Catalyst selection enables direct ROMP incorporation of diazirines into polyolefin backbones while preserving their reactivity, providing efficient macromolecular crosslinkers for hydrocarbon polymers.

The influence of network topology on adhesive performance is reported by leveraging the controlled synthesis of polymers with precisely placed cross-linking sites. Copolymers based on n-butyl acrylate and acrylic acid were synthesized using reverse addition-fragmentation chain-transfer (RAFT) polymerization to define the placement of active esters at (1) both chain ends, (2) the center of the backbone, or (3) statistically distributed throughout. After displacement of the active esters with a norbornene-functionalized amine and UV-initiated cross-linking via thiol-ene click chemistry, the properties of each adhesive were compared by 180 degrees peel (adhesive) and lap shear (cohesive) tests. Localizing cross-link points at both chain ends or the center of the backbone improved adhesive properties significantly compared to the statistical copolymer. In summary, these insights suggest new strategies for designing adhesives with tunable and enhanced properties.

Throughout the world, increasing students' awareness of and preparedness for opportunities in STEM fields continues to be a challenge, with international efforts facing additional language and cultural barriers. To address this need, a new outreach experience was developed to stimulate interest in STEM through a passion shared across most of the world: soccer. The curriculum Materials Science of Soccer was developed by a multidisciplinary team at MIT and taught in collaboration with local Brazilian engineering undergraduate and graduate students to high school students in Brazil. Each day of camp consisted of a series of 15-minute lectures interspersed with a variety of active learning activities and laboratories to reinforce STEM-related skills. The active learning portions were cotaught by Brazilian-MIT undergraduate pairs. Original lectures were written for the class, translated, and taught in Portuguese. Feedback was collected daily and at the end of the course. The goals of this report are to (1) provide an overview of the camp structure and copies of the activity guides as a resource for future outreach programs; (2) discuss the benefits and challenges of a multi-tiered, cross-cultural learning model offered by programs like MIT Global Teaching Laboratory; and (3) analyze the impact and outcomes from the experience on each learning group (MIT students, Brazilian college students, and Brazilian high school students) from postcamp survey data.

Solid polymer electrolytes (SPEs) possess several advantages over liquid electrolytes, such as stability and nonflammability, that can enable next-generation batteries with improved performance. However, current SPEs suffer from sluggish Li+ transport and poor mechanical properties. Polymeric ionic liquids (PILs) have emerged as promising electrolyte materials due to their ability to dissociate high concentrations of Li salts. High segmental motion enables fast ion transport, but it has been challenging to find materials that are both rubbery and also have high salt dissolution. We find improved transport properties and rheological behavior for a PIL with a flexible siloxane backbone (designated as PMS-ImTFSI) in the salt-in-polymer (<50 wt % salt) regime. PMS-ImTFSI exhibits a long-lived rubbery plateau in shear rheology at salt loadings up to ca. 20 wt % salt that imparts it with greater elasticity. At the same time, PMS-ImTFSI enables high Li+ conductivity (up to 2 × 10-5 S/cm at 90 °C) due to beneficial ion-ion correlations. A transition from salt-in-polymer to polymer-in-salt regimes as seen consistently across rheology, NMR diffusometry, inverse Haven ratios, and X-ray scattering suggests that PILs with flexible, nonpolar backbones at low salt loading can form ion-rich domains that simultaneously exhibit high Li+ conductivity and robust mechanical properties. At high salt loading, the microstructure disappears, and the optimal properties are lost. These findings guide the design of advanced SPEs for next-generation batteries.

The rapid accumulation of plastic waste underscores the urgent need for effective recycling strategies, yet conventional approaches are hindered by the immiscibility of chemically dissimilar polymers, which phase-separate upon blending and yield poor material properties. This study demonstrates a versatile strategy for electrostatic compatibilization, utilizing acid-base proton transfer between minimally functionalized polymers. Waste-derived polystyrene (PS) was successfully modified with <4 mol % acid groups, while amorphous polybutadiene (PBD) was functionalized with <6 mol % diethylamino base groups and subsequently hydrogenated to yield semicrystalline polyethylene (PE) with the same functionalization level as the PBD. In both cases, blending with functionalized PS produced optically transparent, mechanically robust films. Notably, increasing charge density from 1.0 to 3.5 mol % significantly reduced domain sizes, indicating enhanced compatibilization, while increasing PS molecular weight from 28 to 470 kDa led to a three-order-of-magnitude increase in toughness. In PE/PS blends, the preservation of crystallinity during melt reprocessing was achieved by maintaining low functionalization levels, demonstrating compatibility without sacrificing critical material properties. These findings establish electrostatic compatibilization as a powerful, scalable platform for creating high-performance materials from chemically diverse and mixed plastic waste streams.

