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.

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 a Biobased Epoxy Vitrimer from a Cardanol Derivative Prepared by a Simple Thiol-Epoxy "Click" Reaction

The development of this work lies in the relevant interest in epoxy resins, which, despite their wide use, do not meet the requirements for sustainable materials. Therefore, the proposed approach considers the need to develop environmentally friendly systems, in terms of both the starting material and the synthetic method applied as well as in terms of end-of-life. The above issues were taken into account by (i) using a monomer from renewable sources, (ii) promoting the formation of dynamic covalent bonds, allowing for material reprocessing, and (iii) evaluating the degradability of the material. Indeed, an epoxy derived from cardanol was used, which, for the first time, was applied in the development of a vitrimer system. The exploitation of a diboronic ester dithiol ([2,2'-(1,4-phenylene)-bis[4-mercaptan-1,3,2-dioxaborolane], DBEDT) as a cross-linker allowed the cross-linking reaction to be carried out without the use of solvents and catalysts through a thiol-epoxy "click" mechanism. The dynamicity of the network was demonstrated by gel fraction experiments and rheological and DMA measurements. In particular, the formation of a vitrimer was highlighted, characterized by low relaxation times (around 4 s at 70 °C) and an activation energy of ca. 48 kJ/mol. Moreover, the developed material, which is easily biodegradable in seawater, was found to show promising flame reaction behavior. Preliminary experiments demonstrated that, unlike an epoxy resin prepared from the same monomer and using a classical cross-linker, our boron-containing material exhibited no dripping under combustion conditions, a phenomenon that will allow this novel biobased system to be widely used.

Digital Light Processing 3D Printing of Isosorbide- and Vanillin-Based Ester and Ester-Imine Thermosets: Structure-Property Recyclability Relationships

Four isosorbide-based photocurable resins were designed to reveal correlations between the composition and chemical structure, digital light processing (DLP) three-dimensional (3D) printability, thermoset properties, and recyclability. Especially, the role of functional groups, i.e., the concentration of ester groups vs the combination of ester and imine functionalities, in the recyclability of the resins was investigated. The resins consisted of methacrylated isosorbide alone or in combination with methacrylated vanillin or a flexible methacrylated vanillin Schiff-base. The composition of the resins significantly affected their 3D printability as well as the physical and chemical properties of the resulting thermosets. The results indicated the potential of methacrylated isosorbide to confer rigidity to thermosets with some negative effects on the printing quality and solvent-resistance properties. An increase in the methacrylated vanillin concentration in the resin enabled us to overcome these drawbacks, leading, however, to thermosets with lower thermal stability. The replacement of methacrylated vanillin with the methacrylated Schiff-base resin decreased the rigidity of the networks, ensuring, on the other hand, improved solvent-resistance properties. The results highlighted an almost complete preservation of the elastic modulus after the reprocessing or chemical recycling of the ester-imine thermosets, thanks to the presence of two distinct dynamic covalent bonds in the network; however, the concentration of the ester functions in the ester thermosets played a significant role in the success of the chemical recycling procedure.

High-Performance, Light-Stimulation Healable, and Closed-Loop Recyclable Lignin-Based Covalent Adaptable Networks

In this work, high-performance, light-stimulation healable, and closed-loop recyclable covalent adaptable networks are successfully synthesized from natural lignin-based polyurethane (LPU) Zn2+ coordination structures (LPUxZy). Using an optimized LPU (LPU-20 with a tensile strength of 28.4 ± 3.5 MPa) as the matrix for Zn2+ coordination, LPUs with covalent adaptable coordination networks are obtained that have different amounts of Zn. When the feed amount of ZnCl2 is 9 wt%, the strength of LPU-20Z9 reaches 37.3 ± 3.1 MPa with a toughness of 175.4 ± 4.6 MJ m-3 , which is 1.7 times of that of LPU-20. In addition, Zn2+ has a crucial catalytic effect on "dissociation mechanism" in the exchange reaction of LPU. Moreover, the Zn2+ -based coordination bonds significantly enhance the photothermal conversion capability of lignin. The maximum surface temperature of LPU-20Z9 reaches 118 °C under the near-infrared illumination of 0.8 W m-2 . This allows the LPU-20Z9 to self-heal within 10 min. Due to the catalytic effect of Zn2+ , LPU-20Z9 can be degraded and recovered in ethanol completely. Through the investigation of the mechanisms for exchange reaction and the design of the closed-loop recycling method, this work is expected to provide insight into the development of novel LPUs with high-performance, light-stimulated heal ability, and closed-loop recyclability; which can be applied toward the expanded development of intelligent elastomers.

