Biosourced Acetal and Diels–Alder Adduct Concurrent Polyurethane Covalent Adaptable Network

Dual-Factor-Controlled Dynamic Precursors Enable On-Demand Thermoset Degradation and Recycling

Thermosets are well known for their advantages such as high stability and chemical resistance. However, developing sustainable thermosets with degradability and recyclability faces several principal challenges, including reconciling the desired characteristics during service with the recycling and reprocessing properties required at the end of life, establishing efficient methods for large-scale synthesis, and aligning with current manufacturing process. Here a general strategy is presented for the on-demand degradation and recycling of thermosets under mild conditions utilizing dynamic precursors with dual-factor-controlled reversibility. Specifically, dynamic triazine crosslinkers are introduced through dynamic nucleophilic aromatic substitution (SNAr) into the precursor polyols used in polyurethane (PU) synthesis. Upon removal of the catalyst and alcohol, the reversibility of SNAr is deactivated, allowing for the use of standard PU polymerization techniques such as injection molding, casting, and foaming. The resulting cyanurate-crosslinked PUs maintain high stability and diverse mechanical properties of traditional crosslinked PUs, yet offer the advantage of easy on-demand depolymerization for recycling by activating the reversibility of SNAr under specific but mild conditions-a combination of base, alcohol, and mild heat. It is envisioned that this approach, involving the pre-installation of dual-factor-controlled dynamic crosslinkers, can be broadly applied to current thermosetting plastic manufacturing processes, introducing enhanced sustainability.

Sustainable and Scalable Synthesis of Acetal-Containing Polyols as a Platform for Circular Polyurethanes

Polyurethanes (PUs) are highly versatile polymers widely utilized across industries. However, chemical recycling of PU poses significant challenges due to the harsh conditions required and the formation of complex mixtures of oligomers upon depolymerization. Addressing this inherent lack of recyclability, we developed closed-loop recyclable PU materials by integrating cleavable acetal groups. We present a sustainable and scalable synthesis method for acetal-containing polyols (APs) through aldehyde-diol polycondensation, utilizing reusable heterogeneous catalysts. Three APs with different hydrolytic stabilities depending on the structure of acetal groups were synthesized from formaldehyde, acetaldehyde, and propionaldehyde with 1,6-hexanediol (H16). These APs were employed alongside 4,4'-methylene diisocyanate (MDI) for preparation of PU materials. The resulting PUs exhibited mechanical properties comparable to or surpassing those of conventional PUs, while demonstrating excellent recyclability under acidic conditions. Notably, hydrolysis of PU materials based on acetaldehyde-derived APs yielded remarkable monomer recovery rates, with 89 % for H16 and 84 % for 4,4'-methylenedianiline, a precursor to MDI. Furthermore, we successfully demonstrated closed-loop recycling by synthesizing APs from recovered H16, resulting in PU materials with identical properties to the original PU. This achievement highlights the potential for establishing a closed-loop recycling system for acetal-containing PUs, contributing to the advancement of a sustainable and circular economy.

Recycling of Commercially Available Biobased Thermoset Polyurethane Using Covalent Adaptable Network Mechanisms

Until recently, recycling thermoset polyurethanes (PUs) was limited to degrading methods. The development of covalent adaptable networks (CANs), to which PUs can be assigned, has opened novel possibilities for actual recycling. Most efforts in this area have been directed toward inventing new materials that can benefit from CAN theory; presently, little or nothing has been applied to industrially producible materials. In this study, both an industrially available polyol (Sovermol780®) and isocyanate (Tolonate X FLO 100®) with percentages of bioderived components were employed, resulting in a potentially scalable and industrially producible material. The resultant network could be reworked up to three times, maintaining the crosslinked structure without significantly changing the thermal properties. Improvements in mechanical parameters were observed when comparing the pristine material to the material exposed to three rework processes, with gains of roughly 50% in elongation at break and 20% in tensile strength despite a 25% decrease in Young's modulus and crosslink density. Thus, it was demonstrated that theory may be profitably applied even to materials that are not designed including additional bonds but instead rely just on the dynamic urethane bond that is naturally present in the network.

