We report that differences in ring strain enthalpy between cis and trans isomers of sila-cycloheptene provide a driving force for both polymerization and depolymerization via olefin metathesis. A need for new methods to reintroduce the low-strain isomer into the plastic economy inspired the development of a polymerization based on ring-opening/cross-metathesis step polymerization, which afforded perfect sequence control for an alternating copolymer. The chemical principles are a platform for achieving both efficient polymerization and depolymerization with high mass recovery in functional polymers.

In polymer mechanochemistry, mechanophores are specific molecular units within the macromolecular backbone that are particularly sensitive to tension. To facilitate understanding of this selective responsiveness, we introduce the restoring force triangle (RFT). The RFT is a mnemonic device intended to provide intuitive insight into how external tensile forces (i.e., stretching) can selectively activate scissile bonds, thereby initiating mechanically driven chemical reactions. The RFT utilizes two easily computable parameters: the effective bond stiffness constant, which measures a bond’s resistance to elongation, and the bond dissociation energy, which is the energy required to break a bond. These parameters help categorize reactivity into thermal and mechanical domains, providing a useful framework for developing new mechanophores that are responsive to force but thermally stable. The RFT helps chemists intuitively understand how tensile force contributes to the activation of a putative mechanophore, facilitating the development of mechanochemical reactions and mechano-responsive materials.

We established a strain learning mechanical metamaterial that can not only recover after plastic deformation but also become stronger and stiffer in response to the applied loads. We demonstrate that shape-memory protein–polymer networks can unravel to release their stored length and, after recovery, will reconfigure to accommodate higher loads. These materials are incorporated into lattice frameworks using additive manufacturing which are then shown to become up to 2.5× stiffer after being crushed to 80% strain and recovered. This strain learning behavior enables the creation of metamaterials that are “teachable” and can autonomously remodel in response to applied loads, similar to the processes that occur in natural materials.

Aggregation-induced emission luminogens (AIEgens) that respond to mechanical force are increasingly used as force probes, memory devices, and advanced security systems. Most of the known mechanisms to modulate mechanoresponsive AIEgens have been based on changes in aggregation states, involving only physical alterations. Instances that employ covalent bond cleavage are still rare. We have developed a novel mechanochemical uncaging strategy to unveil AIEgens with diverse emission characteristics using engineered norborn-2-en-7-one (NEO) mechanophores. These NEO mechanophores were covalently integrated into polymer molecules and activated in both the solution and solid states. This activation resulted in highly tunable fluorescence upon immobilization through solidification or aggregation, producing blue, green, yellow, and orange-red emissions. By designing the caged and uncaged forms as donor–acceptor pairs for Förster resonance energy transfer (FRET), we achieved multicolor mechanofluorescence, effectively broadening the color spectrum to include white emission. Additionally, we computationally explored the electronic structures of activated NEOs, providing insights into the observed regiochemical effects of the substituents. This understanding, together with the novel luminogenic characteristics of the caged and activated species, provides a highly tunable reporter that traces progress with continuous color evolution. This advancement paves the way for future applications of mechanoresponsive materials in areas like damage detection and bioimaging.

Thermoset toughness and deconstructability are often opposing features; simultaneously improving both without sacrificing other mechanical properties (e.g., stiffness and tensile strength) is difficult, but, if achieved, could enhance the usage lifetime and end-of-life options for these materials. Here, a strategy that addresses this challenge in the context of photopolymer resins commonly used for 3D printing of glassy, acrylic thermosets is introduced. It is shown that incorporating bis-acrylate “transferinkers,” which are cross-linkers capable of undergoing degenerative chain transfer and new strand growth, as additives (5–25 mol%) into homemade or commercially available photopolymer resins leads to photopolymer thermosets with substantially improved tensile toughness and triggered chemical deconstructability with minimal impacts on Young's moduli, tensile strengths, and glass transition temperatures. These properties result from a transferinker-driven topological transition in network structure from the densely cross-linked long, heterogeneous primary strands of traditional photopolymer networks to more uniform, star-like networks with few dangling ends; the latter structure more effectively bear stress yet is also more easily depercolated via solvolysis. Thus, transferinkers represent a simple and effective strategy for improving the mechanical properties of photopolymer thermosets and providing a mechanism for their triggered deconstructability.

In this work, we introduce a 3D-printable virus-like particle (VLP)-enhanced cross-linked biopolymer system. VLPs displaying surface-available acrylate groups were prepared through aza-Michael addition to serve as resins. The VLP resins were then photopolymerized into a poly(ethylene glycol) diacrylate (PEGDA) network following DLP 3D printing. This approach represents a convergence of disciplines, where the synergistic interaction between virology and additive manufacturing unlocks new frontiers in biotechnology.