Influence of Catalyst Content and Epoxy/Carboxylate Ratio on Isothermal Creep of Epoxy Vitrimers

In the present work, a commercial epoxy based on epoxy anhydride and tertiary amine was modified by a metallic catalyst (Zn2+) to induce vitrimeric behavior by promoting the transesterification reaction. The effect of two different epoxy/acid ratios (1 and 0.6) at two different zinc acetate amounts (Zn(Ac)2) on the thermomechanical and viscoelastic performances of the epoxy vitrimers were investigated. Creep experiments showed an increase in molecular mobility above the critical "Vitrimeric" temperature (Tv) of 170 °C proportionally to the amount of Zn(Ac)2. A procedure based on Burger's model was set up to investigate the effect of catalyst content on the vitrimer ability to flow as the effect of the dynamic exchange reaction. The analysis showed that in the case of a balanced epoxy/acid formulation, the amount of catalyst needed for promoting molecular mobility is 5%. This system showed a value of elastic modulus and dynamic viscosity at 170 °C of 9.50 MPa and 2.23 GPas, respectively. The material was easily thermoformed in compression molding, paving the way for the recyclability and weldability of the thermoset system.

An Investigation of the Healing Efficiency of Epoxy Vitrimer Composites Based on Zn2+ Catalyst

The need to recycle carbon-fibre-reinforced composite polymers (CFRP) has grown significantly to reduce the environmental impact generated by their production. To meet this need, thermoreversible epoxy matrices have been developed in recent years. This study investigates the performance of an epoxy vitrimer made by introducing a metal catalyst (Zn2+) and its carbon fibre composites, focusing on the healing capability of the system. The dynamic crosslinking networks endow vitrimers with interesting rheological behaviour; the capability of the formulated resin (AV-5) has been assessed by creep tests. The analysis showed increased molecular mobility above a topology freezing temperature (Tv). However, the reinforcement phase inhibits the flow capability, reducing the flow. The fracture behaviour of CFRP made with the vitrimeric resin has been investigated by Mode I and Mode II tests and compared with the conventional system. The repairability of the vitrimeric CFRP has been investigated by attempting to recover the delaminated samples, which yielded unsatisfactory results. Moreover, the healing efficiency of the modified epoxy composites has been assessed using the vitrimer as an adhesive layer. The joints were able to recover about 84% of the lap shear strength of the pristine system.

High-Performance Reversible Furan-Maleimide Resins Based on Furfuryl Glycidyl Ether and Bismaleimides

Two reversible furan-maleimide resins, in which there are rigid -Ph-CH2-Ph- structures and flexible -(CH2)6- structures in bismaleimides, were synthesized from furfuryl glycidyl ethers (FGE), 4,4'-diaminodiphenyl ether (ODA), N,N'-4,4'-diphenylmethane-bismaleimide (DBMI), and N,N'-hexamethylene-bismaleimide (HBMI). The structures of the resins were confirmed using Fourier transform infrared analysis, and the thermoreversibility was evidenced using differential scanning calorimetry (DSC) analysis, as well as the sol-gel transformation process. Mechanical properties and recyclability of the resins were preliminarily evaluated using the flexural test. The results show the Diels-Alder (DA) reaction occurs at about 90 °C and the reversible DA reaction occurs at 130-140 °C for the furan-maleimide resin. Thermally reversible furan-maleimide resins have high mechanical properties. The flexural strength of cured FGE-ODA-HBMI resin arrives at 141 MPa. The resins have a repair efficiency of over 75%. After being hot-pressed three times, two resins display flexural strength higher than 80 MPa.

