Macromolecules
Article
thermoreversible dynamic-covalent bonds that can bring about
self-healing behavior. However, the radical products of
alkoxyamine dissociation are also sensitive to oxygen and
high temperatures (90−130 °C), which are typically required to
induce dissociation of the labile O−C bond.30,31 There have
been only limited reports of healing via photoirradiation of
alkoxyamines at room temperature.32 Like thiol−disulfide
exchange, the efficiency of alkoxyamine healing is expected to
decrease with time, in this case, due to the inevitable
irreversible combination of carbon-centered radicals. Alter-
natively, in one of the earliest reports that relied on a light
trigger, Chung and co-workers achieved covalent healing by the
photoinduced [2 + 2] cycloaddition of cinnamoyl groups to
reversibly form a cyclobutane derivative.33 More recently,
trithiocarbonate moieties were employed by Matyjaszewski and
co-workers in the first example of macroscopic fusion from UV-
induced healing.34
While photo- and heat-induced dynamic-covalent chemistries
have proven valuable in many self-healing systems, autonomous
healing with no external trigger is often desirable. Fewer
examples exist of bulk systems being healed with no significant
outside stimulus being necessary. In most of these cases, such
healing typically occurs as a result of the stimuli being present
under ambient conditions (i.e., ambient light or heat). For
example, the relatively stable radicals from thiuram disulfide
have been employed for visible-light self-healing over a 24 h
period, which succeeded efficiently in air as long the damaged
pieces had not been separated for an extended time.35 Ghosh
and Urban developed a UV self-repairing polyurethane based
on oxetane rings capable of scratch healing within an hour, only
needing power densities similar to sunlight.36,37 Other
previously reported materials rely on the heat present at
room temperature to induce healing. However, some of these
systems require the presence of catalysts in the bulk matrix. For
example, Lehn and co-workers designed a double dynamic bis-
imino carbohydrazide polymer infused with acid catalyst that
healed at room temperature in bulk.38 Similarly, disulfide
metathesis can also cause healing at room temperature with the
aid of an aliphatic phosphine catalyst.39 Catalyst-free
approaches comprise an even smaller subset of strategies to
achieve bulk healing at room temperature. Preliminary
investigations qualitatively suggest room-temperature self-
healing can occur through tailor-made Diels−Alder moieties.40
Furthermore, Odriozola and co-workers have demonstrated a
compelling example of room-temperature intrinsic self-healing
in the solid state due to aromatic disulfide metathesis;41
however, this material relies largely on reversible supra-
molecular hydrogen bonds for healing instead of reversible
covalent bonds. We are interested in exploiting new dynamic-
covalent chemistries for intrinsic self-healing in bulk under
ambient conditions.
demonstrated that boronic ester-based macromolecular stars
can be rendered dynamic in organic solutions.48 Boronate
esters have also been employed to prepare self-healing
hydrogels, wherein covalent healing can be effected by
formation of new boronate ester bonds along the interface of
damage.21,49−54 Esterification of boronic acids can also be
exploited to bring about mending in the absence of water.
Lavigne and co-workers have prepared dynamic-covalent linear
polymer chains by polymerization of low molecular weight bis-
diols with diboronic acids.43 The resulting polymers were
hydrolyzed in organic solution, isolated by drying, and restored
back to the original molecular weight under vacuum.
As compared to these previous reports, we were interested in
using boronic esters for self-healing of networks in the bulk,
reasoning that these linkages may be ideal for self-healing
because they can be rendered dynamic at room temperature
under ambient conditions. We reasoned that hydrolysis of
surface-exposed boronic esters in a bulk material could occur by
intentionally wetting the surface at the site of damage (or from
water present in the atmosphere under ambient humidity) to
induce exchange of boronic esters to heal the material by
covalent bridge formation across the damage interface.
Accordingly, a boronic ester diene was synthesized and
incorporated into a network by a radical-based thiol−ene
process. The bulk behavior of these networks was investigated
for their self-healing properties that arise from the dynamic-
covalent nature of their boronic ester cross-links. The
polymeric networks were capable of bulk-state healing at
room temperature, suggesting they may hold promise for
various applications, including being used as coatings,
composites, and biological materials.
EXPERIMENTAL SECTION
■
Materials. Divinylbenzene (Sigma-Aldrich, 80%) was passed
through a column of basic alumina. Dimethyl sulfoxide-d6 (d-
DMSO, Cambridge Isotope, 99.9% D) was dried overnight over 4 Å
molecular sieves. Dichloromethane (DCM, Sigma-Aldrich) was dried
using an anhydrous solvent system (Innovative technologies). 4-
Vinylphenylboronic acid (VPBA, Combi-blocks, 98%), 3-allyloxy-1,2-
propanediol (Acros Organics, 98%), pentaerythritol tetrakis(3-
mercaptopropionate) (PTMP, Sigma-Aldrich, 95%), 3,6-dioxa-1,8-
octanedithiol (DODT, TCI America, 95%), 2,2-dimethoxy-2-phenyl-
acetophenone (DMPA, Sigma-Aldrich, 99%), potassium chloride
(BDH, 99%), sodium chloride (Fisher, 99%), potassium acetate
(Macron Chemicals, 99%), deuterium oxide (D2O, Cambridge
Isotope, 99.9% D), and molecular sieves (4 Å, Mallinckrodt) were
used as received.
Instrumentation and Analysis. 1H NMR (500 MHz), 13C NMR
(125 MHz), and 11B NMR (160 MHz) spectra were recorded using an
Inova 500 spectrometer. For 11B NMR spectroscopy, 5 mm thin-
walled quartz NMR tubes were used. Chemical shifts are reported in
parts per million (ppm) downfield relative to tetramethylsilane (TMS,
0.0 ppm). Multiplicities are reported using the following abbreviations:
s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad.
High-resolution mass spectrometry (HRMS) was conducted with an
Agilent 6220 TOF-MS mass spectrometer in the Direct Analysis in
Real Time (DART) mode with the IonSense DART source. Infrared
spectra were collected on a Thermo Nicolet 5700 FTIR spectrometer
equipped with a single bounce diamond stage attenuated total
reflectance (ATR) accessory. Differential scanning calorimetry (DSC)
measurements were performed on a TA Instruments Q1000 equipped
with a liquid nitrogen cooling accessory and calibrated using sapphire
and high-purity indium metal. All samples were prepared in
hermetically sealed pans (4−7 mg/sample) and were referenced to
an empty pan. A scan rate of 10 °C/min was used. Glass transition
temperatures were evaluated as the midpoint of a step change in heat
For this purpose, we were interested in boronic acids, which
are known to form a variety of dynamic-covalent bonds.42−44
For example, the dehydration of boronic acids to form
boroxines is readily reversible by hydrolysis. Boroxine
formation has been employed to prepare a number of
dynamic-covalent assemblies.45−47 The direction of the
boroxine/boronic acid equilibrium can be readily controlled
by temperature, the addition of Lewis bases, or the addition of
water. Boronic acids are also capable of forming dynamic-
covalent bonds by reacting with diols, typically either in basic
aqueous media or in anhydrous organic solutions to form
boronate esters or boronic esters, respectively. Our group has
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