The aza Paternò–Büchi reaction is a [2+2]-cycloaddition reaction between imines and alkenes that produces azetidines, four-membered nitrogen-containing heterocycles. Currently, successful examples rely primarily on either intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò–Büchi reactions, it is essential to extend the reaction to acyclic imine equivalents. Here, we report that matching of the frontier molecular orbital energies of alkenes with those of acyclic oximes enables visible light–mediated aza Paternò–Büchi reactions through triplet energy transfer catalysis. The utility of this reaction is further showcased in the synthesis of epi-penaresidin B. Density functional theory computations reveal that a competition between the desired [2+2]-cycloaddition and alkene dimerization determines the success of the reaction. Frontier orbital energy matching between the reactive components lowers transition-state energy (ΔGǂ) values and ultimately promotes reactivity.

Mechanical force drives distinct chemical reactions; yet, its vectoral nature results in complicated coupling with reaction trajectories. Here, we utilize a physical organic model inspired by the classical Morse potential and its differential forms to identify effective force constant (keff) and reaction energy (ΔE) as key molecular features that govern mechanochemical kinetics. Through a comprehensive experimental and computational investigation with four norborn-2-en-7-one (NEO) mechanophores, we establish the relationship between these features and the force-dependent energetic changes along the reaction pathways. We show that the complex kinetic behavior of the tensioned bonds is generally and quantitatively predicted by a simple multivariate linear regression based on the two easily computed features with a straightforward workflow. These results demonstrate a general mechanistic framework for mechanochemical reactions under tensile force and provide a highly accessible tool for the large-scale computational screening in the design of mechanophores.

Plant virus-like particles (VLPs) are biocompatible, non-infectious nanomaterials with promising applications as immunotherapeutics and vaccines. However, slow-release VLP formulations are needed to achieve long-term efficacy without repeated administration. VLP hydrogels allow the encapsulation and sustained delivery of VLPs, but the particles must covalently bind the hydrogel polymers to avoid premature loss. This has been achieved so far by in situ VLP polymerization, which requires high viral concentrations (5–10 mg/mL, 0.5–1 wt%) to form stable hybrid VLP–hydrogel networks and this complicates scalability and clinical translation. Here, we developed a novel swell-and-click method that led to successful VLP scaffold formation regardless of the viral load used. As a result, VLP-functionalized hydrogels were fabricated with viral concentrations as low as 0.1–1 mg/mL (0.01–0.1 % wt%) without compromising the scaffold stability on the process. The hydrogels incorporate VLPs during swelling, followed by copper-free click chemistry reactions that bind the particles covalently to the polymer. The swell-and-click method also resulted in more than a two-fold enhancement in VLP uptake into the hydrogels and it provides a means of combined burst release and prolonged sustained release, desired traits for cancer immunotherapy treatment. The present work introduces a novel methodology for the design of VLP-based hydrogels, which could facilitate the scalability of the fabrication process and move a significant step forward towards clinical translation of long-term VLP vaccination in cancer disease.

Gels made of telechelic polymers connected by reversible cross-linkers are a versatile design platform for biocompatible viscoelastic materials. Their linear response to a step strain displays a fast, near-exponential relaxation when using low-valence cross-linkers, while larger supramolecular cross-linkers bring about much slower dynamics involving a wide distribution of timescales whose physical origin is still debated. Here, we propose a model where the relaxation of polymer gels in the dilute regime originates from elementary events in which the bonds connecting two neighboring cross-linkers all disconnect. Larger cross-linkers allow for a greater average number of bonds connecting them but also generate more heterogeneity. We characterize the resulting distribution of relaxation timescales analytically and accurately reproduce stress relaxation measurements on metal-coordinated hydrogels with a variety of cross-linker sizes including ions, metal-organic cages, and nanoparticles. Our approach is simple enough to be extended to any cross-linker size and could thus be harnessed for the rational design of complex viscoelastic materials.

Supramolecular polymer networks contain non-covalent cross-links that enable access to broadly tunable mechanical properties and stimuli-responsive behaviors; the incorporation of multiple unique non-covalent cross-links within such materials further expands their mechanical responses and functionality. To date, however, the design of such materials has been accomplished through discrete combinations of distinct interaction types in series, limiting materials design logic. Here we introduce the concept of leveraging “nested” supramolecular crosslinks, wherein two distinct types of non-covalent interactions exist in parallel, to control bulk material functions. To demonstrate this concept, we use polymer-linked Pd2L4 metal–organic cage (polyMOC) gels that form hollow metal–organic cage junctions through metal–ligand coordination and can exhibit well-defined host-guest binding within their cavity. In these “nested” supramolecular network junctions, the thermodynamics of host-guest interactions within the junctions affect the metal–ligand interactions that form those junctions, ultimately translating to substantial guest-dependent changes in bulk material properties that could not be achieved in traditional supramolecular networks with multiple interactions in series.