Malleable, Ultrastrong Antibacterial Thermosets Enabled by Guanidine Urea Structure

Dynamic covalent polymers (DCPs) that strike a balance between high performance and rapid reconfiguration have been a challenging task. For this purpose, a solution is proposed in the form of a new dynamic covalent supramolecular motif-guanidine urea structure (GUAs). GUAs contain complex and diverse chemical structures as well as unique bonding characteristics, allowing guanidine urea supramolecular polymers to demonstrate advanced physical properties. Noncovalent interaction aggregates (NIAs) have been confirmed to form in GUA-DCPs through multistage H-bonding and π-π stacking, resulting in an extremely high Young's modulus of 14 GPa, suggesting remarkable mechanical strength. Additionally, guanamine urea linkages in GUAs, a new type of dynamic covalent bond, provide resins with excellent malleability and reprocessability. Guanamine urea metathesis is validated using small molecule model compounds, and the temperature dependent infrared and rheological behavior of GUA-DCPs following the dissociative exchange mechanism. Moreover, the inherent photodynamic antibacterial properties are extensively verified by antibacterial experiments. Even after undergoing three reprocessing cycles, the antibacterial rate of GUA-DCPs remains above 99% after 24 h, highlighting their long-lasting antibacterial effectiveness. GUA-DCPs with dynamic nature, tuneable composition, and unique combination of properties make them promising candidates for various technological advancements.

Acetal-thiol Click-like Reaction: Facile and Efficient Synthesis of Dynamic Dithioacetals and Recyclable Polydithioacetals

Dithioacetals are heavily used in organic, material and medical chemistries, and exhibit huge potential to synthesize degradable or recyclable polymers. However, the current synthetic approaches of dithioacetals and polydithioacetals are overwhelmingly dependent on external catalysts and organic solvents. Herein, we disclose a catalyst- and solvent-free acetal-thiol click-like reaction for synthesizing dithioacetals and polydithioacetals. High conversion, higher than acid catalytic acetal-thiol reaction, can be achieved. High universality was confirmed by monitoring the reactions of linear and cyclic acetals (including renewable bio-sourced furan-acetal) with aliphatic and aromatic thiols, and the reaction mechanism of monomolecular nucleophilic substitution (SN1) and auto-protonation (activation) by thiol was clarified by combining experiments and density functional theory computation. Subsequently, we utilize this reaction to synthesize readily recyclable polydithioacetals. By simple heating and stirring, linear polydithioacetals withM ‾ ${\bar M}$ w of ~110 kDa were synthesized from acetal and dithiol, and depolymerization into macrocyclic dithioacetal and repolymerization into polydithioacetal can be achieved; through reactive extrusion, a semi-interpenetrating polymer dynamic network with excellent mechanical properties and continuous reprocessability was prepared from poly(vinyl butyral) and pentaerythritol tetrakis(3-mercaptopropionate). This green and high-efficient synthesis method for dithioacetals and polydithioacetals is beneficial to the sustainable development of chemistry.

New Advances in Covalent Network Polymers via Dynamic Covalent Chemistry

Covalent network polymers, as materials composed of atoms interconnected by covalent bonds in a continuous network, are known for their thermal and chemical stability. Over the past two decades, these materials have undergone significant transformations, gaining properties such as malleability, environmental responsiveness, recyclability, crystallinity, and customizable porosity, enabled by the development and integration of dynamic covalent chemistry (DCvC). In this review, we explore the innovative realm of covalent network polymers by focusing on the recent advances achieved through the application of DCvC. We start by examining the history and fundamental principles of DCvC, detailing its inception and core concepts and noting its key role in reversible covalent bond formation. Then the reprocessability of covalent network polymers enabled by DCvC is thoroughly discussed, starting from the significant milestones that marked the evolution of these polymers and progressing to their current trends and applications. The influence of DCvC on the crystallinity of covalent network polymers is then reviewed, covering their bond diversity, synthesis techniques, and functionalities. In the concluding section, we address the current challenges faced in the field of covalent network polymers and speculates on potential future directions.

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.

Polyurethane Adhesives with Chemically Debondable Properties via Diels-Alder Bonds

Covalent adaptable networks (CANs) represent a pioneering advance in polymer science, offering unprecedented versatility in materials design. Unlike conventional adhesives with irreversible bonds, CAN-based polyurethane adhesives have the unique ability to undergo chemical restructuring through reversible bonds. One of the strategies for incorporating these types of reactions in polyurethanes is by functionalisation with Diels-Alder (DA) adducts. By taking advantage of the reversible nature of the DA chemistry, the adhesive undergoes controlled crosslinking and decrosslinking processes, allowing for precise modulation of bond strength. This adaptability is critical in applications requiring reworkability or recyclability, as it allows for easy disassembly and reassembly of bonded components without compromising the integrity of the material. This study focuses on the sustainable synthesis and characterisation of a solvent-based polyurethane adhesive, obtained by functionalising a polyurethane prepolymer with DA diene and dienophiles. The characterisation of the adhesives was carried out using different experimental techniques: nuclear magnetic resonance spectroscopy (NMR), Brookfield viscosity, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and T-peel strength testing of leather/adhesive/rubber joints to determine the adhesive properties, both before and after the application of external stimuli. The conversion of both the DA and retro-Diels-Alder (r-DA) reactions was confirmed by 1H-NMR. The adhesive properties were not altered by the functionalisation of the adhesive prepolymer, showing similar thermal resistance and good rheological and adhesive properties, even exceeding the most demanding technical requirements for upper-to-sole joints in footwear. After the application of an external thermal stimuli, the bonded materials separated without difficulty and without damage, thus facilitating their separation, recovery and recycling.

