A Mechanochemical Reaction Cascade for Controlling Load-Strengthening of a Mechanochromic Polymer

Article abstract of DOI:10.1002/anie.202010043

We demonstrate an intermolecular reaction cascade to control the force which triggers crosslinking of a mechanochromic polymer of spirothiopyran (STP). Mechanochromism arises from rapid reversible force-sensitive isomerization of STP to a merocyanine, which reacts rapidly with activated C=C bonds. The concentration of such bonds, and hence the crosslinking rate, is controlled by force-dependent dissociation of a Diels–Alder adduct of anthracene and maleimide. Because the adduct requires ca. 1 nN higher force to dissociate at the same rate as that of STP isomerization, the cascade limits crosslinking to overstressed regions of the material, which are at the highest rate of material damage. Using comb polymers decreased the minimum concentration of mechanophores required to crosslinking by about 100-fold compared to previous examples of load-strengthening materials. The approach described has potential for controlling a broad range of reaction sequences triggered by mechanical load.

Full text of DOI:10.1002/anie.202010043

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Accepted Article
Title: A mechanochemical reaction cascade for controlling load-
strengthening of a mechanochromic polymer
Authors: Yifei Pan, Huan Zhang, Piaoxue Xu, Yancong Tian, Chenxu
Wang, Shishuai Xiang, Wengui Weng, and Roman Boulatov
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202010043
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A Mechanochemical Reaction Cascade for Controlling Load-
Strengthening of a Mechanochromic Polymer
Yifei Pan+, Huan Zhang+, Piaoxue Xu, Yancong Tian, Chenxu Wang, Shishuai Xiang, Roman Bou-
latov*, and Wengui Weng*
Abstract: We demonstrate an intermolecular reaction cascade to
control the force which triggers crosslinking of a mechanochromic pol-
ymer of spirothiopyran (STP). Mechanochromism arises from rapid
reversible force-sensitive isomerization of STP to a merocyanine,
which reacts rapidly with activated C=C bonds. The concentration of
such bonds, and hence the crosslinking rate, is controlled by force-
dependent dissociation of a Diels-Alder adduct of anthracene and ma-
leimide. Because the adduct requires ~1 nN higher force to dissociate
at the same rate as that of STP isomerization, the cascade limits
crosslinking to overstressed regions of the material, which are at the
highest rate of material damage. Using comb polymers decreased the
minimum concentration of mechanophores required to crosslinking by
~100-fold compared to all previous examples of load-strengthening
materials. The approach described here holds potential for controlling
a broad range of reaction sequences triggered by mechanical load.
zation was accelerated by mechanical load, and the other that re-
acted spontaneously with the isomerization product to form the
crosslinks.[26-29] Load-induced polymerization was achieved when
mechanical load generated a catalyst or initiator for polymeriza-
tion of monomers dissolved in the material prior to loading.[30-34]
Endowing polymers with both damage-signaling and self-re-
inforcing capacity requires either increasing the number of distinct
components, which presents both synthetic challenges and the
need to avoid cross-reactivity in a loaded material, or creating
dual mechanophores. These are reactive sites that are both
mechanochromic and either initiate or catalyze polymerization of
the dissolved monomers, or react stoichiometrically with comple-
mentary sites on adjacent chains for crosslinking. Only the latter
approach was demonstrated to date,[26, 31] albeit at a cost of im-
practically low threshold force for crosslinking onset. In other
words, in the only two examples of polymers that are both dam-
age-signaling and self-reinforcing,[26, 31] crosslinking likely occurs
at a load that is too low to cause significant material damage.
