Malleable and Self-Healing Covalent Polymer Networks through Tunable Dynamic Boronic Ester Bonds

Article abstract of DOI:10.1021/jacs.5b03551

Despite numerous strategies involving dynamic covalent bond exchange for dynamic and self-healing materials, it remains a challenge to be able to tune the malleability and self-healing properties of bulk materials through simple small molecule perturbations. Here we describe the use of tunable rates of boronic ester transesterification to tune the malleability and self-healing efficiencies of bulk materials. Specifically, we used two telechelic diboronic ester small molecules with variable transesterification kinetics to dynamically cross-link 1,2-diol-containing polymer backbones. The sample cross-linked with fast-exchanging diboronic ester showed enhanced malleability and accelerated healing compared to the slow-exchanging variant under the same conditions. Our report demonstrates the possibility of transferring small molecule kinetics to dynamic properties of bulk solid material and may serve as a guide for the rational design of tunable dynamic materials.

Full text of DOI:10.1021/jacs.5b03551

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Communication
Malleable and self-healing covalent polymer networks
through tunable dynamic boronic ester bonds
Olivia R. Cromwell, Jaeyoon Chung, and Zhibin Guan
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 06 May 2015
Downloaded from http://pubs.acs.org on May 6, 2015
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Malleable and self-healing covalent polymer
networks through tunable dynamic boronic ester
bonds
Olivia R. Cromwell , Jaeyoon Chung , and Zhibin Guan*
Department of Chemistry, University of California, Irvine, California 92697, United States
Supporting Information Placeholder
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bond has a high bond dissociation energy (B-O bond 124
ABSTRACT: Despite numerous strategies involving dynamic
covalent bond exchange for dynamic and self-healing materials, it
remains a challenge to be able to tune the malleability and self-
healing properties of bulk materials through simple small
molecule perturbations. Here we describe the use of kinetically
tunable rates of boronic ester transesterification to effectively tune
the malleability and self-healing efficiencies of bulk materials. To
demonstrate the concept, we used two telechelic di-boronic ester
small molecules with variable transesterification kinetics to
dynamically crosslink 1,2-diol-containing polymer backbones.
We found that the sample crosslinked with fast-exchanging di-
boronic ester showed enhanced malleability and accelerated
healing compared to the slow-exchanging variant under the same
conditions. Our report demonstrates the possibility of transferring
small molecule exchange kinetics to malleability and self-healing
ability of bulk solid material, and may serve as a guide for the
bottom-up rational design of tunable dynamic materials.
2
2
kcal/mol) , and, crucially, the rate of boronic ester
transesterification can also be tuned over many orders of
magnitude with simple neighboring group effects, ranging from
2
3
effectively inert to extremely fast. Despite these desirable
properties, applications for dynamic boronic ester exchange has
24-27
been largely limited to solution phase,
often related to sensing
25,28,29
technology for sugars,
self-healing hydrogels.
and more recently, for dynamic and
A very recent report described self-
30-32
healing bulk material utilizing boronic ester, in which the healing
had to be facilitated by water to induce boronic ester hydrolysis
3
3
and re-formation. No prior report has investigated the tunability
of boronic ester exchange kinetics for controlling bulk dynamic
properties. We envisioned that the boronic ester transesterification
reaction provided an untapped potential as a platform for dynamic
material design, allowing us the opportunity to develop of a
library of materials with tunable transesterification exchange
kinetics and emergent malleability and self-healing properties.
Polymeric materials that contain reversible bonds and other
dynamic interactions exhibit interesting properties, such as
adaptability, malleability, and self-healing. In contrast to
traditional thermosets, dynamically crosslinked polymers can be
reprocessed and recycled while still maintaining their thermal and
1
-6
7-11
chemical stability. Moreover, reversible covalent bonds
and
have been employed to design self-
healing materials. While many dynamic motifs have been
12-15
non-covalent bonds
7-15
developed for such applications,
to our knowledge,
systematically tunable motifs whose small molecule kinetics
directly correlate to dynamic properties in bulk solid materials
largely remain to be explored. Achieving such a goal could aid in
the development of novel dynamic materials with tunable
properties by simply adjusting the motif’s kinetics. Whereas
16-18
19
tuning dynamicity of metal-ligand,
imine metathesis, and
20
protein-ligand interactions have been shown to impact various
dynamic properties of crosslinked organogels
16-18
20
and hydrogels
in solution, such a strategy has rarely been demonstrated in bulk
solids for the design of malleable and self-healing materials.
