High Glass-Transition Temperature Polymer Networks Harnessing the Dynamic Ring Opening of Pinacol Boronates

Article abstract of DOI:10.1002/anie.201904559

Differential scanning calorimetry of high molar mass poly(4-vinylphenylboronic acid, pinacol ester)s evidenced unusual reactive events above 120 °C, resulting in a high glass-transition temperature of 220 °C. A reversible ring-opening reactivity of pinaco

Full text of DOI:10.1002/anie.201904559

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Accepted Article
Title: High Glass-Transition Temperature Polymer Networks
harnessing the Dynamic Ring Opening of Pinacol Boronates
Authors: Juliette Brunet, Franck Collas, Matthieu Humbert, Lionel
Perrin, Fabrice Brunel, Emmanuel Lacôte, Damien Montarnal,
and Jean RAYNAUD
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904559
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10.1002/anie.201904559
Angewandte Chemie International Edition
RESEARCH ARTICLE
High glass-transition temperature polymer networks harnessing
the dynamic ring opening of pinacol boronates
Juliette Brunet,[a] Franck Collas,[b] Matthieu Humbert,[a] Lionel Perrin,[c] Fabrice Brunel,[a] Emmanuel
Lacôte,[d] Damien Montarnal,*[a] Jean Raynaud*[a]
Abstract: Differential scanning calorimetry of high molar mass poly(4-
vinylphenylboronic acid, pinacol ester)s evidenced unusual reactive
events above 120°C, resulting in a high glass-transition temperature
of 220°C. We hypothesize a reversible ring-opening reactivity of
pinacol boronates involving a nucleophilic attack on the sp2 boron and
subsequent bridging between boron atoms by interconnected pinacol
moieties to form a densely crosslinked network with high-Tg. FTIR,
solid-state NMR investigations and rheology studies on the polymer
as well as double-tagging analyses on molecular model structures
and theoretical calculations further support this hypothesis and
indicate a ring opening inducing crosslinking. When diluted in an
apolar solvent such as toluene, the polymer network can be
resolubilized via ring closing thus recovering the entropically-favoured
linear chains featuring cyclic boronate esters. The introduction of
boron reactivity in polymer chains may open a path to a new class of
dynamic polymer networks with uncommon and exciting
thermodynamics.
or 1,3-diols, boronic acids are in fast equilibrium at room
temperature with boronate esters and water. This
transesterification equilibrium can be shifted with the help of a
desiccant or exploited in numerous molecular recognition or
chemosensing
applications[8]
such
as
pH-dependent
complexation of various carbohydrates[9] or smart drug delivery
systems.[10] The transesterification of boronate esters is also a key
feature enabling the formation of nanoporous precise organic
structures such as covalent organic frameworks or cages.[11-13]
Following similar approaches, covalent dynamic crosslinking of
organoboron polymers is receiving a growing interest in the
context of materials with high mechanical performances and
facilitated recycling or processing.[14,15]
Such crosslinks with externally adjustable dynamics (e.g. using
temperature or light) are sought-after as they confer both the
benefits of thermosets in conditions of slow dynamics (e.g. creep
and stress-cracking resistance) and malleability, plasticity,
weldability or recyclability in conditions of fast dynamics. A
distinction can be made between dynamic polymer networks
featuring a constant crosslink density with accelerated crosslink
reshuffling upon heating, i.e. vitrimers,[16-18] and networks
Introduction
featuring both
a decreased crosslink density and shorter
Polymers comprising boronic/boronate groups have been the
focus of intensive research due to their unique properties
associated with the electron deficiency of boron conferring
reactivity harnessed via post-polymerization.[1,2] Simple
nucleophilic coordination at the boron vacancy enables further
versatile functionalisations and precise macromolecular
engineering. To perfectly tune the reactivity at boron, synthetic
chemists have developed a wide array of substituents, from
conventional boronates to MIDA esters.[3-7] Challenging metal-
catalysed C-C couplings and orthogonal functionalizations were
achieved by appropriately selecting the boron moieties.[7] For
instance, one can hydrolyse boronate esters, or fluoro-derivatise
them to yield organotrifluoroborates. In both cases, a nucleophilic
attack at the boron vacancy is involved.[3,7] In the presence of 1,2
crosslink-bond lifetimes upon heating or UV-irradiation[19], i.e.
