Covalent Adaptable Networks Using β-Amino Esters as Thermally Reversible Building Blocks

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

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

Revealing the Dynamics of Sustainable Epoxy-Acrylate Networks from Recycled Plastics Blends and Oligomeric Lignin Precursors

The pursuit of carbon circularity in the fabrication of new materials has driven the increased use of recycled and biobased resources, a practice that has become more prevalent in recent years. In epoxy resin systems, alternatives to the use of fossil-based bisphenols have been proposed such as via the production of recycled bisphenol A (r-BPA) or by substitution with lignin derivatives, both of which are recovered from previous processes, promoting circularity. For this study, r-BPA was obtained via the chemical recycling of plastic blends from end-of-life (eol) televisions (TV). Subsequent glycidylation with epichlorohydrin (ECH) and ring-opening using acrylic acid allowed to obtain recycled bisphenol A diglycidyl ether (r-DGEBA) and bisphenol A glycerolate diacrylate (r-DAGBA), respectively. Six thermosets were fabricated by reacting Jeffamine D230 (Jeff D230) with r-DGEBA/r-DAGBA in a diverse range of epoxide:acrylate (E : A) ratios. The addition of acrylates resulted in the formation of β-amino esters (via Aza-Michael addition), which are thermo-reversible and allow the incorporation of dynamic bonds into the otherwise robust epoxy formulation. To evaluate the effect of the increasing biobased content, glycidylated depolymerized lignin (GDL) from hardwood was incorporated into the composition to produce five extra polymers. The crosslinked networks of these materials were extensively characterized, and the structure-property relationship was established by comparing their thermomechanical performance. The dissociative acrylate-amine interactions were identified under specific thermal conditions, applied systematically to program temporary shapes and analyse the crosslink reversibility of the thermosets. In summary, our findings demonstrate that recycled and biobased aromatic monomers can be incorporated to create dynamic crosslinked structures with tuneable properties, representing a step forward towards versatile, reusable, and circular materials.

A general orthogonal functionalization strategy for tailoring zwitterionic polymers with adjustable isoelectric points

Zwitterionic polymers, bearing a pair of oppositely charged groups in their repeat units, have demonstrated significant promise in both biomedical and engineering fields. Tunability of isoelectric points (IEPs) is of great value for bio-applications as it relates to key properties such as the surface charge reversal behavior, biocompatibility and the affinity to biomacromolecules. However, zwitterionic polymers with adjustable IEPs are difficult to obtain due to the fixed combination of ion pairs such as carboxybetaine-, sulfobetaine- and phosphorylcholine-based structures. To address this issue, we present a general approach to tailor zwitterionic polymers with adjustable IEPs. By developing an orthogonal functionalization strategy with sequence-controlled alternating polyesters, a series of zwitterionic polymers featuring customizable ion pairs were synthesized. This strategy, which involves aza-Michael addition and thiol-ene reaction, enables precise control over the alternating sequence of cations and anions, thereby allowing the generation of customizable ion pairs in each repeat unit. By forming block copolyesters with a hydrophobic polycaprolactone block, these polymers self-assemble into nanoparticles with tunable IEPs (e.g., 6.03, 6.37, and 6.54) and surface-charge-reversal properties, responding to physiological (pH 7.4) and tumor microenvironment (pH 6.5 ∼ 6.9) conditions. Notably, PCL54-b-P(MA-alt-AGE-g-Pip/NAC)9 (PPS3) nanoparticles, with the optimal IEP values, exhibited remarkable efficacy in inhibiting murine melanoma tumors in vivo when loaded with curcumin. This innovative approach holds promise for developing biocompatible and biodegradable drug delivery systems with tailored properties for potential clinical applications.

Strategies to Improve the Sustainability of Silicone Polymers

Silicones underpin an enormous range of simple and advanced technologies. Often, only small quantities of silicone are used to enable a technology such that, on a "per use" basis, one might suppose the environmental impact is low. However, silicone preparation processes have a very high carbon footprint, and billions of kg are produced each year. To provide context to the consideration of new strategies to improve silicone sustainability, we first outline traditional silicone chemistry and then describe strategies to improve the degree to which silicones are green, sustainable and circular. One strategy involves dilution of the silicone oil or elastomer by tethering organic entities, particularly natural products, that may provide new properties including facilitated degradation in nature at end-of-life. A greater focus is given to strategies that permit extensive reuse and repurposing of oils and elastomers (e.g., with thermoplastic elastomers), before the silicone undergoes recycling. Each reuse, repurposing or recycling step reduces the net carbon footprint. These mostly involve straightforward, high-yielding organic chemical processes that work efficiently in a silicone milieu. Silicones will eventually end up in the environment, where linear oils are known to rapidly degrade, particularly when compared to organic polymers. Alternative strategies that permit triggered or biological degradation of oils and, more importantly elastomers, are described, including enzymatic degradation and composting.