Thioesters are an essential functional group in biosynthetic pathways, which has motivated their development as reactive handles in probes and peptide assembly. Thioester exchange is typically accelerated by catalysts or elevated pH. Here, we report the use of bifunctional aromatic thioesters as dynamic covalent cross-links in hydrogels, demonstrating that at physiologic pH in aqueous conditions, transthioesterification facilitates stress relaxation on the time scale of hundreds of seconds. We show that intramolecular hydrogen bonding is responsible for accelerated exchange, evident in both molecular kinetics and macromolecular stress relaxation. Drawing from concepts in the vitrimer literature, this system exemplifies how dynamic cross-links that exchange through an associative mechanism enable tunable stress relaxation without altering stiffness.

In recent decades, more than 100 different mechanophores with a broad range of activation forces have been developed. For various applications of mechanophores in polymer materials, it is crucial to selectively activate the mechanophores with high efficiency, avoiding nonspecific bond scission of the material. In this study, we embedded cyclobutane-based mechanophore cross-linkers (I and II) with varied activation forces (fa) in the first network of the double network hydrogels and quantitively investigated the activation selectivity and efficiency of these mechanophores. Our findings revealed that cross-linker I, with a lower activation force relative to the bonds in the polymer main chain (fa-I/fa-chain = 0.8 nN/3.4 nN), achieved efficient activation with 100% selectivity. Conversely, an increase of the activation force of mechanophore II (fa-II/fa-chain = 2.5 nN/3.4 nN) led to a significant decrease of its activation efficiency, accompanied by a substantial number of nonspecific bond scission events. Furthermore, with the coexistence of two cross-linkers, significantly different activation forces resulted in the almost complete suppression of the higher-force one (i.e., I and III, fa-I/fa-III = 0.8 nN/3.4 nN), while similar activation forces led to simultaneous activations with moderate efficiencies (i.e., I and IV, fa-I/fa-IV = 0.8 nN/1.6 nN). These findings provide insights into the prevention of nonspecific bond rupture during mechanophore activation and enhance our understanding of the damage mechanism within polymer networks when using mechanophores as detectors. Besides, it establishes a principle for combining different mechanophores to design multiple mechanoresponsive functional materials.

The advent of covalent adaptable networks (CANs) through the incorporation of dynamic covalent bonds has led to unprecedented properties of macromolecular systems, which can be engineered at the molecular level. Among the various types of stimuli that can be used to trigger chemical changes within polymer networks, light stands out for its remote and spatiotemporal control under ambient conditions. However, most examples of photoactive CANs need to be transparent and they exhibit slow response, side reactions, and limited light penetration. In this vein, it is interesting to understand how molecular engineering of optically active dynamic linkages that offer fast response to visible light can impart “living” characteristics to CANs, especially in opaque systems. Here, the use of carbazole-based thiuram disulfides (CTDs) that offer dual reactivity as photoactivated reshuffling linkages and iniferters under visible light irradiation is reported. The fast response to visible light activation of the CTDs leads to temporal control of shape manipulation, healing, and chain extension in the polymer networks, despite the lack of optical transparency. This strategy charts a promising avenue for manipulating multifunctional photoactivated CANs in a controlled manner.

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.

Second row elements in small- and medium-rings modulate strain. Herein we report the synthesis of two novel oligosilyl-containing cycloalkynes that exhibit angle-strain, as observed by X-ray crystallography. However, the angle-strained sila-cyclooctynes are sluggish participants in cycloadditions with benzyl azide. A distortion-interaction model analysis based on density functional theory calculations was performed.

Hydrogen fluoride (HF) is a versatile reagent for material transformation, with applications in self-immolative polymers, remodeled siloxanes, and degradable polymers. The responsive in situ generation of HF in materials therefore holds promise for new classes of adaptive material systems. Here, we report the mechanochemically coupled generation of HF from alkoxy-gem-difluorocyclopropane (gDFC) mechanophores derived from the addition of difluorocarbene to enol ethers. Production of HF involves an initial mechanochemically assisted rearrangement of gDFC mechanophore to α-fluoro allyl ether whose regiochemistry involves preferential migration of fluoride to the alkoxy-substituted carbon, and ab initio steered molecular dynamics simulations reproduce the observed selectivity and offer insights into the mechanism. When the alkoxy gDFC mechanophore is derived from poly(dihydrofuran), the α-fluoro allyl ether undergoes subsequent hydrolysis to generate 1 equiv of HF and cleave the polymer chain. The hydrolysis is accelerated via acid catalysis, leading to self-amplifying HF generation and concomitant polymer degradation. The mechanically generated HF can be used in combination with fluoride indicators to generate an optical response and to degrade polybutadiene with embedded HF-cleavable silyl ethers (11 mol %). The alkoxy-gDFC mechanophore thus provides a mechanically coupled mechanism of releasing HF for polymer remodeling pathways that complements previous thermally driven mechanisms.