Cerium Methacrylate Assisted Preparation of Highly Thermally Conductive and Anticorrosive Multifunctional Coatings for Heat Conduction Metals Protection

Preparing polymeric coatings with well corrosion resistance and high thermal conductivity (TC) to prolong operational life and ensure service reliability of heat conductive metallic materials has long been a substantive and urgent need while a difficult task. Here we report a multifunctional epoxy composite coating (F-CB/CEP) by synthesizing cerium methacrylate and ingeniously using it as a novel curing agent with corrosion inhibit for epoxy resin and modifier for boron nitride through "cation-π" interaction. The prepared F-CB/CEP coating presents a high TC of 4.29 W m-1 K-1, which is much higher than other reported anti-corrosion polymer coatings and thereby endowing metal materials coated by this coating with outstanding thermal management performance compared with those coated by pure epoxy coating. Meanwhile, the low-frequency impedance remains at 5.1 × 1011 Ω cm2 even after 181 days of immersion in 3.5 wt% NaCl solution. Besides, the coating also exhibits well hydrophobicity, self-cleaning properties, temperature resistance and adhesion. This work provides valuable insights for the preparation of high-performance composite coatings with potential to be used as advanced multifunctional thermal management materials, especially for heat conduction metals protection.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries

The circularity of current and future polymeric materials is a major focus of fundamental and applied research, as undesirable end-of-life outcomes and waste accumulation are global problems that impact our society. The recycling or repurposing of thermoplastics and thermosets is an attractive solution to these issues, yet both options are encumbered by poor property retention upon reuse, along with heterogeneities in common waste streams that limit property optimization. Dynamic covalent chemistry, when applied to polymeric materials, enables the targeted design of reversible bonds that can be tailored to specific reprocessing conditions to help address conventional recycling challenges. In this review, we highlight the key features of several dynamic covalent chemistries that can promote closed-loop recyclability and we discuss recent synthetic progress towards incorporating these chemistries into new polymers and existing commodity plastics. Next, we outline how dynamic covalent bonds and polymer network structure influence thermomechanical properties related to application and recyclability, with a focus on predictive physical models that describe network rearrangement. Finally, we examine the potential economic and environmental impacts of dynamic covalent polymeric materials in closed-loop processing using elements derived from techno-economic analysis and life-cycle assessment, including minimum selling prices and greenhouse gas emissions. Throughout each section, we discuss interdisciplinary obstacles that hinder the widespread adoption of dynamic polymers and present opportunities and new directions toward the realization of circularity in polymeric materials.

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.

Acid-catalysed reactions of amines with dimethyl carbonate

Highly effective acid-catalysed reactions of amines with dimethyl carbonate (DMC) have been conducted with significant yields and selectivity of carboxymethylation or methylation products. Lewis acids (FeCl₃, ZnCl₂, and AlCl₃·6H₂O), Brønsted acids (PTSA, acetic, and formic acids), and acids supported on silica (silica sulfuric and silica perchlorate) resulted in carboxymethylation of primary aliphatic amines with high conversions. It was found that the Lewis acid FeCl₃ also promoted carboxymethylation of primary aromatic amines and secondary amines. At both 90 °C or an elevated temperature of 150 °C under pressure, AlCl₃·6H₂O demonstrated highly selective monomethylation of aromatic amines. In addition, both silica sulfuric acid and silica perchlorate at 90 °C exhibited no conversion for secondary amines but enhanced carboxymethylation with high conversions of 80.7-87.5% and selectivity of >99.00% at 150 °C in a pressure reactor. At 1.0 equivalent, both promoted excellent conversion and selectivity of primary aliphatic amines at 90 °C. In addition, they were easily recovered and reused for at least four additional reactions without significant loss of efficiency with consistent conversions and selectivity. Green metrics evaluation for the silica sulfuric acid-catalysed reaction highlighted the sustainability features of the process. Silica-supported catalysts are highly stable, making them ideal alternative catalysts for the methylation and carbonylation of various amines with DMC. Acid-catalysed DMC reactions of amines may expand the substrate scope and offer new opportunities for developing sustainable organic synthetic methodologies.