Heterocycle-based dynamic covalent chemistry for dynamic functional materials

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

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

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

Taking dynamic covalent chemistry out of the lab and into reprocessable industrial thermosets

Dynamic covalent chemistry (DCC) allows the development of thermally (re)processable and recyclable polymer networks, which is a highly attractive feature for new generations of thermoset materials. However, despite a surge in academic interest wherein soon almost any imaginable DCC platform may have been applied in a thermoset formulation, dynamic or reversible covalent polymer networks have so far found only few industrial applications. This Review provides a perspective on the main strategies for the application of DCC in the design and development of bulk thermoset materials, and it presents some of the key hurdles for their industrial implementation. The polymer design strategies and associated chemistries are viewed from the perspective of how 'close to market' their development pathway is, thus providing a roadmap to achieve high-volume breakthrough applications.

Activated Phenyl Ester Vitrimers

Aromatic esters are amongst the oldest known chemical motifs that allow for thermal (re)processing of thermosetting polymers. Moreover, phenyl esters are generally known as activated esters that do not require a catalyst to undergo acyl transfer reactions. Even though dynamic aromatic esters find applications in commercialized thermoset formulations, all-aromatic esters have found limited use so far in the design of covalent adaptable networks (CAN) as a result of their high glass transition temperature (Tg) and specific curing process. Here, a strategy to include partly aromatic esters as dynamic cross-links inside low Tg (-40 °C) thermosetting formulations, using aliphatic esters derived from para-hydroxybenzoic acid, which serves as a highly activated phenol or as a reactive "phenylogous anhydride" is reported. A small molecule study shows that the activated phenyl ester bonds can readily exchange with free phenol moieties at 200 °C under catalyst-free conditions, while the addition of a catalyst allows for a faster exchange. Robust and hydrophobic polymer networks are conveniently prepared via rapid thiol-ene UV-curing of unsaturated phenol esters. The obtained networks show high thermal stability (350 °C), fast processability, good water resistance, and low creep up to 120 °C, thus showing good promise as a platform for CAN.

Direct restoration of photocurable cross-linkers for repeated light-based 3D printing of covalent adaptable networks

Light-based processing of thermosets has gained increasing attention because of its broad application field including its use in digital light processing (DLP) 3D printing. This technique offers efficient design and fabrication of complex structures but typically results in non-recyclable thermoset-based products. To address this issue, we describe here a photocurable, dynamic β-amino ester (BAE) based cross-linker that is not only suitable for DLP printing but can also be chemically degraded via transesterification upon the addition of 2-hydroxyethyl methacrylate (HEMA) as a decross-linker. This conceptually new protocol allows for efficient depolymerization with the direct restoration of curable monomers in a single step without the addition of external catalysts or solvents. By implementing this protocol, we have established a chemical recycling loop for multiple cycles of photo-cross-linking and restoration of cross-linkers, facilitating repeatable DLP 3D printing without generating any waste. The recycled materials exhibit full recovery of thermal properties and Young's modulus while maintaining 75% of their tensile strength for at least three cycles. Simultaneously, the presence of BAE moieties enables the (re)processability of these materials through compression molding.

Debondable Epoxy-Acrylate Adhesives using β-Amino Ester Chemistry

The reuse of multilayered materials, which are held together by structural epoxy adhesives, is a major challenge since the bonded substrates cannot be easily separated for recycling. In this research, we explore a one-pot strategy based on β-amino ester chemistry for the development of modified epoxy adhesives with on-demand debonding potential. For this, a formulation of commercially available acrylate, epoxy and amine compounds is used. The research starts with a systematic study, demonstrating the influence of the different compounds on the thermal and adhesive properties of the materials. Subsequently, the potential for debonding is demonstrated using rheological measurements and tensile tests. The fast, catalyst-free Aza-Michael reaction enables the straightforward preparation of such epoxy-based adhesives, while the reverse reaction allows for debonding at 120 °C. In general, a chemical design is demonstrated for producing an industrially attractive generation of debondable epoxy-based adhesives.