Macrocyclic dimeric lactones have pharmacological activities that make them attractive synthetic targets, but typical synthetic strategies employ an iterative approach to construct the macrocycle. Herein, we report a visible-light-mediated approach that enables facile access to 1- and 2-azetine-based dimeric lactones of up to 30-membered ring macrocycles. These products form via four consecutive triplet energy transfers for 1-azetine dimeric products and two consecutive triplet energy transfers for 2-azetine dimeric products. Computational investigations provide insights into the mechanism of this reaction, consistent with an unexpected initial intermolecular [2 + 2]-cycloaddition being preferred under nonstandard Curtin–Hammett conditions over the corresponding intramolecular reaction, which ultimately enables an efficient reaction pathway for macrocyclic dimerization.

Flexible and lightweight sensors can assess their environment for a broad range of applications that include wearables for health monitoring and soft robotics. While 2D and 3D printing enables control over sensor design in multiple dimensions, customizability of a sensor toward different individual use cases is still limited because each sensor requires a new design and manufacturing process. Thus, there is a need for methodologies that produce modular sensor components that can be assembled and customized by an individual user. Herein, we demonstrate 3D printed, elastomeric ionogels comprising covalent adaptable networks (CANs) for modular sensor assemblies. Reversible Diels–Alder connections incorporated into the network can occur at the interface between two 3D printed objects in physical contact with each other. As a result, modular components can be combined and assembled on-demand into customized piezoionic sensors. Thermal curing of these modular blocks triggered the dynamic remodeling of the polymer networks that caused them to become fused together. Three different configurations (linear, cyclic, and box assemblies) were demonstrated to afford piezoionic sensors from the same set of 3D printed building blocks. This study highlights the benefits of dynamic covalent networks toward decentralized manufacturing, wherein a modular approach enables customization of 3D printed parts without the need for modifying the original design.

Embedding dynamic covalent bonds into polymer compositions transforms static thermosets into active materials with the reprocessability of thermoplastics and the bulk properties of cross-linked networks. This class of next-generation materials, called covalent adaptable networks, shows significant promise in composites, soft optoelectronics, and robotics. Herein, we synthesized two oligosiloxane-based epoxy networks that provide fast dynamic bond exchange. Oligosiloxane diepoxides were cured with stoichiometric amounts of 1,2-phenylenediacetic acid to generate epoxy acid networks with two dynamic covalent bonding mechanisms. The resulting polymer networks provided access to fast stress-relaxation times (1–10 min) at temperatures of only 130 °C with excellent reprocessability.

Many virus-like particles (VLPs) have good chemical, thermal, and mechanical stabilities compared to those of other biologics. However, their stability needs to be improved for the commercialization and use in translation of VLP-based materials. We developed an endoskeleton-armored strategy for enhancing VLP stability. Specifically, the VLPs of physalis mottle virus (PhMV) and Qβ were used to demonstrate this concept. We built an internal polymer “backbone” using a maleimide–PEG15–maleimide cross-linker to covalently interlink viral coat proteins inside the capsid cavity, while the native VLPs are held together by only noncovalent bonding between subunits. Endoskeleton-armored VLPs exhibited significantly improved thermal stability (95 °C for 15 min), increased resistance to denaturants (i.e., surfactants, pHs, chemical denaturants, and organic solvents), and enhanced mechanical performance. Single-molecule force spectroscopy demonstrated a 6-fold increase in rupture distance and a 1.9-fold increase in rupture force of endoskeleton-armored PhMV. Overall, this endoskeleton-armored strategy provides more opportunities for the development and applications of materials.

The term 'stimuli-responsive' (SR) refers to materials that undergo a meaningful change in properties (the response) when subjected to a change in external environment (the stimulus). The term is intrinsically redundant—a response by definition has a stimulus that triggered it, and the delivery of energy or matter is only a stimulus if it triggers a response. The broad, if linguistically uneconomical, use of 'stimuli-responsive' likely reflects a desire to emphasise the breadth available on either side of the stimulus–response relationship. Stimuli include various forms of energy (e.g. thermal, photons, electromagnetic waves and fields, or mechanical) and the introduction or removal of matter (e.g. solvents, reagents, acids/bases, salt, or electrons), and each can in principle be coupled to an ever-increasing range of responses (e.g. change in shape, assembly, colour, luminescence, mechanical properties, thermal transport, material transport, conduction, or uptake/release of cargo). The interest in, and study of, such materials has captured commercial and academic interest for decades. Excellent reviews of stimuli-responsive materials are available.