covalent adaptable networks (CANs). [20-23]
Based on both the transesterification equilibrium and the
aforementioned reactivity at boron, CANs gels in toluene have
been obtained by adding bis(boronate ester) crosslinkers in
polycyclooctene-containing vicinal diol moieties. The exchanges
can be tuned by strategically positioning secondary amines in the
vicinity of the boron atoms, which greatly accelerates exchange
dynamics.[24] Recently, Nicolaÿ, Leibler and coworkers disclosed
fascinating covalent exchangeable vitrimers crosslinked by
dynamic exchange of boronate ester linkages, albeit the actual
exchange mechanism still remains unsolved.[25-27]
To the best of our knowledge, none of the dynamically crosslinked
materials described so far reach the very high glass transition
temperatures (Tg above 200°C) required for adhesives, protective
coatings, low-wear or low-friction materials when exposed to high
temperatures (contact with hot parts, composites for aerospace,
electronics, bearings…). For such demanding applications, only a
few thermoplastics constituted of very rigid segments, with
various degrees of solubility and processability are commercially
[a]
J. Brunet, M. Humbert, Dr. F. Brunel, Dr. D. Montarnal, Dr. J.
Raynaud.
Univ Lyon, Université Claude Bernard Lyon 1, CPE Lyon, CNRS,
Laboratory of Chemistry, Catalysis, Polymers and Processes (C2P2,
UMR 5265)
43 Bd. Du 11 Novembre 1918, F-69616 Villeurbanne, France
E-mail: damien.montarnal@univ-lyon1.fr, jean-raynaud@univ-
lyon1.fr
[b]
[c]
Dr. F. Collas
Mettler-Toledo SAS
available
(e.g.
polyaryletherketones,
polyethersulfones,
polyaramides, polyetherimides or polybenzimidazoles).[28]
Alternatively and historically, one can use non-reprocessable,
very densely crosslinked thermosetting resins such as maleimide
or phenolic formaldehyde, or specialty epoxies.[29]
18/20 avenue de la Pépinière, 78222 Viroflay cedex, France
Dr. L. Perrin
Univ Lyon, Universitꢀ Claude Bernard Lyon 1, CPE Lyon, INSA
Lyon, ICBMS, CNRS UMR 5246, Equipe ITEMM
43 Bd. du 11 Novembre 1918, 69622 Villeurbanne, France
Dr. E. Lacôte
Univ Lyon, Université Claude Bernard Lyon 1, CNRS, CNES,
ArianeGroup, Laboratoire Hydrazines et Composés Energétiques
Polyazotés (LHCEP, UMR 5278)
[d]
All vitrimers disclosed so far have moderate crosslink densities,
and exhibit therefore two clearly distinguishable relaxations: the
conventional glass-to-rubber transition involving cooperative
mobility at the polymer-segment scale and a second rubber-to-
liquid transition dictated by crosslink exchanges and dynamic
rearrangements of the network topology at the network-mesh
scale, and thus with a viscous flow activation energy (e.g. the
Bât. Raulin, 2 rue Victor Grignard, F-69622 Villeurbanne, France
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201904559
Angewandte Chemie International Edition
RESEARCH ARTICLE
temperature-dependence of the viscosity) matching the activation
energy of the exchange reaction. Although the rubber-to-liquid
transition can occur at very high temperatures in systems with
sluggish exchange kinetics, the glass-to-rubber transition is
contingent on the rigidity of the backbone in-between crosslinks
and was not reported so far beyond 166°C.|30] Some CANs feature
very rigid and densely crosslinked backbones, but their Tg is
limited by de-crosslinking of the network at elevated
temperatures: rigid furane-maleimide networks undergo a unique
glass-to-liquid transition around 120°C.[20] The viscous flow
activation energies are typically much higher in CANs than in
vitrimers, as they result both from the dynamics of reversible
bonds and the de-crosslinking of the network.
well-defined, Tg with mid-point at 220°C. Interestingly, similar
behaviours, though with slightly lower Tg ~210°C, were observed
when lower molar-mass PSBPin were utilized (see comparison
between Fig. S9 & S10).[31-33] Additionally, it is possible to finely
tune the Tg of the final material by simple copolymerization with
styrene using similar methodology (see SI., Fig. S7 & S8 for NMR
and Fig. S10 & S11 for DSC). Thermogravimetric analysis (TGA)
of the fresh sample did not reveal any significant weight loss in
the 25-200°C range (Fig. S15 in SI), which ruled out a mere
evaporation of plasticizers. We therefore surmised that the
polymer had been chemically transformed, resulting in a very high
Tg polymer after the initial heating step. Additional experiments
ran in high-pressure sealed pans enabled to suppress most
evaporation endotherms (Figures S9 & S11, SI). In such
conditions, we observed a unique endotherm between 90 and
110°C. In this case the Tg is lowered to 160°C, even after multiple
heating cycles.
