundamental Research Funds for the Central Universities
(xjh012019024). We acknowledge the Instrument Analysis
Center of Xi’an Jiaotong University for material characterization.
We thank graduate student Wen Wang for helpful assistance in
the synthesis of model compounds. We gratefully acknowledge
helpful discussions with Prof. Mingming Zhang from Xi’an
Jiaotong University.
REFERENCES
■
(1) Long, T. E. Toward Recyclable Thermosets. Science 2014, 344
(6185), 706−707.
(2) Wojtecki, R. J.; Meador, M. A.; Rowan, S. J. Using the Dynamic
Bond to Access Macroscopically Responsive Structurally Dynamic
Polymers. Nat. Mater. 2011, 10 (1), 14−27.
(3) Zhang, Z. P.; Rong, M. Z.; Zhang, M. Q. Polymer Engineering
based on Reversible Covalent Chemistry: A Promising Innovative
Pathway towards New Materials and New Functionalities. Prog. Polym.
Sci. 2018, 80, 39−93.
(4) Wang, S.; Yang, Y.; Ying, H.; Jing, X.; Wang, B.; Zhang, Y.; Cheng,
J. Recyclable, Self-Healable, and Highly Malleable Poly(urethane-
urea)s with Improved Thermal and Mechanical Performances. ACS
Appl. Mater. Interfaces 2020, 12 (31), 35403−35414.
(5) Denissen, W.; Winne, J. M.; Du Prez, F. E. Vitrimers: Permanent
Organic Networks with Glass-Like Fluidity. Chem. Sci. 2016, 7 (1), 30−
38.
(6) Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. Silica-Like
Malleable Materials from Permanent Organic Networks. Science 2011,
334 (6058), 965−968.
(7) Yang, Y.; Pei, Z.; Zhang, X.; Tao, L.; Wei, Y.; Ji, Y. Carbon
Nanotube−Vitrimer Composite for Facile and Efficient Photo-Welding
of Epoxy. Chem. Sci. 2014, 5 (9), 3486−3492.
(8) Hendriks, B.; Waelkens, J.; Winne, J. M.; Du Prez, F. E.
Poly(thioether) Vitrimers via Transalkylation of Trialkylsulfonium
Salts. ACS Macro Lett. 2017, 6, 930−934.
(9) Fortman, D. J.; Brutman, J. P.; Cramer, C. J.; Hillmyer, M. A.;
Dichtel, W. R. Mechanically Activated, Catalyst-Free Polyhydroxyur-
ethane Vitrimers. J. Am. Chem. Soc. 2015, 137 (44), 14019−14022.
(10) Zheng, N.; Fang, Z.; Zou, W.; Zhao, Q.; Xie, T. Thermoset
Shape-Memory Polyurethane with Intrinsic Plasticity Enabled by
Transcarbamoylation. Angew. Chem., Int. Ed. 2016, 55, 11421−11425.
(11) Wang, S.; Xing, X.; Zhang, X.; Wang, X.; Jing, X. Room-
Temperature Fully Recyclable Carbon Fibre Reinforced Phenolic
Composites through Dynamic Covalent Boronic Ester Bonds. J. Mater.
Chem. A 2018, 6 (23), 10868−10878.
(30) Ogden, W.; Guan, Z. Recyclable, Strong, and Highly Malleable
Thermosets Based on Boroxine Networks. J. Am. Chem. Soc. 2018, 140
(20), 6217−6220.
(31) Marsden, S. Boronic Acids. Preparation and Applications in
Organic Synthesis and Medicine. Herausgegeben von Dennis G. Hall.
Angew. Chem. 2006, 118 (13), 2057−2058.
(32) Gigant, N.; Honraedt, A.; Gras, E.; Gillaizeau, I. Efficient Cross-
Coupling of Dioxazaborocanes with α-Phosphate Enamides. Eur. J. Org.
Chem. 2014, 2014 (35), 7889−7894.
(12) Rottger, M.; Domenech, T.; van der Weegen, R.; Breuillac, A.;
Nicolay, R.; Leibler, L. High-Performance Vitrimers from Commodity
Thermoplastics Through Dioxaborolane Metathesis. Science 2017, 356
(6333), 62−65.
(13) Zheng, P.; Mccarthy, T. J. A Surprise from 1954: Siloxane
Equilibration is a Simple, Robust, and Obvious Polymer Self-Healing
Mechanism. J. Am. Chem. Soc. 2012, 134, 2024−2027.
(14) Capelot, M.; Montarnal, D.; Tournilhac, F.; Leibler, L. Metal-
Catalyzed Transesterification for Healing and Assembling of
Thermosets. J. Am. Chem. Soc. 2012, 134 (18), 7664−7667.