Non-Isocyanate Synthesis of Covalent Adaptable Networks Based on Dynamic Hindered Urea Bonds: Sequential Polymerization and Chemical Recycling

The development of environmentally sustainable processes for polymer recycling is of paramount importance in the polymer industry. In particular, the implementation of chemical recycling for thermoset polymers via covalent adaptable networks (CANs), particularly those based on the dynamic hindered urea bond (HUB), has garnered intensive attention from both the academic and industrial sectors. This interest stems from its straightforward chemical structure and reaction mechanism, which are well-suited for commercial polyurethane and polyurea applications. However, a substantial drawback of these CANs is the requisite use of toxic isocyanate curing agents for their synthesis. Herein, we propose a new HUB synthesis pathway involving thiazolidin-2-one and a hindered amine. This ring-opening reaction facilitates the isocyanate-free formation of a HUB and enables sequential reactions with acrylate and epoxide monomers via thiol-Michael and thiol-epoxy click chemistry. The CANs synthesized using this methodology exhibit superior reprocessability, chemical recyclability, and reutilizability, facilitated by specific catalytic and solvent conditions, through the reversible HUB, thiol-Michael addition, and transesterification processes.

In situ self-assembly of pulp microfibers and nanofibers into a transparent, high-performance and degradable film

Despite the significant properties of fossil plastics, the current unsustainable methods employed in production, usage and disposal present a grave threat to both energy and environment. The development of degradable biomass materials as substitutes for fossil plastics can effectively address the energy-environment paradox at the source. Here, we prepared novel micro-nano multiscale composite films through assembling and crosslinking nanocellulose with coniferous wood pulp microfibers. The composite film combines the advantages of microfibers and nanocellulose, achieving a maximum transmittance of 91 %, foldability, excellent mechanical properties (tensile strength: 51.3 MPa, elongation at break: 4 %, young's modulus: 3.4 GPa), high thermal stability and complete degradation within 40 days. The composite film exhibits mechanochemical self-healing and retains properties even after fracture. Such exceptional performance fully meets the requirements for substituting petroleum plastics. By incorporating CaAlSiN3:Eu2+ into the composite film, it enables dual emission of red and blue light, thereby being able to promote plant growth and presenting potential as a novel sustainable alternative for agricultural films. By assembling microfiber and nanocellulose, such novel strategy is presented for the fabrication of high-quality biomass materials, thereby offering a promising avenue towards environment-friendly resource-sustainable new materials.

Carbohydrate-Based Reprocessable and Healable Covalent Adaptable Biofoams

Polymeric foams derived from bio-based resources and capable of self-healing and recycling ability are of great demand to fulfill various applications and address environmental concerns related to accumulation of plastic wastes. In this article, a set of polyester-based covalent adaptable biofoams (CABs) synthesized from carbohydrates and other bio-derived precursors under catalyst free conditions to offer a sustainable alternative to conventional toxic isocyanate-based polyurethane foams is reported. The dynamic β-keto carboxylate linkages present in these biofoams impart self-healing ability and recyclability to these samples. These CABs display adequate tensile properties especially compressive strength (≤123 MPa) and hysteresis behavior. The CABs swiftly stress relax at 150 °C and are reprocessable under similar temperature conditions. These biofoams have displayed potential for use as attachment on solar photovoltaics to augment the output efficiency. These CABs with limited swellability in polar protic solvents and adequate mechanical resilience are suitable for other commodity applications.

Internal catalysis significantly promotes the bond exchange of covalent adaptable polyurethane networks

Self-healing covalent adaptable networks (CANs) are not only of fundamental interest but also of practical importance for achieving carbon neutrality and sustainable development. However, there is a trade-off between the mobility and cross-linking structure of CANs, making it challenging to develop CANs with excellent mechanical properties and high self-healing efficiency. Here, we report the utilization of a highly dynamic four-arm cross-linking unit with an internally catalyzed oxime-urethane group to obtain CAN-based ionogel with both high self-healing efficiency (>92.1%) at room temperature and superior mechanical properties (tensile strength 4.55 MPa and toughness 13.49 MJ m-3). This work demonstrates the significant potential of utilizing the synergistic electronic, spatial, and topological effects as a design strategy for developing high-performance materials.

"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.

Energetic Nitrate-Based Polymer-Bonded Explosives Derived from Sustainable Aza-Michael Reactions

A novel nontoxic method for processing energetic binder, namely, polyglycidyl nitrate (PGN), using Aza-Michael reactions for deriving high-performance explosive formulations is being reported. The polyol binders used in polymer-bonded explosives (PBX) including PGN are usually cross-linked using isocyanate leading to polyurethane (PU)-based cured solid networks. These reactions require mild reaction conditions and yield good mechanical properties for the PBX but remain challenging due to extraneous reactions of isocyanate resulting in defects in the cured blocks. In addition, the presence of nitrato groups in the vicinity of terminal hydroxyl groups of PGN results in the decuring of cross-linked urethane that affects the storage life of PBX, though PGN-based binder can provide an 18% improvement in the velocity of detonation of PBX at lower solid loadings of 70%. This prevents researchers from exploiting the major performance advantage of using PGN for PBX compositions. This article herein features a green and mild aza-Michael reaction for functional modification of PGN using readily available substrates and triethylene tetramine to form a cross-linked β-aminocarbonyl network. The methodology ensures a void-free, stable, cured network and offers an effective replacement for toxic cure chemistry currently employed for processing of PBX.

