Mechanistic Insights into the Sonochemical Activation of Multimechanophore Cyclopropanated Polybutadiene Polymers

A Thermally Stable SO2-Releasing Mechanophore: Facile Activation, Single-Event Spectroscopy, and Molecular Dynamic Simulations

Polymers that release small molecules in response to mechanical force are promising candidates as next-generation on-demand delivery systems. Despite advancements in the development of mechanophores for releasing diverse payloads through careful molecular design, the availability of scaffolds capable of discharging biomedically significant cargos in substantial quantities remains scarce. In this report, we detail a nonscissile mechanophore built from an 8-thiabicyclo[3.2.1]octane 8,8-dioxide (TBO) motif that releases one equivalent of sulfur dioxide (SO2) from each repeat unit. The TBO mechanophore exhibits high thermal stability but is activated mechanochemically using solution ultrasonication in either organic solvent or aqueous media with up to 63% efficiency, equating to 206 molecules of SO2 released per 143.3 kDa chain. We quantified the mechanochemical reactivity of TBO by single-molecule force spectroscopy and resolved its single-event activation. The force-coupled rate constant for TBO opening reaches ∼9.0 s-1 at ∼1520 pN, and each reaction of a single TBO domain releases a stored length of ∼0.68 nm. We investigated the mechanism of TBO activation using ab initio steered molecular dynamic simulations and rationalized the observed stereoselectivity. These comprehensive studies of the TBO mechanophore provide a mechanically coupled mechanism of multi-SO2 release from one polymer chain, facilitating the translation of polymer mechanochemistry to potential biomedical applications.

Mechanically triggered on-demand degradation of polymers synthesized by radical polymerizations

Polymers that degrade on demand have the potential to facilitate chemical recycling, reduce environmental pollution and are useful in implant immolation, drug delivery or as adhesives that debond on demand. However, polymers made by radical polymerization, which feature all carbon-bond backbones and constitute the most important class of polymers, have proven difficult to render degradable. Here we report cyclobutene-based monomers that can be co-polymerized with conventional monomers and impart the resulting polymers with mechanically triggered degradability. The cyclobutene residues act as mechanophores and can undergo a mechanically triggered ring-opening reaction, which causes a rearrangement that renders the polymer chains cleavable by hydrolysis under basic conditions. These cyclobutene-based monomers are broadly applicable in free radical and controlled radical polymerizations, introduce functional groups into the backbone of polymers and allow the mechanically gated degradation of high-molecular-weight materials or cross-linked polymer networks into low-molecular-weight species.

Bicyclo[2.2.0]hexene: A Multicyclic Mechanophore with Reactivity Diversified by External Forces

Polymer mechanochemistry has been established as an enabling tool in accessing chemical reactivity and reaction pathways that are distinctive from their thermal counterparts. However, eliciting diversified reaction pathways by activating different constituent chemical bonds from the same mechanophore structure remains challenging. Here, we report the design of a bicyclo[2.2.0]hexene (BCH) mechanophore to leverage its structural simplicity and relatively low molecular symmetry to demonstrate this idea of multimodal activation. Upon changing the attachment points of pendant polymer chains, three different C-C bonds in bicyclo[2.2.0]hexene are specifically activated via externally applied force by sonication. Experimental characterization confirms that in different scenarios of polymer attachment, the regioisomers of BCH undergo different activation reactions, entailing retro-[2+2] cycloreversion, 1,3-allylic migration, and retro-4π ring-opening reactions, respectively. Control experiments with small-molecule analogues reveal that the observed diversified reactivity of BCH regioisomers is possible only with mechanical force. Theoretical studies further elucidate that the differences in the positions of substitution between regioisomers have a minimal impact on the potential energy surface of the parent BCH scaffold. The mechanochemical selectivity between different C-C bonds in each constitutional isomer is a result of selective and effective coupling of force to the aligned C-C bond in each case.

