Vitrimers: directing chemical reactivity to control material properties

Carbon Fiber-Based Vitrimer Composites: A Path toward Current Research That Is High-Performing, Useful, and Sustainable

In the polymeric material industry, thermosets and related composites have played a substantial role in the production of rubber and plastics. One important subset of these is thermoset composites with carbon reinforcement. The incorporation of carbon fillers and fibers gives polymeric materials improved electrical and mechanical properties, among other benefits. However, the covalently crosslinked network of thermosets presents significant challenges for recycling and reprocessing because of its intractable nature. The introduction of vitrimer materials opens a new avenue to produce biodegradable and recyclable thermosets. Carbon-reinforced vitrimer composites are pursued for high-performance, long-lasting materials with attractive physical properties, the ability to be recycled and processed, and other features that respond uniquely to stimuli. The development of carbon-reinforced vitrimer composites over the last few years is summarized in this article. First, an overview of vitrimers and the methods used to prepare carbon fiber-reinforced vitrimer composites is provided. Because of the vitrimer nature of such composites, reprocessing, healing, and recycling are viable ways to greatly extend their service life; these approaches are thoroughly explained and summarized. The conclusion is our prediction for developing carbon-based vitrimer composites.

Mechanical Properties and Recyclability of Fiber Reinforced Polyester Composites

Fiber reinforced polymer composites (FRPs) are valuable construction materials owing to their strength, durability, and design flexibility; however, conventional FRPs utilize petroleum-based polymer matrices with limited recyclability. Furthermore, fiber reinforcements are made from nonrenewable feedstocks, through expensive and energy intensive processes, making recovery and reuse advantageous. Thus, FRPs that use biobased and degradable or reprocessable matrices would enable a more sustainable product, as both components can be recovered and reused. We previously developed a family of degradable and reprocessable cross-linked polyesters from bioderived cyclic esters (l-lactide, δ-valerolactone, and ε-caprolactone) copolymerized with a bis(1,3-dioxolan-4-one) cross-linker. We now incorporate these networks into FRPs and demonstrate degradability of the matrix into tartaric acid and oligomers, enabling recovery and reuse of the fiber reinforcement. Furthermore, the effect of varying comonomer structure, catalyst, reinforcement type, and lay-up method on mechanical properties of the resultant FRPs is explored. The FRPs produced have tensile strengths of up to 202 MPa and Young's moduli up to 25 GPa, promising evidence that sustainable FRPs can rival the mechanical properties of conventional high performance FRPs.

"Cloth-to-Clothes-Like" Fabrication of Soft Actuators

Inspired by adaptive natural organisms and living matter, soft actuators appeal to a variety of innovative applications such as soft grippers, artificial muscles, wearable electronics, and biomedical devices. However, their fabrication is typically limited in laboratories or a few enterprises since specific instruments, strong stimuli, or specialized operation skills are inevitably involved. Here a straightforward "cloth-to-clothes-like" method to prepare soft actuators with a low threshold by combining the hysteretic behavior of liquid crystal elastomers (LCEs) with the exchange reaction of dynamic covalent bonds, is proposed. Due to the hysteretic behavior, the LCEs (resemble "cloth") effectively retain predefined shapes after stretching and releasing for extended periods. Subsequently, the samples naturally become soft actuators (resemble "clothes") via the exchange reaction at ambient temperatures. As a post-synthesis method, this strategy effectively separates the production of LCEs and soft actuators. LCEs can be mass-produced in bulk by factories or producers and stored as prepared, much like rolls of cloth. When required, these LCEs can be customized into soft actuators as needed. This strategy provides a robust, flexible, and scalable solution to engineer soft actuators, holding great promise for mass production and universal applications.

Tuning Reprocessing Temperature of Aliphatic Polyurethane Networks by Alkoxyamine Selection

Recent studies have shown that the largest employed thermoset family, polyurethanes (PUs), has great potential to be reprocessed due to the dynamic behavior of carbamate linkage. However, it requires high temperatures, especially in the case of aliphatic PUs, which causes side reactions besides the desired exchange reaction. To facilitate the reprocessing of aliphatic PUs, in this work, we have explored the dynamic potential of alkoxyamine bonds in PU networks to facilitate the reprocessing under mild conditions considering their fast recombination ability. Taking advantage of the structural effect of the nitroxide and alkyl radicals on the dissociation energy, two different alkoxyamine-based diols have been designed and synthesized to generate PU networks. Our study shows that replacing 50 mol % of a nondynamic diol chain extender with these dynamic blocks boosts the relaxation times of the networks, enabling reprocessing at temperatures as low as 80 °C.

Segmental and Chain Dynamics of Polyisoprene-Based Model Vitrimers

Polymer vitrimers are a new class of materials that combine the advantages of thermoplastics and thermosets. This is due to the dynamic nature of the chemical bonds linking different chains. However, how this property affects the polymer dynamics at different length scales is still an open question. Here, we investigate the dynamics of model vitrimers based on well-defined polyisoprene (PI) chains using broadband dielectric spectroscopy. In this way, we study the polymer dynamics from the segmental to the whole chain scale, taking advantage of the fact that PI belongs to the class of molecules that exhibit a net dipole moment associated with the end-to-end vector. Three distinct relaxation phenomena are identified. The fastest relaxation is attributed to the segmental PI dynamics with a small influence of the cross-linking. An intermediate relaxation attributed to the dipolar character of the cross-linker is also observed. The slower identified relaxation component, corresponding to limited fluctuations of the end-to-end PI chains, is found to be determined by the dynamics of the clusters formed by the cross-linkers with an average time scale orders of magnitude faster than that of the terminal relaxation as inferred from the viscous flow.

Knoevenagel C═C Metathesis Enabled Glassy Vitrimers with High Rigidity, Toughness, and Malleability

Thermosets, characterized by their permanent cross-linked networks, present significant challenges in recyclability and brittleness. In this work, we explore a polarized Knoevenagel C═C metathesis reaction for the development of rigid yet tough and malleable thermosets. Initial investigation on small molecule model reactions reveals the feasibility of conducting the base-catalyzed C═C metathesis reaction in a solvent-free environment. Subsequently, thermosetting poly(α-cyanocinnamate)s (PCCs) were synthesized via Knoevenagel condensation between a triarm cyanoacetate star and a dialdehyde. The thermal and mechanical properties of the developed PCCs can be easily modulated by altering the structure of the dialdehyde. Remarkably, the introduction of ether groups into the PCC leads to a combination of high rigidity and toughness with Young's modulus of ∼1590 MPa, an elongation at break of ∼79%, and a toughness reaching ∼30 MJ m3. These values are competitive to traditional thermosets, in Young's modulus but far exceed them in ductility and toughness. Moreover, the C═C metathesis facilitates stress relaxation within the bulk polymer networks, thus rendering PCCs excellent malleability and reprocessability. This work overcomes the traditional limitations of thermosets, introducing groundbreaking insights for the design of rigid yet tough and malleable thermosets, and contributing significantly to the sustainability of materials.

Dynamic Chalcogen Squares for Material and Topological Control over Macromolecules

Herein we introduce chalcogen squares via selenadiazole motifs as a new class of dynamic supramolecular bonding interactions for the modification and control of soft matter materials. We showcase selenadiazole motifs in supramolecular networks of varying primary chain length prepared through polymerization using tandem step-growth/Passerini multicomponent reactions (MCRs). Compared to controls lacking the selenadiazole motif, these networks display increased glass transition temperatures and moduli due to the chalcogen bonding linkages formed between chains. These elastomeric networks were shown to autonomously heal at room temperature, retaining up to 83 % of the ultimate tensile strength. Lastly, we use post-polymerization modification via the Biginelli MCR to add selenadiazole motifs to narrowly dispersed polymers for controlled topology in solution. Chalcogen squares via selenadiazoles introduce an exciting exchange mechanism to the realm of dynamic materials.