Deciphering the Self-Catalytic Mechanisms of Polymerization and Transesterification in Polybenzoxazine Vitrimers

The use of internal catalysts has emerged as a pivotal design principle to facilitate dynamic exchanges within covalent adaptable networks (CANs). Polybenzoxazines, specifically, have shown considerable potential in generating vitrimers through thermally induced transesterification reactions catalyzed internally by tertiary amines. This study aims to investigate the chemical complexities of transesterification reactions within benzoxazine vitrimers. To achieve this, model molecules using various phenolic acids and amino-alcohol derivatives were synthesized as precursors. The structure of these model molecules was fully elucidated by using nuclear magnetic resonance (NMR). Differential scanning calorimetry (DSC) and rheology experiments evidenced the accelerated network formation of the precursors due to the presence of aliphatic -OH groups. Thermogravimetric analysis coupled with microcomputed gas chromatography (TGA-μGC) was used to provide evidence of transesterification reactions. The results showed that the spatial proximity between tertiary amine and hydroxyl groups significantly enhances the rate exchange, attributed to a neighboring group participation (NGP) effect. Interestingly, kinetic experiments using complementary NMR techniques revealed the thermal latency of the tertiary amine of benzoxazine toward transesterification reactions as its opening is needed to trigger the dynamic exchange. The study highlights the crucial role of steric hindrance and tertiary amine basicity in promoting the dynamic exchange in an internally catalyzed system.

New Prospects Arising from Dynamically Crosslinked Polymers: Reprogramming Their Properties

Dynamically crosslinked polymers (DCPs) have gained significant attention owing to their applications in fabricating (re)processable, recyclable, and self-healable thermosets, which hold great promise in addressing ecological issues, such as plastic pollution and resource scarcity. However, the current research predominantly focuses on redefining and/or manipulating their geometries while replicating their bulk properties. Given the inherent design flexibility of dynamic covalent networks, DCPs also exhibit a remarkable potential for various novel applications through postsynthesis reprogramming their properties. In this review, the recent advancements in strategies that enable DCPs to transform their bulk properties after synthesis are presented. The underlying mechanisms and associated material properties are overviewed mainly through three distinct strategies, namely latent catalysts, material-growth, and topology isomerizable networks. Furthermore, the mutual relationship and impact of these strategies when integrated within one material system are also discussed. Finally, the application prospects and relevant issues necessitating further investigation, along with the potential solutions are analyzed.

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.

Direct and Catalyst-Free Ester Metathesis Reaction for Covalent Adaptable Networks

Thermosetting polymers possess excellent environmental resistance and mechanical properties but cannot be reprocessed due to their covalently cross-linked structures. Recycling of thermosets via the implantation of dynamic covalent bonds offers a promising solution. Here, we report the direct and catalyst-free ester metathesis of N-acyloxyphthalimide (NAPI) at about 100 °C without the requirement of hydroxyl groups and its utilization for the fabrication of covalent adaptable networks (CANs). NAPI metathesis has interesting sigmoid kinetics with a fast exchange rate, which proceeds via a free radical chain mechanism, guaranteeing a fast associative exchange under a rather low dissociation. The bifunctional molecule of NAPI as both the radical precursor and substrate is the key to the dissociatively initiated associative (DAssociative) mechanism and kinetic behavior. Based on the efficient NAPI metathesis, polyester networks, poly(N-acyloxyphthalimides) (PNAPIs), show excellent malleability. Notably, PNAPIs exhibit exceptional solvent resistance and mechanical stability at elevated temperatures owing to the unique DAssociative mechanism, suggesting exciting opportunities for designing recyclable thermosetting polymers.