Although it is not yet known how to design monomers in
which desired chemistry is triggered by a load of any desired mag-
nitude, mechanochemical reaction cascades[35-38] offer a general
strategy of decoupling the chemistry from the triggering force by
using a sacrificial molecular moiety. The sacrificial moiety blocks
the desired chemistry until it itself reacts mechanochemically by
sequestering (or “masking”) a reactant needed for the desired
chemistry to occur. The force necessary to affect the sacrificial
reaction is independent of the mechanochemical kinetics of the
target reaction. As long as the sacrificial reaction requires higher
force than the target reaction, the kinetics is determined by the
sacrificial reaction, while the target reaction delivers the desired
chemistry. Mechanochemical cascades that increased threshold
force of a target reaction are limited to a single intramolecular ex-
ample.[37]
Here we describe an intermolecular reaction cascade that
increases the threshold force which triggers crosslinking of a
mechanochromic polymer by >1 nN. This cascade comprises
three reactions (Scheme 1): force-dependent isomerization of spi-
rothiopyran, STP, to thiomerocyanine (TMC), which is responsible
for mechanochromism; spontaneous addition of TMC to the C=C
bond of maleimide, which crosslinks the material, and force-de-
pendent dissociation of an anthracene/maleimide Diels-Alder ad-
duct, DA, which controls the force that triggers this crosslinking.
Computational and experimental evidence reported below indi-
cate that at low load (single-chain force <0.5 nN) the material is
inert on practical timescales; at moderate loads (~0.5 - ~1.5 nN)
the stress response is reversible and exclusively mecha-
nochromic; at high loads the material crosslinks irreversibly.
Masking a component of the crosslinking pair also eliminates un-
desired light- and heat-induced crosslinking, and introduce a sec-
ond mechanochromic reaction, thanks to anthracene absorption
at 260 - 400 nm and fluorescence at 375 - 500 nm.[17]
A major aspiration of contemporary polymer mechanochemistry
is to reproduce aspects of the sophisticated response of living tis-
sue to mechanical load in synthetic materials.[1] Both types of ma-
terials degrade under mechanical load, but only the living tissue
can sense and signal this damage (e.g., as pain), and trigger its
repairs.[2-4]
Damage-signaling in synthetic materials is generally
achieved by exploiting mechanochromism, or mechanically-trig-
gered changes in the absorption or emission properties of a ma-
terial. Numerous mechanochromic polymers are known,[5-6] in-
cluding those containing spiro- or benzopyrans,[7-9] rhodamine,[10]
dimers of stable radicals,[11] Diels-Alder adducts,[12-13] cin-
namate,[14-15] coumarin[16] and anthracene[17] dimers, metal com-
plexes,[18] dioxetanes,[19-22] energy transfer systems,[23-24] and an
indene derivative.[25]
The closest that synthetic polymers have come to auto-
nomic self-repair are load-induced crosslinking or polymerization.
To date the former was demonstrated in four separate polymers
each comprised of two types of monomers: one whose isomeri-
[*]
Yifei Pan[+], Dr. Huan Zhang[+], Piaoxue Xu, Shishuai Xiang, Prof.
Wengui Weng
Department of Chemistry, College of Chemistry and Chemical Engi-
neering
Xiamen University
422 South Siming Road, Xiamen, Fujian, 361005, PRC
E-mail: wgweng@xmu.edu.cn
Dr. Yancong Tian, Dr. Chenxu Wang, Prof. Roman Boulatov
Department of Chemistry
University of Liverpool and Donnan Lab
G31, Crown St. Liverpool, L69 7ZD, GB
E-mail: boulatov@liverpool.ac.uk
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202010043
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Scheme 1. An intermolecular mechanochemical reaction cascade for controlling load induced crosslinking of a mechanochromic material. In this implementation,
spirothiopyran (STP, yellow) and anthracene/maleimide adduct (DA, white) are located in long side chains of comb polymers. STP is in a rapid equilibrium with a
green merocyanine form (TMC, green), which reacts spontaneously with activated C=C bonds, such as those in maleimide (MAL, orange). The position of the
equilibrium is affected by temperature, light and mechanical load, with the latter favoring the TMC form. A polymer containing both STP and maleimide crosslinks
slowly in the dark and rapidly under irradiation or modest mechanical load. Masking maleimide as a mechanochemically labile adduct with anthracene (ATC, blue)
eliminates crosslinking in the absence of load and increases the onset force of mechanochemical crosslinking by 1 nN because the adduct is inert unless is stretched
to force >1.5 nN. At this force, rapid dissociation of the adduct releases maleimide, enabling fast crosslinking.