In this article we report that poly(4-vinylphenyl-pinacol-boronate
ester) (PSBPin, Figure 1) exhibits a very high Tg up to 220°C.
Although the bulky pinacol ester group is expected to reduce the
chain mobility in comparison to polystyrene, the rigidity in the
polymer structure and the polar interactions cannot account for
such a high Tg. This compelled us to further explore the dynamics
of this polymer linked to the reactivity of the pendant cyclic
boronate. Herein we unveil a reaction mechanism involving
dynamic and reversible ring openings of the cyclic boronate
moieties at high temperatures.
Results and Discussion
The phenylboronate polymer was prepared via free radical
polymerization of 4-vinylphenyl-pinacol-boronate ester.[1,31-33] The
organoboron monomer was prepared via esterification with
pinacol of the corresponding phenylboronic acid. It is important to
note that the monomer always contains minute amounts of
1
pinacol (<1 mol % by H NMR, see SI, Fig. S1), and we carried
out polymerizations without further purification. This
contamination will be discussed later on as a putative source of
nucleophiles for initiating the ring opening mechanism. Careful
optimization of polymerization temperatures, (non-transferable)
solvents and initiator concentrations allowed us to obtain PSBPin
with Mw as high as 200 kg.mol-1 (see Supporting Information, SEC
discussion and Fig. S16 to S19). This high-mass material was
used for the rest of the study, though comparisons are drawn with
lower molar-mass PSBPin in SI (Fig S9 & S10).
Figure 2. DSC thermograms (Exo up) of PSBPin at 20 K/min. H1-4 and C1-4
are the successive heating and cooling ramps, respectively. Traces are cut at
the beginning of each cooling and heating ramp for clarity.
In order to better characterize the polymer after the heating step,
we investigated its viscoelastic properties from 230 to 260°C by
shear rheology, using small amplitude oscillations. Frequency-
dependent storage and loss moduli at different temperatures were
shifted by conventional time-temperature superposition in order to
build a mastercurve using 230°C as reference temperature
(Figure 3, right). The proximity to the glass transition was
evidenced by the strong increase of G’ and G” at high frequencies,
while the upturn of G’ at lower frequencies suggested the
presence of entanglements or crosslinks. The dependency of the
shift factors with temperature could not be fitted with the classical
Williams-Landel-Ferry model (Fig. S14 in SI), but followed instead
an Arrhenius model with a very high activation energy, about 500
kJ/mol. These uncommon features suggest that the mechanical
relaxations associated to the glass transition are not governed by
cooperative segmental motions, but rather by chemical
exchanges.[34] This polymer behaves therefore as a dynamic
Figure 1. Synthesis of poly(4-vinylphenyl-pinacol-boronate ester) (PSBPin).
Freshly synthesized high-mass PSBPin was re-precipitated in
pentane and thoroughly vacuum dried. Its thermal behaviour was
first analysed by differential scanning calorimetry (DSC) (Figure
2). The thermogram showed upon first heating a shift in the
baseline near 80°C, followed by a large endothermic event from
network with
a unique glass-to-liquid transition. As lower
frequencies could not be probed because of degradations starting
to occur above 260°C, we rather chose to further study highly
plasticized PSBPin as a 50 wt % solution in xylenes (bp ≈ 140°C).
Monitoring in-situ the viscoelastic properties of this solution by
130 to 180°C and
a second baseline shift near 220°C.
Subsequent multiple cooling/heating cycles all revealed a single,
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10.1002/anie.201904559
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small amplitude oscillations at 100°C (Figure 3, right) indicated
initially a low-viscosity, followed by a strong increase in viscosity
after about 10 h. Upon shearing with large deformations, the
viscosity suddenly dropped but grew again within 4 h, showing
this time a clear gelation with G’/G” crossover and an elastic
modulus increasing up to 50 kPa. The slow and reversible build-
up of elasticity here rather indicates a reversible crosslinking
phenomenon than entanglement.
Figure 3. Left: Master curve of dynamic storage and loss moduli of PSBPin in bulk, referenced at 230°C. Inset: Arrhenius-like dependency of the shift factors. Right:
time monitoring of storage and loss moduli of PSBPin at 50 wt % in xylenes
before and after heating. 11B ss-NMR is not straightforward
because of the quadrupolar nature of the 11B nucleus (spin 3/2).