Catalyst-Free α-Acetyl Cinnamate/Acetoacetate Exchange to Enable High Creep-Resistant Vitrimers

Vitrimers represent an emerging class of polymeric materials that combine the desirable characteristics of both thermoplastics and thermosets achieved through the design of dynamic covalent bonds within the polymer networks. However, these materials are prone to creep due to the inherent instability of dynamic covalent bonds. Consequently, there are pressing demands for the development of robust and stable dynamic covalent chemistries. Here, we report a catalyst-free α-acetyl cinnamate/acetoacetate (α-AC/A) exchange reaction to develop vitrimers with remarkable creep resistance. Small-molecule model studies revealed that the α-AC/A exchange occurred at temperatures above 140 °C in bulk, whereas at 120 °C, this reaction was absent. For demonstration in the case of polymers, copolymers derived from common vinyl monomers were crosslinked with terephthalaldehyde to produce α-AC/A vitrimers with tunable thermal and mechanical performance. All resulting α-AC/A vitrimers exhibited high stability, especially in terms of creep resistance at 120 °C, while retaining commendable reprocessability when subjected to high temperatures. This work showcases the α-AC/A exchange reaction as a novel and robust dynamic covalent chemistry capable of imparting both reprocessability and high stability to cross-linked networks.

Molecular Simulation of Covalent Adaptable Networks and Vitrimers: A Review

Covalent adaptable networks and vitrimers are novel polymers with dynamic reversible bond exchange reactions for crosslinks, enabling them to modulate their properties between those of thermoplastics and thermosets. They have been gathering interest as materials for their recycling and self-healing properties. In this review, we discuss different molecular simulation efforts that have been used over the last decade to investigate and understand the nanoscale and molecular behaviors of covalent adaptable networks and vitrimers. In particular, molecular dynamics, Monte Carlo, and a hybrid of molecular dynamics and Monte Carlo approaches have been used to model the dynamic bond exchange reaction, which is the main mechanism of interest since it controls both the mechanical and rheological behaviors. The molecular simulation techniques presented yield sufficient results to investigate the structure and dynamics as well as the mechanical and rheological responses of such dynamic networks. The benefits of each method have been highlighted. The use of other tools such as theoretical models and machine learning has been included. We noticed, amongst the most prominent results, that stress relaxes as the bond exchange reaction happens, and that at temperatures higher than the glass transition temperature, the self-healing properties are better since more bond BERs are observed. The lifetime of dynamic covalent crosslinks follows, at moderate to high temperatures, an Arrhenius-like temperature dependence. We note the modeling of certain properties like the melt viscosity with glass transition temperature and the topology freezing transition temperature according to a behavior ruled by either the Williams-Landel-Ferry equation or the Arrhenius equation. Discrepancies between the behavior in dissociative and associative covalent adaptable networks are discussed. We conclude by stating which material parameters and atomistic factors, at the nanoscale, have not yet been taken into account and are lacking in the current literature.

Revealing Dynamic Behavior in High Dielectric Poly(thiourethane)-Based Vitrimer-like Materials

Here, we have explored covalent adaptable networks (CANs) comprising poly(thiourethane)-based systems (PTUs). The PTUs were synthesized through the combination of thiol and isocyanate monomers in stoichiometric proportions, in the presence of dibutyltin dilaurate (DBTDL) as catalyst. Dynamic mechanical analysis (DMA) provided detailed insight into the vitrimeric behavior. Through these investigations, we evaluated the viscoelastic, thermomechanical, and vitrimeric properties. Additionally, broadband dielectric spectroscopy (BDS) revealed the various relaxation processes inherent in such vitrimer-like materials. We methodically examined the evolution of each relaxation in every prepared sample to comprehend the operational mechanisms in these vitrimer-like systems. Our findings underscore that depending on the PTU formulation, the glass transition temperature (Tg) and the topology freezing transition temperature (Tv) can be effectively distinguished and studied. Considering the high dipole moment of the dynamic bonds present in these systems, there is potential for utilizing them as dielectric materials working under the concept of dipolar glass polymers. Furthermore, the reversibility exhibited by their inner chemical structures positions them as promising candidates for active layers in capacitor devices, particularly for energy-related applications, with the ability to be recyclable while maintaining almost invariant both their mechanical and dielectric properties, thus promoting the extension of the lifespan of electronic devices.

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.

Polymerized and Colloidal Ionic Liquids─Syntheses and Applications

The breadth and importance of polymerized ionic liquids (PILs) are steadily expanding, and this review updates advances and trends in syntheses, properties, and applications over the past five to six years. We begin with an historical overview of the genesis and growth of the PIL field as a subset of materials science. The genesis of ionic liquids (ILs) over nano to meso length-scales exhibiting 0D, 1D, 2D, and 3D topologies defines colloidal ionic liquids, CILs, which compose a subclass of PILs and provide a synthetic bridge between IL monomers (ILMs) and micro to macro-scale PIL materials. The second focus of this review addresses design and syntheses of ILMs and their polymerization reactions to yield PILs and PIL-based materials. A burgeoning diversity of ILMs reflects increasing use of nonimidazolium nuclei and an expanding use of step-growth chemistries in synthesizing PIL materials. Radical chain polymerization remains a primary method of making PILs and reflects an increasing use of controlled polymerization methods. Step-growth chemistries used in creating some CILs utilize extensive cross-linking. This cross-linking is enabled by incorporating reactive functionalities in CILs and PILs, and some of these CILs and PILs may be viewed as exotic cross-linking agents. The third part of this update focuses upon some advances in key properties, including molecular weight, thermal properties, rheology, ion transport, self-healing, and stimuli-responsiveness. Glass transitions, critical solution temperatures, and liquidity are key thermal properties that tie to PIL rheology and viscoelasticity. These properties in turn modulate mechanical properties and ion transport, which are foundational in increasing applications of PILs. Cross-linking in gelation and ionogels and reversible step-growth chemistries are essential for self-healing PILs. Stimuli-responsiveness distinguishes PILs from many other classes of polymers, and it emphasizes the importance of segmentally controlling and tuning solvation in CILs and PILs. The fourth part of this review addresses development of applications, and the diverse scope of such applications supports the increasing importance of PILs in materials science. Adhesion applications are supported by ionogel properties, especially cross-linking and solvation tunable interactions with adjacent phases. Antimicrobial and antifouling applications are consequences of the cationic nature of PILs. Similarly, emulsion and dispersion applications rely on tunable solvation of functional groups and on how such groups interact with continuous phases and substrates. Catalysis is another significant application, and this is an historical tie between ILs and PILs. This component also provides a connection to diverse and porous carbon phases templated by PILs that are catalysts or serve as supports for catalysts. Devices, including sensors and actuators, also rely on solvation tuning and stimuli-responsiveness that include photo and electrochemical stimuli. We conclude our view of applications with 3D printing. The largest components of these applications are energy related and include developments for supercapacitors, batteries, fuel cells, and solar cells. We conclude with our vision of how PIL development will evolve over the next decade.