Quantum-chemical calculations of force-dependent activa-
tion free energies of STP isomerization and the anthracene/ma-
leimide adduct dissociation (Figure 1) explain the rate-determin-
ing role of the latter reaction in the load-induced crosslinking cas-
cade. Calculations at the CAM-B3LYP/6-311+G(d) level of DFT
with THF-parameterized CPCM model of the reaction solvent
suggest that mechanochemical dissociation of the DA adduct
used in our study follows the same mechanism as previously re-
ported for other anthracene/maleimide derivatives.[39-40] The dis-
sociation is concerted up to 0.5 nN of force, above which a two-
step mechanism traversing two open-shell singlet transition states
dominates (Figures 1 and S1). As expected,[41] the bridgehead,
C9-bound CH2 group increases the thermal stability of the DA ad-
duct compared to C9-alkoxy derivatives studied previously,[42] but
does not change the sensitivity of the reaction to force. The mech-
anism of isomerization of STP has not been reported and is likely
complex.[43] Extrapolating from the assumed mechanism of spiro-
maleimide increases the minimum force at which crosslinking oc-
curs at practical rate by ~1 nN.
We tested our mechanochemical cascade using comb pol-
ymers, each containing either multiple DA adducts (M2) or STP
moieties (S2, Schemes 1 and 2). A comb polymer contains a rel-
atively short main chain with multiple three-way branch points at
which longer linear side chains are connected. In our polymers,
the mechanophores were confined to the side-chains in the prox-
imity of the branch points. We chose comb polymers to maximize
the efficiency of transducing external mechanical load to the force
acting on individual mechanophores.[46-47] Doing so is expected to
reduce the minimum concentration of mechanophores needed to
elicit a detectable stress response, which is critical for practical
applications of such polymers.
We prepared multifunctional initiators M1 and S1 by poly-
merizing mixtures of methyl acrylate and either M0 (2:1 mol) or
S0 (10:1 mol) under standard conditions.[48] The low fraction of S0
in polymerization mixtures reflects our assumption that fairly de-
manding syntheses of spiropyrans necessitate minimizing their
contents in polymers for practical applications, while maintaining
an acceptable intensity of mechanochemical response. A combi-
pyran isomerization,[44-45] its activation free energy, G‡ (green
f
curve, Figure 1) is calculated to be ~10 kcal/mol lower than that
for the DA dissociation (blue curve, Figure 1) at the same force of
up to 2 nN. The rate of bimolecular addition of TMC to maleimide
(grey line, Figure 1) is force independent and faster than DA dis-
sociation (blue curve, Figure 1) at <~2 nN (depending on the
steady-state concentration of the two reactants). Consequently,
up to this force the kinetics of crosslinking is determined by the
rate of mechanochemical dissociation of the DA adduct, whereas
at higher force, the kinetics of TMC/maleimide addition becomes
rate-determining. The ~10 kcal/mol difference of the two force-
1
nation of H NMR spectroscopy and size exclusion chromatog-
raphy (SEC) suggests that an average chain of M1 contained
18±3 DA adducts, branching and initiator moieties (ƉM = 1.6, see
Table S1 for further details); the equivalent number for S1 was
15±2 (ƉM = 1.7). Living radical polymerization of methyl acrylate
yielded comb polymers M2 and S2 (Scheme 2 and Supporting
Information) from multifunctional initiators M1 and S1. The
polymerization increased Mn of M1 ~100-fold (M2: Mn = 1.1 MDa,
dependent G‡ means that using the DA adduct as the source of
f
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A similar estimate of the side-chain size is derived from the
changes in the molecular mass distributions of these polymers
during sonication (Table S2 and below).