Closed-Loop Recyclable Poly(imine-acetal)s with Dual-Cleavable Bonds for Primary Building Block Recovery

Chemical recycling offers a promising solution for the end-of-life treatment of synthetic polymers. However, the efficient recovery of well-defined recycled building blocks continues to be a major challenge, especially for crosslinked thermosets. Here, we developed vanillin-based polymer networks functionalized with dual-cleavable imine and acetal bonds that facilitate chemical recycling to primary building blocks and their convenient separation at the molecular level. A library of crosslinked poly(imine-acetal)s was synthesized by combining the in-bulk synthesized liquid di-vanillin acetal monomer (DVA) with commercially available liquid di- and triamines under solvent-free conditions. These thermosets showed tailor-made thermal and mechanical properties along with outstanding chemical recyclability. Under aqueous acidic conditions, poly(imine-acetal)s selectively and completely disintegrate into small molecules. During the polymer design stage, these compounds were carefully selected to enable facile separation without tedious techniques. As a result, the primary building blocks were isolated in high yields and purity and immediately reused to produce fresh polymers with identical thermomechanical properties. Since our "design for recycling" concept aims at obtaining the primary building blocks rather than monomers after depolymerization, a plethora of possibilities are unlocked to utilize these chemical resources, including closed-loop recycling as portrayed.

Chemically Recyclable Biobased Non-Isocyanate Polyurethane Networks from CO2 -Derived Six-membered Cyclic Carbonates

Non-isocyanate polyurethanes (NIPUs) are widely studied as sustainability potential, because they can be prepared without using toxic isocyanates in the synthesis process. The aminolysis of cyclic carbonate to form NIPUs is a promising route. In this work, a series of NIPUs is prepared from renewable bis(6-membered cyclic carbonates) (iEbcc) and amines. The resulting NIPUs possess excellent mechanical properties and thermal stability. The NIPUs can be remolded via transcarbamoylation reactions, and iEbcc-TAEA-10 (the molar ratio of tris(2-aminoethyl)amine in amines is 10%) still get a recovery ratio of 90% in tensile stress after three cycles of remolding. In addition, the obtained materials can be chemically degraded into bi(1,3-diol) precursors with high purity (>99%) and yield (>90%) through alcoholysis. Meanwhile, the degraded products can be used to regenerate NIPUs with similar structures and properties as the original samples. The synthetic strategy, isocyanate-free and employing isoeugenol and carbon dioxide (CO2 ) as building blocks, makes this approach an attractive pathway to NIPU networks taking a step toward a circular economy.

Hemiaminal dynamic covalent networks with rapid stress relaxation, reprocessability and degradability endowed by the synergy of disulfide and hemiaminal bonds

This work proposes a strategy to address the challenge of achieving rapid reprocessability of vitrimers at mild temperatures by introducing dynamic disulfide and hemiaminal bonds into hemiaminal dynamic covalent networks (HDCNs). The resulting HDCNs, termed HDCNs-DTDA, were prepared through a facile polycondensation between formaldehyde and 4,4'-dithiodianiline. The dual dynamic bond system in the HDCNs-DTDA enables rapid stress relaxation under mild temperature (65 °C for 54 s), which is significantly faster than that observed in HDCNs containing a single dynamic bond (HDCNs-DDM). The HDCNs-DTDA also exhibit a glass transition temperature of 96 °C, excellent solvent resistance and high recovery rates (97%) of tensile strength after reprocessing. In addition, HDCNs-DTDA can be easily degraded in HCl and thiol solutions at room temperature to enable chemical recyclability. Finally, HDCNs-DTDA demonstrates fast shape memory behaviors using thermal stimulation.

Improving Sustainability through Covalent Adaptable Networks in the Recycling of Polyurethane Plastics

The global plastic waste problem has created an urgent need for the development of more sustainable materials and recycling processes. Polyurethane (PU) plastics, which represent 5.5% of globally produced plastics, are particularly challenging to recycle owing to their crosslinked structure. Covalent adaptable networks (CANs) based on dynamic covalent bonds have emerged as a promising solution for recycling PU waste. CANs enable the production of thermoset polymers that can be recycled using methods that are traditionally reserved for thermoplastic polymers. Reprocessing using hot-pressing techniques, in particular, proved to be more suited for the class of polyurethanes, allowing for the efficient recycling of PU materials. This Review paper explores the potential of CANs for improving the sustainability of PU recycling processes by examining different types of PU-CANs, bond types, and fillers that can be used to optimise the recycling efficiency. The paper concludes that further research is needed to develop more cost-effective and industrial-friendly techniques for recycling PU-CANs, as they can significantly contribute to sustainable development by creating recyclable thermoset polymers.