Figure 1. Design concept. a. Tuning neighboring group to control
the exchange kinetics of boronic ester. b. Design of di-boronic
ester crosslinkers with tunable exchange kinetics. c. Dynamic
exchange of boronic ester crosslinkers affords dynamic materials.
2
1
In this study, we used a divalent crosslinker with adjustable
exchange kinetics to tune the properties of bulk polymer
networks. The dynamics of the functional groups on the small
molecule crosslinkers could be kinetically tuned thereby
controlling the dynamic and self-healing emergent properties of
the resulting networks. In our search for a suitable dynamic motif,
the boronic ester bond and its transesterification reaction attracted
our attention because of its unique combination of high
thermodynamic stability and kinetic tunability. The boronic ester
Specifically, we demonstrated our concept with a simple
polymer embedded with 1,2-diol moieties as attachment sites for
boronic esters, crosslinked by telechelic divalent boronic esters.
For our initial proof-of-concept, two kinetic variants of di-boronic
esters were chosen to serve as dynamically mobile crosslinkers for
self-healing (Fig. 1). Consistent with our hypothesis, both
crosslinked polymer networks exhibited malleability and
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4
reprocessability. However, the two different variants showed
variable efficiencies of self-healing, with the faster exchanging
linker showing a significant extent of healing at 50 °C, while the
slower exchanging linker showed minimal healing. To our
knowledge, our report provides the first direct demonstration of
tunable malleability and self-healing efficiency through variations
in small molecule design.
temperatures (Fig. 2c). The rate of tranesterification was
determined to be ~3000/s with an activation energy of 12.6
kcal/mol. The rate of transesterification of the slower exchanging
boronic ester 1 could not be monitored through coalescence, and
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was instead determined via 2D EXchange Spectroscopy (EXSY)
as 0.016 ± 0.004/s (Fig. 2b), five orders of magnitude slower than
variant 2 which has a neighboring o-dimethylaminomethyl group,
confirming the earlier observation of strong influence of
neighboring group on boronic ester exchange kinetics. It is
believed that the nitrogen atom of the o-aminomethyl acts as a
proximal base to facilitate the proton transfer between the leaving
group diol on the boronate and the protonated ammonium during
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3,36,37
transesterification.
We then moved to transfer of this molecular design to
dynamically crosslinked polymers. First, di-boronic ester
crosslinkers of the slow (3) and fast (4) variants were synthesized.
Crosslinker 3 was synthesized starting from S 2 substitution of
N
3
8
both ends of diethylene glycol with 4-bromobenzyl bromide
followed by Miyaura coupling with bis(pinacolato)diboron to
form a telechelic di-boronic ester species. The kinetically inert
39
3
9
pinacol esters were deprotected with sodium meta-periodate and
4
0
replaced with the more labile propylene glycol ester for dynamic
crosslinking (Scheme 1a). Faster-exchanging crosslinker 4 was
prepared via reductive amination of 2-formyl boronic acid with
methylamine followed by nucleophilic substitution of 1,6-
dibromohexane, and finally esterification with propylene glycol
Scheme 1b) (full details in Supporting Information).
We chose to synthesize the 1,2-diol containing polymer
backbone using ring-opening metathesis polymerization of
4
0
(
Figure 2. NMR kinetic study of boronic ester transesterification.
a. The two model compounds, slower exchanging boronic ester 1
and faster exchanging boronic ester 2. b. EXSY results for
compound 1 shows k = 0.016 ± 0.004/s. c. Coalescence for
compound 2 shows k = ~3000/s.