Bio-Based Vitrimers from 2,5-Furandicarboxylic Acid as Repairable, Reusable, and Recyclable Epoxy Systems

In this work, a series of bio-based epoxy vitrimers were developed by reacting diglycidyl ether of bisphenol A (DGEBA) and bio-based 2,5-furandicarboxylic acid (FDCA) at different molar ratios. Triazabicyclodecene was used as a transesterification catalyst to promote thermally induced exchange reactions. Differential scanning calorimetry, gel content measurements, and Fourier transform infrared spectroscopy were used to study the FDCA-DGEBA crosslinking reaction. The transesterification exchange reaction kinetics of such crosslinked systems was characterized via stress relaxation tests, evidencing an Arrhenius-type dependence of the relaxation time on temperature, and an activation energy of the dynamic rearrangement depending on the molar composition. In addition, self-healing, thermoformability, and mechanical recycling were demonstrated for the composition showing the faster topology rearrangement, namely, the FDCA/DGEBA molar ratio equal to 0.6. This work provides the first example of bio-based epoxy vitrimers incorporating FDCA, making these systems of primary importance in the field of reversible, high-performance epoxy materials for future circular economy scenarios.

Optimization and Testing of Hybrid 3D Printing Vitrimer Resins

The quality of photocure-based 3D printing greatly depends on the properties of the photoresin. There are still many challenges to be overcome at the material level before such additive manufacturing methods dominate the manufacturing industry. To contribute to this exciting re-search, an acrylate-epoxy hybrid and vitrimeric photoresin was studied to reveal the formulation parameters that could be leveraged to obtain improved processability, mechanical performance, and repairability/reprocessability. As the network becomes more lightly or densely crosslinked as a result of changing monomer compositions, or as its components are compatibilized to different extents by varying the types and loadings of the coupling agents, its thermomechanical, tensile, and vitrimeric behaviors are impacted. Using a particular formulation with a high concentration of dynamic β-hydroxyester linkages, samples are 3D printed and tested for repair and recyclability. When processed at sufficiently high temperatures, transesterification reactions are triggered, allowing for the full recovery of the tensile properties of the repaired or recycled materials, despite their inherently crosslinked structure.

Unlocking Molecular Secrets in a Monomer-Assembly-Promoted Zn-Metalated Catalytic Porous Organic Polymer for Light-Responsive CO2 Insertion

Anthropogenic carbon dioxide (CO₂) emission is soaring day by day due to fossil fuel combustion to fulfill the daily energy requirements of our society. The CO₂ concentration should be stabilized to evade the deadly consequences of it, as climate change is one of the major consequences of greenhouse gas emission. Chemical fixation of CO₂ to other value-added chemicals requires high energy due to its stability at the highest oxidation state, creating a tremendous challenge to the scientific community to fix CO₂ and prevent global warming caused by it. In this work, we have introduced a novel monomer-assembly-directed strategy to design va isible-light-responsive conjugated Zn-metalated porous organic polymer (Zn@MA-POP) with a dynamic covalent acyl hydrazone linkage, via a one-pot condensation between the self-assembled monomer 1,3,5-benzenetricarbohydrazide (TPH) and a Zn complex (Zn@COM). We have successfully explored as-synthesized Zn@MA-POP as a potential photocatalyst in visible-light-driven CO₂ photofixation with styrene epoxide (SE) to styrene carbonate (SC). Nearly 90% desired product (SC) selectivity has been achieved with our Zn@MA-POP, which is significantly better than that for the conventional Zn@TiO₂ (∼29%) and Zn@gC₃N₄ (∼26%) photocatalytic systems. The excellent light-harvesting nature with longer lifetime minimizes the radiative recombination rate of photoexcited electrons as a result of extended π-conjugation in Zn@MA-POP and increased CO₂ uptake, eventually boosting the photocatalytic activity. Local structural results from a first-shell EXAFS analysis reveals the existence of a Zn(N₂O₄) core structure in Zn@MA-POP, which plays a pivotal role in activating the epoxide ring as well as capturing the CO₂ molecules. An in-depth study of the POP-CO₂ interaction via a density functional theory (DFT) analysis reveals two feasible interactions, Zn@MA-POP-CO₂-A and Zn@MA-POP-CO₂-B, of which the latter has a lower relative energy of 0.90 kcal/mol in comparison to the former. A density of states (DOS) calculation demonstrates the lowering of the LUMO energy (EL) of Zn@MA-POP by 0.35 and 0.42 eV, respectively, for the two feasible interactions, in comparison to Zn@COM. Moreover, the potential energy profile also unveils the spontaneous and exergonic photoconversion pathways for the SE to SC conversion. Our contribution is expected to spur further interest in the precise design of visible-light-active conjugated porous organic polymers for CO₂ photofixation to value-added chemicals.