The fracture of polymer networks is tied to the molecular behavior of strands within the network, yet the specific molecular-level processes that determine the mechanical limits of a network remain elusive. Here, the question of reactivity-guided fracture is explored in otherwise indistinguishable end-linked networks by tuning the relative composition of strands with two different mechanochemical reactivities. Increasing the substitution of less mechanochemically reactive (“strong”) strands into a network comprising more reactive (“weak”) strands has a negligible impact on the fracture energy until the strong strand content reaches approximately 45%, at which point the fracture energy sharply increases with strong strand content. This aligns with the measured strong strand percolation threshold of 48 ± 3%, revealing that depercolation, or the loss of a percolated network structure, is a necessary criterion for crack propagation in a polymer network. Coarse-grained fracture simulations agree closely with the tearing energy trend observed experimentally, confirming that weak strand scissions dominate the failure until the strong strands approach percolation. The simulations further show that twice as many strands break in a mixture than in a pure network.

The formation of end-linked polymer networks is commonly modeled as idealized chemical reactions, resulting in defect-free networks. However, many widely used industrial processes including platinum-catalyzed vinyl-silane cross-linking of poly(dimethylsiloxane) (PDMS) are mechanistically complex and involve a variety of side reactions. Here, a kinetic graph theory (KGT) model was updated to account for off-stoichiometric reactive groups and side reactions by adding two fitting parameters representing the relative rate of competing side reactions and the probability of side cross-linking events. The updated KGT outputs the population of each junction type from which the reaction fates of both starting materials are calculated. The elastic effectiveness of the resulting network is calculated with the nonlinear Miller–Macosko theory (MMT), updated to account for side reactions and side cross-linking. The MMT was validated on off-stoichiometric data and was chosen here for its ability to account for a range of effective junction functionalities. Combined, the updated KGT and MMT provide elasticity estimates that capture the experimental peak in elastic modulus observed at an off-stoichiometric silane/alkene ratio in PDMS networks. Both the Lake Thomas and micronetwork fracture theories were subsequently used to estimate the tearing energy, showing a similar peak at off-stoichiometric ratios in qualitative agreement with experimental data. This model is useful in systems where the cross-linking chemistry yields more complex reaction networks, making it relevant to many classes of polymer network chemistry where classical theories may not adequately capture network behavior.

Gelation has long been conceptualized and modeled as a percolation process in which bond formation or destruction events are random. Percolation assumes that connections are created or destroyed randomly such that the critical point should occur at the same point when approached from either direction. Here, the gel point of an end-linked poly(ethylene glycol) gel was measured during forward (bond forming) and reverse (bond breaking) gelation and degelation processes to interrogate how the gel point scales with synthesis concentration, where decreased concentration leads to an increased prevalence of inelastic loops. Forward gel points, measured with combined kinetic nuclear magnetic resonance (NMR) and diffusing wave spectroscopy (DWS) experiments, were identical to results generated from a kinetic Monte Carlo (KMC) simulation, demonstrating the expected gel point suppression as the concentration decreased. Reverse gel points, measured with a selective degradation technique, were within the error of forward gel points at high concentrations but displayed a lesser degree of suppression as the concentration decreased. This deviation between forward and reverse gel points at low concentrations was qualitatively reproduced in the KMC simulation. These experiments and simulations show that forward and reverse gel points diverge as the gel system becomes more dilute, suggesting that kinetic effects cause a departure from the percolation behavior in defect-rich gels.

Polyethylene (PE) is the most widely produced synthetic polymer. By installing chemically cleavable bonds into the backbone of PE, it is possible to produce chemically deconstructable PE derivatives; to date, however, such designs have primarily relied on carbonyl- and olefin-related functional groups. Bifunctional silyl ethers (BSEs; SiR2(OR′2)) could expand the functional scope of PE mimics as they possess strong Si−O bonds and facile chemical tunability. Here, we report BSE-containing high-density polyethylene (HDPE)-like materials synthesized through a one-pot catalytic ring-opening metathesis polymerization (ROMP) and hydrogenation sequence. The crystallinity of these materials can be adjusted by varying the BSE concentration or the steric bulk of the Si-substituents, providing handles to control thermomechanical properties. Two methods for chemical recycling of HDPE mimics are introduced, including a circular approach that leverages acid-catalyzed Si−O bond exchange with 1-propanol. Additionally, despite the fact that the starting HDPE mimics were synthesized by chain-growth polymerization (ROMP), we show that it is possible to recover the molar mass and dispersity of recycled HDPE products using step-growth Si−O bond formation or exchange, generating high molecular weight recycled HDPE products with mechanical properties similar to commercial HDPE.

Polymers comprising polybutadiene backbones with 2,6-bis(1′-methyl-benzimidazolyl)pyridine (MeBip) sidechains were crosslinked by complexation with two different metal salts, either with copper(II) trifluoromethanosulfonate or with iron(II) trifluoromethanosulfonate. Dynamic mechanical analysis (DMA) and small-angle X-ray scattering (SAXS) data indicate that the crosslinking density and topology of the two materials are the same. The material crosslinked with copper ions, however, exhibits a higher extensibility and fracture energy than the polymer crosslinked with iron. These differences are attributed to differing mechanochemical responses of the metal complexes to applied stress. Computational results further indicate that the copper complexes are more labile, both in the stress-free state as well as upon application of force, and that the “open” complex in which only one MeBip ligand coordinates copper binds fewer counter-ions than the iron-coordinated analog. Both these factors enable easier re-binding of a second MeBip ligand. The computations further suggest that mechanochemically coupled spin-crossover behavior must be considered to fully understand the response of these metal-ligand complexes to mechanical stimuli. The data presented here furthers the facile manipulation of a material's strain response via metal species modulation, and the results offer a way to understand the relationship between bulk and molecular strain response.