41
cyclooctene-based monomers (ROMP).
Dihydroxylated
cyclooctene (dHCO) monomer was prepared via
5
stoichometrically controlled epoxidation of cyclooctadiene
followed by acid catalyzed epoxide ring opening, which was then
copolymerized with cyclooctene to afford 1,2-diol containing
polycyclooctene polymer (20%-diol PCO) (Scheme 1c, with full
details in Supporting Information.)
To confirm that the rate of boronic ester transesterification can
be tuned by neighboring groups, we synthesized small molecule
model compounds and measured the variable exchange rates in
2
3
solution. We prepared neopentyl glycol esters from phenyl
boronic acid and o-(dimethylaminomethyl)phenylboronic acid
Next, we used solution rheology to monitor the solution
dynamic properties of polymer networks crosslinked by different
boronic ester crosslinkers (Fig. 3a). A solution of 20%-diol PCO
in toluene was crosslinked with compound 3 or 4. This caused
gelation in sample with crosslinker 3, while the sample
crosslinked by 4 showed only a mild increase in viscosity. This is
in qualitative agreement with the small molecule exchange
kinetics (Fig. 2), and is reflected in the observed moduli of the
samples. The sample with slower crosslinker 3 shows a noticeably
higher elastic (G’) than viscous (G”) modulus throughout the
range of frequencies tested, showing a high resistance to flow
corroborating the relatively inert nature of the crosslinker. On the
other hand, the sample crosslinked by 4 shows comparable elastic
and viscous moduli in the range of tested frequencies, showing a
much lower resistance to flow consistent with rapidly shuffling
crosslinkers. Finally, the crossover frequency of G’ and G” of
crosslinker 3 sample, corresponding to its gel point, is beyond the
lowest frequency tested (0.01 Hz), which is several orders of
magnitude lower than the ~1 Hz crossover frequency of sample
crosslinked by 4. These results corroborate the difference in rates
determined in small molecule exchange and demonstrate the
applicability of the concept to the gel state.
(compounds 1 and 2, respectively, Fig. 2a), and their rates of self
transesterification were monitored in the presence of 1 equivalent
excess of neopentyl glycol. The rate of faster-exchanging boronic
ester 2 could be monitored through coalescence between bound
1
and unbound glycol methyl resonances in H NMR at varied
Scheme 1. Synthesis of di-boronic ester crosslinkers and 1,2-diol
a
containing polycyclooctene polymer
1
) NaH,
O
O
O
O
a.
Br
O
BPin
O
B
B
HO
OH
1) NaIO , HCl
4
88%
6
2%
2)
HO
2
) B2Pin2, KOAc
PdCl2dppf
OH
O
O
Br
O
O
2
81%
98%
'slow' crosslinker 3
Br
Br
O
O
O
O
6
b.
c.
B(OH)2
CHO
MeNH2, NaBH4
0%
B(OH)2
1)
85%
B
B
6
NH
N
N
9
2)
OH
HO
'
fast' crosslinker 4
Quant.
HO
OH
OH
1) mCPBA
To further correlate the small molecule kinetics with dynamic
properties of the solid polymer, we investigated the malleability
of the samples by performing stress-relaxation studies of 20%-
diol PCO crosslinked with 1.0% of either crosslinker 3 or 4 at
various temperatures ranging from 35 to 55 degrees. We found,
given identical crosslinking percentages, that solid polymer
samples crosslinked with crosslinker 4 (Fig. 3b) released stress
much faster (within 5 min for 2% strain) than samples crosslinked
2) cat. H2SO4
H2O
x
Grubbs G2
ROMP
OH
y
dHCO 5
20%-diol PCO
82%
95%
a
(a) Synthesis of ‘slow’ di-phenylboronic ester crosslinker 3. (b)
Synthesis of ‘fast’ di-o-(dimethylaminomethyl) phenylboronic
ester crosslinker 4. (c) Synthesis of 1,2-diol containing
polycyclooctene (20%-diol PCO).