Practical Applications of Self-Healing Polymers Beyond Mechanical and Electrical Recovery

Self-healing polymeric materials, which can repair physical damage, offer promising prospects for protective applications across various industries. Although prolonged durability and resource conservation are key advantages, focusing solely on mechanical recovery may limit the market potential of these materials. The unique physical properties of self-healing polymers, such as interfacial reduction, seamless connection lines, temperature/pressure responses, and phase transitions, enable a multitude of innovative applications. In this perspective, the diverse applications of self-healing polymers beyond their traditional mechanical strength are emphasized and their potential in various sectors such as food packaging, damage-reporting, radiation shielding, acoustic conservation, biomedical monitoring, and tissue regeneration is explored. With regards to the commercialization challenges, including scalability, robustness, and performance degradation under extreme conditions, strategies to overcome these limitations and promote successful industrialization are discussed. Furthermore, the potential impacts of self-healing materials on future research directions, encompassing environmental sustainability, advanced computational techniques, integration with emerging technologies, and tailoring materials for specific applications are examined. This perspective aims to inspire interdisciplinary approaches and foster the adoption of self-healing materials in various real-life settings, ultimately contributing to the development of next-generation materials.

Trapping bond exchange phenomenon revealed for off-stoichiometry cross-linking of phase-separated vitrimer-like materials

Vitrimer materials combined with nano-phase separated structures have attracted attention, expanding the tuning range of physical properties, such as flow and creep properties. We recently demonstrated a preparation of vitrimer-like materials with phase-separated nanodomains in which dissociative bond exchange via trans-N-alkylation of quaternized pyridine was operated. In this study, we demonstrate a new finding about the bond exchange mechanism: that is, the trapping bond exchange phenomenon. The component polymer is a poly(acrylate) containing pyridine side groups randomly along the chain, which is cross-linked by diiodo molecules via pyridine-iodo quaternization, where the quaternized pyridines are aggregated to form nano-size domains. When the cross-linking reaction is performed at an off-stoichiometric pyridine : iodo ratio (i.e., an excess of pyridine groups), free pyridine groups are located in the matrix phase. Since the bond exchange in the present system progresses in an inter-domain manner, the dissociated unit bearing pendant iodo is trapped by the free pyridine groups in the matrix, which generates other small aggregates. This trapping phenomenon greatly affects the relaxation and creep properties, which are very different from those found in conventional knowledge about vitrimer physics.

Intramolecular Catalytic Hydrogen Atom Transfer (CHAT)

Intramolecular catalysis (IntraCat) is the acceleration of a process at one site of a molecule catalyzed by a functional group in the same molecule; an external agent such as a solvent typically facilitates it. Here, we report a general first-principles-based IntraCat mechanism, which strictly occurs within a single molecule with no coreagent being involved─we call it intramolecular catalytic transfer of hydrogen atoms (CHAT). A reactive part of a molecule (chat catalyst moiety or chat agent, represented by -OOH, -COOH, -SH, -CH2OH, -HPO4, or another bifunctional H-donor/acceptor group) catalyzes an interconversion process, such as keto-enol or amino-imino tautomerization, and cyclization in the same molecule, while being regenerated in the process. It can thus be regarded as an intramolecular version of the intermolecular H atom transfer processes mediated by an external molecular catalyst, e.g., dihydrogen, water, or a carboxylic acid. Earlier, we proposed a general mechanistic systematization of intermolecular processes, illustrated in the simplest case of the H2-mediated reactions classified as dihydrogen catalysis [Asatryan, R.; et al. Catal. Rev.: Sci. Eng., 2014, 56, 403-475]. Following this systematization, the CHAT catalysis belongs to the category of relay transfer of H atoms, albeit in an intramolecular manner. A broader class of intramolecular processes includes all types of H-transfer reactions stimulated by an H-migration, which we call self-catalyzed H atom transfer (SC-HAT). The CHAT mechanism comprises a subset of SC-HAT in which the catalytic moiety is regenerated (i.e., acts as a true catalyst and not a reagent). We provide several characteristic examples of CHAT mechanism based on detailed analysis of the corresponding potential energy surfaces. All such cases showed a dramatically reduced activation barrier relative to the corresponding uncatalyzed H-transfer reactions. For example, we show that CHAT can facilitate long-range H-migration in larger molecules and can occur multiple times in one molecule with multiple interconverting groups. It also facilitates amino-imino tautomerization of unsaturated GABA-analogues and peptides, as well as intramolecular cyclization processes to form heterocycles, e.g., oxygenated rings. CHAT pathways may also explain the pH-dependent increase of mutarotation rate of glucose-6-phosphate demonstrated in pioneering experiments that introduced the classical IntraCat concept. In addition, we identify a ground electronic state CHAT pathway as an alternative to the UV-promoted long-range molecular crane keto-enol conversion with a remarkably low activation energy. To initially assess the possible impact of the new keto-enol conversion pathway on combustion of n-alkanes, we present a detailed kinetic analysis of isomerization and decomposition of pentane-2,4-ketohydroperoxide (2,4-KHP). The results are compared with key alternative reactions, including direct dissociation and Korcek channels (for which a new alkyl group migration channel is also identified), revealing the competitiveness of the CHAT pathway across a range of conditions. Taken together, this work provides insight into a general class of reaction pathways that has not previously being systematically considered and that may occur in a broad range of contexts from combustion to atmospheric chemistry to biochemistry.

Tuning the Mechanical and Dynamic Properties of Elastic Vitrimers by Tailoring the Substituents of Boronic Ester

Elastic vitrimers, i.e., elastic polymers with associative dynamic covalent bonds, can afford elastomers with recyclability while maintaining their thermal and chemical stability. Herein, we report a series of boronic ester-based vitrimers with tunable mechanical properties and recyclability by varying the substitute groups of boronic acid in polymer networks. The dynamic polymer networks are formed by reacting diol-containing tetra-arm poly(amidoamine) with boronic acid-terminated tetra-arm poly(ethylene glycol), which possesses different substituents adjacent to boronic acid moieties. Varying the substituent adjacent to the boronic ester unit will significantly affect the binding strength of the boronic ester, therefore affecting their dynamics and mechanical performance. The electron-withdrawing substituents noticeably suppress the dynamics of boronic ester exchange and increase the activation energy and relaxation time while enhancing the mechanical strength of the resulting elastic vitrimers. On the other hand, the presence of electron-rich substituent affords relatively reduced glass transition temperature (Tg), faster relaxation, and prominent recyclability and malleability at lower temperatures. The developed pathway will guide the rational design of elastomers with well-tunable dynamics and processabilities.

Programmable and Shape-Color Synchronous Dual-Response Wood with Thermal Stimulus

Stimuli-responsive materials exhibit huge potential in sensors, actuators, and electronics; however, their further development for reinforcement, visualization, and biomass-incorporation remains challenging. Herein, based on the impregnation of thermochromic microcapsule (TCM)-doped dynamic covalent vitrimers, a programmable shape-color dual-responsive wood (SRW-TC) was demonstrated with robust anisotropic structures and exchangeable covalent adaptable networks. Under mild conditions, the resultant SRW-TC displays feasible shape memorability and programmability, resulting from the rigidity-flexibility shift induced by the glass-transition temperature (34.99 °C) and transesterification reaction triggered by the topology freezing transition temperature (149.62 °C). Furthermore, the obtained SRW-TC possesses satisfactory mechanical performance (tensile strength of 45.70 MPa), thermal insulation (thermal conductivity of 0.27 W/m K), anisotropic light management, and benign optical properties (transmittance of 51.73% and haze of 99.67% at 800 nm). Importantly, the incorporation of compatible TCM enables SRW-TC to visualize shape memory feasibility and rigidity/flexibility switching and respond to the external thermal stimulus through the thermal-induced shape-color synchronous dual-responsiveness, which successfully demonstrates the applications of sensing temperature, grasping objects, encrypting/decoding icon messages, and so on. The proposed facile and highly effective strategy could serve as a guideline for developing high-performance multifunctional wood composite with promising intelligent applications in performance visualization, environmental sensing, materials interactivity, information dual-encryption, local precision shape and color regulation, etc.