We confirmed mechanochemical generation of multiple an-
thracene-terminated macromolecules from each chain of M2 in
sonicated dilute solutions of M2 in THF under previously reported
conditions (Figure 2 and Supporting Information).[26] Aliquots of
the sonicated solution were periodically withdrawn and analyzed
1
by SEC, absorption, fluorescence and H NMR spectroscopies.
Several pieces of evidence unambiguously support mechano-
chemical cleavage of the DA adducts. First, SECs of sonicated
solutions manifested a new peak at ~57 kDa, whose intensity rel-
ative to that of the reactant peak increased with the sonication
time (Figure S13). Second, sonicated solutions absorbed at 260 -
400 nm and fluoresced at 375 - 500 nm under 256 nm excitation
(Figure S14), with the intensities increasing during sonication.
Both spectra were characteristic of anthracene.[17] Third, 1H NMR
spectra of sonicated M2 contained new resonance signals at 6.58,
and 8.05, 8.35, 8.52 ppm, consistent with those of maleimide and
anthracene (Figure 2b). Under the same conditions, sonication of
the precursor to M2, M1, in which the DA adducts terminate very
short side chains generated neither new 1H NMR resonances, ab-
sorbance nor fluorescence (Figure S15). The difference supports
mechanochemical rather than purely thermal mechanism[39] of
dissociation of the DA adducts of M2, which is precluded in M1
whose adducts are not strained when the chain in stretched in an
elongational flow.
Figure 1. The standard activation free energy of anthracene/maleimide DA ad-
duct dissociation, STP isomerization and the addition of TMC and maleimide at
CAM-B3LYP/6-311+G(d) in THF-parameterized CPCM and the molecular
structures of the reactants used. The pulling axis is shown by the arrows. See
Figure S1 for the geometries of the transition states.
The average molar mass of sonicated M2 decreased during
sonication (Figure 2c and Table S2) due to both gradual loss of
side-chains by dissociation of the DA adducts and non-selective
fragmentation of the backbone, which accompanies sonication of
any polymer.[39] The latter was evidenced by the appearance of a
SEC peak at half the mass of intact M2 (Figure S13a), with negli-
gible absorbance at >300 nm. To understand how both selective
and non-selective fragmentation of M2 affects the kinetics of ma-
leimide production, which controls the rate of bimolecular cross-
linking, we analyzed the changes in Mn, and the fraction of dis-
ƉM = 1.3, Table S1) and that of S1 ~30-fold (S2: Mn = 580 kDa,
ƉM = 1.7) or approximately proportionally to the estimated density
of the initiator moieties in the two precursors, suggesting that the
side chain in both M2 and S2 are of comparable size of ~50 kDa.
Scheme 2. Syntheses of comb polymers M2 and S2. AIBN: 2,2'-azobis(2-methylpropionitrile), DMF: N,N-dimethylformamide, Me6TREN: tris[2-(dimethyla-
mino)ethyl]amine, DMSO: dimethylsulfoxide.
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Figure 2. (a) Mechanochemical retro-DA reaction of comb polymer M2 with anthracene-maleimide moieties in solution. (b) 1H NMR spectra of M2 before and after
120 min sonication (5 mg/mL, THF, 10 W/cm2, 0 - 5 oC, 1 s on and 1 s off), showing the products of the retro-DA reaction as indicated by grey rectangles. (c) The
fraction of dissociated DA adducts, DA, and the number-average molar mass of sonicated solution of M2, relative to that of intact M2, as a function of sonication
time. The dashed lines are guide for eyes.
taining n side chains (and hence n DA adducts), Rn (n>1), frag-
ments either selectively to a chain with one fewer arms, Rn-1, and
a linear, anthracene terminated chain, PAn, or non-selectively to
two comb macromolecules, each with half the original number of
side chains (or, for odd n, into two macromolecules whose num-
ber of side chains differed by 1 and totalled n). The measured
correlation between the number average molar mass and the
fraction of dissociated DA adducts during sonication was repro-
duced accurately (solid line, Figure 3a) by the simplest version
of this scheme in which the rate constant for fragmentation of Rn
by mechanochemical dissociation of one of its n DA adducts was
proportional to n (n>1), and non-selective fragmentation contrib-
utes <7% to the total reaction.