(15) Chao, A.; Negulescu, I.; Zhang, D. Dynamic Covalent Polymer
Networks Based on Degenerative Imine Bond Exchange: Tuning the
Malleability and Self-Healing Properties by Solvent. Macromolecules
2016, 49 (17), 6277−6284.
(16) Brooks, W. L. A.; Sumerlin, B. S. Synthesis and Applications of
Boronic Acid-Containing Polymers: From Materials to Medicine.
Chem. Rev. 2016, 116 (3), 1375−1397.
(17) Guan, Y.; Zhang, Y. Boronic Acid-Containing Hydrogels:
Synthesis and Their Applications. Chem. Soc. Rev. 2013, 42 (20), 8106−
8121.
(18) Xu, J.; Yang, D.; Li, W.; Gao, Y.; Chen, H.; Li, H.
Phenylboronate-Diol Crosslinked Polymer Gels with Reversible Sol-
Gel Transition. Polymer 2011, 52 (19), 4268−4276.
(33) Ito, Y.; Kida, J.; Aoki, D.; Otsuka, H. Modification of Amine-
Cured Epoxy Resins by Boronic Acids based on Their Reactivity with
Intrinsic Diethanolamine Units. Chem. Commun. 2018, 54, 12930−
12933.
(34) Franzen, S.; Ni, W.; Wang, B. Study of the Mechanism of
Electron-Transfer Quenching by Boron−Nitrogen Adducts in
Fluorescent Sensors. J. Phys. Chem. B 2003, 107 (47), 12942−12948.
(35) Goldberg, A. R.; Northrop, B. H. Spectroscopic and Computa-
tional Investigations of The Thermodynamics of Boronate Ester and
Diazaborole Self-Assembly. J. Org. Chem. 2016, 81 (3), 969−980.
(36) Rekondo, A.; Martin, R.; Ruizdeluzuriaga, A.; Cabanero, G.;
Grande, H.; Odriozola, I. Catalyst-Free Room-Temperature Self-
Healing Elastomers Based on Aromatic Disulfide Metathesis. Mater.
Horiz. 2014, 1 (2), 237−240.
(37) Lee, S. H.; Shin, S. R.; Lee, D. S. Self-Healing of Cross-linked PU
via Dual-Dynamic Covalent Bonds of a Schiff Base from Cystine and
Vanillin. Mater. Des. 2019, 172, 107774.
(38) Erice, A.; Azcune, I.; Ruiz de Luzuriaga, A.; Ruiperez, F.;
Irigoyen, M.; Matxain, J. M.; Asua, J. M.; Grande, H.-J.; Rekondo, A.
Effect of Regioisomerism on Processability and Mechanical Properties
of Amine/Urea Exchange Based Poly(urea-urethane) Vitrimers. ACS
Appl. Polym. Mater. 2019, 1 (9), 2472−2481.
H
https://dx.doi.org/10.1021/jacs.0c10244
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(39) Shi, J.; Zheng, T.; Zhang, Y.; Guo, B.; Xu, J. Reprocessable Cross-
Linked Polyurethane with Dynamic and Tunable Phenol−Carbamate
Network. ACS Sustainable Chem. Eng. 2020, 8 (2), 1207−1218.
(40) Yuan, C. E.; Rong, M. Z.; Zhang, M. Q. Self-Healing
Polyurethane Elastomer with Thermally Reversible Alkoxyamines as
Crosslinkages. Polymer 2014, 55 (7), 1782−1791.
(41) Chen, Y.; Tang, Z.; Zhang, X.; Liu, Y.; Wu, S.; Guo, B. Covalently
Cross-Linked Elastomers with Self-Healing and Malleable Abilities
Enabled by Boronic Ester Bonds. ACS Appl. Mater. Interfaces 2018, 10,
24224−24231.
(42) Dyre, J. C. Colloquium: The Glass Transition and Elastic Models
of Glass-Forming Liquids. Rev. Mod. Phys. 2006, 78 (3), 953−972.
(43) Angell, C. A. Formation of Glasses from Liquids and
Biopolymers. Science 1995, 267 (5206), 1924−1935.
(44) Ediger, M. D.; Angell, C. A.; Nagel, S. R. Supercooled Liquids and
Glasses. J. Phys. Chem. 1996, 100 (31), 13200−13212.
(45) Capelot, M.; Unterlass, M. M.; Tournilhac, F.; Leibler, L.
Catalytic Control of the Vitrimer Glass Transition. ACS Macro Lett.
2012, 1 (7), 789−792.
(46) Brutman, J. P.; Delgado, P. A.; Hillmyer, M. A. Polylactide
Vitrimers. ACS Macro Lett. 2014, 3 (7), 607−610.
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J. Am. Chem. Soc. XXXX, XXX, XXX−XXX