Multimechanophore Polymers for Mechanically Triggered Small Molecule Release with Ultrahigh Payload Capacity

Polymers that release small molecules in response to mechanical force are promising for a variety of applications including drug delivery, catalysis, and sensing. While a number of mechanophores have been developed for the release of covalently bound payloads, existing strategies are either limited in cargo scope or, in the case of more general mechanophore designs, are restricted to the release of one or two cargo molecules per polymer chain. Herein, we introduce a nonscissile mechanophore based on a masked 2-furylcarbinol derivative that enables the preparation of multimechanophore polymers with ultrahigh payload capacity. We demonstrate that polymers prepared via ring-opening metathesis polymerization are capable of releasing hundreds of small-molecule payloads per polymer chain upon ultrasound-induced mechanochemical activation. This nonscissile masked 2-furylcarbinol mechanophore overcomes a major challenge in cargo loading capacity associated with previous 2-furylcarbinol mechanophore designs, enabling applications that benefit from much higher concentrations of delivered cargo.

Mechanochemistry in Block Copolymers: New Scission Site due to Dynamic Phase Separation

Mechanochemistry can lead to the degradation of the properties of covalent macromolecules. In recent years, numerous functional materials have been developed based on block copolymers (BCPs), however, like homopolymers, their chains could undergo mechanochemical damage during processing, which could have crucial impact on their performance. To investigate the mechanochemical response of BCPs, multiple polymers comprising different ratios of butyl acrylate and methyl methacrylate were prepared with similar degree of polymerization and stressed in solution via ultrasonication. Interestingly, all BCPs, regardless of the amount of the methacrylate monomer, presented a mechanochemistry rate constant similar to that of the methacrylate homopolymer, while a random copolymer reacted like the acrylate homopolymer. Size-exclusion chromatography showed that, in addition to the typical main peak shift towards higher retention times, a different daughter fragment was produced indicating a secondary selective scission site, situated around the covalent connection between the two blocks. Molecular dynamics modeling using acrylate and methacrylate oligomers were carried out and indicated that dynamic phase separation occurs even in a good solvent. Such non-random conformations can explain the faster polymer mechanochemistry. Moreover, the dynamic model for end-to-end chain overstretching supports bond scission which is not necessarily chain-centered.

Bioorthogonal chemistry for prodrug activation in vivo

Prodrugs have emerged as a major strategy for addressing clinical challenges by improving drug pharmacokinetics, reducing toxicity, and enhancing treatment efficacy. The emergence of new bioorthogonal chemistry has greatly facilitated the development of prodrug strategies, enabling their activation through chemical and physical stimuli. This "on-demand" activation using bioorthogonal chemistry has revolutionized the research and development of prodrugs. Consequently, prodrug activation has garnered significant attention and emerged as an exciting field of translational research. This review summarizes the latest advancements in prodrug activation by utilizing bioorthogonal chemistry and mainly focuses on the activation of small-molecule prodrugs and antibody-drug conjugates. In addition, this review also discusses the opportunities and challenges of translating these advancements into clinical practice.

Experimental quantitation of molecular conditions responsible for flow-induced polymer mechanochemistry

Fragmentation of macromolecular solutes in rapid flows is of considerable fundamental and practical importance. The sequence of molecular events preceding chain fracture is poorly understood, because such events cannot be visualized directly but must be inferred from changes in the bulk composition of the flowing solution. Here we describe how analysis of same-chain competition between fracture of a polystyrene chain and isomerization of a chromophore embedded in its backbone yields detailed characterization of the distribution of molecular geometries of mechanochemically reacting chains in sonicated solutions. In our experiments the overstretched (mechanically loaded) chain segment grew and drifted along the backbone on the same timescale as, and in competition with, the mechanochemical reactions. Consequently, only <30% of the backbone of a fragmenting chain is overstretched, with both the maximum force and the maximum reaction probabilities located away from the chain centre. We argue that quantifying intrachain competition is likely to be mechanistically informative for any flow fast enough to fracture polymer chains.

Covalent Mechanochemistry and Contemporary Polymer Network Chemistry: A Marriage in the Making

Over the past 20 years, the field of polymer mechanochemistry has amassed a toolbox of mechanophores that translate mechanical energy into a variety of functional responses ranging from color change to small-molecule release. These productive chemical changes typically occur at the length scale of a few covalent bonds (Å) but require large energy inputs and strains on the micro-to-macro scale in order to achieve even low levels of mechanophore activation. The minimal activation hinders the translation of the available chemical responses into materials and device applications. The mechanophore activation challenge inspires core questions at yet another length scale of chemical control, namely: What are the molecular-scale features of a polymeric material that determine the extent of mechanophore activation? Further, how do we marry advances in the chemistry of polymer networks with the chemistry of mechanophores to create stress-responsive materials that are well suited for an intended application? In this Perspective, we speculate as to the potential match between covalent polymer mechanochemistry and recent advances in polymer network chemistry, specifically, topologically controlled networks and the hierarchical material responses enabled by multi-network architectures and mechanically interlocked polymers. Both fundamental and applied opportunities unique to the union of these two fields are discussed.