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a. 100000
Only the sample containing the fast crosslinker 4 was able to
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1000
self-heal, almost quantitatively recovering the material’s
mechanical properties including Young’s modulus, yield strength,
ultimate tensile strain and strength (Fig. 4a&c, and Table 1). On
the other hand, the sample containing the slow-exchange
crosslinker 3 showed minimal or no healing (Fig. 4b&d, and
Table 1). Furthermore, both uncrosslinked and permanently
crosslinked (via 1,4-diisocyanatobutane) 20%-diol PCO samples
did not display any healing, verifying that healing was not due to
other mechanism, such as through hydrogen bonding of the diol
moieties in the PCO backbone (see Figs. S3-4). Together, these
results indicate that dynamic shuffling of the boronic esters is
directly responsible for self-healing of our materials. Crucially,
the variability in healing efficiencies follows the expected trend
based on kinetics of the small molecule boronic ester
transesterification reaction, directly demonstrating the effect of
small molecule dynamics on emergent bulk self-healing.
1
00
1
0
1
'Fast' crosslinker 4 G'
'Slow' crosslinker 3 G'
'Fast' crosslinker 4 G''
'Slow' crosslinker 3 G''
0.1
0
.01
0.01
0.1
1
10
100
Frequency (Hz)
b.
c.0.12
‘
Fast’ Crosslinker 4
‘Slow’ Crosslinker 3
0.06
0.04
0.02
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5C
0.08
0.04
0
35C
40C
45C
50C
55C
40C
4
5C
0C
5
55C
0
10
20
0
10
20
Time (min)
Time (min)
0
.5 mol% ‘Fast’ Crosslinker 4
0.5 mol% ‘Slow’ Crosslinker 3
a.
3
2
1
0
b.
2
1.5
1
Figure 3. Solution rheology and stress relaxation data. a.
Rheological data showing storage (diamond) and loss (square)
moduli for samples crosslinked with compounds 4 (unfilled) and 3
filled). b & c. Stress relaxation data for 20%-diol PCO
crosslinked with di-boronic esters 4 (b) or 3 (c).
0
.5
0
(
0
2
4
6
0
2
4
6
by 3 (>20 min for 2% strain; Fig. 3c) at all temperatures,
providing good evidence that variations in small molecule
properties can manifest in tunable malleability for materials in the
solid state. The sample containing the fast exchange boronic ester
(crosslinker 4) is significantly more malleable than the system
crosslinked by the slow exchange boronic ester.
Strain (mm/mm)
Strain (mm/mm)
1.0 mol% ‘Fast’ Crosslinker 4
1.0 mol% ‘Slow’ Crosslinker 3
c.
2
d. 1.5
1
1
0
.5
1
0
.5
0
.5
0
Finally, we prepared dynamically crosslinked diol PCO
samples and performed self-healing experiments on these
samples. Crosslinked bulk samples of 20%-diol PCO polymers
were prepared using both the fast and slow di-boronic ester
crosslinkers. To ensure that there were enough dynamic units
present without making the material too stiff through
overcrosslinking, crosslinking percentages were empirically
chosen to be 0.5 and 1.0% with respect to monomer. Solution cast
polymer films were concentrated in a vacuum oven and melt
pressed to afford elastomeric samples (details of sample
preparation and mechanical property studies are in the Supporting
Information). The mechanical properties of the samples are
summarized in Table 1. Despite of dramatic difference in dynamic
properties, the static mechanical properties are comparable for the
networks having same percentage of fast and slow exchange
crosslinkers. Whereas B-N dative interaction may exist in the
network with fast crosslinker 4, given the relatively weak B-N
0
0.5
1
1.5
2
0
0.5
1
1.5
2
Strain (mm/mm)
Strain (mm/mm)
Figure 4. Self-healing tests for crosslinked 20%-diol PCO. All
red curves are healed samples and blue curves are pristine
samples. a. Sample crosslinked by 0.5-mol% compound 4. b.