High-performance polyimine vitrimers from an aromatic bio-based scaffold

Bio-based vitrimers represent a promising class of thermosetting polymer materials, pairing the recyclability of dynamic covalent networks with the renewability of non-fossil fuel feedstocks. Vanillin, a low-cost lignin derivative, enables facile construction of polyimine networks marked by rapid exchange and sensitivity to acid-catalyzed hydrolysis. Furthermore, the aromatic structure makes it a promising candidate for the design of highly aromatic networks capable of high-performance thermal and dimensional stability. Such properties are paramount in polymeric thermal protection systems. Here, we report on the fabrication of polyimine networks with particularly high aromatic content from a novel trifunctional vanillin monomer prepared from the nucleophilic aromatic substitution of perfluoropyridine (PFP) on a multi-gram scale (>20 g) in high yield (86%). The trifunctional aromatic scaffold was then crosslinked with various diamines to demonstrate tunable viscoelastic behavior and thermal properties, with glass transition temperatures (Tg) ranging from 9 to 147 °C, degradation temperatures (5% mass loss) up to approximately 370 °C, and excellent char yields up to 68% at 650 °C under nitrogen. Moreover, the vitrimers displayed mechanical reprocessability over five destruction/healing cycles and rapid chemical recyclability following acidic hydrolysis at mild temperatures. Our findings indicate that vitrimers possessing tunable properties and high-performance thermomechanical behavior can be easily constructed from vanillin and electrophilic aromatic scaffolds for applications in heat-shielding materials and ablative coatings.

Dynamic PEG-PLA/Hydroxyurethane Networks Based on Imine Bonds as Reprocessable Elastomeric Biomaterials

The development of dynamic covalent chemistry opens the way to the design of materials able to be reprocessed by an internal exchange reaction under thermal stimulus. Imine exchange differs from other exchange reactions by its relatively low temperature of activation. In this study, amine-functionalized star-shaped PEG-PLA and an aldehyde-functionalized hydroxyurethane modifier were combined to produce PEG-PLA/hydroxyurethane networks incorporating imine bonds. The thermal and mechanical properties of these new materials were evaluated as a function of the initial ratio of amine/aldehyde used during synthesis. Rheological analyses highlighted the dynamic behavior of these vitrimers at moderate temperature (60-85 °C) and provided the flow activation energies. Additionally, the reprocessability of these PEG-PLA/hydroxyurethane vitrimers was assessed by comparing the material properties before reshaping and after three reprocessing cycles (1 ton, 1 h, 70 °C). Hence, these materials can easily be designed to satisfy a specific medical application without properties loss. This work opens the way to the development of a new generation of dynamic materials combining degradable PEG-PLA copolymers and hydroxyurethane modifiers, which could find applications in the shape of medical devices on-demand under mild conditions.

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.

Spiroborate-Linked Ionic Covalent Adaptable Networks with Rapid Reprocessability and Closed-Loop Recyclability

Covalent adaptable networks (CANs) represent a novel class of polymeric materials crosslinked by dynamic covalent bonds. Since their first discovery, CANs have attracted great attention due to their high mechanical strength and stability like conventional thermosets under service conditions and easy reprocessability like thermoplastics under certain external stimuli. Here, we report the first example of ionic covalent adaptable networks (ICANs), a type of crosslinked ionomers, consisting of negatively charged backbone structures. More specifically, two ICANs with different backbone compositions were prepared through spiroborate chemistry. Given the dynamic nature of the spiroborate linkages, the resulting ionomer thermosets display rapid reprocessability and closed-loop recyclability under mild conditions. The materials mechanically broken into smaller pieces can be reprocessed into coherent solids at 120 °C within only 1 min with nearly 100% recovery of the mechanical properties. Upon treating the ICANs with dilute hydrochloric acid at room temperature, the valuable monomers can be easily chemically recycled in almost quantitative yield. This work demonstrates the great potential of spiroborate bonds as a novel dynamic ionic linkage for development of new reprocessable and recyclable ionomer thermosets.

Poly(ether)s derived from oxa-Michael polymerization: a comprehensive review

Poly(ether)s represent an important class of polymers and are typically formed by ring-opening polymerization, Williamson ether synthesis, or self-condensation of alcohols. The oxa-Michael reaction presents another method to form poly(ether)s with additional functional groups in the polymer backbone starting from di- or triols and electron deficient olefins such as acrylates, sulfones, or acrylamides. However, research on oxa-Michael polymerization is still limited. Herein, we outline the principles of the oxa-Michael polymerization and focus on the synthesis and preparation of poly(ether-sulfone)s, poly(ether-ester)s, poly(ether)s, and poly(ether-amide)s. Further, challenges as well as future perspectives of the oxa-Michael polymerization are discussed.

Graphical abstract

Closed-loop recyclable and biodegradable thioester-based covalent adaptable networks

Closed-loop recyclable and biodegradable aliphatic covalent adaptable networks (CANs) based on dynamic β-CO thioester linkages that exhibit a service temperature beyond 100 °C are reported. These CANs possessing tensile strength and modulus values of up to 0.3 and 3 MPa, respectively, effectively undergo stress relaxation above 100 °C. The samples exhibit creep resistance ability and low hysteresis loss, and are repeatedly reprocessable at 120 °C. These CANs are depolymerizable to monomers under mild conditions and lose notable mechanical strength (92.4%) and weight (76.5%) within ∼35 days under natural biodegradation conditions.

