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

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.

Mechanochemical Backbone Editing for Controlled Degradation of Vinyl Polymers

The chemically inert nature of fully saturated hydrocarbon backbones endows vinyl polymers with desirable durability, but it also leads to their significant environmental persistence. Enhancing the sustainability of these materials requires a pivotal yet challenging shift: transforming the inert backbone into one that is degradable. Here, we present a versatile platform for mechanochemically editing the fully saturated backbone of vinyl polymers towards degradable polymer chains by integrating cyclobutene-fused succinimide (CBS) units along backbone through photo-iniferter reversible addition-fragmentation chain-transfer (RAFT) copolymerization. Significantly, the evenly insertion of CBS units does not compromise thermal or chemical stability but rather offers a means to adjust the properties of polymethylacrylate (PMA). Meanwhile, reactive acyclic imide units can be selectively introduced to the backbone through mechanochemical activation (pulse ultrasonication or ball-milling grinding) when required. Subsequent hydrolysis of the acyclic imide groups enables efficient degradation, yielding telechelic oligomers. This approach holds promise for inspiring the design and modification of more environmentally friendly vinyl polymers through backbone editing.

Mechanistic Studies of Stimulus-Response Integrated Catalysis of Single-Atom Alloys under Electric Fields for Electrochemical Nitrogen Reduction

The present work introduces a novel catalytic strategy to promote the nitrogen reduction reaction (NRR) by employing a cooperative Cu-based single-atom alloy (SAA) and oriented external electric fields (OEEFs) as catalysts. The field strength (F)-dependent reaction pathways are investigated by means of first-principles calculations. Different dipole-induced responses of intermediates to electric fields break the original scaling relationships and effectively tune not only the activity but also the product selectivity of the NRR. When the most active Os1Cu SAA is taken as an example, in the absence of an OEEF, the overpotential (η) of the NRR is 0.62 V, which is even larger than that of the competitive hydrogen evolution reaction (HER). A negative field not only reduces η but switches the preference to the NRR over the HER. In particular, η at F = -1.14 V/Å reaches the bottom of 0.18 V, which is 70% lower than that in the field-free state.

Mechanically Triggered Polymer Deconstruction through Mechanoacid Generation and Catalytic Enol Ether Hydrolysis

Polymers that amplify a transient external stimulus into changes in their morphology, physical state, or properties continue to be desirable targets for a range of applications. Here, we report a polymer comprising an acid-sensitive, hydrolytically unstable enol ether backbone onto which is embedded gem-dichlorocyclopropane (gDCC) mechanophores through a single postsynthetic modification. The gDCC mechanophore releases HCl in response to large forces of tension along the polymer backbone, and the acid subsequently catalyzes polymer deconstruction at the enol ether sites. Pulsed sonication of a 61 kDa PDHF with 77% gDCC on the backbone in THF with 100 mM H2O for 10 min triggers the subsequent degradation of the polymer to a final molecular weight of less than 3 kDa after 24 h of standing, whereas controls lacking either the gDCC or the enol ether reach final molecular weights of 38 and 27 kDa, respectively. The process of sonication, along with the presence of water and the existence of gDCC on the backbone, significantly accelerates the rate of polymer chain deconstruction. Both acid generation and the resulting triggered polymer deconstruction are translated to bulk, cross-linked polymer networks. Networks formed via thiol-ene cross-linking and subjected to unconstrained quasi-static uniaxial compression dissolve on time scales that are at least 3 times faster than controls where the mechanophore is not covalently coupled to the network. We anticipate that this concept can be extended to other acid-sensitive polymer networks for the stress-responsive deconstruction of gels and solvent-free elastomers.

Mechanoluminescent Materials Enable Mechanochemically Controlled Atom Transfer Radical Polymerization and Polymer Mechanotransduction

Organic mechanophores have been widely adopted for polymer mechanotransduction. However, most examples of polymer mechanotransduction inevitably experience macromolecular chain rupture, and few of them mimic mussel's mechanochemical regeneration, a mechanically mediated process from functional units to functional materials in a controlled manner. In this paper, inorganic mechanoluminescent (ML) materials composed of CaZnOS-ZnS-SrZnOS: Mn2+ were used as a mechanotransducer since it features both piezoelectricity and mechanolunimescence. The utilization of ML materials in polymerization enables both mechanochemically controlled radical polymerization and the synthesis of ML polymer composites. This procedure features a mechanochemically controlled manner for the design and synthesis of diverse mechanoresponsive polymer composites.