Vitrimers are associative covalent adaptable networks that undergo reversible bond-exchange reactions while maintaining a fixed cross-linking density with changing temperature. To date, experimental studies that rely on macroscopic rheology have not been able to reveal topological changes and microscopic dynamics in these materials. Here, coarse-grained molecular dynamics simulations combined with a Monte Carlo method are implemented to investigate the topological structural changes, microscopic dynamics, and linear rheology of unentangled side-chain-linked vitrimers in conjunction with the sticky Rouse model (SRM). We find that there is a minor variation in the topological structure with temperature. The dynamic heterogeneities of the bond-exchange behavior and the system dynamics increase remarkably when approaching the topological freezing transition temperature Tv. Quantitative agreement between the simulation results and the SRM predictions is observed for the stress relaxation, elastic and loss moduli, and the relative mean-squared displacement, especially at the intermediate- and long-time or low-frequency regimes, where the time–temperature superposition principle is satisfied. We obtain a scaling collapse curve for the dynamic bond relaxation time, the zero-shear viscosity, and the horizontal shift factors without introducing any parameters, suggesting that the microscopic and macroscopic dynamics exhibit a similar relaxation behavior even in the presence of loop defects. Moreover, these results are in good agreement with those predicted by the SRM, indicating that the linear rheology of unentangled vitrimers with a fast bond-exchange rate can be analyzed via a single-chain approach based on the SRM.

Metal–organic cages/polyhedra (MOCs) are versatile building blocks for advanced polymer networks with properties that synergistically blend those of traditional polymers and crystalline frameworks. Nevertheless, constructing polyMOCs from very stable Pt(II)-based MOCs or mixtures of metal ions such as Pd(II) and Pt(II) has not, to our knowledge, been demonstrated, nor has exploration of how the dynamics of metal–ligand exchange at the MOC level may impact bulk polyMOC energy dissipation. Here, we introduce a new class of polymer metal–organic cage (polyMOC) gels featuring polyethylene glycol (PEG) strands of varied length cross-linked through bis-pyridyl-carbazole-based M6L12 cubes, where M is Pd(II), Pt(II), or mixtures thereof. We show that, while polyMOCs with varied Pd(II) content have similar network structures, their average stress–relaxation rates are tunable over 3 orders of magnitude due to differences in Pd(II)- and Pt(II)-ligand exchange rates at the M6L12 junction level. Moreover, mixed-metal polyMOCs display relaxation times indicative of intrajunction cooperative interactions, which stands in contrast to previous materials based on point metal junctions. Altogether, this work (1) introduces a novel MOC architecture for polyMOC design, (2) shows that polyMOCs can be prepared from mixtures of Pd(II)/Pt(II), and (3) demonstrates that polyMOCs display unique relaxation behavior due to their multivalent junctions, offering a strategy for controlling polyMOC properties independently of their polymer components.

Four unsaturated poly(carbooligosilane)s (P1–P4) were prepared via acyclic diene metathesis polycondensation of new oligosilane diene monomers (1–4). These novel polymers with varying main-chain Si incorporation have high trans internal olefin stereochemistry (ca. 80%) and molecular weights (9500–21,700 g mol–1). Postpolymerization epoxidation converted all alkene moieties to epoxides and rendered the polymers (P5–P8) more electrophilic, which allowed for single-molecule force spectroscopy studies via a modified atomic force microscope setup with a silicon tip and cantilever. The single-chain elasticity of the polycarbooligosilanes decreased with increasing numbers of Si–Si bonds, a finding reproduced by quantum chemical calculations.

While Si-containing polymers can often be deconstructed using chemical triggers such as fluoride, acids, and bases, they are resistant to cleavage by mild reagents such as biological nucleophiles, thus limiting their end-of-life options and potential environmental degradability. Here, using ring-opening metathesis polymerization, we synthesize terpolymers of (1) a “functional” monomer (e.g., a polyethylene glycol macromonomer or dicyclopentadiene); (2) a monomer containing an electrophilic pentafluorophenyl (PFP) substituent; and (3) a cleavable monomer based on a bifunctional silyl ether Image ID:d3sc02868b-t1.gif. Exposing these polymers to thiols under basic conditions triggers a cascade of nucleophilic aromatic substitution (SNAr) at the PFP groups, which liberates fluoride ions, followed by cleavage of the backbone Si–O bonds, inducing polymer backbone deconstruction. This method is shown to be effective for deconstruction of polyethylene glycol (PEG) based graft terpolymers in organic or aqueous conditions as well as polydicyclopentadiene (pDCPD) thermosets, significantly expanding upon the versatility of bifunctional silyl ether based functional polymers.

