Engineering Supramolecular Hydrogen Bonding Interactions into Dynamic Covalent Polymers To Obtain Double Dynamic Biomaterials

Inspired by dynamic systems in nature, we can introduce dynamics into synthetic biomaterials through dynamic covalent bonds or supramolecular interactions. Combining both types of dynamic interactions may allow for advanced and innovative networks with multiple levels of dynamicity. Here we present two types of solid materials consisting of either dynamic covalent imine bonds or a combination of these dynamic covalent bonds with supramolecular hydrogen bonding ureido-pyrimidinone (UPy) units to obtain double dynamic materials. We showed the facile synthesis and formulation of both materials at room temperature. The thermal and physical properties of each material are highly tunable by altering the ratio and type of cross-linker. Interestingly, we showed that minimal amounts of UPy units result in a drastic increase in material mechanics. Furthermore, we show that both types of materials are suitable as biomaterials through functionalization with cell-adhesive peptides, through either a dynamic covalent imine bond or a supramolecular UPy moiety.

'Vitrimer nanocomposites' derived from graphene oxide and post-consumer recycled polypropylene

Post-consumer recycled polypropylene (PCR PP) is promising for sustainable applications, yet its limitations in electrical conductivity and mechanical properties require modifications. This study develops a vitrimer nanocomposite by modifying PCR PP via styrene-assisted maleic anhydride grafting and incorporating a molecule containing multiple epoxide groups facilitating effective crosslinking. Graphene oxide (GO) is added as a nanofiller, improving rheological, thermal, electrical and infrared thermal properties. Characterization techniques confirm structural enhancements, while tensile testing shows significant gains in strength and modulus. The vitrimer nanocomposite demonstrates recyclability and high performance, offering a sustainable path for advanced engineering applications within a circular economy framework.

Toward Circular Polymer Materials and Manufacturing: Dynamic Bonding Strategies for Upcycling Thermoplastics and Thermosets

The global production of plastics has reached unprecedented levels, with <10% being recycled and even fewer recycled more than once. This lack of circularity poses critical environmental threats. However, upcycling-recycling materials while improving their properties and functionality-through dynamic bonding strategies offers a promising approach to enhancing polymer sustainability. Dynamic bonds enable polymeric structures to reconfigure under specific conditions, improving thermal, chemical, and mechanical resilience and controllability while facilitating recyclability. This review specifically takes the viewpoint of upcycling existing thermoplastics and thermosets to develop sustainable dynamic covalent networks (DCNs). Integrating these DCN upcycling strategies into the design of additive manufacturing (AM) feedstocks creates unique benefits compared to traditional polymer systems. This approach is briefly highlighted in extrusion-based and light-based AM, assessing the potential for improved material processability, recyclability, and the creation of high-value customized products. The combination of upcycling technologies and AM techniques presents a significant opportunity to advance sustainability in macromolecular science.

A simulation-based comparative study on the reaction-controlled terminal relaxation of associative and dissociative CANs using a mesoscopic coarse-grained single-chain model

Covalent adaptable networks (CANs) are polymer networks that engage in chemical reactions. Their dynamic covalent linkages permit topology fluctuations, making them processable. Here, we demonstrate the reaction-controlled terminal relaxation of unentangled CANs by using a mesoscopic coarse-grained single-chain model based on Gaussian strands. The association dynamics is incorporated to reproduce the features of reversible or bond-exchange reactions in CANs. With this model, the dependence of terminal relaxation on cross-ink density [i.e., the number of associated stickers (Nas) for this model] is comparatively studied for dissociative and associative CANs, in terms of stress-relaxation behavior, plateau modulus, as well as terminal relaxation times. Both dissociative and associative model CANs exhibit plateau moduli and exponential terminal relaxations. Their slow and fast relaxation modes are of different Nas dependences, inducing the stress-relaxation curves to undergo a change in shape with Nas. The temperature dependence of terminal relaxation is also examined for both model CANs by considering the kinetics of intrinsic reaction and segmental motion. The engagement of segmental motion forces the horizontal shift factor of time-temperature superposition (TTS) to depart from the Arrhenius-like equation. For dissociative model CANs, the shape of stress-relaxation curve changes with temperature, causing the TTS principle not to hold.

Repurposing Post-Consumer Polyethylene to Access Cross-Linked Polyethylene with Reprocessability, Recyclability, and Tunable Properties

Polyethylene (PE) is the most widely produced plastic but accumulation and resistance to degradation has significantly contributed to the plastic waste crisis. Upcycling has presented promising solutions to transform PE waste into value-added products. In this study, mixed post-consumer PE was successfully repurposed into reprocessable and chemically recyclable cross-linked polyethylene (XLPE). This process involved converting PE into telechelic oligomers, followed by repolymerization using a hybrid cross-linking system consisting of a dynamic cross-linker 2,4,6-triethoxy-1,3,5-triazine (TETA) and non-dynamic cross-linker tris(6-isocyanatohexyl)isocyanurate (Tri-HDI). In the resulting XLPE, TETA facilitated iterative reprocessing with minimal property degradation across cycles, whereas Tri-HDI helped preserve functional performance throughout service life. Compared to PE, XLPE exhibited enhanced mechanical properties, reduced creep deformation under application-relevant temperatures, and high temperature structural stability. Notably, copolymerizing PE oligomers with commercial macrodiols was employed to create composite XLPEs, enabling tuning material properties. After use, XLPE can be efficiently and selectively depolymerized under mild conditions, even when mixed with commercial insulator cables. This depolymerization allows for the recovery of the constituent building blocks, enabling purification and subsequent repolymerization for reuse. This approach demonstrates the potential of repurposing plastic waste into sustainable materials and fostering the development of a circular economy.

Siloxane-Mediated Schiff Base Bio-Based Curing Agent: Achieving Epoxy Vitrimer with Excellent Mechanical Properties, Low Dielectric Constant and Rapid Degradation Characteristics

Epoxy resin is indispensable in various applications due to its outstanding properties. However, its limited recyclability and associated environmental issues pose significant challenges for sustainable development. To address this issue, integrating recyclable Schiff base groups into epoxy resin systems to construct epoxy vitrimer with dynamic properties has become a promising strategy. Herein, a rapid degradation, enhanced mechanical properties, and low dielectric constant epoxy vitrimer (EP-BOB) is proposed through a unique rigid-flexible structure bio-based curing agent (BOB). BOB is synthesized using siloxane as a flexible chain to bridge with vanillin in a one-pot process. The incorporation of the Schiff base structure imparted exceptional degradability to EP-BOB, allowing it to fully degrade within 45 min. In addition, due to the unique rigid-flexible structure, EP-BOB exhibited lower dielectric constant (1.2-2.6) and outstanding mechanical properties (60.5 MPa tensile strength). Furthermore, Raman spectroscopy and scanning electron microscopy shows that EP-BOB can be completely degraded in the amine solution to recycle carbon fibers (CFs) without damage. Especially, the Schiff base can endow EP-BOB UV-shielding and antibacterial properties. This work opens up a new strategy for designing a rigid-flexible structure epoxy vitrimer using silicone to achieve multifunctional and high-performance EP.