Oxime metathesis: tuneable and versatile chemistry for dynamic networks

Oxime chemistry has emerged as a versatile tool for use in a wide range of applications. In particular, the combination of oximes with esters and urethanes has enabled the realisation of Covalent Adaptable Networks (CANs) with improved and tunable dynamic properties. Nevertheless, an exclusively oxime-based chemistry has not yet been explored in the fabrication of CANs. In this work, we investigate the mechanism of the acid-catalysed dynamic exchange of oximes. We propose a metathesis mechanism that is well supported by both experimental and computational studies, which highlight the importance of the substituent effect on the exchange reaction kinetics. Then, as a proof of concept, we incorporate oxime groups into a cross-linked polymeric material and demonstrate the ability of oxime-based polymers to be reprocessed under acid catalysis while maintaining their structural integrity.

Next-Generation Vitrimers Design through Theoretical Understanding and Computational Simulations

Vitrimers are an innovative class of polymers that boast a remarkable fusion of mechanical and dynamic features, complemented by the added benefit of end-of-life recyclability. This extraordinary blend of properties makes them highly attractive for a variety of applications, such as the automotive sector, soft robotics, and the aerospace industry. At their core, vitrimer materials consist of crosslinked covalent networks that have the ability to dynamically reorganize in response to external factors, including temperature changes, pressure variations, or shifts in pH levels. In this review, the aim is to delve into the latest advancements in the theoretical understanding and computational design of vitrimers. The review begins by offering an overview of the fundamental principles that underlie the behavior of these materials, encompassing their structures, dynamic behavior, and reaction mechanisms. Subsequently, recent progress in the computational design of vitrimers is explored, with a focus on the employment of molecular dynamics (MD)/Monte Carlo (MC) simulations and density functional theory (DFT) calculations. Last, the existing challenges and prospective directions for this field are critically analyzed, emphasizing the necessity for additional theoretical and computational advancements, coupled with experimental validation.

The Kinetic Study of the Influence of Common Modifiers on the Curing Process of Epoxy Vitrimers

An analysis of the influence of common modifiers on the kinetics of the curing process of epoxy-anhydride vitrimers was carried out. As common modifiers to enhance the "vitrimeric" nature of the material, zinc acetylacetonate as a transesterification catalyst and glycerol as a modifier of hydroxyl group content were chosen. The curing process of all obtained compositions was studied by differential scanning calorimetry (DSC) followed by the application of the isoconversional approach. It was shown that additives significantly affect the curing process. The resulting cured polymers were shown to be chemically recyclable by dissolution in the mixture of ethylene glycol and N-methylpirrolidone in a volume ratio of nine to one. The introduction of both zinc acethylacetonate and glycerol to the neat formulation led to a decrease in the dissolution time by 85.7% (from 35 h for the neat epoxy-anhydride formulation to 5 h for the modified formulation). In order to show the opportunity of the secondary use of recyclates, the mixtures based on the basic composition containing 10 wt. % of secondary polymers were also studied. The introduction of a recycled material to neat composition led to the same curing behavior as glycerol-containing systems.

On Lightweight Shape Memory Vitrimer Composites

Lightweight materials are highly desired in many engineering applications. A popular approach to obtain lightweight polymers is to prepare polymeric syntactic foams by dispersing hollow particles, such as hollow glass microbubbles (HGMs), in a polymer matrix. Integrating shape memory vitrimers (SMVs) in fabricating these syntactic foams enhances their appeal due to the multifunctionality of SMVs. The SMV-based syntactic foams have many potential applications, including actuators, insulators, and sandwich cores. However, there is a knowledge gap in understanding the effect of the HGM volume fraction on different material properties and behaviors. In this study, we prepared an SMV-based syntactic foam to investigate the influence of the HGM volume fractions on a broad set of properties. Four sample groups, containing 40, 50, 60, and 70% HGMs by volume, were tested and compared to a control pure SMV group. A series of analyses and various chemical, physical, mechanical, thermal, rheological, and functional experiments were conducted to explore the feasibility of ultralight foams. Notably, the effect of HGM volume fractions on the rheological properties was methodically evaluated. The self-healing capability of the syntactic foam was also assessed for healing at low and high temperatures. This study proves the viability of manufacturing multifunctional ultralightweight SMV-based syntactic foams, which are instrumental for designing ultralightweight engineering structures and devices.

Chemical upcycling of commodity thermoset polyurethane foams towards high-performance 3D photo-printing resins

Polyurethane thermosets are indispensable to modern life, but their widespread use has become an increasingly pressing environmental burden. Current recycling approaches are economically unattractive and/or lead to recycled products of inferior properties, making their large-scale implementation unviable. Here we report a highly efficient chemical strategy for upcycling thermoset polyurethane foams that yields products of much higher economic values than the original material. Starting from a commodity foam, we show that the polyurethane network is chemically fragmented into a dissolvable mixture under mild conditions. We demonstrate that three-dimensional photo-printable resins with tunable material mechanical properties-which are superior to commercial high-performance counterparts-can be formulated with the addition of various network reforming additives. Our direct upcycling of commodity foams is economically attractive and can be implemented with ease, and the principle can be expanded to other commodity thermosets.

Microscopic Evidence for the Topological Transition in Model Vitrimers

In addition to the glass transition, vitrimers undergo a topological transition from viscoelastic liquid to viscoelastic solid behavior when the network rearrangements facilitated by dynamic bond exchange reactions freeze. The microscopic observation of this transition is elusive. Model polyisoprene vitrimers based on imine dynamic covalent bonds were synthesized by reaction of α,ω-dialdehyde-functionalized polyisoprenes and a tris(2-aminoethyl)amine. In these dynamic networks nanophase separation of polymer and reactive groups leads to the emergence of a relevant length scale characteristic for the network structure. We exploited the scattering sensitivity to structural features at different length scales to determine how dynamical and topological arrests affect correlations at segmental and network levels. Chains expand obeying the same expansion coefficient throughout the entire viscoelastic region, i.e., both in the elastomeric regime and in the liquid regime. The onset of liquid-like behavior is only apparent at the mesoscale, where the scattering reveals the reorganization of the network triggered by bond exchange events. The such determined "microscopic" topological transition temperature is compared with the outcome of "conventional" methods, namely viscosimetry and differential scanning calorimetry. We show that using proper thermal (aging-like) protocols, this transition is also nicely revealed by the latter.

Highly Recyclable and Tough Elastic Vitrimers from a Defined Polydimethylsiloxane Network

Despite intensive research on sustainable elastomers, achieving elastic vitrimers with significantly improved mechanical properties and recyclability remains a scientific challenge. Herein, inspired by the classical elasticity theory, we present a design principle for ultra-tough and highly recyclable elastic vitrimers with a defined network constructed by chemically crosslinking the pre-synthesized disulfide-containing polydimethylsiloxane (PDMS) chains with tetra-arm polyethylene glycol (PEG). The defined network is achieved by the reduced dangling short chains and the relatively uniform molecular weight of network strands. Such elastic vitrimers with the defined network, i.e., PDMS-disulfide-D, exhibit significantly improved mechanical performance than random analogous, previously reported PDMS vitrimers, and even commercial silicone-based thermosets. Moreover, unlike the vitrimers with random network that show obvious loss in mechanical properties after recycling, those with the defined network enable excellent thermal recyclability. The PDMS-disulfide-D also deliver comparable electrochemical signals if utilized as substrates for electromyography sensors after the recycling. The multiple relaxation processes are revealed via a unique physical approach. Multiple techniques are also applied to unravel the microscopic mechanism of the excellent mechanical performance and recyclability of such defined network.