A mean-field equilibrium theory for reversible network formation due to heterotypic pairwise interactions in mixtures of associative polymers is extended via a weak inhomogeneity expansion to account for spatial fluctuations due to chemical incompatibility. We consider solutions and blends of polymers of types A and B with many associating groups per chain, and consider only A–B association between these groups. The structural correlations of the reversibly bonded polymers are accounted for by considering the Gaussian 4-arm star-like chain conformations between cross-links, which is analogous to an affine-network assumption.

An equilibrium statistical mechanical theory for the formation of reversible networks in two-component solutions of associative polymers is presented to account for the phase behavior due to hydrogen-bonding, metal–ligand, electrostatic, or other pairwise heterotypic associative interactions. We derive explicit analytical expressions for the binding statistics, gelation condition, and free energy, in which we consider polymers of types A and B with many associating groups per chain and consider only A–B association between the groups. The free energy is approximated at the mean-field level, considering overlapping polymer chains with an ideal gas of “stickers” capable of intermolecular association.

Extending polymer chains results in a positive chain tension, fch, primarily due to conformational restrictions. At the level of individual bonds, however, tension, fb, is either negative or positive and depends on both chain tension and bulk pressure. Typically, the chain and bond tension are assumed to be directly related. In specific systems, however, this dependence may not be intuitive, whereby fch increases while fb decreases; i.e., the entire chain is extended while bonds are compressed. Specifically, increasing the grafting density of a polymer brush results in chain extension along the direction perpendicular to the grafting surface while the underlying bonds are compressed. Similarly, upon compression of polymer networks, the extension of chains oriented in the “free” direction increases while their bonds are getting more compressed. We demonstrate this phenomenon in molecular dynamics simulations and explain it by the fact that the pressure contribution to fb is dominant over a wide range of network deformations and brush grafting densities.

The mechanical properties of covalent polymer networks often arise from the permanent end-linking or cross-linking of polymer strands, and molecular linkers that break more easily would likely produce materials that require less energy to tear. We report that cyclobutane-based mechanophore cross-linkers that break through force-triggered cycloreversion lead to networks that are up to nine times as tough as conventional analogs. The response is attributed to a combination of long, strong primary polymer strands and cross-linker scission forces that are approximately fivefold smaller than control cross-linkers at the same timescales. The enhanced toughness comes without the hysteresis associated with noncovalent cross-linking, and it is observed in two different acrylate elastomers, in fatigue as well as constant displacement rate tension, and in a gel as well as elastomers.

We develop a suite of hydrogel cross-links that employs an associative exchange mechanism, allowing stress relaxation to be tuned independently from stiffness. We present a structure-reactivity-property relationship that enables reactivity and stress relaxation timescales to be estimated in silico for new structures. This work, therefore, opens new avenues to design and control associative dynamic hydrogels with targeted properties, overcoming the limitations of previous systems that have largely relied on dissociative exchange mechanisms.

We investigate polyethylene glycol (PEG) coatings on tobacco mosaic virus (TMV), which was used as a model nanocarrier system, to evaluate the effects of linear and multivalent PEG coatings at varying chain lengths on serum protein adsorption, antibody recognition, and macrophage uptake.

The cis- and trans-isomers of a silacycloheptene were selectively synthesized by the alkylation of a silyl dianion, a novel approach to strained cycloalkenes. The trans-silacycloheptene (trans-SiCH) was significantly more strained than the cis isomer, as predicted by quantum chemical calculations and confirmed by crystallographic signatures of a twisted alkene. Each isomer exhibited distinct reactivity toward ring-opening metathesis polymerization (ROMP), where only trans-SiCH afforded high-molar-mass polymer under enthalpy-driven ROMP. Hypothesizing that the introduction of silicon might result in increased molecular compliance at large extensions, we compared poly(trans-SiCH) to organic polymers by single-molecule force spectroscopy (SMFS). Force-extension curves from SMFS showed that poly(trans-SiCH) is more easily overstretched than two carbon-based analogues, polycyclooctene and polybutadiene, with stretching constants that agree well with the results of computational simulations.

Here we address the question how the deformation and tension experienced by a strand is influenced by strand length through the use of mechanophore force probes with discrete molecular weights.

Here we report a polyelectrolyte handle for single-molecule force spectroscopy that offers a combination of high (several hundred pN) attachment forces, good (~4%) success in obtaining a high-force (>200 pN) attachment, a non-fouling detachment process that allows for repetition, and specific attachment locations along the polymer analyte.

We present a modified Lake–Thomas theory that accounts for the molecular details of network connectivity upon crack propagation in polymer networks.

