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Mechanochemical generation of acid-degradablepoly(enol ether)s†
Jinghui Yangand Yan Xia*

In an eﬀort to develop polymers that can undergo extensive backbone degradation in response tomechanical stress, we report a polymer system that is hydrolytically stable but unmasks easilyhydrolysable enol ether backbone linkages when force is applied. These polymers were synthesized byring-opening metathesis polymerization (ROMP) of a novel mechanophore monomer consisting ofcyclic ether fused bicyclohexene. Hydrogenation of the resulting polymers led to signiﬁcantly enhanced
thermal stability (Td > 400  C) and excellent resistance toward acidic or basic conditions. Solution
ultrasonication of the polymers resulted in up to 65% activation of the mechanophore units andconversion to backbone enol ether linkages, which then allowed facile degradation of the polymers to

Received 1st January 2021
Accepted 8th February 2021

generate small molecule or oligomeric species under mildly acidic conditions. We also achieved solidstate mechano-activation and polymer degradation via grinding the solid polymer. Force-inducedhydrolytic polymer degradability can enable materials that are stable under force-free conditions but

DOI: 10.1039/d1sc00001b

readily degrade under stress. Facile degradation of mechanically activated polymechanophores also

rsc.li/chemical-sciencefacilitates the analysis of mechanochemical products.

Introduction
Polymer mechanochemistry explores the use of mechanicalforce to direct selective chemical transformations in forceresponsive molecules called mechanophores.1,2 Embedding
properly designed mechanophores in polymer backbones can
induce nondestructive covalent bond transformations andgenerate materials with a wide range of force-responses.3–11
Mechanical activation of mechanophores is usually accompanied by nonselective chain scission, and chain scission can halt
further activation of mechanophores.12 While it has long been
known that high molecular weight (MW) polymers undergo
undesired mechanical degradation under shear stress,13,14
mechanically triggered chemical degradation over an extended
strand of polymer (rather than a single-point chain degradation) is an interesting and desired mechanical response. It canenhance the extent of polymer degradation to result in muchsmaller species. Such polymers may enable eﬀective selfdestruction and minimize their environmental impact. For
fundamental study of mechanochemistry, incorporating forcetriggered degradable units in polymechanophores may facilitate the analysis of the mechanochemical products and provide

Department of Chemistry, Stanford University, Stanford, California, 94305, USA.
E-mail: yanx@stanford.edu† Electronic supplementary information (ESI) available: Detailed descriptions ofsynthetic procedures, 1H and 13C NMR spectra of new compounds, GPC traces
and single crystal structure. Crystallographic data for compound S2. CCDC
2023034. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d1sc00001b© 2021 The Author(s). Published by the Royal Society of Chemistry

information on the continuity (or discontinuity) of polymechanophore activation.
In 2020, the Craig and Wang groups simultaneously reported
strategies for mechanically “unlocking” polymer degradability
by fusing hydrolysable lactone and cyclic ketal linkages withcyclobutane mechanophores in the polymer backbone. Uponring-opening of cyclobutane, these hydrolysable moieties become
backbone linkages and enable hydrolytic degradation of themechanochemically activated polymers (Scheme 1A and B).15,16
Subsequently, the Craig group reported a clever alternative designof a tetrasubstituted [4.2.0]bicyclooctene based mechanophore
that, upon mechanochemical ring-opening into 1,3-cyclooctadiene, can spontaneously undergo intramolecular lactonization and result in backbone cleavage (Scheme 1C).17 These recent
examples showcased mechanically enhanced polymer degradation as a new modality of response, but in these designs, thehydrolysable functionalities already exist in the polymers before
mechanoactivation. As a result, deleterious hydrolysis prior to thedesired mechanoactivation would inevitably hydrolyze thesesame functionalities in the precursor polymers and change theirproperties, leading to the generation of undesired scissile cyclobutane linkages or premature polymer cleavage that
disrupts the transduction of force and thus limits theextent of mechanoactivation. We herein report a new design ofpolymechanophores that overcomes these limitations and only
contains completely chemically inert linkages, but upon mechanoactivation, easily hydrolysable and degradable linkages areunmasked. Taking advantage of the olens generated frommechanochemical ring-opening of cyclobutane, we sought toChem. Sci., 2021, 12, 4389–4394 | 4389