Overcoming the barrier: designing novel thermally robust shape memory vitrimers by establishing a new machine learning framework

Shape memory vitrimers (SMVs) are an emerging class of advanced materials that have garnered significant interest from researchers in the past five to six years. These materials can return to their original shape when exposed to a stimulus, while also healing damage they have sustained. However, achieving both high healing/recycling efficiency and a high glass transition temperature (Tg) in SMVs has been challenging, due to the conflicting requirements between molecular chain mobility and the formation and reaction of dynamic covalent bond exchange. Based on the understanding of chemo-physical properties, this study first leverages machine learning (ML), involving supervised and unsupervised learning approaches, to navigate this complex design space of SMVs. Furthermore, we elaborated the basic mathematical frameworks of ML approaches and comprehensively compared their performances. Based on the best performing model, we designed four types of thermally robust shape memory vitrimers (TRSMVs), which boast high recycling efficiency, elevated Tg, and exemplary shape memory effects, overcoming conventional barriers. One of the discovered samples exhibited outstanding performance with a Tg of 233.5 °C, a recycling efficiency of 84.1%, and a recovery stress of 33 MPa in experiments. It aligns well with ML predictions, showcasing the potential of our ML framework in driving innovative materials design and advancing the field of smart polymers.

On-Demand Activation of Transesterification by Chemical Amplification in Dynamic Thiol-Ene Photopolymers

Chemical amplification is a well-established concept in photoresist technology, wherein one photochemical event leads to a cascade of follow-up reactions that facilitate a controlled change in the solubility of a polymer. Herein, we transfer this concept to dynamic polymer networks to liberate both catalyst and functional groups required for bond exchange reactions under UV irradiation. For this, we exploit a photochemically generated acid to catalyse a deprotection reaction of an acid-labile tert-butoxycarbonyl group, which is employed to mask the hydroxy groups of a vinyl monomer. At the same time, the released acid serves as a catalyst for thermo-activated transesterifications between the deprotected hydroxy and ester moieties. Introduced in an orthogonally cured (450 nm) thiol-click photopolymer, this approach allows for a spatio-temporally controlled activation of bond exchange reactions, which is crucial in light of the creep resistance versus reflow ability trade-off of dynamic polymer networks.

Reprocessable Polyurethane Foams Using Acetoacetyl-Formed Amides

Like any other thermosetting material, polyurethane foams (PUFs) contain permanent cross-links that hinder their reprocessability and make their recyclability a tedious and environmentally unfriendly process. Herein, we introduce acetoacetyl-formed amides, formed by the reaction of isocyanates with acetoacetate groups, as dynamic units in the backbone of PUFs. By extensive variation of the foam composition, optimum parameters have been found to produce malleable foams above temperatures of 130 °C, without the requirement of any solvent during the foaming process. The PU cross-linked material can be compression-molded at least three times, giving rise to PU elastomers and thus maintaining a cross-linked network structure. Characterization of the original foams shows comparable properties to standard PUFs, for example, having a density of 32 kg/m3, while they show similar chemical and thermal properties upon reprocessing to strong PU elastomers, exhibiting Tg ranging from -42 to -48 °C. This research provides a straightforward method to produce thermally reprocessable PUFs as a promising pathway to address the recycling issues of end-of-life foams.

DFT Study of Imine-Exchange Reactions in Iron(II)-Coordinated Pincers

The imine bond is among the most applied motifs in dynamic covalent chemistry. Although its uses are varied and often involve coordination to a transition metal for stability, mechanistic studies on imine exchange reactions so far have not included metal coordination. Herein, we investigated the condensation and transimination reactions of an Fe2+ -coordinated diimine pyridine pincer, employing wB97XD/6-311G(2d,2p) DFT calculations in acetonitrile. We first experimentally confirmed that Fe2+ is strongly coordinated by these pincers, and is thus a justified model ion. When considering a four-membered ring-shaped transition state for proton transfers, the required activation energies for condensation and transimination reaction exceeded the values expected for reactions known to be spontaneous at room temperature. The nature of the incoming and exiting amines and the substituents on the para-position of the pincer had no effect on this. Replacing Fe2+ with Zn2+ or removing it altogether did not reduce it either. However, the addition of two ethylamine molecules lowered the energy barriers to be compatible with experiment (19.4 and 23.2 kcal/mol for condensation and transimination, respectively). Lastly, the energy barrier of condensation of a non-coordinated pincer was significantly higher than found for Fe2+ -coordinating pincers, underlining the catalyzing effect of metal coordination on imine exchange reactions.

Diaroyl-S,N-ketene Acetals: Red-Shifted Solid-State and Aggregation-Induced Emitters from a One-Pot Synthesis

Symmetric and unsymmetric diaroyl-S,N-ketene acetals can be readily accessed in consecutive syntheses in good to excellent yields by exploiting the inherent nucleophilic character of the methine position. Different aroyl-S,N-ketene acetals as well as acid chlorides yield a library of 19 diaroyl compounds with substitution and linker pattern-tunable emission properties, leading to a significant red-shift of emission in the solid and aggregated state, which was thoroughly investigated. Additionally, the stability of the luminescent aggregates is highly increased. In a follow-up one-pot procedure, pyrazolo-S,N-ketene acetals can easily be accessed employing a nucleophilic cyclocondensation.

Exploring the Limits of High-Tg Epoxy Vitrimers Produced through Resin-Transfer Molding

Over the past few years, scientists have developed new ways to overcome the recycling issues of conventional thermosets with the introduction of associative covalent adaptable networks (i.e., vitrimers) in polymer materials. Even though various end-use vitrimers have already been reported, just a few of them have targeted high-performance industrial applications. Herein, we develop a promising high-performance epoxy vitrimer based on a commercially available resin widely used in aeronautics with the highest glass transition temperature (Tg) of 233 °C ever reported for a vitrimer. A complete study of its physicochemical properties and cure kinetics was conducted, enabling the construction of the first time-temperature-transformation (TTT) diagram reported in the literature. This diagram allows a full determination of the processing and curing parameters leading to the manufacturing of vitrimer samples by the resin-transfer molding (RTM) process. The reshapability and limits therefrom of this high-Tg vitrimer were evaluated by three successful thermoforming cycles without degradation.

Improving the Recyclability of an Epoxy Resin through the Addition of New Biobased Vitrimer

In recent decades, the use of thermoset epoxy resins (ER) has spread to countless applications due to their mechanical properties, heat resistance and stability. However, these ERs are neither biodegradable nor recyclable due to their permanent crosslinked networks and usually, they are synthesized from fossil and toxic precursors. Therefore, reducing its consumption is of vital importance to the environment. On the one hand, the solution to the recyclability problems of epoxy resins can be achieved through the use of vitrimers, which have thermoset properties and can be recycled as thermoplastic materials. On the other hand, vitrimers can be made from natural sources, reducing their toxicity. In this work, a sustainable epoxy vitrimer has been efficiently synthesized, VESOV, by curing epoxidized soybean oil (ESO) with a new vanillin-derived Schiff base (VSB) dynamic hardener, aliphatic diamine (1,4-butanediamine, BDA) and using 1,2-dimethylimidazole (DMI) as an accelerator. Likewise, using the same synthesized VSB agent, a commercial epoxy resin has also been cured and characterized as ESO. Finally, different percentages (30, 50 and 70 wt%) of the same ER have been included in the formulation of VESOV, demonstrating that only including 30 wt% of ER in the formulation is able to improve the thermo-mechanical properties, maintaining the VESOV's inherent reprocessability or recyclability. In short, this is the first approach to achieve a new material that can be postulated in the future as a replacement for current commercial epoxy resins, although it still requires a minimum percentage of RE in the formulation, it makes it possible to recycle the material while maintaining good mechanical properties.