Achieving High-Strength Polymer Adhesion Through Bond Exchange at the Interphase

Bond-exchangeable cross-linked materials, including covalent adaptable networks and vitrimers, exhibit numerous advantageous properties such as reprocessability, recyclability, and healability. These features arise from the relaxation and diffusion of network polymers facilitated by bond exchange within the network. The application of these materials in functional adhesives is particularly promising, given the growing demand across various industries. It is well established that vitrimer films can adhere to a wide range of substrates. In this study, a novel concept of bond exchange-based adhesion between different polymers is introduced, specifically noting that each polymer does not inherently possess bond-exchange capabilities. The key feature lies in activating bond exchange exclusively at the interphase. Significant adhesion between commercial thermoplastic polyurethanes and cross-linked poly(acrylate)s with hydroxy side groups randomly is demonstrated, achieved through transcarbomoylation bond exchange at the contact interphase. The incorporation of a small amount of bond exchange catalyst is crucial for enhancing adhesion, and both adhesion strength and fracture behavior can be manipulated through specific heating conditions. Overall, this study explores a new functionalization approach using the bond exchange concept, contributing to the development of a practical adhesion technique that eliminates the need for traditional adhesives.

Photo-Tunable Elastomers Enabling Reversible, Broad-Range Modulation of Mechanical Properties Via Dynamic Covalent Crosslinkers

Modulating the mechanical properties of soft materials with light is essential for achieving customizable functionalities. However, existing photo-responsive materials suffer from limited mechanical performance and a restricted tunable range. Here, a photo-tunable elastomer is developed by incorporating a urethane acrylate network with selenosulfide-based dynamic covalent crosslinkers, achieving high tensile strength exceeding 1.2 MPa in their stiff state and variable Young's modulus within a 0.8 MPa range. These crosslinkers undergo selenosulfide photo-metathesis, gradually breaking under ultraviolet light and reforming under visible light, enabling fine control over the modulus, strength, and stretchability of the elastomer. In terms of controllability, the design supports multiple tunable states, which allow for the use of intermediate mechanical properties. Moreover, by modeling the crosslinking density changes with reaction kinetics, modulus variation is predicted as a function of light exposure time. The light-induced modulation facilitates localized mechanical property adjustments, generating transformable multi-material structures and enhancing fracture resistance. Integrating these crosslinkers into different polymer networks provides a strategy for creating various photo-tunable elastomers and gels.

Eliminating creep in vitrimers using temperature-resilient siloxane exchange chemistry and N-heterocyclic carbenes

This study explores a novel N-heterocyclic carbene-mediated siloxane exchange mechanism, laying the foundation for designing covalent adaptable networks (CANs) with high temperature stability (>200 °C) for dynamic covalent chemistry. Small molecule siloxane compounds, obtained by hydrosilylation reactions, are used to demonstrate siloxane-exchange via a mechanism supported by density functional theory. The proposed mechanism presents an equilibrium, at elevated temperatures, between an imidazolium salt and its free carbene form, which is the catalytically active species. Following this mechanistic insight, a tetra-substituted ester-terminated siloxane cross-linker was synthesized and cured with a commercial amine hardener. The ensuing ester-amine reaction yields thermally stable, non-dynamic amide bonds, thereby enhancing material stability. The resulting CANs exhibit rapid stress relaxation at elevated temperatures and demonstrate successful recycling through compression molding without any significant loss of material properties. Remarkably, the synthesized material showcases high creep resistance, even up to 150 °C, indicating the benefits of having a thermally reversible catalyst system for siloxane activation. This ground-up design of dynamic chemistry and material synthesis not only presents innovative material design but also suggests avenues for exploring thermally stable, fast-exchanging and yet creep-resistant CANs.

Heterocycle-based dynamic covalent chemistry for dynamic functional materials

Dynamic covalent chemistry, which renders reusable and degradable thermoset polymers, is a promising tool for solving the global problem of plastic pollution. Although dynamic covalent chemistry can construct dynamic polymer networks, it rarely introduces other functions into polymers, which limits the development of dynamic functional materials. Herein, we develop heterocycle-based dynamic covalent chemistry and demonstrate the reversibility of the aza-Michael addition reaction between functional heterocycle dihydropyrimidin-2(1H)-thione and electron-deficient olefins. Our method produces a degradable linear polymer and recyclable and self-healable crosslinked polymers similar to traditional dynamic covalent chemistry, but the heterocycles endow the polymer with excellent ultraviolet-blocking and high-energy blue light-blocking abilities, and tunable fluorescence and phosphorescence properties. These are difficult to create with ordinary dynamic covalent chemistry. This proof-of-concept study provides insights into heterocycle-based dynamic reactions, and may prompt the development of dynamic chemistry and dynamic functional materials.

Multi-Generation Recycling of Thermosets Enabled by Fragment Reactivation

Thermosets are used in numerous industrial applications due to their excellent stabilities and mechanical properties; however, their covalently cross-linked structures limit chemical circularity. Cleavable comonomers (CCs) offer a practical strategy to impart new end-of-life opportunities, such as deconstructability or remoldability, to thermosets without altering critical properties, cost, or manufacturing workflows. Nevertheless, CC-enabled recycling of thermosets has so far been limited to one cycle with a 25% recycled content. Here, we introduce a "fragment reactivation" strategy, wherein the oligomeric fragments obtained from CC-enabled thermoset deconstruction are activated with functional groups that improve fragment solubility and reactivity for subsequent rounds of recycling. Using polydicyclopentadiene (pDCPD), an industrial hydrocarbon thermoset material, containing low loadings of a siloxane-based CC, we first demonstrate two rounds of chemical recycling by incorporating 40 wt % norbornene silyl ether-reactivated fragments derived from the prior generation's deconstruction. Then, we show that the two-step sequence of deconstruction and reactivation can be unified into a single-step process, referred to as "deconstructive reactivation." Using this approach, we demonstrate three rounds of chemical recycling with 40-45 wt % fragments incorporated per cycle while maintaining key material properties and deconstructability. These three generations of recycling effectively extend the lifespan of deconstructable pDCPD thermosets by ∼2.6 times. Combined with CCs, fragment reactivation presents a promising and potentially generalizable strategy to improve the chemical recycling efficiency of thermosets.

Extrudable Thermosets Based on Dynamic Covalent Polymers

Dynamic covalent polymers (DCPs) have attracted considerable attention for fabricating self-healing, recyclable, adaptive, and malleable thermosets. However, their processing has largely depended on compression molding, which requires extended time due to the crosslinked nature of these materials. This limitation poses significant challenges for applications in continuous processes like extrusion, a technique highly valued in industry for producing long, uninterrupted products. Recently, continuous processing methods for DCPs have emerged, offering the potential to streamline their production by providing scalability and consistency-factors that could transform sectors such as automotive, aerospace, and consumer goods. Yet, developing extrudable DCPs remains a complex task, demanding the precise design of dynamic crosslinks with rapid kinetics, high thermal stability, and cost-effectiveness, all while ensuring compatibility with suitable molecular backbones. In this review, we summarize the key requirements for designing extrudable DCPs and discuss recent advancements in continuous processing techniques aligned with various design strategies. We also highlight the challenges and outline future directions for optimizing high-performance DCP extrusion, aiming to offer a valuable resource for researchers and industry professionals looking to leverage these techniques.