The synthesis and application of multifunctional diazirine-containing polymers for on-demand cross-linking of unfunctionalized commodity polymers through C-H bond insertion is demonstrated. While small-molecule diazirine cross-linkers have seen important applications such as plastic compatibilization and photopatterning, the high degree of functionalization of polymer-based diazirine cross-linkers offers promise for enhanced compatibility based on polymer blending and increased efficiency due to controllable multivalency. As a demonstrative example, unfunctionalized linear poly(n-butyl acrylate) (PnBA) can be cross-linked using various polymeric cross-linkers with diazirine contents as low as 0.8 wt % in 1 min under photochemical conditions. With gel fractions up to 95%, tunable rheological behavior is observed with increasing cross-linker loadings, consistent with a transition from entangled branched polymers to a cross-linked network. Moreover, the synthetic stability of the diazirine units can be exploited to prepare diazirine-containing polymers based on a variety of different backbones, from vinyl copolymers to poly(dimethylsiloxane) (PDMS), which allows successful photopatterning using a commercial 3D printer.

The design and functionality of polymeric materials hinge on failure resistance. While molecular-level details drive crack evolution in polymer networks, the connection between individual chain scission and bulk failure remains unclear and difficult to probe. In this work, we systematically study the fracture mechanics of polymer-like networks with hybrid bond strengths. We reveal that varying the ratio of strong and weak strands within otherwise identical networks gives a non-monotonic relationship between intrinsic fracture energy and strong strand fraction. Networks with some weak strands can counterintuitively outperform those with exclusively strong strands. Experiments on poly(ethylene glycol) gels and architected polymer-like lattices together with simulations unveil these properties. We show through computational visualization that strand type concentrations impact crack growth patterns and fracture energy trends. Cracks propagate through weak layers at low strong strand fractions. Aggregate clusters deflect or pin cracks at similar concentrations of strong and weak strands. Cracks blunt due to dispersed weak strand failure at high strong strand fractions. The sacrificial weak strands can notably deconcentrate stress near the crack tip, which toughens by delaying crack advancement. The interplay between concentration and clustering of strand types in networks with hybrid bond strengths, combined with crack growth phenomena and nonlocal energy release, provides insights into unusual fracture characteristics. Results shed light on fracture in polymer networks and percolated lattices.

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.

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.

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. The prestrain increased from 1.06 ± 0.01 to 1.16 ± 0.02 as gel synthesis concentration decreased near the overlap concentration, indicating that the chains are slightly more stretched in the network than in solution. Dilute gels with higher loop fractions were found to be spatially homogeneous. Form factor and volumetric scaling analyses independently confirmed that elastic strands stretch by 2-23% from Gaussian conformations to create a space-spanning network, with increased stretching as network synthesis concentration decreases. Prestrain measurements reported here serve as a point of reference for network theories that rely on this parameter for the calculation of mechanical properties.

Non-covalent BigSMILES enables the representation of donor/acceptor interactions and delocalized bonds for polymer assemblies.

Here, we report covalent polymer gels in which the macroscopic fracture “reaction” is controlled by mechanophores embedded within mechanically active network strands. We synthesized poly(ethylene glycol) (PEG) gels through the end-linking of azide-terminated tetra-arm PEG (M n = 5 kDa) with bis-alkyne linkers. Networks were formed under identical conditions, except that the bis-alkyne was varied to include either a cis -diaryl ( 1 ) or cis -dialkyl ( 2 ) linked cyclobutane mechanophore that acts as a mechanochemical “weak link” through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane ( 3 ) was also synthesized. The networks show the same linear elasticity (G' = 23~24 kPa, 0.1 – 100 Hz) and equilibrium mass swelling ratios (Q = 10~11 in tetrahydrofuran), but they exhibit tearing energies that span a factor of 8 (3.4 J∙m -2 , 10.5 J∙m -2 , and 27.1 J∙m -2 for networks with 1 , 2 , and 3 , respectively). The difference in fracture energy is well aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of 1 as a key determinant of the relative ease with which its network is torn. The connection between macroscopic fracture and small molecule reaction mechanism suggests opportunities for molecular understanding and optimization of polymer network behavior.

The Flory-Rehner and Bray-Merrill swelling theories are venerable theories for calculating the swelling of polymer networks and are widely applied across polymer materials. Here, these theories are revised to include cyclic topological defects present in polymer networks by using a modified phantom network model. These closed-form equations assume defect contributions to the swelling elasticity to be linear and additive and allow different assumptions regarding prestrain of larger loops to be incorporated. To compare to the theories, swelling experiments are performed on end-linked poly(ethylene glycol) gels in which the topological defects (primary and secondary loops) have been previously measured. Gels with higher loop densities exhibit higher swelling ratios. An equation is derived to compare swelling models independent of knowledge of the Flory-Huggins χ parameter, showing that the revised swelling models for loop defects are more accurate than both the phantom network model that neglects loops and the Bray-Merrill equation.