The combination of DSC and rheology experiments thus indicates
Thus, when comparing signals, one should not conclude
immediately from the maximum of intensity of the observable
massif, but instead derive the isotropic chemical shift (δiso), the
ηQ values (related to the mobility of the corresponding boron
atom) and observe the relative asymmetries/anisotropies of the
signals. In our case, when superimposed, the two signal
envelopes were distinct, suggesting an altered boron environment
once PSBPin was exposed to high temperatures (Figure 4). To
evaluate whether only one or several types of boron chemical
environments were present, we used simulations from the “dmfit”
program, which was specifically developed to account for
quadrupolar couplings.[35] One type of boron was sufficient to
perfectly fit the experimental signal before heating. Conversely,
two types of boron were necessary to achieve perfect match
between simulation and experimental spectrum when fitting the
signal after heating. Furthermore, the simulations established that
the new type of boron formed after heating (~16 mol %) had more
symmetrical features, in particular quadrupolar couplings with
lower ηQ value (see Figure 4), and a different δiso of 32.5 ppm.
This is indicative of an enhanced mobility and is compatible with
boronate esters with pendant or intermolecular bridging
pinacolates. The main environment was common to the samples
before and after heating, with a δiso of 30.5 ppm before (~30 after)
and similar relatively high ηQ values (0.71 and 0.63, respectively)
thus indicative of a constrained geometry. It was attributed to
native pinacol phenyl boronate cyclic species. The slight
differences in δiso might come from residual solvent evaporation
(toluene traces or adsorbed moisture no longer present after
heating for 4 h at 200°C). The newly formed type of boron displays
an enhanced mobility, as can be expected when the ring opens.
Therefore, 11B ss-NMR corroborates our hypothesis of ring
opening of cyclic pinacol boronates. Interestingly, the low value of
ηQ for the new species is not consistent with the formation of
boroxine moieties (δiso ~25 ppm) as they typically display much
higher ηQ values (> 0.7), characteristic of constrained geometries
around boron.[36,37]
a dynamic crosslinking of PSBPin occurring very slowly in
plasticized PSBPin at 100°C. The presence of an endothermic
reaction in bulk PSBPin in the 130-180°C range points towards
an analogous, faster crosslinking. Despite our efforts, complete
characterization of the dynamic-crosslinking density in bulk is not
possible as the transition associated to chemical exchanges
appears coincident with the segmental glass transition.
While we have established that PSBPin undergoes apparent
crosslinking upon heating up to 200°C at 20K/min, and even as
concentrated solution over 10h at 100°C, we observed that the
polymer could be completely resolubilised in most organic
solvents after these heating treatments. Figure S19 in SI displays
the SEC traces in toluene of fresh PSBPin and PSBPin taken from
the DSC pan after multiple heating cycles. As no significant
change occurred, it appears that the crosslinking reaction is
dynamic, and can be fully reversed upon dilution (see Fig. 20 in
SI, for two complete cycles).
To better understand the reaction involved in the dynamic
crosslinking, we carried out temperature-dependent FTIR
measurements under inert atmosphere from 25 to 250°C (10°C
increments per hour) using a diffuse reflectance cell. The fresh
PSBPin powder was used directly rather than samples obtained
by hot or cold pressing. A few noticeable variations appeared in
the 100-170°C range in the spectra (Fig. S22 & S23 in SI). Among
them, two main changes raised our attention: a shouldering
around 1370 cm-1 and a band shift from ca. 1170 to 1160 cm-1.
These vibration modes involve torsions/elongations of the O–B–
O as well as C–O bonds of the pinacol motif as assigned by
frequency calculations performed at the DFT level
(M06/def2TZVP) on model pinacol phenyl boronate (see SI). This
drove us to hypothesize a ring opening of the boronate cycles.
Solid-state boron NMR spectroscopy was therefore used to detect
differences in the environment of the boron atoms of PSBPin
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Figure 4. Solid-state 11B NMR study of poly(4-vinylphenylboronic acid, pinacol ester) (PSBPin). (Up) Putative mechanism, (Down, left) 11B NMR spectrum of native
PSBPin and corresponding fit, (Down, right) 11B NMR spectrum of PSBPin after heating at 200°C for 4 hours and corresponding fits.
Taking into account the body of evidence obtained so far, we
suggest that the ring opening of the boronates releases a free
pinacolate moiety that can further reach neighbouring boron
atoms thus acting as a rigid crosslink between PSBPin chains.