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that can be synthesized from commercially available chemicals
at gram scales as outlined in Scheme 2. Thermally induced [2 +2] cycloaddition between dimethyl acetylenedicarboxylate and2,3-dihydrofuran produced bicyclic compound 2,25 which was
subsequently hydrogenated and reduced to aﬀord diol 3. 3 wasfound to have an exclusive endo conguration (Fig. S1†) due tothe high facial selectivity in the hydrogenation step. Treatmentof 3 with MsCl aﬀorded dimesylate 4, which was then treatedwith Na2S followed by oxidation with H2O2 to yield sulfoxide 5.
Chlorination of 5 by one equivalent of SO2Cl2 generated achlorosulfoxide as a mixture of diﬀerent regioisomers andstereoisomers, which underwent sulfoxide Ramberg–Bäcklandolenation to generate 1 as a volatile liquid.
The ring opening metathesis polymerization (ROMP) of 1
was then explored using the fast-initiating Grubbs catalyst G3.
G3 was added into a solution of 1 in CHCl3 ([1]0 ¼ 1 M) at room
temperature with a target degree of polymerization (DP) of 1000.
1
H NMR analysis of the crude reaction mixture indicatedcomplete monomer conversion within 15 min. Despite the fastpolymerization, gel permeation chromatography (GPC) of thepolymerization solution showed an asymmetric peak skewed to
the low MW side and the measured MW was lower than the
expected value (Fig. S2†). We have previously observed reduced
stability of the propagating Ru in ROMP of ladderenes, but thestability is improved at lower temeratures.22 Delightfully,
lowering the polymerization temperature to 0  C yieldeda symmetric peak with expected MW and low dispersity. The

Scheme 1 Designs of polymers that can mechanochemicallygenerate degradable backbone linkages.

mechanically convert chemically inert alkyl ether linkages to acidlabile enol ether linkages. The resultant poly(enol ether)s can be
rapidly degraded under mildly acidic conditions but remain
stable under basic conditions.18–20

Results and discussions
We have developed a general strategy to directly polymerizemechanophore monomers in a controlled fashion to generate
polymechanophores, and designed such polymechanophores toundergo dramatic mechanochemical transformation from nonconjugated structures to conjugated structures.21–24 Leveraging
this strategy, we designed a tricyclic mechanophore monomer 1(a) ROMP of monomer 1 and subsequent hydrogenation (thespeciﬁed stereochemistry refers to that of the repeat unit, not suggesting the polymer tacticity). (b) GPC traces of poly(1) at diﬀerent DPs
and their hydrogenated polymers. (c) TGA traces of poly(1) andhydrogenated poly(1).

Fig. 1

Scheme 2

Synthesis of monomer 5.

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Table 1

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Summary of synthesized polymers

PolymerDPtheory

Mn,theory (kDa)

Mn,MALLSa (kDa)

P1
P2
P3
P1-H
P2-H
P3-H

1000
500
1000b
1000
500
1000b

122
61.1
186
124
62.1
188

121
63.1
172
141
74.0
179

Đ
1.14
1.04
1.10
1.17
1.10
1.13

a
Determined by GPC MALLS analysis in THF. b Equal composition of 1
and NBE-EtHex.

measured MWs agreed with the theoretical values and could becontrolled by the ratio of [1]0/[G3]0 (P1 and P2 in Fig. 1b andTable 1). However, if the quenched polymer solution was leunder ambient conditions, low MW polymers were progressively formed over a few hours (Fig. S3†), suggesting decomposition of the ROMP polymer. Diﬀerential scanningcalorimetry (DSC) analysis of the isolated ROMP polymer
showed two overlapping irreversible exothermic peaks startingfrom 46  C (Fig. S4†). The limited thermal stability was also
corroborated by thermogravimetric analysis (TGA), whichshowed gradual mass loss starting from 100  C (Fig. 1c). To
probe the thermal decomposition, poly(1) was dissolved in
CDCl3 and heated at 65  C. Additional olenic peaks at 6.0–
6.2 ppm emerged, which were tentatively assigned to conjugated olens generated from ring-opening of cyclobutanes(Fig. S8†). Interestingly, the peak intensity of new olens quickly
saturated aer 3 h of heating and remained unchanged aer
additional 37 h of heating at the same temperature. Further
increasing the temperature to 90  C for another 24 h did not
result in much change in the peak intensity (Fig. S9†). This
result implies that a fraction of cyclobutanes in the polymer areless thermally stable than the rest, which also agrees with thetwo overlapping exothermic peaks at diﬀerent temperaturesfrom the DSC analysis (Fig. S4†). To investigate whether theconguration of backbone olens would inuence the thermal
stability of cyclobutanes, we synthesized a Z-olen rich polymer
using a Z-selective Grubbs catalyst26 and examined the thermal
stability using the same 1H NMR analysis. Surprisingly, almost
identical thermal decomposition prole was observed despite
the distinct diﬀerence in backbone olen congurations(Fig. S10†). At this point, the reason behind this unusual
thermal behavior remains unknown.
Recently, Bielawski and coworkers reported poly(dewar
benzene), which has the same divinyl cyclobutane backbone
structure.27 This polymer was also found to be unstable, but wasstabilized aer hydrogenation of the backbone olens. Therefore, we hydrogenated the backbone olens of poly(1) using
tosylhydrazide to improve the thermal stability. Hydrogenation
also simplies the analysis of mechanically generated olens byeliminating the formation of conjugated backbone olens,
which have complex congurations and broad 1H NMR signals.
Freshly prepared poly(1) was hydrogenated using tosyl hydrazide (5 equiv. to olen) with added NEt3. 1H NMR analysis of thehydrogenated polymers (P1-H and P2-H) showed complete© 2021 The Author(s). Published by the Royal Society of Chemistry