Sample crosslinked by 0.5-mol% compound 3. c. Sample
crosslinked by 1.0-mol% compound 4. d. Sample crosslinked by
1.0-mol% compound 3. Absence of red (self-healed) curve is due
to complete lack of healing in this particular sample.
We further envisioned that the dynamic boronic ester crosslinks
could allow for reprocessing of the samples. To test this, the
2
0%-diol PCO crosslinked sample by 0.5 mol % 4 was cut into
o
small millimeter sized pieces and then melt pressed at 80 C to
reform the bulk materials (Fig. 5b). Static tensile tests proved that
after multiple cycles, the materials were able to recover most of
their mechanical properties (Fig. 5a). The thermal stability of the
materials (Fig. S2) is high and remains unchanged as a result of
the crosslinker, suggesting that the incorporation of boronic esters
into the polymer does not decrease the thermal stability.
4
2
bond in this system and low density of boronic ester
crosslinkers, B-N interactions should not play a significant role in
the material properties. For self-healing test, we completely cut
through the sample with a razor blade and gently placed the cut
interfaces together for one minute and allowed them to heal for 16
hours. Due to the semicrystalline nature of the 20%-diol PCO
(
See Fig. S1 in the SI), a slightly elevated temperature (50 °C)
was required to produce effective healing.
Table 1. Static tensile data for dynamically crosslinked samples.
Sample
0.5%
Fast (4)
1.0%
0.5%
Slow (3)
1.0%
Fast (4)
Slow (3)
Uncrosslinked
3.23 ± 0.52 2.23 ± 0.28
a
E (pristine) 4.68 ± 0.39 4.86 ± 0.86 4.55 ± 0.45
E (healed)
ε (pristine) 345 ± 80
ε (healed)
4.63 ± 0.32 4.81 ± 0.40 4.15 ± 0.13
--
--
b
174 ± 10
158 ± 21
446 ± 27
121 ± 31
--
1030 ± 66
--
344 ± 160
28.3 ± 7.1
c
Figure 5. Reprocessing (three cycles) of 20%-diol PCO
crosslinked by 0.5-mol% compound 4. a. Static mechanical
testing of reprocessed samples. b. The image on the top was the
sample cut into mm sized pieces and on the bottom was the
reprocessed sample after the third repetition.

(pristine) 1.85 ± 0.38 1.51 ± 0.27 1.97 ± 0.36
1.12 ± 0.18 0.821 ± 0.030
 (healed)
1.75 ± 0.35 1.47 ± 0.51 0.723 ± 0.10 -- --
a
Young’s modulus calculated from the initial slope of static stress strain
curves in (MPa). Ultimate tensile strain (mm/mm). Ultimate tensile
strength (MPa).
b
c
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One potential issue of our design is the hydrolytic stability of
boronic ester linkages. To test for this, we immersed our
crosslinked samples in water overnight and then monitored any
change of mass and mechanical properties. The mass change was
negligible for samples before and after water submersion,
indicating no appreciable transesterification and dissolution of the
resulting small molecules (Table S2). Importantly mechanical
properties of the samples after submersion in water over night
remained unchanged (Fig. S5), further confirming the hydrolytic
stability of boronic ester embedded in our polymer system.
Despite the fact that small molecule boronic esters are susceptible
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33
boronic ester network reported recently.
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In conclusion, we have demonstrated that the dynamic boronic
ester linkage can be successfully used to prepare malleable, self-
healing, and reprocessable covalent network polymers. The
dynamic exchange of boronic esters bonds afforded the observed
dynamic properties. Significantly, tuning the rates of trans-
esterification in the crosslinkers varied the malleability and the
efficiency of self-healing, demonstrating a direct link between
small molecule kinetics and rate of self-healing. This work shows
the possibility of bottom-up rational design of dynamic materials
with tunable dynamic properties through simple perturbations of
small molecule structure and kinetics, which may give rise to
materials with a variety of applications, ranging from robust self-
healing elastomers to processable thermosets.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
zguan@uci.edu
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Author Contributions
‡
These authors contributed equally.
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Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
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We acknowledge the financial support of the US Department of
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acid purification.
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