Off-center Mechanophore Activation in Block Copolymers

Block copolymers (BCPs) are used in numerous applications in modern materials science. Yet, like homopolymers, BCPs can undergo covalent bond scission when mechanically stressed (mechanochemistry), which could lead to unexpected consequences in such applications. BCPs' heterogeneity may affect force transduction, perhaps changing force distribution and localization. To verify this, a gem-dichlorocyclopropane (gDCC) embedded linear chain is prepared and extended with a poly(methyl methacrylate) block. When stressed in solution, the mechanochemical ring-opening of gDCC is accelerated compared to homopolymers, even though the mechanophores are at the chain ends. Moreover, a higher mechanophore activation selectivity is obtained. These results indicate that mechanochemical response outside, and even far from the chain center is quite prominent in BCPs, and that forces along the polymer chain can efficiently activate multi-mechanophores regions, even when far from the polymer midchain.

Examining the Impact of Relative Mechanophore Activity on the Selectivity of Ultrasound-Induced Mechanochemical Chain Scission

Despite recent advances in polymer mechanochemistry, a more complete understanding of the factors that dictate the ultrasound-induced mechanochemical activation efficiency of mechanophores is necessary. Here, we examine how the identity of a mechanophore, and hence its unique force-coupled reactivity, affects the competition between mechanophore activation and nonspecific polymer backbone scission. Polymers incorporating distinct mechanophores but with putatively similar "chain-centeredness" exhibit widely different mechanochemical activation efficiencies. Furthermore, we employ mechanophores that can be orthogonally cleaved following ultrasonication using heat or light to report on the degree of nonspecific backbone scission that occurs for different mechanophore-containing polymers subjected to ultrasound-induced mechanical force. Our results illustrate that the identity of the mechanophore as well as its position in the polymer chain are inextricably important parameters that together control the selectivity of mechanophore activation during ultrasonication.

Accelerated Mechanochemistry in Helical Polymers

Polymer chains, if long enough, are known to undergo bond scission when mechanically stressed. While the mechanochemical response of random coils is well understood, biopolymers and some key synthetic chains adopt well-defined secondary structures such as helices. To understand covalent mechanochemistry in such structures, poly(γ-benzyl glutamates) are prepared while regulating the feed-monomer chirality, producing chains with similar molecular weights and backbone chemistry but different helicities. Such chains are stressed in solution and their mechanochemistry rates compared by following molecular weight change and using a rhodamine mechanochromophore. Results reveal that while helicity itself is not affected by the covalent bond scissions, chains with higher helicity undergo faster mechanochemistry. Considering that the polymers tested differ only in conformation, these results indicate that helix-induced chain rigidity improves the efficiency of mechanical energy transduction.

Mechanically Triggered Carbon Monoxide Release with Turn-On Aggregation-Induced Emission

Polymers that release functional small molecules under mechanical stress potentially serve as next-generation materials for catalysis, sensing, and mechanochemical dynamic therapy. To further expand the function of mechanoresponsive materials, the discovery of chemistries capable of small molecule release are highly desirable. In this report, we detail a nonscissile bifunctional mechanophore (i.e., dual mechano-activated properties) based on a unique mechanochemical reaction involving norborn-2-en-7-one (NEO). One property is the release of carbon monoxide (CO) upon pulsed solution ultrasonication. A release efficiency of 58% is observed at high molecular weights (M_n = 158.8 kDa), equating to ∼154 molecules of CO released per chain. The second property is the bright cyan emission from the macromolecular product in its aggregated state, resulting in a turn-on fluorescence readout coincident with CO release. This report not only demonstrates a unique strategy for the release of small molecules in a nonscissile way but also guides future designs of force-responsive aggregation-induced emission (AIE) luminogens.

Harnessing the Power of Force: Development of Mechanophores for Molecular Release

Polymers that release small molecules in response to mechanical force are promising materials for a variety of applications ranging from sensing and catalysis to targeted drug delivery. Within the rapidly growing field of polymer mechanochemistry, stress-sensitive molecules known as mechanophores are particularly attractive for enabling the release of covalently bound payloads with excellent selectivity and control. Here, we review recent progress in the development of mechanophore-based molecular release platforms and provide an optimistic, yet critical perspective on the fundamental and technological advancements that are still required for this promising research area to achieve significant impact.