disappearance of olenic peaks and unnoticeable ring-openingof cyclobutane (Fig. S46†). Their GPC traces exhibited narrowdisperse peaks with the same distribution as the ROMP polymers and MWs close to the theoretical values (Fig. 1b, Table 1).
The hydrogenated polymers showed signicantly improved
thermal stability with a Td over 400  C (Fig. 1c). DSC analysis
showed that P1-H is amorphous with a Tg of 94  C (Fig. S5†).
Hydrogenated poly(1) also exhibited excellent stability
towards acidic or basic conditions. Treating their solutions witheither 100 mM HCl or KOH resulted in no change in GPC traces(Fig. 3c) or 1H NMR spectra (Fig. S13†) over multiple days. The
absence of any labile linkages in both polymer backbone andside chain is attributed to the high chemical stability prior tothe mechanoactivation of the polymers.
To study the mechanoactivation, we subjected a dilute
solution of P1-H in THF (1 mg mL1) to pulsed ultrasonication.
Aliquots removed at regular time intervals were analyzed byboth GPC and 1H NMR spectroscopy. The GPC traces showeda rapid decrease of the original polymer peak and appearance oflower MW peak (Fig. 2b). The Mn decreased steadily andreached an ultimate MW of 18.0 kDa aer 480 min of ultrasonication. Sonicating a shorter polymer P2-H under the same
conditions resulted in a similar ultimate Mn of 18.7 kDa. 1H
NMR analysis showed a continuous decrease in cyclobutane(a) Mechanochemical activation of P1-H to generate poly(enolether). (b) GPC traces of P1-H at diﬀerent sonication times. (c) 1H NMR
spectrum of activated P1-H after 240 min of sonication.
Fig. 2

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Fig. 3 (a) Acid degradation of mechanically activated polymer. (b) 1H NMR spectra of mechanically activated poly(1) before (top, red trace) andafter (bottom, blue trace) 10 mM TFA treatment. (c) GPC traces of hydrogenated poly(1) showing resistance to acidic and basic hydrolysis. (d) GPC
traces of activated P1-H at diﬀerent times after 10 mM TFA treatment. (e) Comparison of GPC traces of P1-H and P2-H after 480 min ofsonication and 10 mM TFA treatment.

signals along with a concomitant increase in two sets of alkenyl
peaks at the same intensity, corresponding to the generated
enol ether olen and the other olen as assigned in Fig. 2c. The
activation ratio increased steadily overtime and reached up to
65% and 55% for P1-H and P2-H, respectively, aer 480 min ofultrasonication based on the 1H NMR integrations (Fig. S14,
S15, Tables S2 and S3†). These degrees of activation are notably
higher than our other polymechanophore systems that weresimilarly synthesized by ROMP of mechanophore monomers
but generated conjugated polymers,21–24 which typically showa nal degree of activation around 40%. The lower degree ofactivation observed in our other systems may be attributed to
the variable extent of microscale aggregation and potentiallyoccasional chemical crosslinking of the generated conjugatedsegments, which may disrupt the force transduction to someregions of the polymer strand. The absence of extended conjugation in the activated P1-H also gives them well resolved 1H
NMR signals, in contrast to the broad and partly suppressedsignals for the mechanically generated conjugated polymers.
The generated poly(enol ether)s consisted of nearly all Z
olens with an E/Z ratio of 3 : 100 regardless of the MW andremained unchanged over the course of ultrasonication(Fig. 2c). The predominant formation of Z olens represents
stereochemical retention in the mechanochemical ringopening. Interestingly, other studies of cyclobutane mechanochemical cycloreversion by Craig28 and Wang15 groups reported
higher fractions of stereochemical inversion. The stereochemical distribution of the mechanochemically generated diene can4392 | Chem. Sci., 2021, 12, 4389–4394