Monodisperse Cyclic Polymer Mechanochemistry: Scission Kinetics and the Dynamic Memory Effect with Ultrasonication and Ball-Mill Grinding

A series of monodisperse cyclic and linear poly(d,l-lactide)s (c-PLA and l-PLA, respectively) were prepared with various degrees of polymerization (DP) using an iterative convergent synthesis approach. The absence of a molecular weight distribution provided us a chance to study their mechanochemical reactivity without obstructions arising from the size distribution. Additionally, we prepared l- and c-PLAs with identical DPs, which enabled us to attribute differences in scission rates to the cyclic polymer architecture alone. The polymers were subjected to ultrasonication (US) and ball-mill grinding (BMG), and their degradation kinetics were explored. Up to 9.0 times larger scission rates were observed for l-PLA (compared to c-PLA) with US, but the difference was less than 1.9 times with BMG. Fragmentation requires two backbone scission events for c-PLA, and we were able to observe linear intermediates (formed after a single scission) for the first time. We also developed a new method of studying the dynamic memory effect in US by characterizing and comparing the daughter fragment molecular weight distributions of l- and c-PLAs. These results provide new insights into the influence of the cyclic polymer architecture on mechanochemical reactions as well as differences in reactivity observed with US and BMG.

Tribochemical nanolithography: selective mechanochemical removal of photocleavable nitrophenyl protecting groups with 23 nm resolution at speeds of up to 1 mm s-1

We describe the mechanochemical regulation of a reaction that would otherwise be considered to be photochemical, via a simple process that yields nm spatial resolution. An atomic force microscope (AFM) probe is used to remove photocleavable nitrophenyl protecting groups from alkylsilane films at loads too small for mechanical wear, thus enabling nanoscale differentiation of chemical reactivity. Feature sizes of 20-50 nm are achieved repeatably and controllably at writing rates up to 1 mm s^-1. Line widths vary monotonically with the load up to 2000 nN. To demonstrate the capacity for sophisticated surface functionalisation provided by this strategy, we show that functionalization of nanolines with nitrilo triacetic acid enables site-specific immobilization of histidine-tagged green fluorescent protein. Density functional theory (DFT) calculations reveal that the key energetic barrier in the photo-deprotection reaction of the nitrophenyl protecting group is excitation of a π-π* transition (3.1 eV) via an intramolecular charge-transfer mechanism. Under modest loading, compression of the adsorbate layer causes a decrease in the N-N separation, with the effect that this energy barrier can be reduced to as little as 1.2 eV. Thus, deprotection becomes possible via either absorption of visible photons or phononic excitation transfer, facilitating fast nanolithography with a very small feature size.

Rapid and Selective Photo-degradation of Polymers: Design of an Alternating Copolymer with an o-Nitrobenzyl Ether Pendant

The development of polymers with on-demand degradability is required to alleviate the current global issues on polymer-waste pollution. Therefore, we designed a vinyl ether monomer with an o-nitrobenzyl (oNBn) group as a photo-deprotectable pendant (oNBnVE) and synthesized an alternating copolymer with an oNBn-capped acetal backbone via cationic copolymerization with p-tolualdehyde (pMeBzA). The resultant alternating copolymer could be rapidly degraded into lower-molecular-weight compounds upon simple exposure to UV irradiation without any reactants or catalysts, while it was sufficiently stable toward heat and ambient light. This degradation proceeds via cleavage of the hemiacetal structure generated upon photo-deprotection of the oNBn pendant. The oNBn-peculiar degradability allowed the exclusive photo-degradation of the oNBnVE/pMeBzA segments in a diblock copolymer composed of oNBnVE/pMeBzA and benzyl vinyl ether (BnVE)/pMeBzA segments.

Mechanochemically accessing a challenging-to-synthesize depolymerizable polymer

Polymers with low ceiling temperatures (T_c) are highly desirable as they can depolymerize under mild conditions, but they typically suffer from demanding synthetic conditions and poor stability. We envision that this challenge can be addressed by developing high-T_c polymers that can be converted into low-T_c polymers on demand. Here, we demonstrate the mechanochemical generation of a low-T_c polymer, poly(2,5-dihydrofuran) (PDHF), from an unsaturated polyether that contains cyclobutane-fused THF in each repeat unit. Upon mechanically induced cycloreversion of cyclobutane, each repeat unit generates three repeat units of PDHF. The resulting PDHF completely depolymerizes into 2,5-dihydrofuran in the presence of a ruthenium catalyst. The mechanochemical generation of the otherwise difficult-to-synthesize PDHF highlights the power of polymer mechanochemistry in accessing elusive structures. The concept of mechanochemically regulating the T_c of polymers can be applied to develop next-generation sustainable plastics.