Development and Digital Light Processing 3D Printing of a Vitrimer Composed of Glycerol 1,3-Diglycerolate Diacrylate and Tetrahydrofurfuryl Methacrylate

The development of biobased reshapable and repairable vitrimers has received extensive attention due to the growing focus on an environmentally friendly society. Therefore, the objective of this research was to synthesize sustainable polymers with an environmentally friendly strategy combining the benefits of renewable resources, UV curing, and vitrimers. Two biobased monomers, glycerol 1,3-diglycerolate diacrylate and tetrahydrofurfuryl methacrylate, were chosen for the preparation of UV-curable resins and tested by real-time photorheometry and RT-FTIR spectroscopy to determine their suitability for digital light processing (DLP) 3D printing. DLP 3D-printed polymer showed shape memory, weldability, and repairability capabilities by triggering the dynamic transesterification process at high temperatures. The vitrimer with a weight ratio of 60:40 of glycerol 1,3-diglycerolate diacrylate and tetrahydrofurfuryl methacrylate showed shape memory properties with a recovery ratio of 100% and a 7-fold improved tensile strength compared to the original sample, confirming efficient weldability and repairability.

Editing of polymer backbones

Polymers are at the epicentre of modern technological progress and the associated environmental pollution. Considerations of both polymer functionality and lifecycle are crucial in these contexts, and the polymer backbone - the core of a polymer - is at the root of these considerations. Just as the meaning of a sentence can be altered by editing its words, the function and sustainability of a polymer can also be transformed via the chemical modification of its backbone. Yet, polymer modification has primarily been focused on the polymer periphery. In this Review, we focus on the transformations of the polymer backbone by defining some concepts fundamental to this topic (for example, 'polymer backbone' and 'backbone editing') and by collecting and categorizing examples of backbone editing scattered throughout a century's worth of chemical literature, and outline critical directions for further research. In so doing, we lay the foundation for the field of polymer backbone editing and hope to accelerate its development.

Interfacial Compatibility of Core-Shell Cellulose Nanocrystals for Improving Dynamic Covalent Adaptable Networks' Fracture Resistance in Nanohybrid Vitrimer Composites

The development of polymeric nanocomposites with dynamic covalent adaptable networks and biobased nanomaterials has been a promising approach toward sustainable advanced materials, enabling reprogramming and recycling capabilities. Herein, a core-shell nanohybrid of functionalized cellulose nanocrystals (CNCs) is explored to provide crucial interfacial compatibility for improving the covalent adaptable networks of epoxy-thiol vitrimers in fracture resistance. The poly(ε-caprolactone) (PCL) shells grafted from CNC surfaces can be cross-linked with the covalent adaptable networks via a hot-pressing transesterification process. According to the additive concentration and annealing temperature, the stress relaxation behavior of nanohybrid vitrimer composites can be effectively regulated by the core-shell PCL-grafted CNC (CNC-PCL) nanohybrids from a dispersed to cross-linked interaction. The addition of 15 wt % of the core-shell CNC-PCLs exhibits the reinforced improvement of nanohybrid vitrimer composites in the average Young's modulus of 2.5×, fracture stress of 5.4×, and fracture strain of 2.0×. The research findings might have profound implications for developing synergistic interfacial compatibility between dynamic vitrimer networks and functional nanoparticles for advanced polymeric nanocomposites.

Functional Polymer Nanocomposites with Increased Anticorrosion Properties and Wear Resistance for Water Transport

Corrosive destruction and hydroabrasive wear is a serious problem in the operation of machine parts and water transport mechanisms. It is promising to develop new composite materials with improved properties to increase the reliability of transport vehicles. In this regard, the use of new polymer-based materials, which are characterized by improved anticorrosion properties and wear resistance, is promising. In this work, therefore, for the formation of multifunctional protective coatings, epoxy dian oligomer brand ED-20, polyethylene polyamine (PEPA) hardener, a mixture of nanodispersed compounds with a dispersion of 30-90 nm, fillers Agocel S-2000 and Waltrop with a dispersion of 8-12 μm, and particles of iron slag with a dispersion of 60-63 μm are used for the formation of multifunctional protective coatings. Using the method of mathematically planning the experiment, the content of additives of different physico-chemical natures in the epoxy binder is optimized to obtain fireproof coatings with improved operational characteristics. A mathematical model is developed for optimizing the content of components in the formation of protective anticorrosion and wear-resistant coatings for means of transport as a result of the complex effect of a mixture of nanodispersed compounds, iron scale, and Waltrop. Based on the mathematical planning of the experiment, new regularities of increasing the corrosion resistance and resources of the means of transport are established through the formation of four different protective coatings, which are tested for resistance to aggressive environments (technical water-CAS No. 7732-18-5, gasoline-CAS No. 64742-82-1, acetone-CAS No. 67-64-1, I-20A lubricant-CAS No. 64742-62-7, sodium solutions-CAS No. 1310-73-2, and sulfuric acid-CAS No. 7664-93-9) and hydroabrasive wear resistances. A study of the change in the permeability index in aggressive environments is additionally carried out, taking into account the rational ratio of dispersive fillers in the epoxy binder, which made it possible to create an effective barrier to the penetration of aggressive water molecules into the base. A decrease in the permeability of protective coatings by 2.0-3.3 times relative to the epoxy matrix is achieved. In addition, the wear resistance of the developed materials under the action of hydroabrasion is investigated. The relative resistance of the CM to the action of hydroabrasion was found by the method of materials and coatings testing on the gas-abrasive wear with a centrifugal accelerator. This method enables one to model the real process of the wear of mechanism parts under the hydroabrasive action. It is shown that the coefficient of the wear resistance of the developed materials is 1.3 times higher than that of the polymer matrix, which indicates the resistance of the composites to the influence of hydroabrasive environment. As a result, modified epoxy composite protective coatings with improved anticorrosion properties and wear resistance under hydroabrasive conditions are developed. It is established that the protective coating filled with particles of a mixture of nanodispersed compounds (30-90 nm), iron scale (60-63 μm), and Waltrop (8-12 μm) has the lowest permeability indicators. The permeability in natural conditions of such a coating during the time t = 300 days of the study is χ = 0.5%, which is 3.6 times less than the similar indicators of the epoxy matrix. It is substantiated that the protective coating filled with particles of a mixture of nanodispersed compounds (30-90 nm), iron scale (60-63 μm), and Agocel S-2000 (8-12 μm) is characterized by the highest indicators of wear resistance. The coefficient of wear resistance under the action of hydroabrasion of such a coating is K = 1.75, which is 1.3 times higher than the similar indicators of the original epoxy matrix.