Release of Molecular Cargo from Polymer Systems by Mechanochemistry

The design and manipulation of (multi)functional materials at the nanoscale holds the promise of fuelling tomorrow's major technological advances. In the realm of macromolecular nanosystems, the incorporation of force‐responsive groups, so called mechanophores, has resulted in unprecedented access to responsive behaviours and enabled sophisticated functions of the resulting structures and advanced materials. Among the diverse force‐activated motifs, the on‐demand release or activation of compounds, such as catalysts, drugs, or monomers for self‐healing, are sought‐after since they enable triggering pristine small molecule function from macromolecular frameworks. Here, we highlight examples of molecular cargo release systems from polymer‐based architectures in solution by means of sonochemical activation by ultrasound (ultrasound‐induced mechanochemistry). Important design concepts of these advanced materials are discussed, as well as their syntheses and applications.

Mechanochemical Reactivity of Bottlebrush and Dendronized Polymers: Solid vs. Solution States

We explored the mechanochemical degradation of bottlebrush and dendronized polymers in solution (with ultrasonication, US) and solid states (with ball-mill grinding, BMG). Over 50 polymers were prepared with varying backbone length and arm architecture, composition, and size. With US, we found that bottlebrush and dendronized polymers exhibited consistent backbone scission behavior, which was related to their elongated conformations in solution. Considerably different behavior was observed with BMG, as arm architecture and composition had a significant impact on backbone scission rates. Arm scission was also observed for bottlebrush polymers in both solution and solid states, but only in the solid state for dendronized polymers. Motivated by these results, multi-mechanophore polymers with bottlebrush and dendronized polymer architectures were prepared and their reactivity was compared. Although dendronized polymers showed slower arm-scission, the selectivity for mechanophore activation was much higher. Overall, these results have important implications to the development of new mechanoresponsive materials.

Mechanically Triggered Release of Functionally Diverse Molecular Payloads from Masked 2-Furylcarbinol Derivatives

Polymers that release functional small molecules in response to mechanical force are appealing targets for drug delivery, sensing, catalysis, and many other applications. Mechanically sensitive molecules called mechanophores are uniquely suited to enable molecular release with excellent selectivity and control, but mechanophore designs capable of releasing cargo with diverse chemical functionality are limited. Here, we describe a general and highly modular mechanophore platform based on masked 2-furylcarbinol derivatives that spontaneously decompose under mild conditions upon liberation via a mechanically triggered reaction, resulting in the release of a covalently installed molecular payload. We identify key structure-property relationships for the reactivity of 2-furylcarbinol derivatives that enable the mechanically triggered release of functionally diverse molecular cargo with release kinetics being tunable over several orders of magnitude. In particular, the incorporation of an electron-donating phenoxy group on the furan ring in combination with an α-methyl substituent dramatically lowers the activation barrier for fragmentation, providing a highly active substrate for molecular release. Moreover, we find that phenoxy substitution enhances the thermal stability of the mechanophore without adversely affecting its mechanochemical reactivity. The generality and efficacy of this molecular design platform are demonstrated using ultrasound-induced mechanical force to trigger the efficient release of a broad scope of cargo molecules, including those bearing alcohol, phenol, alkylamine, arylamine, carboxylic acid, and sulfonic acid functional groups.

Flyby reaction trajectories: Chemical dynamics under extrinsic force

Dynamic effects are an important determinant of chemical reactivity and selectivity, but the deliberate manipulation of atomic motions during a chemical transformation is not straightforward. Here, we demonstrate that extrinsic force exerted upon cyclobutanes by stretching pendant polymer chains influences product selectivity through force-imparted nonstatistical dynamic effects on the stepwise ring-opening reaction. The high product stereoselectivity is quantified by carbon-13 labeling and shown to depend on external force, reactant stereochemistry, and intermediate stability. Computational modeling and simulations show that, besides altering energy barriers, the mechanical force activates reactive intramolecular motions nonstatistically, setting up "flyby trajectories" that advance directly to product without isomerization excursions. A mechanistic model incorporating nonstatistical dynamic effects accounts for isomer-dependent mechanochemical stereoselectivity.