This leads to a strong decrease in the chain mobility, consistently
with the very high Tg observed. DSC and rheology data
corroborate this ring opening occurring at high temperatures (130-
180°C) and the dynamic character of the crosslinks formed.
the net endothermic reaction measured by DSC (Fig. S9 through
S13, SI) and with crosslinking observed above 100°C. We infer
that this system, with a peculiar thermodynamic balance between
the enthalpic and entropic contributions, is reminiscent of floor-
temperature polymerizations already demonstrated in several
low-strained cyclic monomers.[38,39] Kinetically, the enthalpy (resp.
Gibbs energy) barriers corresponding to ring opening is
calculated at 27 (resp. 30) kcal.mol-1 above the water adduct, with
a clear pyramidalisation induced by H2O nucleophilic attack at B
through O. Interestingly, the energy barrier corresponding to a
direct metathesis has been estimated to 70 kcal.mol-1 relative to
free pinacolboronate. This mechanism can thus be confidently
ruled out.
Regarding the ring-opening initiation, involvement of
a
nucleophile and assistance by an acid seem compulsory:
adventitious water or residual pinacol molecules remaining from
the synthesis could fill both roles. The nucleophilic attack at boron
and the subsequent ring opening would lead to singly-attached
pinacol group readily available to initiate ring opening of a
proximal pinacol boronate, ultimately triggering bridge formation.
We resorted to computational mechanistic investigation
performed at the DFT level in order to assess the enthalpy of ring
opening of these pinacol boronates (details in SI). In this
investigation, PSBPin units have been represented by phenyl
pinacol boronate and a water molecule has been used as
prototype nucleophile. At room temperature, the interaction of
H2O with a pinacol oxygen atom via H-bonding is exothermic by
4.4 kcal.mol-1 at 293 K. This adduct will be taken as the energy
reference hereafter. From this adduct, the subsequent ring
opening to yield a singly-attached pinacol substituted boronic acid
is endothermic by 3 kcal.mol-1 (Fig. S21, SI). In Gibbs energy, the
reaction is endergonic by 4.0 kcal.mol-1 at 293 K. Consequently,
the ring opening is thermodynamically disfavoured at room
temperature but becomes less disfavoured upon heating, thus
slightly shifting the equilibrium from essentially closed to partially
open boronates. This thermodynamic trend is consistent both with
Even though the net endergonic thermodynamics of the system
would likely limit the extent of oligomerisation, bridging between
boron atoms appears sufficient to trigger dense crosslinking in
these highly functional polymers at high temperature.
Furthermore, the de-crosslinking induced at high dilutions is likely
entropically driven by the formation of the more favoured
intramolecular 5-membered cycle in the presence of a high
concentration of solvent molecules. This de-crosslinking should
also be favoured at low temperatures but is most probably
kinetically frozen by the glass transition in PSBPin.
To verify this mechanism, we performed double-tagging
experiments. If the dynamic character of the ring-opening
oligomerization is effectively due to the living character of the
pendant pinacolate groups, we should not only observe
oligomerisation and depolymerization of the model molecules, but
also backbiting or chain shuffling that could result in more
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10.1002/anie.201904559
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RESEARCH ARTICLE
complex rearrangements and effective exchange of substituents
between boronate esters (Figure 5). These exchanges should be
evidenced by double tagging: we therefore prepared two
phenylboronate esters A and B (Figure 5, section 5 of SI). A has
a phenyl ring and its diol component is 1,2-ethanediol while B has
a fluorine tag on the phenyl part and pinacol as its diol. The
fluorine substituent enables straightforward quantification of any
diol exchanged by 19F NMR, while only marginally affecting the
intrinsic reactivity at the boron atom. The boronates A and B were
mixed without additional solvent in a NMR tube and heated at
100°C. Periodically, the NMR tube was cooled at room
temperature, equipped with an insert filled with Toluene-d8 to lock
the NMR apparatus and analysed with 19F NMR. While only A
could initially be observed, a new species appeared with time that
was attributed to the F-tagged phenyl boronate bearing an
ethylene glycol group (C, Figure 5). The attribution of the new 19
F
NMR signal to C was confirmed by separately synthesizing the 4-
fluoro-phenyl boronate of ethylene glycol (Fig. S24 & S25, SI).
Integration of the signals corresponding to A and C at –107.8 ppm
Figure 5. Double-tagging experiments using 19F NMR spectroscopy.
and –108.2 ppm, respectively, showed that
a nearly 1:1
equilibrium was reached after 50 h.