Acetate exchange mechanism on a Zr12 oxo hydroxo cluster: relevance for reshaping Zr-carboxylate coordination adaptable networks

The kinetics and mechanism of the acetate ligand exchange with free acetic acid in [Zr6O4(OH)4(O2CCH3)12]2, used as a molecular model of crosslink migration in [Zr6O4(OH)4(carboxylate)12-n(OH)n]-based coordination adaptable networks with vitrimer-like properties, has been thoroughly investigated by dynamic 1H NMR and DFT calculations. The compound maintains its C2h-symmetric Zr12 structure in CD2Cl2 and C6D6, while it splits into its Zr6 subunits in CD3OD and D2O. In the Zr12 structure, the topologically different acetates (3 chelating, 6 belt-bridging, 2 intercluster-bridging and 1 inner-face-bridging) of the Zr6 subunits behave differently in the presence of free CH3COOH: very fast exchange for the chelating (coalesced resonance at room temperature), slower exchange for the belt-bridging (line broadening upon warming), no observable exchange up to 65 °C (by EXSY NMR) for the intercluster- and inner-face-bridging. The rates of the first two exchange processes have zero-order dependence on [CH3COOH]. Variable-temperature line broadening studies yielded ΔH‡ = 15.0 ± 0.4 kcal mol-1, ΔS‡ = 8 ± 1 cal mol-1 K-1 (-30 to +25 °C range in CD2Cl2) for the chelating acetates and ΔH‡ = 22.7 ± 1.6, 22.9 ± 2.1 and 20.6 ± 1.0 kcal mol-1 and ΔS‡ = 13 ± 5, 14 ± 6 and 9 ± 3 cal mol-1 K-1, respectively (+25 to +70 °C range in C6D6), for three distinct resonances of magnetically inequivalent belt-bridging acetates. With support of DFT calculations, these results point to an operationally associative mechanism involving a rate-determining partial dissociation to monodentate acetate, followed by rapid acid coordination and proton transfer. The cluster μ3-OH ligands accelerate the exchange processes through H-bonding stabilization of the coordinatively unsaturated intermediate. The lower exchange barrier for the chelated vs. bridging acetates is associated to the release of ring strain. The results presented in this investigation may help the interpretation of carboxylate exchange phenomena in other systems and the design of new carboxylate-based materials.

Chemically Recyclable Dithioacetal Polymers via Reversible Entropy-Driven Ring-Opening Polymerization

In a sustainable circular economy, polymers capable of chemical recycling to monomers are highly desirable. We report an efficient monomer-polymer recycling of polydithioacetal (PDTA). Pristine PDTAs were readily synthesized from 3,4,5-trimethoxybenzaldehyde and alkyl dithiols. They then exhibited depolymerizability via ring-closing depolymerization into macrocycles, followed by entropy-driven ring-opening polymerization (ED-ROP) to reform the virgin polymers. High conversions were obtained for both the forward and reverse reactions. Once crosslinked, the network exhibited thermal reprocessability enabled by acid-catalyzed dithioacetal exchange. The network retained the recyclability into macrocyclic monomers in solvent which can repolymerize to regenerate the crosslinked network. These results demonstrated PDTA as a new molecular platform for the design of recyclable polymers and the advantages of ED-ROP for which polymerization is favored at higher temperatures.

Thermally Latent Bases in Dynamic Covalent Polymer Networks and their Emerging Applications

A novel strategy allowing temporal control of dynamic bond exchange in covalently crosslinked polymer networks via latent transesterification catalysts is introduced. Obtained by a straightforward air- and water-tolerant synthesis, the latent catalyst is designed for an irreversible temperature-mediated release of a strong organic base. Its long-term inactivity at temperatures below 50 °C provides the unique opportunity to equip dynamic covalent networks with creep resistance and high bond-exchange rates, once activated. The presented thermally latent base catalyst is conveniently introducible in readily available building blocks and, as proof of concept, applied in a radically polymerized thiol-ene network. Light-mediated curing is used for 3D-printing functional objects, on which the possibility of spatially controlled reshaping and welding based on dynamic transesterification is illustrated. Since the catalyst is thermally activated, limitations regarding sample geometry and optical transparency do not apply, which facilitates a transfer to well-established industrial technologies. Consequently, fiber-reinforced and highly filled magneto-active thiol-ene polymer composites are fabricated by a thermal curing approach. The on-demand activation of dynamic transesterification is demonstrated by (magneto-assisted) reshaping experiments, highlighting a wide range of potential future applications offered by the presented concept.

A bibliometric survey of research trends in vitrimer

The recent trends of vitrimer studies enhance the thermoset material with superior properties, therefore, it is particularly important to address the critical scientific inquiries in this area using their research metrics. The reported vitrimer systems have been highly required for future real-time applications; however, the inquisitiveness of material exchange mechanisms extends the research studies further. Significantly, more scientific information's are required to achieve the evident prospective outcomes via these materials. This article highlights the trends and developments of the most relevant publications, authors, articles, countries, and keywords in the vitrimer research field over the past 10 years. The represented bibliometric survey would elevate the basic understanding of the current vitrimer research stats and also help follow the particular research community to learn and develop insight. To generate bibliometric networks, bibliometric data has obtained from Scopus and visualised in VOS-viewer; as an overview of that, the highest number of publications were from China, United States, France, United Kingdom, and Spain.

Dynamic Ablative Networks: Shapeable Heat-Shielding Materials

Thermoset materials sacrifice recyclability and reshapeability for increased chemical and mechanical robustness because of an immobilized, cross-linked polymeric matrix. The robust material properties of thermosets make them well-suited for applications such as heat-shielding materials (HSMs) or ablatives where excellent thermal stability, good mechanical strength, and high charring ability are paramount. Many of these material properties are characteristic of covalent adaptable networks (CANs), where the static connectivity of thermosets has been replaced with dynamic cross-links. This dynamic connectivity allows network mobility while retaining cross-link connectivity to permit damage repair and reshaping that are traditionally inaccessible for thermoset materials. Herein, we report the synthesis of hybrid inorganic-organic enaminone vitrimers that contain an exceptionally high weight percent of polyhedral oligomeric silsesquioxane (POSS)-derivatives. Polycondensation of β-ketoester-containing POSS with various diamine cross-linkers led to materials with facile tunability, shapeability, predictable glass transition temperatures, good thermal stability, and high residual char mass following thermal degradation. Furthermore, the char materials show notable retention of their preordained shape following decomposition, suggesting their future utility in the design of HSMs with complex detailing.

Biobased Photopolymer Resin for 3D Printing Containing Dynamic Imine Bonds for Fast Reprocessability

Acrylic photopolymer resins are widely used in stereolithographic 3D printing. However, the growing demand for such thermosetting resins is weighing on global issues such as waste management and fossil fuel consumption. Therefore, there is an increasing demand for reactive components that are biobased and enable recyclability of the resulting thermoset products. In this work, the synthesis of a photo-cross-linkable molecule containing dynamic imine bonds based on biobased vanillin and dimer fatty diamine is described. Using the biobased building blocks, formulations containing reactive diluent and a photoinitiator were prepared. The mixtures could be rapidly cross-linked under UV light, yielding vitrimers. Using digital light processing, 3D-printed parts were prepared, which were rigid, thermally stable, and reprocessed within 5 min at elevated temperature and pressure. The addition of a building block containing a higher concentration of imine bonds accelerated the stress relaxation and improved the mechanical rigidity of the vitrimers. This work will contribute to the development of biobased and recyclable 3D-printed resins to facilitate the transition to a circular economy.

Solvent and catalyst free vitrimeric poly(ionic liquid) electrolytes

Polymer electrolytes (PEs) are a promising alternative to overcome shortcomings of conventional lithium ion batteries (LiBs) and make them safer for users. Introduction of self-healing features in PEs additionally leads to prolonged life-time of LIBs, thus tackling cost and environmental issues. We here present solvent free, self-healable, reprocessable, thermally stable, conductive poly(ionic liquid) (PIL) consisting of pyrrolidinium-based repeating units. PEO-functionalized styrene was used as a co-monomer for improving mechanical properties and introducing pendant OH groups in the polymer backbone to act as a transient crosslinking site for boric acid, leading to the formation of dynamic boronic ester bonds, thus forming a vitrimeric material. Dynamic boronic ester linkages allow reprocessing (at 40 °C), reshaping and self-healing ability of PEs. A series of vitrimeric PILs by varying both monomers ratio and lithium salt (LiTFSI) content was synthesized and characterized. The conductivity reached 10^-5 S cm^-1 at 50 °C in the optimized composition. Moreover, the PILs rheological properties fit the required melt flow behavior (above 120 °C) for 3D printing via fused deposition modeling (FDM), offering the possibility to design batteries with more complex and diverse architectures.