Practical Route for Catalytic Ring-Opening Metathesis Polymerization

Norbornene derivatives are typical monomers for ring-opening metathesis polymerization (ROMP) for synthesizing highly functional polymers. However, the lack of catalytic methods, that is, the lack of readily available chain transfer agents (CTAs) for these monomers has been a significant cost limitation when large-scale syntheses are required. Here, we report commercially available styrene and its derivatives as efficient regioselective CTAs for the catalytic synthesis of metathesis polymers requiring up to 1000 times less ruthenium than in classical ROMP experiments. The molecular weight of the synthesized polymers was controlled by the monomer-to-CTA ratio. Low molecular weight ROMP polymers known for their antimicrobial properties were also synthesized on a gram scale in this report. Polymers were characterized by SEC, ¹H NMR spectroscopy, and isotopically resolved MALDI-TOF MS. This approach describes a greener, more cost-effective, and eco-friendly methodology for the preparation of metathesis-based materials on the multigram scale.

CD14-positive extracellular vesicles in bronchoalveolar lavage fluid as a new biomarker of acute respiratory distress syndrome

Recent studies have indicated that extracellular vesicles (EVs) may play a role in the pathogenesis of acute respiratory distress syndrome (ARDS). EVs have been identified as potential biomarkers of disease severity and prognosis in other pulmonary diseases. We sought to characterize the EV phenotype within bronchoalveolar lavage (BAL) fluid of patients with ARDS, and to determine whether BAL EV could be used as a potential biomarker in ARDS. BAL was collected from patients with sepsis with and without ARDS, and from esophagectomy patients postoperatively (of whom a subset later developed ARDS during hospital admission). BAL EVs were characterized with regard to size, number, and cell of origin. Patients with sepsis-related ARDS had significantly higher numbers of CD14⁺/CD81⁺ monocyte-derived BAL EV than patients with sepsis without ARDS (P = 0.015). However, the converse was observed in esophagectomy patients who later developed ARDS (P = 0.003). Esophagectomy patients who developed ARDS also had elevated CD31⁺/CD63⁺ and CD31⁺/CD81⁺ endothelial-derived BAL EV (P ≤ 0.02) compared with esophagectomy patients who did not develop ARDS. Further studies are required to determine whether CD31⁺ BAL EV may be a predictive biomarker for ARDS in esophagectomy patients. CD14⁺/CD81⁺ BAL EV numbers were significantly higher in those patients with sepsis-related ARDS who died during the 30 days following intensive care unit admission (P = 0.027). Thus, CD14⁺/CD81⁺ BAL EVs are a potential biomarker for disease severity and mortality in sepsis-related ARDS. These findings provide the impetus to further elucidate the contribution of these EVs to ARDS pathogenesis.

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.

Metal-Catalyzed Switching Degradation of Vinyl Polymers via Introduction of an "In-Chain" Carbon-Halogen Bond as the Trigger

In this work, we achieved switching degradation of vinyl polymers made of a carbon-carbon bonded backbone. Crucial in this strategy was a small feed of methyl α-chloroacrylate (MCA) as the comonomer in radical polymerization of methyl methacrylate (MMA) so that the carbon-halogen bonds were introduced as the triggers for degradation. The "in-chain" trigger was activated by a one-electron redox metal catalyst as the chemical stimulus to generate the carbon-centered radical species, and subsequently, the neighboring carbon-carbon bond was cleaved via an electron transfer of the radical species giving the terminal olefin. Particularly, an iron complex (FeCl₂) in conjunction with tributylamine (n-Bu₃N) was effective as the chemical stimulus to allow the switching degradation, where the molecular weight was gradually decreased over time. The switching feature was confirmed by some control experiments.

Heterocyclic Mechanophores in Polymer Mechanochemistry

This Account covers the recent progress made on heterocyclic mechanophores in the field of polymer mechanochemistry. In particular, the types of such mechanophores as well as the mechanisms and applications of their force-induced structural transformations are discussed and related perspectives and future challenges proposed.

1 Introduction

2 Types of Mechanophores

3 Methods to Incorporate Heterocycle Mechanophores into Polymer Systems

4 Mechanochemical Reactions of Heterocyclic Mechanophores

4.1 Three-Membered-Ring Mechanophores

4.2 Four-Membered-Ring Mechanophores

4.3 Six-Membered-Ring Mechanophores

4.4 Bicyclic Mechanophores

5 Applications

5.1 Cross-Linking of Polymer

5.2 Degradable Polymer

5.3 Mechanochromic Polymer

6 Concluding Remarks and Outlook

Backbone-Photodegradable Polymers by Incorporating Acylsilane Monomers via Ring-Opening Metathesis Polymerization

Materials capable of degradation upon exposure to light hold promise in a diverse range of applications including biomedical devices and smart coatings. Despite the rapid access to macromolecules with diverse compositions and architectures enabled by ring-opening metathesis polymerization (ROMP), a general strategy to introduce facile photodegradability into these polymers is lacking. Here, we report copolymers synthesized via ROMP that can be degraded by cleaving the backbone in both solution and solid states under irradiation with a 52 W, 390 nm Kessil LED to generate heterotelechelic low-molecular-weight fragments. To the best of our knowledge, this work represents the first instance of the incorporation of acylsilanes into a polymer backbone. Mechanistic investigation of the degradation process supports the intermediacy of an α-siloxy carbene, formed via a 1,2-photo Brook rearrangement, which undergoes insertion into water followed by cleavage of the resulting hemiacetal.