Dynamic gelation of a 50 wt % concentrated PSBPin solution in
toluene in presence of 2 mol % of benzyl alcohol is illustrated in
Figure 6 (& Fig. S20, SI). After heating the viscous solution for 72
h at 100°C, gelation was observed. Upon cooling to room
temperature and further addition of dry toluene (the final
concentration is 25 wt % PSBPin), immediate swelling was visible.
After 2 min, the gel was fully broken and a low-viscosity solution
was obtained. Two full cycles were performed on the material to
evidence the dynamic character of the crosslinking and its
complete reversibility (see SI, Fig. S20).
In order to support our hypothesis that the crosslinks were
initiated by the adventitious nucleophiles still present in the
sample, we added 50 mol % of benzyl alcohol - a non-volatile
nucleophile - in the initial mixture. 19F NMR monitoring at 100°C
showed a much faster exchange rate, as the A / C 1:1 equilibrium
was reached within 12 h. In addition, smaller new signals could
be observed on the 19F NMR spectra, which we believe are those
of the benzyl-alcohol derived phenyl boronates and several other
products/intermediates of the exchange reaction (e.g. the
tetragonal intermediates), which was partially confirmed by 19F
DOSY NMR experiments (section 5 of SI, Fig. S24 to S29). These
results are consistent with previously reported double-tagging
experiments[25] using GC as an analytical method of choice. Our
method relies however on
a
very-straightforward NMR
complementary strategy and clearly evidence the advantageous
nucleophilic assistance authorizing the exchange derived from
boronate-ester ring opening.
In light of these experiments, adventitious nucleophiles present in
bulk PSBPin are thus expected to accelerate the dynamics of
equilibration between cyclic pinacol boronates and bridging
pinacol units. Additional DSC experiments on fresh PSBPin
sample ran in high-pressure sealed pans enabled to suppress
evaporation endotherms (Fig. S9 & S11, SI). In such conditions,
we also observed a unique endotherm between 90 and 110°C,
but the Tg is dramatically lowered to 160°C, even after multiple
heating cycles. We believe that water adsorbed and minute
amounts of pinacol remaining in the PSBPin sample are
responsible for faster dynamics and thus a lower Tg of the material,
possibly also acting to a lesser extent as a plasticizer.
Figure 1. Macroscopic observations of the reversible crosslinking of an
originally 50 wt % concentrated PSBPin solution in toluene. (a) Initial solution of
poly(4-vinylphenylboronic pinacolate) at 50 wt % in toluene with 2 % molar
equivalent of benzyl alcohol, (b) After 1 night at 110°C, (c) t=2 min after the
addition of 2 mL of dry toluene.
Conclusion
We have demonstrated through a variety of thermomechanical
and spectroscopic techniques (DSC, rheology, FTIR, 11B ss-
NMR) that above 100°C, boronate-ester ring opening occurs in
poly(4-vinylphenylboronic pinacolate)s and related copolymers of
various molar masses, leading to interchain bridging between
boron moieties and high-Tg (up to 220°C) polymer networks.
Direct exchange of 1,2-diol substituents was evidenced on model
compounds by 19F NMR spectroscopy, and confirms the dynamic
character of the ring opening and the importance of nucleophilic
assistance to trigger it. This phenomenon appears to be
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10.1002/anie.201904559
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reversible by dilution in apolar solvents. The peculiar
thermodynamics of this ring opening/closing sequence, supported
by DFT calculations, favour depolymerization at room
temperature and oligomerisation at high temperatures (100-
150°C). The exact mechanism governing the glass-to-liquid
transition in PSBPin at ca. 220°C is difficult to ascertain as
diagnostic spectroscopic characterization of ring-opening such as
11B ss-NMR cannot be conducted at such temperatures. Judging
from the very high viscous flow activation energies measured by
rheology, it is likely that the dynamic network is not purely driven
by associative exchanges like vitrimers, but undergoes
depolymerization like CANs. The very fact that bridging between
boron atoms lead to high glass transition temperatures in our
system and subsequently froze the depolymerization kinetics was
critical in evidencing these dynamic crosslinking phenomena.
When comparing our system to vitrimers and CANs, that have
either constant crosslinking densities or continuously decreasing
crosslinking densities with temperature, it is exciting to note that
polymers bearing cyclic boronate esters may constitute the first
example of a distinct class of dynamic networks, with a
crosslinking density increasing with temperature, at least within a
defined temperature range.