3D printing of dynamic covalent polymer network with on-demand geometric and mechanical reprogrammability

Delicate geometries and suitable mechanical properties are essential for device applications of polymer materials. 3D printing offers unprecedented versatility, but the geometries and mechanical properties are typically fixed after printing. Here, we report a 3D photo-printable dynamic covalent network that can undergo two independently controllable bond exchange reactions, allowing reprogramming the geometry and mechanical properties after printing. Specifically, the network is designed to contain hindered urea bonds and pendant hydroxyl groups. The homolytic exchange between hindered urea bonds allows reconfiguring the printed shape without affecting the network topology and mechanical properties. Under different conditions, the hindered urea bonds are transformed into urethane bonds via exchange reactions with hydroxyl groups, which permits tailoring of the mechanical properties. The freedom to reprogram the shape and properties in an on-demand fashion offers the opportunity to produce multiple 3D printed products from one single printing step.

In-situ forming dynamic covalently crosslinked nanofibers with one-pot closed-loop recyclability

Polymeric nanofibers are attractive nanomaterials owing to their high surface-area-to-volume ratio and superior flexibility. However, a difficult choice between durability and recyclability continues to hamper efforts to design new polymeric nanofibers. Herein, we integrate the concept of covalent adaptable networks (CANs) to produce a class of nanofibers ⎯ referred to dynamic covalently crosslinked nanofibers (DCCNFs) via electrospinning systems with viscosity modulation and in-situ crosslinking. The developed DCCNFs possess homogeneous morphology, flexibility, mechanical robustness, and creep resistance, as well as good thermal and solvent stability. Moreover, to solve the inevitable issues of performance degradation and crack of nanofibrous membranes, DCCNF membranes can be one-pot closed-loop recycled or welded through thermal-reversible Diels-Alder reaction. This study may unlock strategies to fabricate the next generation nanofibers with recyclable features and consistently high performance via dynamic covalent chemistry for intelligent and sustainable applications.

Stress-Induced Shape-Shifting Materials Possessing Autonomous Self-Healing and Scratch-Resistant Ability

Covalent adaptable networks (CANs) capable of both shape-shifting and self-healing ability offer a viable alternative to 4D printing technology to gain access to various complex shapes in a simplified manner. However, most of the reported CANs exhibit shape-shifting ability in the presence of temperature, light or chemical stimuli, which restricts their further utilization as realization of such a controlled environment is not feasible under complex scenarios. Herewith, we report a set of CANs based on a room-temperature exchangeable thia-Michael adduct, which undergoes rearrangement in network topology on application of external stress. These CANs with tensile strength (≤6 MPa) and modulus (≤71.4 MPa) adopt to any programmed shape under application of nominal stress. The CANs also exhibit stress-induced recyclability, self-welding and self-healing ability under ambient conditions. The transparency and ambient condition self-healing ability render these CANs to be utilized as scratch-resistant coatings on display items.

Thia-Michael Reaction: The Route to Promising Covalent Adaptable Networks

While the Michael addition has been employed for more than 130 years for the synthesis of a vast diversity of compounds, the reversibility of this reaction when heteronucleophiles are involved has been generally less considered. First applied to medicinal chemistry, the reversible character of the hetero-Michael reactions has recently been explored for the synthesis of Covalent Adaptable Networks (CANs), in particular the thia-Michael reaction and more recently the aza-Michael reaction. In these cross-linked networks, exchange reactions take place between two Michael adducts by successive dissociation and association steps. In order to understand and precisely control the exchange in these CANs, it is necessary to get an insight into the critical parameters influencing the Michael addition and the dissociation rates of Michael adducts by reconsidering previous studies on these matters. This review presents the progress in the understanding of the thia-Michael reaction over the years as well as the latest developments and plausible future directions to prepare CANs based on this reaction. The potential of aza-Michael reaction for CANs application is highlighted in a specific section with comparison with thia-Michael-based CANs.

Renewable Vanillin-Based Thermoplastic Polybutadiene Rubber: High Strength, Recyclability, Self-Welding, Shape Memory, and Antibacterial Properties

The vast majority of traditional vulcanized rubber products are insoluble and infusible, which is difficult to reprocess and biodegrade, resulting in black pollution. In addition, although most rubber materials based on covalent adaptive networks (CANs) can achieve structural reconstruction, the lack of traditional vulcanization system leads to a decline in strength. In this study, biobased vanillin derivatives (PV) were synthesized to cross-link the commercially available 1,2-polybutadiene rubber precursor to construct imine-based CANs, thereby fabricating a resource-renewable, recyclable, and degradable high-performance rubber material. Due to the rigid tripod structure of the PV, the tensile strength of the material can achieve as high as 16.24 MPa, ranking among the best in the field of recyclable polybutadiene-based materials. Benefiting from the dynamic imine unit, the "dynamic covalent bridge" can be re-established to repair the damaged network and endow the material with excellent weldability. And, shape memory faculty of the material was proved and depicted. Moreover, this material displayed excellent antibacterial property originates from the introduced Schiff-base structure. By mixing with graphene, the application of action sensors can also be achieved.