Our modeling illustrates the advantage of using a comb
polymer rather than a linear polymer for activation of scissile
mechanophores whose products are used as reactants in a
Figure 3. Modeling of mechanochemical kinetics of dissociation of the DA ad-
downstream reaction. The rate at which this reactant is gener-
duct in comb polymers. (a) The measured correlation between the fraction of
ated by dissociation of a comb polymer decreases linearly with
the fraction of the reacted moieties (solid line, Figure 3b) and is
largely insensitive to non-selective fragmentation of the polymer.
In contrast, this rate decreases exponentially with every frag-
mentation of a linear polymer (dash line, Figure 3b), whether se-
lective (i.e., it generates the desired precursor for the subsequent
downstream reaction) or non-selective. The reason is that frag-
mentation of linear polymers either by dissociation of a scissile
mechanophore or non-selective homolysis of a backbone bond
dissociated adducts, DA, and the average molar mass of the sonicated material,
Mn (red dots) is reproduced by a simple model (black line). (b) The relative rate
of maleimide production by mechanochemical activation of a comb polymer
(black line) is much less sensitive to the degree of conversion than that of a
linear analog (red line).
sociated DA adducts, DA, (Figures 2c-3 and S16-21) during son-
ication. We used a kinetic scheme whereby a comb polymer con-
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COMMUNICATION
between mechanophores yields two fragments, each approxi-
mately half of length of the original chain. Because mechano-
chemical kinetics scales quadratically with the polymer contour
length,[39] activation of the remaining mechanophores in these
fragments is considerably less efficient than in the original chain.
In contrast, selective fragmentation of a multi-mechanophore
comb macromolecule produces another comb macromolecule,
which is only marginally less sensitive to further mechanochem-
ical activation because its spanning length remains intact. More-
over, non-selective fragmentation of a comb polymer produces
two new comb polymers whose combined reactivity matches that
of the original polymer, according to our model.
a film made of 1:1 mixture of M2 and S2 turned green upon UV
irradiation, suggesting the generation of TMC, but remained fully
soluble in DCM (Figure S31). Both observations indicate the lack
of crosslinking when M2 is not subject to conditions necessary
for mechanochemical dissociation of its DA adducts. In contrast,
irradiating a mixture of sonicated M2 (30 min sonication) and un-
sonicated S2 at 365 nm for 30 min yielded insoluble materials
(Figure S32). A mixture of sonicated M2 (30 min sonication) and
unsonicated S2 also became insoluble after 7 days at room tem-
perature in the dark. The latter suggests that at room tempera-
ture the STP/TMC equilibrium maintains sufficient concentration
of TMC to react with maleimide at an observable rate even in
solids. Masking maleimide in the form of the DA adduct elimi-
nates this spontaneous undesired crosslinking of the unloaded
material.
The high cost of most reported mechanophores limits their
potential practical applications, and requires strategies to mini-
mize mechanophore content without sacrificing the desired
stress response. Although this problem has not been studied
systematically, in all reported examples of crosslinking in soni-
cated solutions, the total concentration of mechanophores was
on the order of ~100 mM.[26-29] In contrast, sonicated solutions of
S2 and M2 crosslink readily when the concentration of the re-
spective moiety is as low as 1.6 mM and 0.35 mM, or about two
orders of magnitude lower than previous examples. We assume
that the primary reason is how sensitive the kinetics of mechano-
phore activation is to chain fragmentation in linear and comb pol-
ymers as discussed above.