Recyclable Dynamic Covalent Networks Derived from Isocyanate Chemistry: The Critical Role of Electronic and Steric Effects in Reversibility

The dynamic covalent networks (DCNs), featuring dynamic covalent bonds (DCBs) formed through isocyanate-involved chemistry, potentially contributes to a circular economy in polyurea and polyurethane industries, due to the inherent recyclability of DCNs. Over the past decade, remarkable progress has been made in the development of isocyanate-derived DCBs (IdDCBs) for the synthesis of recyclable DCNs, aiming to substitute conventional, non-recyclable materials. Herein, the fundamental aspect of the IdDCB-related chemistries reported to date is investigated, and it is found that their reversibility is governed by electronic and steric effects. This discovery encourages us to structure the review into three sections. The first section examines the reversibility of various IdDCBs through the lens of electronic and steric influences. The findings show that the reversibility of some IdDCBs is driven by a single chemical effect, with the examples of steric effect contributing to the dynamic behavior of thiourethanes and hindered ureas, while other cases of reversibility arise from a combination of two or more chemical effects. The knowledge thus established allows to categorize and discuss the technologically relevant DCNs, with particular emphasis on how these chemical effects influence their recyclability. Finally, the review concludes by highlighting several potentially impactful research directions that merit further exploration.

Benzyl ether: a dynamic covalent motif for designing a trans-ether based covalent adaptable network (CAN)

This report introduces benzyl ether-based trans-etherification as a new robust chemistry for designing CANs. The dynamic ether exchange and its dissociative nature are demonstrated using small molecule model studies. Along with reprocessability, the CAN also exhibits efficient stress relaxation. This study expands the library of chemistries for the development of CAN.

Adding Value into Elementary Sulfur for Sustainable Materials

Sulfur-rich copolymers, characterized by high sulfur contents and dynamic disulfide bonds, show significant promise as sustainable alternatives to conventional carbon-based plastics. Since the advent of inverse vulcanization in 2013, numerous synthesis strategies have emerged - ranging from thermopolymerization and photoinduced polymerization to the use of crosslinkers such as mercaptans, episulfides, benzoxazines, and cyclic disulfides. These advancements coupled with the rising demand for degradable plastics have driven research for diverse applications, including optical windows, metal uptake, and adhesives. Due to the unique electronic properties of sulfur-rich materials, they are promising candidates for cathodes in Li-S batteries and triboelectric nanogenerators. This review highlight the latest exciting ways of synthesis strategy in which sulfur and sulfur-based reactions are bing utilized to produce sustainable materials in energy, optics, engeneering material, environemtal, and triboelectric nanogenerators. Finally, this review provides a forward-looking perspective on the opportunities and challenges shaping this rapidly evolving field.

Composition-structure-property relationships of polyethylene vitrimers crosslinked by 8-arm polyhedral oligomeric silsesquioxane

Transforming polyolefins (POs), such as polyethylene (PE), into vitrimers is a promising research field due to their low cost, high availability, and excellent chemical resistance and mechanical properties. In these systems, the introduction of dynamic crosslinking can affect the degree of crystallinity in POs and may lead to phase separation due to incompatibility between the PO matrix and crosslinking agents, both of which can impact mechanical performance. This study investigates the relationship between crystallinity, crosslinking, and thermal-mechanical properties in commodity PE-derived vitrimers utilizing reactive 8-arm polyhedral oligomeric silsesquioxane (POSS) nanoparticles by deconvoluting the crosslinked and non-crosslinked components. Specifically, the insoluble crosslinked components displayed a lower modulus and increased brittleness, while the non-crosslinked phase performed similarly to neat PE. Together, the PE-vitrimer, crosslinked with 8-arm POSS, exhibited reduced toughness, elongation at break, and a slight increase in ultimate tensile strength. These behaviors were consistent when comparing the crosslinking density and gel fraction with a bifunctional crosslinker analogue. This work demonstrates the influence of multi-arm, nanoparticle-based crosslinker content on the mechanical properties of semi-crystalline PO-vitrimers, elucidating the roles of network density and crystallinity in determining their performance.

Efficient preparation and properties of PBAT/starch covalent a daptable network composites

Poly (butylene adipate-co-terephthalate) (PBAT)/starch composites have captured great attentions due to their favorable biodegradability and cost-effectiveness. However, it is still a challenge to prepare PBAT/starch composites with superior mechanical performance, solvent and creep resistance, and gas barrier. This study used a chain breaking-crosslinking strategy to prepare covalent adaptable network (CAN) composites from PBAT and unmodified starch via reactive extrusion. In the chain breaking stage, dipentaerythritol was used to break PBAT via transesterification and capped more hydroxyl groups on PBAT, achieving favorable compatibility with unmodified starch, which is confirmed via infrared spectroscopy analysis and scanning electron microscopy among other experimental methods. In the crosslinking stage, the hydroxyl groups of chain broken PBAT and starch were reacted with styrene-maleic anhydride copolymer (SMA) to form CAN composites. The superior compatibility and network feature gave the composites excellent thermal properties, strength and modulus, creep resistance, solvent resistance and gas barrier performance. Moreover, fast stress relaxation can be achieved based on the transesterification, allowing for extrusion processing and thermal compression molding, demonstrating excellent reprocessing recyclability of the PBAT/starch CAN composites. Overall, this work provides a simple, green and efficient method for preparing high-performance PBAT/starch CAN composites, which should also be suitable for other polymer/filler composites.

Mechanically Robust and Recyclable Polyurea Networks Enabled by Dynamic Caprolactam-Urea Bonds

The integration of dynamic covalent bonds into polymer design offers transformative potential for sustainable materials.  Herein, we report a catalyst-free dynamic caprolactam-urea (CAU) bond that enables the construction of poly(caprolactam-ureas) (PCAU) networks with dual mechanical and chemical recyclability. The resulting PCAU networks exhibit remarkable mechanical robustness, solvent resistance, and hydrolytic stability. Owing to the rapid dynamic exchange of CAU bonds, PCAU networks are successfully mechanically recycled via hot-pressing over multiple cycles with >92% mechanical property retention. Furthermore, chemical recycling is achieved through selective depolymerization into caprolactam monomers under mild catalyst-free conditions, which are directly repolymerized into new generations of PCAU networks. This work establishes CAU chemistry as a versatile platform for designing high-performance dynamic polymers with applications in circular manufacturing.

Stereochemistry-Tuned Hydrogen-Bonding Synergistic Covalent Adaptable Networks: Towards Recycled Elastomers with Excellent Creep-Resistant Performance

Covalent adaptable networks (CANs) offer innovative solutions for the reprocessing and recycling of thermoset polymers. However, achieving a balance between easy reprocessing and creep resistance remains a challenge. This study focuses on designing and synthesizing polyurethane (PU) materials with tailored properties by manipulating the stereochemistry of diamine chain extenders. By employing cis- and trans-configurations of diamine extenders, we developed a series of PU materials with varying mechanical properties and creep resistance. The trans-configured materials (R,R-DAC-PU or S,S-DAC-PU) exhibited superior creep resistance and mechanical strength due to dense hydrogen bonding networks. The cis-configured materials (Cis-DAC-PU) exhibited enhanced processability and elasticity. Under 0.1 MPa stress, R,R-DAC-PU showed a mere 3.5 % strain change at 170 °C over 60 minutes, highlighting its superior creep resistance. Both configurations can be recycled via urea bond exchange reactions using hot pressing or solvothermal methods. Density Functional Theory (DFT) calculations indicate that both the (R,R-DCA-UB-U)2 and (S,S-DCA-UB-U)2 segments form six hydrogen bonds with shorter bond lengths, leading to stronger hydrogen-bonding interactions. Conversely, the (Cis-DCA-UB-U)2 segment forms four hydrogen bonds with longer bond lengths, resulting in weaker interactions. This work highlights the critical role of stereochemistry in designing high-performance, recyclable polymer materials with tailored properties.