References
[1]
[2]
F. Jäkle. Chemical Reviews 2010, 110, 3985-4022.
W. L. A. Brooks, B. S. Sumerlin. Chemical Reviews 2016, 116, 1375-
1397.
[3]
[4]
[5]
G. A. Molander, N. Ellis. Accounts of Chemical Research 2007, 40, 275-
286.
E. P. Gillis, M. D. Burke. Journal of the American Chemical Society 2008,
130, 14084-14085.
D. M. Knapp, E. P. Gillis, M. D. Burke. Journal of the American Chemical
Society 2009, 131, 6961-6963.
[6]
[7]
D. V. Partyka. Chemical Reviews 2011, 111, 1529-1595.
A. Biffis, P. Centomo, A. Del Zotto, M. Zecca. Chemical Reviews 2018,
118, 2249-2295.
[8]
[9]
J. N. Cambre, B. S. Sumerlin. Polymer 2011, 52, 4631-4643.
W. Yang, X. Gao, B. Wang in Boronic Acids: Preparation and
Applications in Organic Synthesis and Medicine, (Ed.: D. G. Hall), Wiley-
VCH, 2006, pp. 481-512.
[10] K. Kataoka, H. Miyazaki, M. Bunya, T. Okano, Y. Sakurai. Journal of the
American Chemical Society 1998, 120, 12694-12695.
[11] N. Huang, P. Wang, D. Jiang, Nature Reviews 2016, 1, 16068.
[12] M. Mastalerz, Acc. Chem. Res. 2018, 51, 2411.
[13] F. Beuerle, B. Gole, Angew. Chem. Int. Ed. 2018, 57, 4850.[14]
O.
R. Cromwell, J. Chung, Z. Guan. Journal of the American Chemical
Society 2015, 137, 6492-6495.
[15] J. J. Cash, T. Kubo, A. P. Bapat, B. S. Sumerlin. Macromolecules 2015,
48, 2098-2106.
[16] D. Montarnal, M. Capelot, F. Tournilhac, L. Leibler. Science 2011, 334,
965-969.
[17] W. Denissen, J. M. Winne, F. E. Du Prez. Chemical Science 2016, 7, 30-
Experimental Section
38.
[18] L. Imbernon, S. Norvez. European Polymer Journal 2016, 82, 347-376.
[19] B. T. Worrell, M. K. McBride, B. L. Gayla, L. M. Cox, C. Wang, S. Mavila,
C.-H. Lim, H. M. Coley, C. B. Musgrave, Y. Ding, C. N. Bowman. Nature
Communications 2018, 9, 2804-2814.
[20] X. Chen, M. A. Dam, O. Kanji, A. Mal, H. Shen, S. R. Nutt, K. Sheran, F.
Wudl. Science 2002, 295, 1698-1702.
[21] C. N. Bowman, C. J. Kloxin. Angew. Chem. 2012, 124, 4346-4348. C. N.
Bowman, C. J. Kloxin. Angew. Chem. Int. Ed. 2012, 51, 4272-4274.
[22] R. J. Wojtecki, M. A. Meador, S. J Rowan. Nature Materials 2011, 10,
14-27.
[23] W. Zou, J. Dong, Y. Luo, Q. Zhao, T. Xie. Advanced Materials 2017, 29,
1606100.
Unless otherwise stated, all reactions were conducted under inert
atmosphere (argon) using standard techniques for manipulating air-
sensitive compounds. All glassware was stored in an oven or was flame-
dried prior to establishing an argon atmosphere. The reagents were
purchased from Combi Blocks and Sigma Aldrich and used without any
further purification. The polymerization solvents were purified by drying
columns and freeze-pump-thaw technique. 1) 4-vinylphenylboronic acid
(10,0g, 67.6 mmol) and pinacol (8.03g, 68.0 mmol) as well as 0.05 mL
distilled water were stirred in THF (100 mL) at room temperature for 30
min. Then 2 g of anhydrous magnesium sulfate were added to the solution
and stirred at room temperature for 2 h. The mixture was filtered and
volatiles were removed under vacuum to yield a clear yellow oil. Yield:
14.7g, 95% with 0.9 mol% of residual pinacol, see SI. Fig.S1. 2) 4-
vinylphenylboronic acid, pinacol ester (3.8 g, 16 mmol) and benzoyl
peroxide (0.02 eq, 0.33 mmol, 80 mg) were introduced in a Schlenk vessel
with 3.8 mL toluene (degassed by three freeze-pump-thawing cycles). The
polymerization was conducted at 70°C during 72h. Then, the reaction
media was precipitated in pentane and the solid was obtained by filtration
and dried under vacuum for one night. The polymer was obtained as a
white powder. Yield: 87%. In order to minimize any residual functionality
in the final polymer, we used benzoyl peroxide (BPO) as a nitrogen-free
initiator (Figure 1; see sections 2, 3 and 4 of Supporting Information for
further details & characterization).