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 in structural proteins. Here we report the force-dependent kinetics of stored length release in a family of covalent domain polymers based on cis-1,2-substituted cyclobutane mechanophores. The stored length is determined by the size (n) of a fused ring in an [n.2.0] bicyclic architecture, and it can be made sufficiently large (>3 nm per event) that individual unravelling events are resolved in both constant-velocity and constant-force single-molecule force spectroscopy (SMFS) experiments. Replacing a methylene in the pulling attachment with a phenyl group drops the force necessary to achieve rate constants of 1 s(-1) from ca. 1970 pN (dialkyl handles) to 630 pN (diaryl handles), and the substituent effect is attributed to a combination of electronic stabilization and mechanical leverage effects. In contrast, the kinetics are negligibly perturbed by changes in the amount of stored length. The independent control of unravelling force and extension holds promise as a probe of molecular behavior in polymer networks and for optimizing the behaviors of materials made from covalent domain polymers.

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 that each reveal partial information on the distribution. However, in practice, the exact set of characterizations that are carried out, as well as how the characterization data are aggregated and reported, is largely nonstandard across the polymer community. This scenario makes polymer characterization data highly disparate, thereby significantly slowing down the development of polymer informatics. In this work, a proposal on how structural characterization data can be organized is presented. To ensure that the system can apply universally across the entire polymer community, the proposed schema, PolyDAT, is designed to embody a minimal congruent set of vocabulary that is common across different domains. Unlike most chemical schemas, where only data pertinent to the species of interest are included, PolyDAT deploys a multi-species reaction network construct, in which every characterization on relevant species is collected to provide the most comprehensive profile on the polymer species of interest. Instead of maintaining a comprehensive list of available characterization techniques, PolyDAT provides a handful of generic templates, which align closely with experimental conventions and cover most types of common characterization techniques. This allows flexibility for the development and inclusion of new measurement methods. By providing a standard format to digitalize data, PolyDAT serves not only as an extension to BigSMILES that provides the necessary quantitative information but also as a standard channel for researchers to share polymer characterization data.

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 insight into polymer networks themselves is limited by the statistical and poorly defined nature of network structures. As a result, it is challenging, if not currently impossible, to extrapolate from the molecular behavior of components to the full range of performance and properties of the entire polymer network. Polymer networks therefore present an unrealized, important, and interdisciplinary opportunity to exert molecular-level, chemical control on material macroscopic properties. A barrier to sophisticated molecular approaches to polymer networks is that the techniques for characterizing the molecular structure of networks are often unfamiliar to many scientists. Here, we present a critical overview of the current characterization techniques available to understand the relation between the molecular properties and the resulting performance and behavior of polymer networks, in the absence of added fillers. We highlight the methods available to characterize the chemistry and molecular-level properties of individual polymer strands and junctions, the gelation process by which strands form networks, the structure of the resulting network, and the dynamics and mechanics of the final material. The purpose is not to serve as a detailed manual for conducting these measurements but rather to unify the underlying principles, point out remaining challenges, and provide a concise overview by which chemists can plan characterization strategies that suit their research objectives. Because polymer networks cannot often be sufficiently characterized with a single method, strategic combinations of multiple techniques are typically required for their molecular characterization.

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 are often limited by the covalent chemical structure of the polymer strands between cross-links, which is typically fixed during the material synthesis. Here, we report polymer networks in which the constituent strands lengthen through force-coupled reactions that are triggered as the strands reach their nominal breaking point. Reactive strand extensions of up to 40% lead to hydrogels that stretch 40-50% further than, and exhibit tear energies twice that of, networks made from analogous control strands. The enhancements are synergistic with those provided by double network architectures, and complement other existing toughening strategies.

The failure properties of a polymer network, including toughness, ultimate strain, and ultimate stress, are some of the most critical properties for network performance. The polymer networks often contain various topological defects, such as primary loops and dangling ends, which have a noticeable effect on these properties. This work focuses on understanding the effect of these defects on the fracture strength of a material by expanding the classical Lake-Thomas theory to account for such defects under the assumption that each defect is unaffected by the presence of other defects in its environment. A Flory-Stockmayer gel point criterion is combined with the improved theory to identify the incipience of fracture. The predictions demonstrate that although the presence of defects weakens the material by reducing the tearing energy, the overall network elongation depends strongly on the primary loop fraction. Specifically, a transition from a low ultimate-strain to a significantly high ultimate-strain behavior is predicted. The addition of a kinetic theory for bond scission predicts that the sharpness of this transition is a strong function of the strain rate. To experimentally test these predictions, a series of poly(ethylene glycol) (PEG) gels with previously characterized primary loop fractions were synthesized. Remarkably, the measured tearing energies agree quite well with the theoretical predictions and also suggest the onset of the low to high extensibility transition.