Environmentally ion-dissociable high-performance supramolecular polyelectrolyte plastics

Robust and stiff polymeric materials usually rely on dense covalent crosslinking, which endows them with excellent properties such as high durability and outstanding thermal stability. However, because of the strong covalent bonds within the network, these polymeric materials are not easily degraded or recycled, giving rise to uncontrolled accumulation of end-of-life plastics in seawater or soil. Here, we present a general strategy to fabricate high-performance supramolecular polyelectrolyte plastics with environmentally ion-dissociable properties in a facile manner. By combining dynamic supramolecular hydrogen bonding and multiple electrostatic crosslinking with hydrophobic interactions, the resulting stable supramolecular polyelectrolyte plastic possesses a tensile strength of 93.6 ± 3.3 MPa and a Young's modulus of 2.3 ± 0.3 GPa, outperforming most of the commercial plastics. More importantly, the unique supramolecular dynamic network structures endow the polyelectrolyte plastics with excellent remoldability, good recyclability, and efficient dissociation in seawater and soil under ambient conditions. The simple fabrication strategy developed herein for robust sustainable polyelectrolyte plastics appears to be applicable to other bio-sourced and synthetic polyelectrolytes. This work provides a practical way for fabricating sustainable high-performance plastics by elegantly designing the supramolecular networks of polyelectrolytes.

Covalent Adaptable Poly[2]rotaxane Networks via Dynamic C-N Bond Transalkylation

Covalent adaptable networks (CANs), a novel class of crosslinked polymers with dynamic covalent bonds, have gained significant attention for combining the durability of thermosets with the reprocessability of thermoplastics, making them promising for emerging applications. Here, we report the first example of poly[2]rotaxane-type covalent adaptable networks (PRCANs), in which oligo[2]rotaxane backbones characterized by densely packed mechanical bonds, are cross-linked through dynamic C-N bonds. The oligo[2]rotaxane backbones could guarantee the mechanical properties of the CANs. Under an external force, the synergy of numerous microscopic motions of the cascade [2]rotaxane units, progressively introducing the initially hidden short chains, expands the polymer network, imparting good stretchability to the PRCANs (217 %). On the other hand, the dissociation of host-guest recognition, followed by the motion of mechanical bonds, constitutes a unique energy dissipation pathway, ultimately enhancing the toughness of PRCANs (7.6 MJ/m3). In contrast, the control CAN, which lacks movable mechanical bonds, demonstrates significantly lower stretchability (40 %) and toughness (1.5 MJ/m3). Moreover, the dynamic C-N bond can undergo high efficiency of 1,2,3-triazole alkylation and trans-N-alkylation exchanges at 1,2,3-triazolium sites at elevated temperatures, with good reprocessability and without compromising their mechanical performance. This work demonstrates the great potential of oligo[2]rotaxanes as a novel polymer backbone for the development of sustainable materials with excellent mechanical properties.

Polypropylene Covalent Adaptable Networks with Full Cross-Link Density Recovery after Reprocessing: Development by Free-Radical Reactive Processing with Resonance-Stabilized, Aromatic Disulfide Cross-Linkers

A single-step method that produces percolated, dynamic covalent cross-links integrated into the PP homopolymer has not been previously demonstrated. Here, we synthesized covalent adaptable networks (CANs) from polypropylene (PP) homopolymers using 180 °C, radical-based, reactive processing with a free-radical initiator, dicumyl peroxide (DCP), and resonance-stabilized, aromatic disulfide cross-linkers, one methacrylate-based and another phenyl acrylate-based. Both cross-linkers yielded networks when reactively processed at 4 wt % with relatively high molecular weight (MW) PP (melt flow index (MFI) = 12) and 4 wt % DCP. The phenyl acrylate-based cross-linker also yielded PP networks at other studied DCP/cross-linker concentrations and with relatively low MW PP (MFI = 35). Notably, our highest cross-link density PP CAN exhibited full recovery of cross-link density after three reprocessing steps by compression molding; that PP CAN also exhibited full cross-link density recovery within experimental uncertainty after reprocessing by melt extrusion.

Molecular Insights into Interfacial Stress Amplification and Network Reinforcement in Extrudable Multiphase Vitrimers

Incorporating dynamic covalent bonds (DCBs) into elastomers provides a seminal solution for the upcycling of traditional thermoset elastomers. Recently, engineering a multiphase network with various cross-linking uniformity and phase structures has been proven to be an effective strategy to overcome the bottleneck of continuous and high-throughput recycling (e.g., extrusion reprocessing) of vitrimeric elastomers. However, all of the relevant studies only focused on revealing the influences of network structures on the macroscopic properties of the systems. As for the microscopic mechanism of the multiphase network at the molecular level, it is still lacking. Herein, based on coarse-grained molecular dynamics (CGMD) simulation, a modeled DCBs-cross-linked elastomer with a multiphase network was established, which was subsequently subjected to in situ tensile or shear forces to simulate the evolution of local chain segment motion and stress/strain distributions in various microregions of the network under the complex extrusion/injection force field. The results indicate that phase domains with different cross-link densities feature distinct chain segment motion behavior and local stress/strain distribution evolution during tensile/shear deformation, and the interfacial phase exhibits significant high stresses. Therefore, incorporating heterogeneously cross-linked multiphase networks into elastomeric vitrimers can enable the system to have significant network reinforcement and unique interfacial stress amplification effects, which are critical for determining extrusion/injection reprocessability. Therefore, we envisage that the present study can provide a molecular-level theoretical explanation for the extrusion/injection reprocessability of multiphase elastomeric vitrimers, thereby guiding the rational network/performance design of these seminal materials.

Epoxy-based vitrimeric semi-interpenetrating network/MXene nanocomposites for hydrogen gas barrier applications

Herein, we report MXene-filled epoxy-based vitrimeric nanocomposites featuring a semi-interpenetrating network (S-IPN) to develop a hydrogen gas (H2) barrier coating with self-healing characteristics for compressed H2 storage applications. The reversible epoxy network was formed by synthesizing linear epoxy chains with pendent bis-hydroxyl groups using amino diol, which were then crosslinked with 1,4-benzenediboronic acid to generate dynamic boronic ester linkages. To achieve the S-IPN-type molecular arrangement, the epoxy chains were in situ crosslinked in the presence of poly(ethylene-co-vinyl alcohol) (EVOH), giving rise to a self-healing network (EEP) with a healing efficiency of 87%. Into the S-IPN vitrimer (EEP), a 2D platelet-type nanofiller MXene was incorporated to introduce a tortuous path for H2 gas diffusion along with improved mechanical properties. The nanocomposite coating was applied to nylon 6 liner material, which is conventionally used in all-composite H2 storage vessels. The application of a 2 wt% MXene/EEP nanocomposite coating showed a permeability coefficient of 0.062 cm3 mm m-2 d-1 atm-1 exhibiting ∼96% reduction in gas permeability compared to uncoated nylon 6. The same nanocomposite exhibited a healing efficiency of 79%. Increasing the MXene loading to 10 wt% further reduced the permeability coefficient to 0.002 cm3 mm m-2 d-1 atm-1; however, the healing efficiency decreased due to restricted chain mobility. In essence, the current work highlights the potential of vitrimeric S-IPN nanocomposite coatings for H2 gas-barrier applications, enhancing safety and performance.