Identifying strategies to enable simultaneous control over
mechanochemical reactivity and the threshold force at which it
occurs at practical rate remains a major unresolved problem in
contemporary polymer mechanochemistry. The task is particu-
larly challenging for materials that display multiple productive re-
sponses to load, because it requires integration of several mech-
anochemical reactions while avoiding cross-reactivity. Above we
described an intermolecular reaction cascade to increase by ~1
nN the threshold force that triggers crosslinking of a mecha-
nochromic material containing spirothiopyran and Diels-Alder
adducts of anthracene and maleimide. The onset of reversible
green mechanochromism is determined by the mechanochemi-
cal kinetics of isomerization of spirothiopyran to thiomerocyanine,
with half-life of ~1 min at ~1 nN. The dye is reactive towards ac-
tivated C=C double bonds, crosslinking spontaneously in their
presence. Masking these bonds as DA adducts allows the onset
of crosslinking to be governed by the dissociation kinetics of the
DA adduct instead of the more labile STP-to-TMC isomerization.
As a result, this cascade provides a means of limiting the occur-
rence of irreversible crosslinking to overstressed volumes of the
loaded material that are at the highest risk of failure. The diverse
reactivity of maleimide, including as a Michael’s receptor and as
a dienophile, and the relatively simple means of adjusting the
dissociation kinetics of its DA adducts by substitution[42] suggests
that the approach described here is suitable for controlling a
broad range of reaction sequences triggered by mechanical load.
Behavior of S2 in sonicated solutions resembled that of its
linear analogs,[26] including reduction in the molar mass (Figures
S22-S23), rapid reaction with free N-ethyl maleimide accompa-
nied by a loss of the STP-characteristic absorption (Figure S24)
and reversible photochromism (Figure S25).
Sonication of a mixed solution of the two polymers (M2 at
20 mg/mL and S2 at 50 mg/mL initial concentrations) in DMF for
5 h yielded a suspension of a dark-green material (Figures 4b
and S26), which when isolated weighted 28% of the total mass
of the initially dissolved polymers. The material was insoluble in
all solvents tested (Figure S27), including those in which M2 and
S2 dissolved readily even upon heating, suggesting extensive
crosslinking (Figure S28). The remaining solution manifested a
fluorescence spectrum characteristic of anthracene (Figures 4d
and S29), the expected product of mechanochemical dissocia-
tion of the DA adducts of M2. The existence of the minimum con-
centration of S2 below which crosslinking does not occur in soni-
cated solutions (Figure S30) suggests a kinetic competition be-
tween a bimolecular addition of TMC to maleimide, whose rate
depends on the concentration of S2, and unimolecular isomeri-
zation of TMC to STP which is inert to reaction with maleimide.
Figure 4. Mechanochemical cross-linking of polymers M2 and S2 in sonicated
THF solution. Optical (a) and fluorescence (c) images before sonication; Opti-
cal (b) and fluorescence (d) images after 5 h sonication.
Acknowledgements
A mixed solution of M2 and S2 at saturation concentrations
that was not sonicated yielded no precipitate even under irradia-
tion at 360 nm to photoisomerize STP to reactive TMC. Likewise,
YP, HZ, PX, SX and WW are funded by National Natural Science
Foundation of China (No. 21774106, No. 21574108). YT, CW
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Keywords: mechanochromism • crosslinking • mechanophore •
masking • spirothiopyran • maleimide
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This article is protected by copyright. All rights reserved.

10.1002/anie.202010043
Angewandte Chemie International Edition
COMMUNICATION
Table of Contents
Mechanochemical reaction cascade for controlling load-strengthening of a mechanochromic polymer is realized by using two comb
polymers, each containing either spirothiopyran or a Diels-Alder adduct in the proximity of the branch points. The threshold force which
triggers the crosslinking is increased by ~1 nN and the minimum concentration of mechanophores required for crosslinking is decreased
by ~100-fold compared to previous examples.
This article is protected by copyright. All rights reserved.