An Effective Approach to Improve the Thermal Conductivity, Strength, and Stress Relaxation of Carbon Nanotubes/Epoxy Composites Based on Vitrimer Chemistry

An effective method was developed to improve the interfacial interaction between Mutiwalled carbon nanotubes (MWCNTs) and epoxy matrix. The performance of thermal conductivity and strength of the epoxy vitrimer were enhanced by polydopamine (PDA) coating. Polydopamine is a commonly used photothermal agent, which of course, was effective in modifying MWCNTs used in photoresponsive epoxy resin. The surface temperature of the epoxy composite with 3% MWCNTs@PDA fillers added increased from room temperature to 215 °C in 48 s. The metal–catechol coordination interactions formed between the catechol groups of PDA and Zn2+ accelerated the stress relaxation of epoxy vitrimer. Moreover, the shape memory, repairing, and recycling of epoxy vitrimer were investigated. Therefore, dopamine coating is a multifunctional approach to enhance the performance of epoxy vitrimer.

Vitrimerization of Crosslinked Unsaturated Polyester Resins: A Mechanochemical Approach to Recycle and Reprocess Thermosets

Unsaturated polyester resins (UPRs) are expansively used in different applications and recycling the significant amounts of UPR waste is still a universal problem. Vitrimerization is a feasible, environmental-friendly, cost effective, and operative method, which can be used for recycling the crosslinked UPRs. In this method, the thermoset permanent network is changed into a dynamic network similar to the vitrimer-type polymers. The results show that the existence of a transesterification catalyst in the system significantly enhances the efficiency of vitrimerization. The vitrimerized UPR thermosets can be reprocessed three times with mechanical properties comparable to the initial UPR. The results show that the excess of external hydroxyl groups in the system can prevent the formation of zinc ligand complexes in the network and consequently reduce the crosslinked density and mechanical properties of vitrimerized samples. The vitrimerized thermoset powder can be reprocessed through injection molding, extrusion, and compression molding which are conventional thermoplastic processing techniques. The unrecyclable UPR thermoset wastes can be recycled and reused through vitrimerization with the least loss in mechanical properties.

Catalyst-Free Mechanochemical Recycling of Biobased Epoxy with Cellulose Nanocrystals

Mechanochemical vitrimerization, as a method to recycle cross-linked thermosets by converting the permanent network into a recyclable and reprocessable vitrimer network, inevitably requires a catalyst to accelerate the bond exchange reactions. Here, we demonstrate a catalyst-free approach to achieve the recycling of a cross-linked biobased epoxy into high-performance nanocomposites with cellulose nanocrystals (CNCs). CNCs provide abundant free hydroxyl groups to promote the transesterification exchange reactions while also acting as reinforcing fillers for the resultant nanocomposites. This technique introduces an effective way to fabricate high-performance thermoset nanocomposites based on recycled polymers in an ecofriendly way, promoting the recycle and reuse of thermosets as sustainable nanocomposites for different applications.

Transesterification in Vitrimer Polymers Using Bifunctional Catalysts: Modeled with Solution-Phase Experimental Rates and Theoretical Analysis of Efficiency and Mechanisms