Precise Carboxylic Acid-Functionalized Polyesters in Reprocessable Vitrimers

Thermosets are valued for their exceptional dimensional stability, mechanical properties, and resistance to creep and chemicals. Their permanent molecular structures limit reshaping, reprocessing, and recycling. Incorporating exchangeable chemical bonds into cross-linked polymer networks provides materials with thermoset-like properties that are also reprocessable. Here, ring-opening copolymerization (ROCOP) of unpurified, commercially available epoxides and succinic anhydride is employed to synthesize well-defined, low molecular weight polyesters with controlled functionalization. Polymer networks are then formed through the catalyzed reaction of these copolymers with the epoxy-containing cross-linker diglycidyl ether of bisphenol A. Catalyst mixtures of zinc bis(2-ethylhexanoate) and 1,8-diazabicyclo(5.4.0)undec-7-ene are used to assess the role of the catalysts in the curing and dynamic bond exchange reactions. Varying the catalyst ratios results in polymer networks with tunable mechanical properties (90% < εb < 450%, 0.30 MPa < UTS < 24 MPa), high creep recovery (%recovery > 90% after five creep cycles), and good reprocessability.

Structure-Property Relationships of Elastomeric Vinylogous Urethane Thermosets and Their Application as Closed-Loop Recyclable Strain Sensors

Developing closed-loop recyclable thermosets and understanding their structure-property relationships are essential steps in advancing a circular materials economy. Here, we present a vinylogous urethane (VU) thermoset with closed-loop recyclability, synthesized through the reaction of polytetrahydrofuran bisacetoacetate (aPTHF) and tris(2-aminoethyl)amine (TREN). These VU polymers exhibit high elasticity, with only a 3-9% residual strain observed after cyclic tensile testing at a maximum strain of 100%, depending on the molecular weight of aPTHF and network cross-link density. The two structural parameters also allow modulation of the mechanical and stress-relaxation properties of VU elastomers. To investigate the hydrolysis of the VU linkages within the hydrophobic aPTHF matrix, we employed a heterogeneous system using a biphasic mixture of HCl and CDCl3. Our findings show that the hydrophobic VUs remain stable in pure water but can be dissociated under acidic conditions, with the dissociation rate accelerated at higher temperatures and/or in the presence of higher HCl concentrations. These detailed investigations indicate the potential of VU elastomers as sustainable substrates for wearable sensors. We therefore conduct a case study of synthesizing a strain sensor through the incorporation of multiwalled carbon nanotubes (MCNs) into the VU elastomer matrix. The sensor can robustly detect various movements. Moreover, acidic treatment of both the neat polymer and the sensor composite using a HCl and diethyl ether solvent mixture allows for the excellent recovery of aPTHF (>90%) and TREN (86%), without discernible damage to the MCNs reclaimed from the latter.

Towards cell-adhesive, 4D printable PCL networks through dynamic covalent chemistry

In recent years, the development of biodegradable, cell-adhesive polymeric implants and minimally invasive surgery has significantly advanced healthcare. These materials exhibit multifunctional properties like self-healing, shape-memory, and cell adhesion, which can be achieved through novel chemical approaches. Engineering of such materials and their scalability using a classical polymer network without complex chemical synthesis and modification has been a great challenge, which potentially can be resolved using biobased dynamic covalent chemistry (DCC). Here, we report a scalable, self-healable, biodegradable, and cell-adhesive poly(ε-caprolactone) (PCL)-based vitrimer scaffold, using imine exchange, free from the limitations of melting transitions and supramolecular interactions in 4D-printed PCL. PCL's typical hydrophobicity hinders cell adhesion; however, our design, based on photopolymerization of PCL-dimethacrylate and methacrylate-terminated vanillin-based imine, achieves a water contact angle of 64°. The polymer network, fabricated in varying proportions, exhibited a co-continuous phase morphology, achieving optimal shape fixity (91 ± 1.7%) and shape recovery (92.5 ± 0.1%) at physiological temperature (37 °C). Additionally, the scaffold promoted cell adhesion and proliferation and reduced oxidative stress at the defect site. This multifunctional material shows the potential of DCC-based research in developing smart biomedical devices with complex geometries, paving the way for novel applications in regenerative medicine and implant design.

Current State-of-the-Art and Perspectives in the Design and Application of Vitrimeric Systems

To fulfill the current circular economy concept, the academic and industrial communities are devoting significant efforts to plastic materials' end-of-life. Unlike thermoplastics, which are easy to recover and re-valorize, recycling thermosets is still difficult and challenging. Conversely, because of their network structure, thermosetting polymer systems exhibit peculiar features that make these materials preferable for several applications where high mechanical properties, chemical inertness, and thermal stability, among others, are demanded. In this view, vitrimers have quite recently attracted the attention of the scientific community, as they can form dynamic covalent adaptive networks that provide the properties typical of thermosets while keeping the possibility of being processed (and, therefore, mechanically recycled) beyond a certain temperature. This review aims to provide an overview of vitrimers, elucidating their most recent advances and applications and posing some perspectives for the forthcoming years.

Supramolecular modification of sustainable high-molar-mass polymers for improved processing and performance

The plastic waste crisis is among humanity's most urgent challenges. However, widespread adoption of sustainable plastics is hindered by their often inadequate processing characteristics and performance. Here, we introduce a bio-inspired strategy for the modification of a representative high molar mass, biodegradable aliphatic polyester that helps overcome these limitations and remains effective at molar masses far greater than the entanglement molar mass. We use co-assembly of oligopeptide-based polymer end groups and a low molar mass additive to create a hierarchical structure characterized by regularly spaced nanofibrils interconnected by entangled polymer segments. The modified materials show a rubbery plateau at temperatures above their melting point, associated with strongly increased melt strength, extraordinary melt extensibility, improved dimensional stability, and accelerated crystallization. These thermomechanical property changes open up otherwise inaccessible processing routes and offer considerable scope for improving solid-state properties, thereby addressing typical shortcomings of sustainable alternatives to conventional plastics.

Self-healing, flame retardant and UV resistant lignin-derived epoxy wood coating with a Schiff base structure

The traditional epoxy resin not only is flammable and non-recyclable and but also heavily dependents on petroleum resources, which cannot meet the requirements of fire prevention and sustainable development. In this study, a vanillin intermediate (VAP) with dynamic imine bond (C=N) was prepared by schiff base reaction between the lignin derivative vanillin (-CHO) and the cage-like polyhedral oligomeric silsesquioxane OA-POSS(-NH2). Then, a biomass-based P-N-Si flame retardant (VAPD) was synthesized by adding 9,10-Dihydro-9-Oxa-10-Phosphaphenanthrene-10-Oxide (DOPO) into the VAP. Subsequently, the VAPD acted as curing agent of epoxy resin to prepare epoxy wood coatings (VAPDs/EP). The results showed that the VAPD-5/EP coating not only exhibited excellent self-healing property and was able to achieve 100 % healing within 35 min, but also presented distinctive UV-shielding performance. In addition, the VAPD-5/EP coating reached V-0 level of UL-94 test and got 29.3 % of limiting oxygen index (LOI). Compared with EP coating, the peak heat release rate (pHRR) decreased by 38 %. Furthermore, the VAPD-5/EP coating reached 6H level of hardness test and 4B level of adhesion test. A simple strategy for preparing self-healing flame retardant epoxy coatings with comprehensive properties for protection of wood materials was proposed in this study.