We develop a single-chain model to account for the redistribution of monomers between network strands of a primary chain.

Small angle neutron scattering was used to measure single chain radii of gyration of end-linked polymer gels before and after cross-linking to calculate the prestrain, which is the ratio of the average chain size in a cross-linked network to that of a free chain in solution...

Here, we show that bifunctional silyl ether, i.e., R′O–SiR2–OR′′, (BSE)-based comonomers generate covalent adaptable network analogues of the industrial thermoset polydicyclopentadiene (pDCPD) through a novel BSE exchange process facilitated by the low-cost food-safe catalyst octanoic acid.

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...

High energy photons (λ < 400 nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge...

In this Perspective, we analyze structure–reactivity–property relationships for several classes of CANs, illustrating both general design principles and the predictive potential of linear free energy relationships (LFERs) applied to CANs. We discuss opportunities in the field to develop quantitative structure–reactivity–property relationships and open challenges.

Thermoset polymers and fiber-reinforced polymer composites possess the chemical, physical, and mechanical properties necessary for energy-efficient vehicles and structures, but their energy-inefficient...

Highly volatile and toxic bromine (Br2) molecules can be utilized safely in various chemical processes when coupled with efficient separation systems. Herein, we present two different N-containing porous organic cages (POCs), covalent cage 3-R (CC3-R) and formaldehyde tied-reduced covalent cage 3 (FT-RCC3), for vapor Br2 capture under ambient conditions. They show outstanding sorption capacities (11.02 mmol g−1 and 11.64 mmol g−1, respectively) compared with previously reported adsorbents. Reversibility of the Br2 sorption process has been elucidated experimentally and computationally by identifying bromine species adsorbed at POCs and calculating their binding energies. The strong charge-transfer interactions between adsorbed Br2 and abundant N atomic sites of the host cages led to the dominant formation of polybromide species (Br3− and Br5−). Further host–guest interaction between POCs and polybromides determined the reversibility of the Br2 sorption process—showing partially reversible (>70% recovery) behavior for CC3-R and irreversible (<10% recovery) behavior for FT-RCC3, both of which were affected by the chemical and structural nature of different POCs. DFT calculations further indicate that the formation of carbocationic species (Br3− and Br5−) and HBr is energetically favorable within the cage, which is in good agreement with the experimental results. This work demonstrates that strong host–guest interactions are essential for highly efficient Br2 capture and storage performance.

The linear rheological properties of supramolecular polymer networks formed by mixtures of two different bis-Pd(II) cross-linkers with poly(4-vinylpyridine) in dimethyl sulfoxide are examined. The changes...

As a machine-recognizable representation of polymer connectivity, BigSMILES line notation extends SMILES from deterministic to stochastic structures. The same framework that allows BigSMILES...

The immobilization of homogeneous catalysts onto supports to improve recyclability while maintaining catalytic efficiency is often a trial-and-error process limited by poor control of the local catalyst...

We have developed a scaling theory of the elasticity of swollen and deswollen polymer networks. The elasticity of unentangled networks is primarily due to cross-links, and the elasticity of entangled...

Cowpea mosaic virus (CPMV) is a potent immunogenic adjuvant and epitope display platform for the development of vaccines against cancers and infectious diseases, including coronavirus disease 2019...

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 utility and lifetime of materials made from polymer networks, including hydrogels, depend on their capacity to stretch and resist tearing. In gels and elastomers, those mechanical properties...

Polymer networks are complex systems consisting of molecular components. Whereas the properties of the individual components are typically well understood by most chemists, translating that chemical...

Mechanochemical reactions that lead to an increase in polymer contour length have the potential to serve as covalent synthetic mimics of the mechanical unfolding of noncovalent “stored length” domains...

The fracture of rubbery polymer networks involves a series of molecular events, beginning with conformational changes along the polymer backbone and culminating with a chain scission reaction. Here...

Polymers are stochastic materials that represent distributions of different molecules. In general, to quantify the distribution, polymer researchers rely on a series of chemical characterizations...

Vitrimers are a class of covalent adaptable networks (CANs) that undergo topology reconfiguration via associative exchange reactions, enabling reprocessing at elevated temperatures. Here, we show...

Photoresponsive materials that change in response to light have been studied for a range of applications. These materials are often metastable during irradiation, returning to their pre-irradiated...

Having a compact yet robust structurally based identifier or representation system is a key enabling factor for efficient sharing and dissemination of research results within the chemistry community...

Metal–organic framework nanoparticles (MOF NPs) have emerged as an important class of materials that display significantly enhanced performance in many applications compared to bulk MOF materials...

Polymer networks, which are materials composed of many smaller components—referred to as “junctions” and “strands”—connected together via covalent or non-covalent/supramolecular interactions...

We present a conceptual framework for adding molecular details of chain extension and force-coupled bond dissociation to the Lake–Thomas model of tear energy in rubbery crack propagation. Incorporating...