Recently, thermoset vitrimer polymers have shown significant promise for structural applications because of their ability to be reshaped and remolded due to their covalent adaptive network (CAN). In these vitrimers, the transesterification reaction is responsible for the CAN, where the efficiency of the reaction is controlled either by organic or by organometallic catalysts. Understanding the mechanism of the transesterification reaction in the bulk phase using direct experimental techniques is extremely difficult due to the highly cross-linked complex structure of thermosetting vitrimers. Therefore, we use solution-phase experiments to investigate the catalytic efficiency and to guide density functional theory (DFT) simulations of the transesterification reaction mechanism with catalysts triazabicyclodecene (TBD), zinc acetate (Zn(OAc)₂), 1-methylimidazole (1-MI), and dibutyltin oxide (DBTO). The estimated catalytic efficiency from the detailed DFT reaction path calculations follows the order TBD ≳ DBTO ≳ Zn(OAc)₂ > 1-MI, which agrees with the experimental results. In addition to reaction path modeling, the mechanism and the relative rates of the transesterification reaction are analyzed with the assistance of Fukui indices as a measure of electrophilicity and nucleophilicity of atomic sites and with partial charges. It was found that the sum of the nucleophilicity index of the base and the electrophilicity index of the acid of the bifunctional catalysts correlates with the S_N2 transition state and tetrahedral intermediate energies, which are related to the barrier of the rate-limiting step. This correlation provides a hypothesis for computational prescreening of potentially better catalysts that have an index in a range of values. These results provide a basis for understanding an important part of the mechanism of transesterification in vitrimer systems and may assist with designing new catalysts.

The Impact of Vitrimers on the Industry of the Future: Chemistry, Properties and Sustainable Forward-Looking Applications

Thermosets are known to be very reliable polymeric materials for high-performance and light-weight applications, due to their retained dimensional stability, chemical inertia and rigidity over a broad range of temperatures. However, once fully cured, they cannot be easily reshaped or reprocessed, thus leaving still unsolved the issues of recycling and the lack of technological flexibility. Vitrimers, introduced by Leibler et al. in 2011, are a valiant step in the direction of bridging the chasm between thermoplastics and thermosets. Owing to their dynamic covalent networks, they can retain mechanical stability and solvent resistance, but can also flow on demand upon heating. More generally, the family of Covalent Adaptable Networks (CANs) is gleaming with astounding potential, thanks to the huge variety of chemistries that may enable bond exchange. Arising from this signature feature, intriguing properties such as self-healing, recyclability and weldability may expand the horizons for thermosets in terms of improved life-span, sustainability and overall enhanced functionality and versatility. In this review, we present a comprehensive overview of the most promising studies featuring CANs and vitrimers specifically, with particular regard for their industrial applications. Investigations into composites and sustainable vitrimers from epoxy-based and elastomeric networks are covered in detail.

Vitrimerization: Converting Thermoset Polymers into Vitrimers

Thermoset polymers with permanently cross-linked networks have outstanding mechanical properties and solvent resistance, but they cannot be reprocessed or recycled. On the other hand, vitrimers with covalent adaptable networks can be recycled. Here we provide a simple and practical method coined as "vitrimerization" to convert the permanent cross-linked thermosets into vitrimer polymers without depolymerization. The vitrimerized thermosets exhibit comparable mechanical properties and solvent resistance with the original ones. This method allows recycling and reusing the unrecyclable thermoset polymers with minimum loss in mechanical properties and enables closed-loop recycling of thermosets with the least environmental impact.

Implantation of Recyclability and Healability into Cross-Linked Commercial Polymers by Applying the Vitrimer Concept

Vitrimers are a new class of cross-linked materials that are capable of network topology alternation through the associative dynamic bond-exchange mechanism, which has recently been invented to solve the problem of conventional cross-linked materials, such as poor recyclability and healability. Thus far, the concept of vitrimers has been applied to various commercial polymers, e.g., polyesters, polylactides, polycarbonates, polydimethylsiloxanes, polydienes, polyurethanes, polyolefins, poly(meth)acrylates, and polystyrenes, by utilizing different compatible bond-exchange reactions. In this review article, the concept of vitrimers is described by clarifying the difference from thermoplastics and supramolecular systems; in addition, the term "associative bond-exchange" in vitrimers is explained by comparison with the "dissociative" term. Several useful functions attained by the vitrimer concept (including recyclability and healability) are demonstrated, and recent molecular designs of vitrimers are classified into groups depending on the types of molecular frameworks. This review specifically focuses on the vitrimer molecular designs with commercial polymer-based frameworks, which provide useful hints for the practical application of the vitrimer concept.