Comparative Study of Polyethylene, Polypropylene, and Polyolefins Silyl Ether-Based Vitrimers

Polyolefins (POs), which constitute over 50% of all plastics, predominantly end up in landfills. To date, there have been no reports on mixtures of PO vitrimers. This study reports the successful synthesis of PO vitrimers from a mixture of 27.7% high-density polyethylene (HDPE), 36.3% linear low-density polyethylene (LLDPE)/low-density polyethylene (LDPE), and 36.0% polypropylene (PP), which is similar to that of Municipal Solid Waste (MSW). This is achieved by using silyl ether-based chemistry, both with and without nitroxides. Additionally, these PO vitrimers are compared with individual vitrimers made of HDPE, LDPE, LLDPE, and PP, as well as vitrimers made from PE blends (comprising HDPE, LLDPE, and LDPE). All vitrimers were prepared via melt extrusion. Their cross-linking density, storage modulus, tensile properties, and reprocessability were assessed. For PO vitrimers, a storage modulus of 0.61 MPa was achieved, indicating a cross-linked network while also maintaining complete melt reprocessability. This study not only provides fundamental insights but also presents a sustainable pathway for recycling PEs and POs into useful materials, hence helping to minimize waste.

Dynamic Boronic Ester Cross-Linked Polymers with Tunable Properties via Side-Group Engineering

The development of dynamic covalent materials with repairability, reprocessability, and recyclability is crucial for sustainable development. In this work, we report a new strategy to adjust the thermomechanical properties of boronic ester cross-linked poly(β-hydroxyl amine)s through side-group engineering. By tuning the side groups of the poly(β-hydroxyl amine)s, we have developed self-healable, reprocessable, and shape-programmable materials. By tuning the side groups of the poly(β-hydroxyl amine)s, the thermomechanical properties can be readily adjusted. Notably, the 3-amino-1,2-propanediol-derived polymer exhibits enhanced thermal (Tg = 95 °C) and mechanical (tensile strength = 34.2 MPa) performance due to increased hydrogen bonding. Benefiting from the dynamic reversibility of the boronic esters, these materials demonstrate solvent-assisted healing, reprocessing, chemical recycling, and shape programming capabilities. Given their straightforward synthesis, tunable properties, and robust dynamic features, the boronic ester cross-linked poly(β-hydroxyl amine)s hold great promise for various applications, including flexible electronic and biomedical materials.

3D Printing of Strong and Room-Temperature Reprocessable Silicone Vitrimers

Silicones find use in a myriad of applications from sealants and adhesives to cooking utensils and medical implants. However, state-of-the-art silicone parts fall short in terms of shape complexity and reprocessability. Advances in three-dimensional printing and the discovery of vitrimers have recently opened opportunities for shaping and recycling of silicone objects. Here, we report the 3D printing via direct ink writing of silicone vitrimers into complex-shaped parts with high strength and room-temperature reprocessability. The reprocessing properties of the printed objects result from the adaptive nature of the silicone vitrimer, which can deform under stress without losing its network connectivity. Rheological and mechanical experiments reveal that printable inks can be tuned to generate strong parts with high creep resistance and room-temperature reprocessability, two properties that are usually challenging to reconcile in vitrimers. By combining printability, high strength, and room-temperature reprocessability, the reported silicone vitrimers represent an attractive sustainable alternative to currently available elastomers in a broad range of established and prospective applications.

Direct Monomer Recovery from Ring-Closing Depolymerization of Thermosets

Recovering monomers from the depolymerization of thermosets presents a significant challenge, which becomes even more daunting if one sets the goal of doing it directly, i.e., without complex chemical separation steps. To this end, we have synthesized a new type of polycarbonate thermoset by first copolymerizing alkyl cyclic carbonates (ACCs) with small amounts of allyloxy cyclic carbonates (AoCCs), followed by cross-linking the resulting allyloxy polycarbonate with excess tetrathiol compounds under UV irradiation. These cross-linked polycarbonates demonstrate enhanced thermal and mechanical properties compared to their linear analogues, while maintaining the linear polymers' capacity for ring-closing depolymerization. The depolymerization process enables the direct recovery of ACC and its dimer, bypassing complex chemical separation steps that are commonly employed in the recycling of conventional chemically recyclable thermosets. The yields range from 74.7% to 91.7% depending on the ratios of AoCC to ACC in the thermosets. Furthermore, the recovered compounds can be repolymerized with AoCCs leading to polycarbonate of the same quality to the initially synthesized one.

Circular Cross-Linked Polyethylene Enabled by In-Chain Ketones

Cross-linked polyethylenes (PEs) are widely employed, but the permanent links between the chains impede recycling. We show that via imine formation with diamines keto-functionalized polyethylenes from both free-radical (keto-low-density PE, keto-LDPE) and catalytic (keto-high-density PE, keto-HDPE) nonalternating ethylene-CO copolymerization can be cross-linked efficiently in the melt, resulting in gel fractions of the formed cross-linked PEs of up to 85% and improved tensile properties. The imine-based cross-links in the material can be hydrolyzed at 140 °C to recycle up to 97% of the initial thermoplastic keto-polyethylene. Low keto contents of ≤1.5 mol % are found ideal to retain PE-like thermal properties, achieve sufficient cross-link density, and maintain circular recyclability.

Wavelength-Dependent Dynamic Behavior in Thiol-Ene Networks Based on Disulfide Exchange

While latent catalysts have become a well-established strategy for locally and temporally controlling bond exchange reactions in dynamic polymer networks, there is a lack of inherently tailorable systems. Herein, we introduce a thiol-ene network based on disulfide exchange that alters its dynamic properties as a function of the color of light used during the curing reaction. For this purpose, selected allyl-bearing disulfides are synthesized, which are transparent at 450 nm but undergo disulfide scission upon 365 nm light irradiation, as confirmed by UV-vis and EPR measurements. Incorporated into a thiol-ene resin, the wavelength used in the curing reaction defines the dynamic properties of the obtained photopolymer. At 450 nm, photocuring yields a dynamic network with disulfide bonds, which relaxes to 63% of its original stress within 112 s at 160 °C (without the requirement of an external catalyst). In contrast, curing with 365 nm light induces disulfide scission yielding photopolymers, which contain predominately monosulfidic links. The permanent nature of the links effectively prevents relaxation of the polymer within a reasonable period of time, confirming the successful alteration of its dynamic properties simply by the color of the light source used.

Photoresponsive Dioxazaborocanes-Containing Oligomers

The synthetic methodology for the preparation of photoresponsive dioxazaborocanes-containing oligomers is developed. It relies on the transformation of the (diisopropylamino)boryl group (-BH(NiPr2)) into a dioxazaborocane unit in the presence of β-aminodiols and involves a bis-borylated dithienylethene photochromic unit. The photophysical properties of the obtained oligomers are evaluated as well as their processability for the preparation of spin-coated films. The photomechanical behavior of the resulting films is assessed via displacement tracking profile.