[24] O. R. Cromwell, J. Chung, Z. Guan. Journal of the American Chemical
Society 2015, 137, 6492-6495.
[25] M. Röttger, T. Domenech, R. van der Weegen, A. Breuillac, R. Nicolaÿ,
L. Leibler. Science 2017, 356, 62-65.
[26] R. G. Ricarte, F. Tournilhac, L. Leibler. Macromolecules 2018, 52, 432-
443.
[27] F. Caffy, R. Nicolaÿ. Polymer Chemistry 2019, in press.
[28] D. Kyriacos, in Brydson's Plastics Materials (8th Edition), (Ed.: M. Gilbert),
Butterworth-Heinemann, 2017, pp. 545-615.
[29] T. Kaiser. Progress in Polymer Science 1989, 14, 373-450.
[30] T. Liu, C. Hao, S. Zhang, X. Yang, L. Wang, J. Han, Y. Li, J. Xin, J. Zhang.
Macromolecules 2018, 51, 5577-5585.
[31] Y. Qin, V. Sukul, D. Pagakos, C. Cui, F. Jäkle. Macromolecules 2005, 38,
8987-8990.
[32] Y. Qin, D. Cheng, O. Achara, K. Parab, F. Jäkle. Macromolecules 2004,
19, 7123-7131.
[33] R. L. Letsinger, S. B. Hamilton. Journal of the American Chemical
Society 1959, 81, 3009-3012.
[34] M. Capelot, M. M. Unterlass, F.Tournilhac, L. Leibler. ACS Macro Letters
2012, 1, 789-792.
[35] D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calvé, B. Alonso, J.-O.
Durand, B. Bujoli, Z. Gan, G. Hoatson. Magnetic Resonance Chemistry
2002, 40, 70-76.
[36] S.-W. Oh, J. W. E. Weiss, P. A. Kerneghan, I. Korobkov, K. E. Malyb, D.
L. Bryce. Magnetic Resonance Chemistry 2012, 50, 388–401.
[37] Y.-T. A. Wong, D. L. Bryce in Annual Reports on NMR Spectroscopy, Vol.
93, (Eds.: G. Webb), Academic Press, 2018, pp. 213-279.
[38] S. Penczek, A. Duda, K. Kaluzynski, G. Lapienis, A. Nyk, R. Szymanski.
Macromolecular Symposia 1993, 73, 91-101.
Acknowledgements
J.B. thanks the University of Claude Bernard Lyon 1 for funding.
D.M. and J.R. gratefully thank the ANR for funding (grants ANR-
17-CE07-0006 and ANR-15-CE06-0001, respectively). L. P.
thanks the CCIR of ICBMS and P2CHP of Universitꢀ Lyon 1 for
providing computational resources and technical support. The
authors thank A. Baudouin and D. Gajan respectively for liquid-
and solid-state NMR spectroscopy, K. Szeto for DRIFT
experiments, and the SEC characterization platform of the LPSE.
[39] S. Strandman, J. E. Gautrot, X. X. Zhu. Polymer Chemistry 2011, 2, 791-
799.
Keywords: boron chemistry • organoboron polymers • high
glass-transition temperature • dynamic networks • reversible
crosslinking
This article is protected by copyright. All rights reserved.

10.1002/anie.201904559
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
Styrenic-based polymers bearing
pendent pinacol-boronate ester
groups are able to crosslink into
dense networks with glass transition
temperature as high as 220°C. This
crosslinking is based on an
Juliette Brunet, Franck Collas, Matthieu
Humbert, Lionel Perrin, Fabrice Brunel,
Emmanuel Lacôte, Damien Montarnal*,
Jean Raynaud*
Page No. – Page No.
overlooked dynamic ring opening of
cyclic boronates promoting bridging
between boron atoms, and is fully
reversible upon dilution at ambient
temperatures.
High glass-transition temperature
polymer networks harnessing the
dynamic ring opening of pinacol
boronates
This article is protected by copyright. All rights reserved.