The influence of polymer architecture in polymer mechanochemistry

Polymer architecture is an important factor in polymer mechanochemistry. In this Feature Article, we summarize recent developments in utilizing polymer architecture to modulate mechanochemical reactions within polymers, or more specifically, the location and rates of bond scission events that lead to polymer fragmentation or mechanophore activation. Various well-defined architectures have been explored, including those of cyclic, intramolecularly cross-linked, dendritic, star, bottlebrush, and dendronized polymers. We primarily focus on describing the enhancement or suppression of mechanochemical reactivity, with respect to analogous linear polymers, as well as differences in solution- and solid-state behavior.

Multicolor Mechanofluorophores for the Quantitative Detection of Covalent Bond Scission in Polymers

The fracture of polymer materials is a multiscale process starting with the scission of a single molecular bond advancing to a site of failure within the bulk. Quantifying the bonds broken during this process remains a big challenge yet would help to understand the distribution and dissipation of macroscopic mechanical energy. We here show the design and synthesis of fluorogenic molecular optical force probes (mechanofluorophores) covering the entire visible spectrum in both absorption and emission. Their dual fluorescent character allows to track non-broken and broken bonds in dissolved and bulk polymers by fluorescence spectroscopy and microscopy. Importantly, we develop an approach to determine the absolute number and relative fraction of intact and cleaved bonds with high local resolution. We anticipate that our mechanofluorophores in combination with our quantification methodology will allow to quantitatively describe fracture processes in materials ranging from soft hydrogels to high-performance polymers.

Substituent Effects in Mechanochemical Allowed and Forbidden Cyclobutene Ring-Opening Reactions

Woodward and Hoffman once jested that a very powerful Maxwell demon could seize a molecule of cyclobutene at its methylene groups and tear it open in a disrotatory fashion to obtain butadiene (Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry. Angew. Chem., Int. Ed. 1969, 8, 781-853). Nearly 40 years later, that demon was discovered, and the field of covalent polymer mechanochemistry was born. In the decade since our demon was befriended, many fundamental investigations have been undertaken to build up our understanding of force-modified pathways for electrocyclic ring-opening reactions. Here, we seek to extend that fundamental understanding by exploring substituent effects in allowed and forbidden ring-opening reactions of cyclobutene (CBE) and benzocyclobutene (BCB) using a combination of single-molecule force spectroscopy (SMFS) and computation. We show that, while the forbidden ring-opening of cis-BCB occurs at a lower force than the allowed ring-opening of trans-BCB on the time scale of the SMFS experiment, the opposite is true for cis- and trans-CBE. Such a reactivity flip is explained through computational analysis and discussion of the so-called allowed/forbidden gap.

Mechanochemical generation of acid-degradable poly(enol ether)s

In an effort to develop polymers that can undergo extensive backbone degradation in response to mechanical stress, we report a polymer system that is hydrolytically stable but unmasks easily hydrolysable enol ether backbone linkages when force is applied. These polymers were synthesized by ring-opening metathesis polymerization (ROMP) of a novel mechanophore monomer consisting of cyclic ether fused bicyclohexene. Hydrogenation of the resulting polymers led to significantly enhanced thermal stability (T_d > 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 and conversion to backbone enol ether linkages, which then allowed facile degradation of the polymers to generate small molecule or oligomeric species under mildly acidic conditions. We also achieved solid-state mechano-activation and polymer degradation via grinding the solid polymer. Force-induced hydrolytic polymer degradability can enable materials that are stable under force-free conditions but readily degrade under stress. Facile degradation of mechanically activated polymechanophores also facilitates the analysis of mechanochemical products.

A mechanically responsive polymer system that is hydrolytically stable without stress, but unmasks enol ether backbone linkages under force to allow facile hydrolytic degradation.

Mechanochemical tools for polymer materials

This review aims to provide a field guide for the implementation of mechanochemistry in synthetic polymers by summarizing the molecules, materials, and methods that have been developed in this field.