Investigation on Polyether Sulfone Toughening Epoxy Vitrimer: Curing and Dynamic Properties

Diglycidyl ether of bisphenol A crosslinking with glutaric anhydride is used to form the conventional "covalent adaptive network", polyether sulfone (PES) by coiling and aggregating on the adaptive network is used to significantly increase the uncured resin viscosity for improving the processability of epoxy resin, but inevitably affecting the curing reaction and dynamic transesterification reaction. This study investigates the crucial roles of PES in curing dynamics and stress relaxation behavior. The results indicate that although PES does not directly participate in the crosslinking reaction of polyester-based epoxy vitrimers. Moreover, the isothermal curing studies reveal that the addition of PES can greatly bring forward the reaction rate peak from conversion α = 0.6 to α = 0.2, meaning that the curing mechanism transfers from chemical control to diffusion control. Dynamic property analysis shows that the addition of PES significantly accelerates stress relaxation, especially at lower temperatures, leading to low viscous flow activation energy Eτ and relatively insensitive stress relaxation behavior to temperature. Introducing PES into vitrimer resin greatly improves crosslinking density (2.31 × 10⁴ mol m- 3), enhancing glass transition temperature (82.68 °C), tensile strength (68.66 MPa), and fracture toughness (6.25%). Additionally, the modified vitrimer resin exhibits satisfying shape memory performance and reprocessing capability.

Polyurethane Vitrimers Engineered with Nitrogen-Coordinating Cyclic Boronic Diester Bonds for Sustainable Bioelectronics

Flexible bioelectronic devices seamlessly interface with organs and tissues, offering unprecedented opportunity for timely prevention, early diagnosis, and medical therapies. However, the majority of flexible substrates utilized in bioelectronics still encounter significant challenges in terms of recyclability and reprocessing, leading to the accumulation of environmentally and biologically hazardous toxic waste. Here, the study reports the design of recyclable polyurethane (PU) vitrimers engineered with internal boron-nitrogen coordination bonds that can reversibly dissociate to boronic acids and hydroxyl, or undergo metathesis reaction following an associative pathway. The study demonstrates the capacity of these recyclable PU vitrimers as flexible substrates in various wearable and implantable bioelectronic applications, achieving high-quality electrophysiological recordings and stimulation. Furthermore, the study establishes a sustainable recycling process by reconstructing a range of bioelectronic devices from the recycled PU vitrimers without compromising the mechanical performance. This closed-loop approach not only addresses the critical challenge of the reclaiming medical electronic waste but also paves the way for the development of sustainable flexible bioelectronics for healthcare applications.

Alternative Concepts for Extruded Power Cable Insulation: from Thermosets to Thermoplastics

The most common type of insulation of extruded high-voltage power cables is composed of low-density polyethylene (LDPE), which must be crosslinked to adjust its thermomechanical properties. A major drawback is the need for hazardous curing agents and the release of harmful curing byproducts during cable production, while the thermoset nature complicates reprocessing of the insulation material. This perspective explores recent progress in the development of alternative concepts that allow to avoid byproducts through either click chemistry type curing of polyethylene-based copolymers or the use of polyolefin blends or copolymers, which entirely removes the need for crosslinking. Moreover, polypropylene-based thermoplastic formulations enable the design of insulation materials that can withstand higher cable operating temperatures and facilitate reprocessing by remelting once the cable reaches the end of its lifetime. Finally, polyethylene-based covalent and non-covalent adaptable networks are explored, which may allow to combine the advantages of thermoset and thermoplastic insulation materials in terms of thermomechanical properties and reprocessability.

Boronic ester-templated pre-rotaxanes as versatile intermediates for rotaxane endo-functionalisation

We report on the synthesis of [2]rotaxanes from vicinal diols through dynamic covalent boronic ester templates, as well as the use of the boronic ester for rotaxane post-functionalisation. A boronic acid pincer ligand with two alkene-appended arms was condensed with a linear diol-containing thread, and ring-closing metathesis established a pre-rotaxane architecture along with a non-entangled isomer. Advanced NMR spectroscopy and mass spectrometry unambiguously assigned the isomers and revealed that the pre-rotaxane was in equilibrium with its hydrolyzed free [2]rotaxane form. The boronic ester handle in the pre-rotaxane could be synthetically addressed in a multitude of ways to obtain different endo-functionalised [2]rotaxanes, including with direct oxidation reactions, protodeboronation, functional group interconversions and Pd-catalysed cross-couplings.

Reprocessable Epoxy-Anhydride Resin Enabled by a Thermally Stable Liquid Transesterification Catalyst

Introducing dynamic ester bonds into epoxy-anhydride resins enhances the reprocessability of the crosslinked network, facilitated by various types of transesterification catalysts. However, existing catalysts, such as metal salts and organic molecules, often struggle with dispersion, volatility, or structural instability issues. Here, we propose to solve such problems by incorporating a liquid-state, thermally stable transesterification catalyst into epoxy resins. This catalyst, an imidazole derivative, can be uniformly dispersed in the epoxy resin at room temperature. In addition, it shows high-temperature structural stability above at least 200 °C as the synergistic effects of the electron-withdrawing group and steric bulk can be leveraged. It can also effectively promote transesterification at elevated temperatures, allowing for the effective release of shear stress. This property enables the thermal recycling and reshaping of the fully crosslinked epoxy-anhydride resin. This strategy not only enhances the functionality of epoxy resins but also broadens their applicability across various thermal and mechanical environments.

Innovative Synthesis of Photo-Responsive, Self-Healing Silicone Elastomers with Enhanced Mechanical Properties and Thermal Stability

Photo-responsive materials have garnered significant interest for their ability to react to non-contact stimuli, though achieving self-healing under gentle conditions remains an elusive goal. In this research, an innovative and straightforward approach for synthesizing silicone elastomers is proposed that not only self-heal at room temperature but also possess unique photochromic properties and adjustable mechanical strength, along with being both transparent and reprocessable. Initially, aldehyde-bifunctional dithiophene-ethylene molecules with dialdehyde groups (DTEM) and isocyanurate (IPDI) is introduced into the aminopropyl-terminated polydimethylsiloxane (H2N-PDMS-NH2) matrix. Subsequently, palladium is incorporated to enhance coordination within the matrix. These silicone elastomers transition to a blue state under 254 nm UV light and revert to transparency under 580 nm light. Remarkably, they demonstrate excellent thermal stability at temperatures up to 100 °C and show superior fatigue resistance. The optical switching capabilities of the silicone elastomers significantly affect both their mechanical characteristics and self-healing abilities. Notably, the PDMS-DTEM-IPDI-@Pd silicone elastomer, featuring closed-loop photo-switching molecules, exhibits a fracture toughness that is 1.3 times greater and a room temperature self-healing efficiency 1.4 times higher than its open-loop counterparts. This novel photo-responsive silicone elastomer offers promising potential for applications in data writing and erasure, UV protective coatings, and micro-trace development.