Masked Primary Amines for a Controlled Plastic Flow of Vitrimers

We present a simple method for increasing the reprocessability of vinylogous urethane (VU) vitrimers while decreasing the possibility of creep deformation at lower temperatures. In particular, varying amounts of triethylenetetramine were added as a comonomer to the curing VU formulation to ensure that all of the primary amines reacted to form enaminone cross-links, resulting in a network without reactive primary amine chain-ends. As a result, transamination was significantly slowed down because secondary amines are much less reactive to VU exchange. On the other hand, at higher temperatures, pendent primary amines can be released via a dynamic, endothermic exchange with a nearby less-reactive secondary amine, thereby (re)activating material flow. As a result, ambivalent viscoelastic behavior could be achieved without depolymerization by dynamically releasing pendent primary amines from vinylogous urethane polymer chains. Through careful comonomer selection, VU vitrimers with low viscosity at processing temperatures and at the same time high viscosity at service temperatures could be prepared without the use of catalysts or additives, leveraging the synergistic effects of mildly reactive functionalities through neighboring group participation.

Fast Dynamic Siloxane Exchange Mechanism for Reshapable Vitrimer Composites

To develop siloxane-containing vitrimers with fast dynamic characteristics, different mechanistic pathways have been investigated using a range of catalysts. In particular, one siloxane exchange pathway has been found to show a fast dynamic behavior in a useful temperature range (180-220 °C) for its application in vitrimers. The mechanism is found to involve 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) as an organic catalyst in the presence of hydroxyl groups. Using this new mechanistic approach, vitrimers with ultrafast stress-relaxation characteristics (relaxation times below 10 s) have been prepared with a readily available epoxy resin and siloxane-amine hardener. Subsequently, the low viscosity siloxane-containing vitrimer resin enabled the preparation of glass fiber-reinforced vitrimer composites using an industrially relevant vacuum-assisted resin infusion technique. The resulting composite was successfully thermoformed into a new shape, which makes it possible to envision a second life for such highly engineered materials.

Intrinsically Re-curable Photopolymers Containing Dynamic Thiol-Michael Bonds

The development of photopolymers that can be depolymerized and subsequently re-cured using the same light stimulus presents a significant technical challenge. A bio-sourced terpenoid structure, l-carvone, inspired the creation of a re-curable photopolymer in which the orthogonal reactivity of an irreversible thioether and a dynamic thiol-Michael bond enables both photopolymerization and thermally driven depolymerization of mechanically robust polymer networks. The di-alkene containing l-carvone was partially reacted with a multi-arm thiol to generate a non-crosslinked telechelic photopolymer. Upon further UV exposure, the photopolymer crosslinked into a mechanically robust network featuring reversible Michael bonds at junction points that could be activated to revert, or depolymerize, the network into a viscous telechelic photopolymer. The regenerated photopolymer displayed intrinsic re-curability over two recycles while maintaining the desirable thermomechanical properties of a conventional network: insolubility, resistance to stress relaxation, and structural integrity up to 170 °C. Our findings present an on-demand, re-curable photopolymer platform based on a sustainable feedstock.

Healable, Recyclable, and Programmable Shape Memory Organogels Based on Highly Malleable Catalyst-Free Carboxylate Linkages

The development of healable and recyclable organogels possessing responsive abilities is mainly hindered by the unavailability of many dynamic covalent linkages that undergo exchange reaction below the boiling temperature of organic swelling medium. Furthermore, the exchange is desired to be effective under catalyst-free conditions to circumvent the issue of catalyst leaching during the swelling process. Especially, imparting swift reversibility to thermostable carboxylate linkages is challenging. In this approach, we have utilized the β-keto anchimeric assistance as the tool to induce swift reversibility into the conventional carboxylate linkage under mild temperature (∼70-90 °C) and catalyst-free conditions. Using this β-keto carboxylate linkage as an associative bond exchange mean, strong (tensile strength = 0.3 MPa) and stretchable (ultimate elongation ≈ 600%) covalent adaptable organogels (CAOs) with anisotropic swelling, remoldable, self-healing, and shape memory ability are derived from commercially available precursors. The shape memory ability of these samples shows dependency on the shape fixing time and can be programmed, targeting further applications. Soft actuators may be fabricated from the CAOs using temperature and solvent as the activating tools. This research demonstrates that the conventional carboxylate linkages can be made labile under mild conditions for further applications.