Enhanced polymer mechanical degradation through mechanochemically unveiled lactonization

The mechanical degradation of polymers is typically limited to a single chain scission per triggering chain stretching event, and the loss of stress transfer that results from the scission limits the extent of degradation that can be achieved. Here, we report that the mechanically triggered ring-opening of a [4.2.0]bicyclooctene (BCOE) mechanophore sets up a delayed, force-free cascade lactonization that results in chain scission. Delayed chain scission allows many eventual scission events to be initiated within a single polymer chain. Ultrasonication of a 120 kDa BCOE copolymer mechanically remodels the polymer backbone, and subsequent lactonization slowly (~days) degrades the molecular weight to 4.4 kDa, > 10× smaller than control polymers in which lactonization is blocked. The force-coupled kinetics of ring-opening are probed by single molecule force spectroscopy, and mechanical degradation in the bulk is demonstrated. Delayed scission offers a strategy to enhanced mechanical degradation and programmed obsolescence in structural polymeric materials.

Oxidative regulation of the mechanical strength of a C-S bond

The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness. Methods by which the intrinsic molecular strength of the constituents of a given polymeric material might be switched are therefore potentially useful both for applications in which triggered property changes are desirable, and as tests of molecular theories for bulk behaviors. Here we report that the sequential oxidation of sulfide containing polyesters (PE-S) to the corresponding sulfoxide (PE-SO) and then sulfone (PE-SO2) first weakens (sulfoxide), and then enhances (sulfone), the effective mechanical integrity of the polymer backbone; PE-S ∼ PE-SO2 > PE-SO. The relative mechanical strength as a function of oxidation state is revealed through the use of gem-dichlorocyclopropane nonscissile mechanophores as an internal standard, and the observed order agrees well with the reported bond dissociation energies of C–S bonds in each species and with the results of CoGEF modeling.

The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness.

Molecular Damage Detection in an Elastomer Nanocomposite with a Coumarin Dimer Mechanophore

Molecular force probes that generate optical responses to critical levels of mechanical stress (mechanochromophores) are increasingly attractive tools for identifying molecular sites that are most prone to failure. Here, a coumarin dimer mechanophore whose mechanical strength is comparable to that of the sulfur-sulfur bonds found in vulcanized rubbers is reported. It is further shown that the strain-induced scission of the coumarin dimer within the matrix of a particle-reinforced polybutadiene-based co-polymer can be detected and quantified by fluorescence spectroscopy, when cylinders of the nanocomposite are subjected to unconstrained uniaxial stress. The extent of the scission suggests that the coumarin dimers are molecular "weak links" within the matrix, and, by analogy, sulfur bridges are likely to be the same in vulcanized rubbers. The mechanophore is embedded in polymer main chains, grafting agent, and cross-linker positions in a polymer composite in order to generate experimental data to understand how macroscopic mechanical stress is transferred at the molecular scale especially in highly entangled cross-linked polymer nanocomposite. Finally, the extent of activation is enhanced by approximately an order of magnitude by changing the regiochemistry and stereochemistry of the coumarin dimer and embedding the mechanophore at the heterointerface of the particle-reinforced elastomer.

Methods for Exerting and Sensing Force in Polymer Materials Using Mechanophores

In recent years, polymer mechanochemistry has evolved as a methodology to provide insights into the action-reaction relationships of polymers and polymer-based materials and composites in terms of macroscopic force application (stress) and subsequent deformation (strain) through a mechanophore-assisted coupling of mechanical and chemical phenomena. The perplexity of the process, however, from the viewpoint of mechanophore activation via a molecular-scaled disruption of the structure that yields a macroscopically detectable optical signal, renders this otherwise rapidly evolving field challenging. Motivated by this, we highlight here recent advancements of polymer mechanochemistry with particular focus on the establishment of methodologies for the efficient activation and quantification of mechanophores and anticipate to aptly pinpoint unresolved matters and limitations of the respective approaches, thus highlighting possible developments.

Mechanically Gated Degradable Polymers

Degradable polymers are desirable for the replacement of conventional organic polymers that persist in the environment, but they often suffer from the unintentional scission of the degradable functionalities on the polymer backbone, which diminishes polymer properties during storage and regular use. Herein, we report a strategy that combats unintended degradation in polymers by combining two common degradation stimuli-mechanical and acid triggers-in an "AND gate" fashion. A cyclobutane (CB) mechanophore is used as a mechanical gate to regulate an acid-sensitive ketal that has been widely employed in acid degradable polymers. This gated ketal is further incorporated into the polymer backbone. In the presence of an acid trigger alone, the pristine polymer retains its backbone integrity, and delivering high mechanical forces alone by ultrasonication degrades the polymer to an apparent limiting molecular weight of 28 kDa. The sequential treatment of ultrasonication followed by acid, however, leads to a further 11-fold decrease in molecular weight to 2.5 kDa. Experimental and computational evidence further indicate that the ungated ketal possesses mechanical strength that is commensurate with the conventional polymer backbones. Single molecule force spectroscopy (SMFS) reveals that the force necessary to activate the CB molecular gate on the time scale of 100 ms is approximately 2 nN.