Rapidly Self-Healable and Melt-Extrudable Polyethylene Reprocessable Network Enabled with Dialkylamino Disulfide Dynamic Chemistry

Catalyst-free, radical-based reactive processing is used to transform low-density polyethylene (LDPE) into polyethylene covalent adaptable networks (PE CANs) using a dialkylamino disulfide crosslinker, BiTEMPS methacrylate (BTMA). Two versions of BTMA are used, BTMA-S2, with nearly exclusively disulfide bridges, and BTMA-Sn, with a mixture of oligosulfide bridges, to produce S2 PE CAN and Sn PE CAN, respectively. The two PE CANs exhibit identical crosslink densities, but the S2 PE CAN manifests faster stress relaxation, with average relaxation times ∼4.5 times shorter than those of Sn PE CAN over a 130 to 160 °C temperature range. The more rapid dynamics of the S2 PE CAN translate into a shorter compression-molding reprocessing time at 160 °C of only 5 min (vs 30 min for the Sn PE CAN) to achieve full recovery of crosslink density. Both PE CANs are melt-extrudable and exhibit full recovery within experimental uncertainty of crosslink density after extrusion. Both PE CANs are self-healable, with a crack fully repaired and the original tensile properties restored after 30 min for the S2 PE CAN or 60 min for the Sn PE CAN at a temperature slightly above the LDPE melting point and without the assistance of external forces.

Reaction of β-Ketoester and 1,3-Diol to Access Chemically Recyclable and Mechanically Robust Poly(vinyl alcohol) Thermosets through Incorporation of β-(1,3-dioxane)ester

The development of mechanically robust, chemically stable, and yet recyclable polymers represents an essential undertaking in the context of advancing a circular economy for plastics. Here, we introduce a novel cleavable β-(1,3-dioxane)ester (DXE) linkage, synthesized through the catalyst-free reaction of β-ketoester and 1,3-diol, to cross-link poly(vinyl alcohol) (PVA) for the formation of high-performance thermosets with inherent chemical recyclability. PVA, modified with β-ketoester groups through the transesterification reaction with excess tert-butyl acetoacetate, undergoes cross-linking reactions with the unmodified 1,3-diols within PVA itself upon thermal treatment. The cross-linking architecture improves PVA's mechanical properties, with Young's modulus and toughness that can reach up to 656 MPa and 84 MJ cm-3, i.e. approximately 3- and 12-fold those of linear PVA, respectively. Thermal treatment of the cross-linked PVA polymers under acid conditions leads to deconstruction of the networks, enabling the excellent recovery (>90 %) of PVA. In the absence of either thermal or acidic treatment, the cross-linked PVA maintains its dimensional stability. We show that the recovery of PVA is also possible when the treatment is performed in the presence of other plastics commonly found in recycling mixtures. Furthermore, PVA-based composites comprising carbon fibers and activated charcoal cross-linked by the DXE linkages are also shown to be recyclable with recovery of the PVA and the fillers.

Cross-Linked Polyolefins: Opportunities for Fostering Circularity Throughout the Materials Lifecycle

Cross-linked polyolefins (XLPOs) constitute a significant portion of the plastics commercial market, with a market size of a similar order of magnitude to those of polystyrene and polyethylene terephthalate. However, few aspects of XLPO materials circularity have been examined relative to thermoplastic polyolefins. The cross-linking of polyolefins imparts superior performance properties, such as impact strength, chemical and electrical resistance, and thermal stability vs thermoplastic analogues, but it also makes the reprocessing of XLPOs to valuable products more challenging, as XLPOs cannot be molten. Thus, most XLPOs are incinerated or landfilled at the end of the first lifecycle, even though XLPO products are commonly collected as a relatively clean waste stream-providing a unique opportunity for valorization. In this review, we discuss approaches to improve XLPO circularity throughout the entire materials lifecycle by examining biobased feedstocks as alternative olefinic monomer sources and by assessing both traditional mechanical and advanced XLPO recycling methods based on industrial feasibility and potential product value. We also consider how advancing materials longevity can reduce environmental impacts and lifecycle costs and how recyclable-by-design strategies can enable better end-of-life opportunities for future generations of XLPO materials. Throughout this review, we highlight XLPO circularity routes that have the potential to balance the performance, circularity, and scalability necessary to impart economic and environmental viability at an industrial scale.

Synthesis and Characterization of Rebondable Polyurethane Adhesives Relying on Thermo-Activated Transcarbamoylation

Dynamic polymer networks combine the noteworthy (thermo)mechanical features of thermosets with the processability of thermoplastics. They rely on externally triggered bond exchange reactions, which induce topological rearrangements and, at a sufficiently high rate, a macroscopic reflow of the polymer network. Due to this controlled change in viscosity, dynamic polymers are repairable, malleable, and reprocessable. Herein, several dynamic polyurethane networks were synthetized as model compounds, which were able to undergo thermo-activated transcarbamoylation for the use in rebondable adhesives. Ethylenediamine-N,N,N',N'-tetra-2-propanol (EDTP) was applied as a transcarbamoylation catalyst, which participates in the curing reaction across its four -OH groups and thus, is covalently attached within the polyurethane network. Both bond exchange rate and (thermo)mechanical properties of the dynamic networks were readily adjusted by the crosslink density and availability of -OH groups. In a last step, the most promising model compound was optimized to prepare an adhesive formulation more suitable for a real case application. Single-lap shear tests were carried out to evaluate the bond strength of this final formulation in adhesively bonded carbon fiber reinforced polymers (CFRP). Exploiting the dynamic nature of the adhesive layer, the debonded CFRP test specimens were rebonded at elevated temperature. The results clearly show that thermally triggered rebonding was feasible by recovering up to 79% of the original bond strength.

Nanocomposites of Poly(n-Butyl Acrylate) with Fe3O4: Crosslinking with Hindered Urea Bonds, Reprocessing and Related Functional Properties

In this contribution, we reported the synthesis of the nanocomposites of poly(n-butyl acrylate) with Fe3O4 nanoparticles (NPs) via dynamic crosslinking of poly(n-butyl acrylate)-grafted Fe3O4 NPs with hindered urea bonds (HUBs). Towards this end, the surfaces of Fe3O4 NPs were grafted with poly(n-butyl acrylate-ran-2-(3-tert-butyl-3-ethylureido)ethyl acrylate) chains [denoted as Fe3O4-g-P(BA-r-TBEA)] via living radical polymerization. Thereafter, 1,2-bis(tert-butyl)ethylenediamine was used as a crosslinker to afford the organic-inorganic networks with variable contents of Fe3O4 NPs and crosslinking densities. It was found that the fine dispersion of Fe3O4 NPs in the matrix of poly(n-butyl acrylate) was achieved. The nanocomposites exhibited excellent reprocessing properties, attributed to the introduction of HUBs. Owing to the crosslinking, the nanocomposites displayed excellent shape memory properties. Further, the nanocomposites possessed photo- and magnetic-thermal properties, which were inherited from Fe3O4 NPs. These functional properties allow triggering the shape shifting of the nanocomposites in an uncontacted fashion.

Effect of dynamic bond concentration on the mechanical properties of vitrimers

The presence of dynamic covalent bonds allows vitrimers to undergo topology alterations and display self-healing properties. Herein, we study the influence of varying the concentration of dynamic bonds on the macroscopic properties of hybrid vitrimer networks by subjecting them to triaxial stretching tests using molecular simulations. Results show that the presence of dynamic bonds allows for continuous stress relaxation in the hybrid networks leading to delayed craze development and higher stretching as compared to permanently crosslinked networks. The work highlights the ability of glassy vitrimer networks to relax tensile stress during deformation successfully.