be ascribed to the instability of the diradical transition state
upon the rst bond scission and dynamic eﬀect,29,30 andwarrants comprehensive computational studies. 22% and 28%
mechanophore activation per scission cycle was achieved for P1H and P2-H, respectively (Fig. S17†), agreeing with the previous
report that shorter polymers are more selective in mechanophore activation over nonselective chain cleavage.31
The activated polymers aer 480 min of sonication were thendegraded by the addition of 10 mM TFA. 1H NMR analysis
showed the complete disappearance of enol ether signals aswell as the appearance of multiple aldehyde peaks at 9.7–
9.8 ppm (Fig. 3b), as expected from several aldehyde-containing
degradation products at diﬀerent MWs. Several small moleculehydrolysis products may be expected due to the regio-irregularplacement of the pendent cyclic ether rings in the polymers(Fig. S21†). GPC analysis of the hydrolyzed polymer showeda bimodal distribution with a relatively narrow-disperse highMW peak representing the unactivated fraction of the polymer
strand (Punactivated), which is presumably located toward theends of pre-activated polymer, and a low MW population corresponding to the generated oligomers and small moleculesfrom hydrolysis(Poligomer), which were formed due to thenoncontinuous activation of mechanophore repeat units.
Theoretically, the distribution of force on a polymer chain
under elongational ow can be described using a bead-rod
model, where force is maximal around the chain center andgradually decreases toward chain ends.32 Presumably, mechanophores are activated more continuously under high forces© 2021 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Fig. 4 (a) Synthesis of copolymer P3-H. (b) Changes of GPC traces ofP3-H before and after grinding for 10 min, followed by treatment with
10 mM TFA in THF.

Chemical Scienceof hydrolytically labile bonds in the polymer backbone undermechanical force. The mechanophore monomer can be rapidly
polymerized by ROMP to yield homopolymers or copolymers.
Hydrogenation of the backbone olens of the ROMP polymers
yielded saturated polymers with greatly enhanced thermal
stability and excellent hydrolytic stability. The degradability ofthese polymers is turned on by mechanochemically transforming a previously non-degradable ether bond into a degradable enol-ether bond under stress applied by either sonicationof the solution or grinding of the solid polymer. This design can
impart additional chemical degradability to polymers beyond
conventional mechanical chain scission and yield low MW
oligomers and small molecules when under both stress andacidic conditions. We envision that the mechanophore monomer 1 can also be directly polymerized with other mechanophore monomers to allow facile degradation of themechanically activated copolymers and facilitate detailed
analysis of mechanochemical products in the future.

Conﬂicts of interestand less continuously as the force approaches activationthreshold (Fig. 2a). Intriguingly, the Mn of both Punactivated andPoligomer decreased gradually over the course of sonication(Fig. 3d). Notably, the Punactivated fraction of P1-H and P2-H aer
480 min sonication had similar Mn of 9.6 and 11.4 kDa,
respectively, despite their diﬀerent original Mn of 141 and 63.1
kDa for the full-length polymers. However, most interestingly,
the MW distribution of the Poligomer fraction for these two
samples were noticeably diﬀerent (Fig. 3e). Longer P1-H resulted in shorter oligomers aer degradation, indicating a shorter
distance between the activated units or less discontinuous
activation for the longer polymer.
We next attempted activation of poly(1) in the solid state. The
facile copolymerization of 1 with substituted norbornenes
provides a simple means to tune the solid state properties of thecopolymers. Poly(1) is a glassy polymer with Tg of 94  C. We
copolymerized 1 with NBE-EtHex to lower Tg and generate moreviscoelastic polymers (Fig. 4a). Copolymer P3 of 1 with NBEEtHex at 1 : 1 ratio was easily synthesized in low dispersity andcontrolled MW (Table 1). P3 was completely hydrogenated using
tosyl hydrazide without side reactions on the ester side chainsas evidenced by 1H NMR analysis (Fig. S25†). The hydrogenatedcopolymer P3-H showed a Tg of 56  C (Fig. S26†). To demonstrate the solid state activation, we ground P3-H in a mortar for
about 10 min. The GPC trace of ground P3-H showed slighttailing resulting from mechanical degradation of the polymer.
10 mM TFA was then added to P3-H before and aer grinding.
While the GPC trace for P3-H without grinding essentiallyremained unchanged, a considerable fraction of lower MW
polymers was observed for the ground P3-H (Fig. 4b), suggesting
the formation of hydrolysable enol ether linkages.

Conclusions
In summary, we have developed a new polymechanophoresystem that is hydrolytically robust but uncovers a large number© 2021 The Author(s). Published by the Royal Society of Chemistry

There are no conicts to declare.

Acknowledgements
This work was supported by the U.S. Army Research Oﬃce
under grant number W911NF-15-1-0525. J. Y. is supported bya Stanford Graduate Fellowship. We thank Matias Horst forhelp with this manuscript. Part of the work was performed at
the Stanford Nano Shared Facilities (SNSF), supported by the
National Science Foundation under award ECCS-1542152. We
thank Zhiao Yu for assistance with DSC and TGA data collection
and Jana Maclaren and Conor M. Galvin for assistance withcrystallography.

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