Mechanical Force for the Transformation of Aziridine into Imine

Force-selective mechanochemical reactions may be important for applications in polymer mechanochemistry, yet it is difficult to achieve such reactions. This paper reports that cis-N-phthalimidoaziridine incorporated into a macromolecular backbone undergoes migration of N-phthalimido group to afford imine under mechanochemical condition and not thermal one. The imine is further hydrolyzed by water bifurcating into amine and aldehyde. These structural transformations are confirmed by ¹ H NMR and FT-IR spectroscopic analyses. Computational simulations are conducted for the aziridine mechanophore to propose the mechanism of reaction and define the substrate scope of reaction.

Synthesis of degradable and chemically recyclable polymers using 4,4-disubstituted five-membered cyclic ketene hemiacetal ester (CKHE) monomers

Novel degradable and chemically recyclable polymers were synthesized using five-membered cyclic ketene hemiacetal ester (CKHE) monomers. The studied monomers were 4,4-dimethyl-2-methylene-1,3-dioxolan-5-one (DMDL) and 5-methyl-2-methylene-5-phenyl-1,3-dioxolan-4-one (PhDL). The two monomers were synthesized in high yields (80-90%), which is an attractive feature. DMDL afforded its homopolymer with a relatively high molecular weight (M n >100 000, where M n is the number-average molecular weight). DMDL and PhDL were copolymerized with various families of vinyl monomers, i.e., methacrylates, acrylates, styrene, acrylonitrile, vinyl pyrrolidinone, and acrylamide, and various functional methacrylates and acrylate. Such a wide scope of the accessible polymers is highly useful for material design. The obtained homopolymers and random copolymers of DMDL degraded in basic conditions (in the presence of a hydroxide or an amine) at relatively mild temperatures (room temperature to 65 °C). The degradation of the DMDL homopolymer generated 2-hydroxyisobutyric acid (HIBA). The generated HIBA was recovered and used as an ingredient to re-synthesize DMDL monomer, and this monomer was further used to re-synthesize the DMDL polymer, demonstrating the chemical recycling of the DMDL polymer. Such degradability and chemical recyclability of the DMDL polymer may contribute to the circular materials economy.

Backbone-Degradable Polymers via Radical Copolymerizations of Pentafluorophenyl Methacrylate with Cyclic Ketene Acetal: Pendant Modification and Efficient Degradation by Alternating-Rich Sequence

This work deals with syntheses of backbone-degradable polymers via the radical copolymerization of pentafluorophenyl methacrylate (PFMA) with 5,6-benzo-2-methylene-1,3-dioxepane (BMDO), which undergoes ring-opening propagation to afford an ester-bonded backbone. The combination of the electron-deficient methacrylate with the electron-rich cyclic monomer allowed high crossover copolymerization, and the electronic effect was clarified by the comparison with the copolymerization of methyl methacrylate (MMA) and BMDO. The PFMA units of the resultant copolymer underwent quantitative alcoholysis or aminolysis transformation into methacrylate or methacrylamide units along with the pendant functionalization. The alternating-rich sequence was achieved by feeding an excess ratio of BMDO, which was supported by MALDI-TOF-MS of the copolymer obtained by the RAFT copolymerization. The methanolysis-transformed copolymer carrying MMA units was decomposed under basic condition, and the degradation efficiency was superior to that of the copolymer obtained via radical copolymerization of MMA with BMDO because of the alternating-rich sequence.

Understanding the Mechanochemistry of Ladder-Type Cyclobutane Mechanophores by Single Molecule Force Spectroscopy

We have recently reported a series of ladder-type cyclobutane mechanophores, polymers of which can transform from nonconjugated structures to conjugated structures and change many properties at once. These multicyclic mechanophores, namely, exo-ladderane/ene, endo-benzoladderene, and exo-bicyclohexene-peri-naphthalene, have different ring structures fused to the first cyclobutane, significantly different free energy changes for ring-opening, and different stereochemistry. To better understand their mechanochemistry, we used single molecule force spectroscopy (SMFS) to characterize their force-extension behavior and measure the threshold forces. The threshold forces correlate with the activation energy of the first bond, but not with the strain of the fused rings distal to the polymer main chain, suggesting that the activation of these ladder-type mechanophores occurs with similar early transition states, which is supported by force-modified potential energy surface calculations. We further determined the stereochemistry of the mechanically generated dienes and observed significant and variable contour length elongation for these mechanophores both experimentally and computationally. The fundamental understanding of ladder-type mechanophores will facilitate future design of multicyclic mechanophores with amplified force-response and their applications as mechanically responsive materials.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.