Polymer mechanochemistry-enabled pericyclic reactions

Polymer mechanochemical pericyclic reactions are reviewed with regard to their structural features and substitution prerequisites to the polymer framework.

The Intrinsic Mechanochemical Reactivity of Vinyl-Addition Polynorbornene

Herein we report the discovery of the intrinsic mechanochemical reactivity of vinyl-addition polynorbornene (VA-PNB), which has strained bicyclic ring repeat units along the polymer backbone. VA-PNBs with three different side chains were found to undergo ring-opening olefination upon sonication in dilute solutions. The sonicated polymers exhibited spectroscopic signatures consistent with conversion of the bicyclic norbornane repeat units into the ring-open isomer typical of polynorbornene made by ring-opening metathesis polymerization (ROMP-PNB). Thermal analysis and evaluation of chain-scission kinetics suggest that sonication of VA-PNB results in chain segments containing a statistical mixture of vinyl-added and ROMP-type repeat units.

Stereochemical effects on the mechanochemical scission of furan–maleimide Diels–Alder adducts

An internal competition between mechanochemical reactions unveils the relative mechanical reactivity of furan–maleimide Diels–Alder (DA) stereoisomers.

Quantitative and Mechanistic Mechanochemistry in Ferrocene Dissociation

Ferrocene is classically regarded as being highly inert owing to the large dissociation energy of metal-cyclopentadienyl (Cp) bonds. We show that the Fe-Cp bond in ferrocene is the preferential site of mechanochemical scission in the pulsed ultrasonication of main-chain ferrocene-containing polybutadiene-derived polymers. Quantitative studies reveal that the Fe-Cp bond is similar in strength to the carbon-nitrogen bond of an azobisdialkylnitrile (bond dissociation energy < -0 kcal/mol), despite the significantly higher Fe-Cp bond dissociation energy (approximately 90 kcal/mol). Mechanistic studies are consistent with a predominately heterolytic mechanism of chain scission. DFT calculations provide insights into the origins of ferrocene's mechanical lability.

Empowering mechanochemistry with multi-mechanophore polymer architectures

Multi-mechanophore polymers provide advantages in characterization and function relative to chain-centered, single mechanophore polymers.

Experimental Polymer Mechanochemistry and its Interpretational Frameworks

Polymer mechanochemistry is an emerging field at the interface of chemistry, materials science, physics and engineering. It aims at understanding and exploiting unique reactivities of polymer chains confined to highly non-equilibrium stretched geometries by interactions with their surroundings. Macromolecular chains or their segments become stretched in bulk polymers under mechanical loads or when polymer solutions are sonicated or flow rapidly through abrupt contractions. An increasing amount of empirical data suggests that mechanochemical phenomena are widespread wherever polymers are used. In the past decade, empirical mechanochemistry has progressed enormously, from studying fragmentations of commodity polymers by simple backbone homolysis to demonstrations of self-strengthening and stress-reporting materials and mechanochemical cascades using purposefully designed monomers. This progress has not yet been matched by the development of conceptual frameworks within which to rationalize, systematize and generalize empirical mechanochemical observations. As a result, mechanistic and/or quantitative understanding of mechanochemical phenomena remains, with few exceptions, tentative. In this review we aim at systematizing reported macroscopic manifestations of polymer mechanochemistry, and critically assessing the interpretational framework that underlies their molecular rationalizations from a physical chemist's perspective. We propose a hierarchy of mechanochemical phenomena which may guide the development of multiscale models of mechanochemical reactivity to match the breadth and utility of the Eyring equation of chemical kinetics. We discuss the limitations of the approaches to quantifying and validating mechanochemical reactivity, with particular focus on sonicated polymer solutions, in order to identify outstanding questions that need to be solved for polymer mechanochemistry to become a rigorous, quantitative field. We conclude by proposing 7 problems whose solution may have a disproportionate impact on the development of polymer mechanochemistry.