Double Networks of Liquid-Crystalline Elastomers with Enhanced Mechanical Strength

Liquid-crystalline elastomers (LCEs) are frequently used in soft actuator development. However, applications are limited because LCEs are prone to mechanical failure when subjected to heavy loads and high temperatures during the working cycle. A mechanically tough LCE system offers larger work capacity and lower failure rate for the actuators. Herein, we adopt the double-network strategy, starting with a siloxane-based exchangeable LCE and developing a series of double-network liquid-crystalline elastomers (DN-LCEs) that are mechanically tougher than the initial elastomer. We incorporate diacrylate reacting monomers to fabricate DN-LCEs, some of which have the breaking stress of 40 MPa. We incorporate thermoplastic polyurethane to fabricate a DN-LCE, achieving an enormous ductility of 90 MJ/m³. We have also attempted to utilize the aza-Michael chemistry to make a DN-LCE that retains high plasticity because of several bond-exchange mechanisms; however, it failed to produce a stable reprocessable LCE system using conventional ester-based reactive mesogens. Each of these DN-LCEs exhibits unique features and characteristics, which are compared and discussed.

Combining vinylogous urethane and β-amino ester chemistry for dynamic material design

This study combines vinylogous urethane (VU) and beta-amino ester chemistry for the synthesis of covalent adaptable networks (CANs). The resulting CANs are synthesised using a range of diacetoacetates and commercially...

Dynamic covalent polymers enabled by reversible isocyanate chemistry

Reversible isocyanate chemistry containing urethane, thiourethane, and urea bonds is valuable for designing dynamic covalent polymers to achieve promising applications in recycling, self-healing, shape morphing, 3D printing, and composites.

Taking advantage of β-hydroxy amine enhanced reactivity and functionality for the synthesis of dual covalent adaptable networks

This study highlights the potential of β-hydroxy amines as building blocks for aza-Michael CANs.

Room-Temperature Self-Healable and Mechanically Robust Thermoset Polymers for Healing Delamination and Recycling Carbon Fibers

The advocacy of carbon neutrality and circular economy encourages people to pursue self-healing and recycling of glassy thermoset polymers in a more realistic and energy-saving manner, the best being intrinsic healing under room temperature. However, the high mechanical robustness and healing ability are mutually exclusive because of their completely opposite requirements for the mobility of the polymer networks. Here, we report a dual-cross-linked network by slightly coupling the low-molecular-weight branched polyethylenimine with an ester-containing epoxy monomer in a nonstoichiometric proportion. The highly mobile and dense noncovalent hydrogen bonds at the chain branches and ends can not only complement the mechanical robustness (tensile strength of 61.6 MPa, elastic modulus of 1.6 GPa, and toughness of 19.2 MJ/m3) but also endow the glassy thermoset polymer (Tg > 40 °C) with intrinsic self-healing ability (healing efficiency > 84%) at 20 °C. Moreover, the resultant covalent adaptive network makes the thermoset polymer stable to high temperatures and solvents, yet it is readily dissolved in ethylene glycol through internal catalyzed transesterification. The application to room temperature delamination healing and carbon fiber recycling was demonstrated as a proof-of-concept.

Reprocessing of Covalent Adaptable Polyamide Networks through Internal Catalysis and Ring-Size Effects

Here, we report the introduction of internally catalyzed amide bonds to obtain covalent adaptable polyamide networks that rely on the dissociation equilibrium between dicarboxamides and imides. While amide bonds are usually considered to be robust and thermally stable, the present study shows that their dynamic character can be activated by a smart choice of available building blocks without the addition of any external catalyst or other additives. Hence, a range of polyamide-based dynamic networks with variable mechanical and viscoelastic properties have been obtained in a systematic study, using a straightforward curing process of dibasic ester and amine compounds. Since the dissociation process involves a cyclic imide formation, the correlation between ring size and the thermomechanical viscosity profile was studied for five- to seven-membered ring intermediates, depending on the chosen dibasic ester monomer. This resulted in a marked temperature response with activation energies in the range of 116-197 kJ mol^-1, yielding a sharp transition between elastic and viscous behavior. Moreover, the ease and versatility of this chemistry platform were demonstrated by selecting a variety of amines, resulting in densely cross-linked dynamic networks with T_g values ranging from -20 to 110 °C. With this approach, it is possible to design amorphous polyamide networks with an acute temperature response, allowing for good reprocessability and, simultaneously, high resistance to irreversible deformation at elevated temperatures.