Fabricating liquid crystal vitrimer actuators far below the normal processing temperature

Liquid crystal vitrimers can be reprocessed, reshaped, welded, and healed due to exchange-reaction-enabled topology changes despite having fully covalently cross-linked network structures. Fabricating liquid crystal (LC) vitrimer actuators is invariably carried out above a characteristic temperature known as the topology freezing transition temperature (T_v). The reason that all exchange-reaction-based operations must be performed above T_v is because the exchange reaction is insignificant below T_v. Here we find that LC vitrimers can be reshaped at temperatures below the measured T_v, whereas non-LC vitrimers cannot. The work here not only makes it possible to create reprogrammable and stable LC vitrimer actuators at low temperatures but also reminds us that both our measurement and understanding of the T_v need further attention to facilitate the use of vitrimers in different areas.

Phase separation in supramolecular and covalent adaptable networks

Phase separation phenomena have been studied widely in the field of polymer science, and were recently also reported for dynamic polymer networks (DPNs). The mechanisms of phase separation in dynamic polymer networks are of particular interest as the reversible nature of the network can participate in the structuring of the micro- and macroscale domains. In this review, we highlight the underlying mechanisms of phase separation in dynamic polymer networks, distinguishing between supramolecular polymer networks and covalent adaptable networks (CANs). Also, we address the synergistic effects between phase separation and reversible bond exchange. We furthermore discuss the effects of phase separation on the material properties, and how this knowledge can be used to enhance and tune material properties.

Endowing Polythioester Vitrimer with Intrinsic Crystallinity and Chemical Recyclability

Technologically important thermosets face a long-standing end-of-life (EoL) problem of non-reprocessability, a more sustainable solution of which has resolved to nascent vitrimers that can merge the robust material properties of thermosets and the reprocessability of thermoplastics. However, the lifecycle of vitrimers is still finite, as they often suffer from significant deterioration of mechanical performance following multiple reprocessing cycles, analogous to mechanical recycling, and they often show undesired creep under working conditions. To address these two key limitations, we have developed a cross-linked semi-crystalline polythioester with both dynamic covalent bonds and intrinsic crystallinity and chemical recyclability, affording a vitrimeric system that exhibits not only reprocessability and crystallinity-restricted creep but also complete chemical recyclability to initial monomer by catalyzed depolymerization in solution or bulk. Therefore, reported herein is an "infinite" vitrimer system that is empowered with a facile closed-loop EoL option once serial reprocessing deteriorates performance and the material can no longer meet the application requirements. Specifically, the polythioester vitrimer was constructed by copolymerization of a bicyclic thioester with a bis-dithiolane, producing dynamically cross-linked polythioesters with excellent property tunability, from amorphous to semi-crystalline states and melting transition temperatures from 91 to 178 °C.

Vanillin-Based Epoxy Vitrimers: Looking at the Cystamine Hardener from a Different Perspective

Epoxy vitrimers encompass many advantages compared to traditional epoxy materials such as recyclability, repairability, and reprocessability. These properties are induced by the incorporation of dynamic reversible covalent bonds. Recently, the incorporation of aromatic disulfide bridges that are dynamic has expanded the development of new eco-friendly epoxy materials. Herein, we studied a bio-based aliphatic disulfide based on cystamine as a hardener with a vanillin-derived bio-sourced epoxy to prepare fully bio-based epoxy vitrimers. This article provides a comparative study between cystamine and an aromatic disulfide benchmark hardener issued from petrol resources. This work demonstrated that the presence of this aliphatic hardener has a significant influence not only on the reactivity, but most importantly on the resulting dynamic properties. An interesting yet counterintuitive accelerating effect of the dynamic exchanges was clearly demonstrated with only 2 to 20% of molar fraction of cystamine added to the aromatic disulfide formulation. A similar glass transition was obtained compared to the purely aromatic analogue, but relaxation times were decreased by an order of magnitude.

One-Pot Synthesis of Depolymerizable δ-Lactone Based Vitrimers

A depolymerizable vitrimer that allows both reprocessability and monomer recovery by a simple and scalable one-pot two-step synthesis of vitrimers from cyclic lactones is reported. Biobased δ-valerolactone with alkyl substituents (δ-lactone) has low ceiling temperature; thus, their ring-opening-polymerized aliphatic polyesters are capable of depolymerizing back to monomers. In this work, the amorphous poly(δ-lactone) is solidified into an elastomer (i.e., δ-lactone vitrimer) by a vinyl ether cross-linker with dynamic acetal linkages, giving the merits of reprocessing and healing. Thermolysis of the bulk δ-lactone vitrimer at 200 °C can recover 85-90 wt% of the material, allowing reuse without losing value and achieving a successful closed-loop life cycle. It further demonstrates that the new vitrimer has excellent properties, with the potential to serve as a biobased and sustainable replacement of conventional soft elastomers for various applications such as lenses, mold materials, soft robots, and microfluidic devices.

Epoxy Vitrimer Materials by Lipase-Catalyzed Network Formation and Exchange Reactions

The preparation and reprocessing of an epoxy vitrimer material is performed in a fully biocatalyzed process wherein network formation and exchange reactions are promoted by a lipase enzyme. Binary phase diagrams are introduced to select suitable diacid/diepoxide monomer compositions overcoming the limitations (phase separation/sedimentation) imposed by curing temperature inferior than 100 °C, to protect the enzyme. The ability of lipase TL, embedded in the chemical network, to catalyze efficiently exchange reactions (transesterification) is demonstrated by combining multiple stress relaxation experiments at 70-100 °C and complete recovery of mechanical strength after several reprocessing assays (up to 3 times). Complete stress relaxation ability disappears after heating at 150 °C, due to enzyme denaturation. Transesterification vitrimers thus designed are complementary to those involving classical catalysis (e.g., using the organocatalyst triazabicyclodecene) for which complete stress relaxation is possible only at high temperature.

Deciphering Siloxane Bond Exchanges: From a Molecular Study to Vitrimerization and Recycling of Silicone Elastomers

The activity of various additives promoting siloxane equilibration reactions is examined and quantified on model compounds. We found in particular that the "superbase" phosphazene derivative P₄ -^t Bu can promote very fast exchanges (a few seconds at 90 °C) even at low concentration (<0.1 wt %). We demonstrate that permanent silicone networks can be transformed into reprocessable and recyclable dynamic networks by mere introduction of such additives. Annealing at high temperature degrades the additives and deactivates the dynamic features of the silicone networks, reverting them back into permanent networks. A simple rheological experiment and the corresponding model allow to extract the critical kinetic parameters to predict and control such deactivations.

Chemically Fueled Reinforcement of Polymer Hydrogels

Carbodiimide-fueled anhydride bond formation has been used to enhance the mechanical properties of permanently crosslinked polymer networks, giving materials that exhibit transitions from soft gels to covalently reinforced gels, eventually returning to the original soft gels. Temporary changes in mechanical properties result from a transient network of anhydride crosslinks, which eventually dissipate by hydrolysis. Over an order of magnitude increase in the storage modulus is possible through carbodiimide fueling. The time-dependent mechanical properties can be modulated by the concentration of carbodiimide, temperature, and primary chain architecture. Because the materials remain rheological solids, new material functions such as temporally controlled adhesion and rewritable spatial patterns of mechanical properties have been realized.