Vitrimers: directing chemical reactivity to control material properties

The development of more sustainable materials with a prolonged useful lifetime is a key requirement for a transition towards a more circular economy. However, polymer materials that are long-lasting and highly durable also tend to have a limited application potential for re-use. This is because such materials derive their durable properties from a high degree of chemical connectivity, resulting in rigid meshes or networks of polymer chains with a high intrinsic resistance to deformation. Once such polymers are fully synthesised, thermal (re)processing becomes hard (or impossible) to achieve without damaging the degree of chemical connectivity, and most recycling options quickly lead to a drop or even loss of material properties. In this context, both academic and industrial researchers have taken a keen interest in materials design that combines high degrees of chemical connectivity with an improved thermal (re)processability, mediated through a dynamic exchange reaction of covalent bonds. In particular vitrimer materials offer a promising concept because they completely maintain their degree of chemical connectivity at all times, yet can show a clear thermally driven plasticity and liquid behavior, enabled through rapid bond rearrangement reactions within the network. In the past decade, many suitable dynamic covalent chemistries were developed to create vitrimer materials, and are now applicable to a wide range of polymer matrices. The material properties of vitrimers, however, do not solely rely on the chemical structure of the polymer matrix, but also on the chemical reactivity of the dynamic bonds. Thus, chemical reactivity considerations become an integral part of material design, which has to take into account for example catalytic and cross-reactivity effects. This mini-review will aim to provide an overview of recent efforts aimed at understanding and controlling dynamic cross-linking reactions within vitrimers, and how directing this chemical reactivity can be used as a handle to steer material properties. Hence, it is shown how a focus on a fundamental chemical understanding can pave the way towards new sustainable materials and applications.

Dicarboxylic acid-epoxy vitrimers: influence of the off-stoichiometric acid content on cure reactions and thermo-mechanical properties

Vitrimers with 1 : 1 to 2 : 1 epoxy/acid ratio and TBD show increased stiffness and gradual transition from an exchangeable to non-exchangeable network.

An acid-degradable biobased epoxy-imine adaptable network polymer for the fabrication of responsive structural color film

A reprocessable, acid-degradable epoxy-imine network polymer was fabricated based on an epoxide of vanillin, and it was used to prepare a composite film with structural color.

Epoxy vitrimers incorporating physical crosslinks produced by self-association of alkyl chains

Vitrimers synthesized from epoxy-carboxylic acid-alkylamine (Cn) formulations exhibit tunable mechanical properties and stress relaxation without using external catalysts.

Sustainable alternative to bisphenol A epoxy resin: high-performance recyclable epoxy vitrimers derived from protocatechuic acid

A series of epoxy vitrimers were prepared based on protocatechuic acid, which showed better thermal and mechanical properties than commercial BPA-based epoxy resins. The vitrimers can be reprocessed in high efficiency and degraded in NaOH solution.

Organoselenium chemistry-based polymer synthesis

Novel synthesis of selenium containing polymers with pre-determined structures and applications thereof.

Adaptable Crosslinks in Polymeric Materials: Resolving the Intersection of Thermoplastics and Thermosets

The classical division of polymeric materials into thermoplastics and thermosets based on covalent network structure often implies that these categories are distinct and irreconcilable. Yet, the past two decades have seen extensive development of materials that bridge this gap through incorporation of dynamic crosslinks, enabling them to behave as both robust networks and moldable plastics. Although their potential utility is significant, the growth of covalent adaptable networks (CANs) has obscured the line between "thermoplastic" and "thermoset" and erected a conceptual barrier to the growing number of new researchers entering this discipline. This Perspective aims to both outline the fundamental theory of CANs and provide a critical assessment of their current status. We emphasize throughout that the unique properties of CANs emerge from the network chemistry, and particularly highlight the role that the crosslink exchange mechanism (i.e., dissociative exchange or associative exchange) plays in the resultant material properties under processing conditions. Predominant focus will be on thermally induced dynamic behavior, as the majority of presently employed exchange chemistries rely on thermal stimulus, and it is simple to apply to bulk materials. Lastly, this Perspective aims to identify current issues and address possible solutions for better fundamental understanding within this field.