Thiol-Aldehyde Polycondensation for Bio-based Adaptable and Degradable Phenolic Polymers

Designing sustainable materials with tunable mechanical properties, intrinsic degradability, and recyclability from renewable biomass through a mild process has become vital in polymer science. Traditional phenolic resins are generally considered to be not degradable or recyclable. Here we report the design and synthesis of linear and network structured phenolic polymers using facile polycondensation between natural aldehyde-bearing phenolic compounds and polymercaptans. Linear phenolic products are amorphous with Tg between -9 °C and 12 °C. Cross-linked networks from vanillin and its di-aldehyde derivative exhibited excellent mechanical strength between 6-64 MPa. The connecting dithioacetals are associatively adaptable strong bonds and susceptible to degradation in oxidative conditions to regenerate vanillin. These results highlight the potential of biobased sustainable phenolic polymers with recyclability and selective degradation, as a complement to the traditional phenol-formaldehyde resins.

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.

Closed-loop recyclable membranes enabled by covalent adaptable networks for water purification

In the state-of-the-art membrane industry, membranes have linear life cycles and are commonly disposed of by landfill or incineration, sacrificing their sustainability. To date, little or no thought is given in the design phase to the end-of-life management of membranes. For the first time, we have innovated high-performance sustainable membranes, which can be closed-loop recycled after long-term usage for water purification. By synergizing membrane technology and dynamic covalent chemistry, covalent adaptable networks (CANs) with thermally reversible Diels-Alder (DA) adducts were synthesized and employed to fabricate integrally skinned asymmetric membranes via the nonsolvent-induced phase separation technique. Due to the stable and reversible features of CAN, the closed-loop recyclable membranes exhibit excellent mechanical properties and thermal and chemical stabilities as well as separation performance, which are comparable to or even higher than the state-of-the-art nonrecyclable membranes. Moreover, the used membranes can be closed-loop recycled with consistent properties and separation performance by depolymerization to remove contaminants, followed by refabrication into new membranes through the dissociation and reformation of DA adducts. This study may fill in the gaps in closed-loop recycling of membranes and inspire the advancement of sustainable membranes for a green membrane industry.

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.

Mechanically Strong and Tough Poly(urea-urethane) Thermosets Capable of Being Degraded under Mild Condition

The development of degradable polymeric materials such as degradable polyurethane or polyurea has been much highlighted for resource conservation and environmental protection. Herein, a facile strategy of constructing mechanically strong and tough poly(urea-urethane) (PUU) thermosets that can be degraded under mild conditions by using triple boron-urethane bonds (TBUB) as cross-linkers is demonstrated. By tailoring the molecular weight of the soft segment of the prepolymers, the mechanical performance can be finely controlled. Based on the cross-linking of TBUB units and hydrogen-binding interactions between TBUB linkages, the as-prepared PUU thermosets have excellent mechanical strength of ≈40.2 MPa and toughness of ≈304.9 MJ m^-3 . Typically, the PBUU900 strip can lift a barbell with 60 000 times its own weight, showing excellent load-bearing capacity. Meanwhile, owing to the covalent cross-linking of TBUB units, all the PUU thermosets show initial decomposition temperatures over 290 °C, which are comparable to those of the traditional thermosets. Moreover, the TBUB cross-linked PUU thermosets can be easily degraded in a mild acid solution. The small pieces of the PBUU sample can be fully decomposed in 1 m HCl/THF solution for 3.5 h at room temperature.

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.

Dual-Dynamic Chemistries-Based Fast-Reprocessing and High-Performance Covalent Adaptable Networks

Covalent adaptable networks (CANs) possess multiple functions including reprocessing (or recyclability), self-healing, welding, shape shifting, 3D printing, etc., due to the network rearrangement from dynamic bonds, and favorable performance from their cross-linked feature, and they are supposed to be as sustainable alternatives to thermosets. However, the thermal and mechanical properties, and stability of CANs are often sacrificed for rapid network rearrangement. In this paper, fast-reprocessing CANs with high performance are facilely constructed by in situ polymerization and dynamic cross-linking of styrene (St), maleic anhydride (MA), and acetal diol (BHAD). The rigid and hydrophobic polymer backbone endow the materials with high glass transition temperatures, mechanical performance, and water resistance. Besides, carboxylic group-catalyzed dual dynamic ester and acetal-based networks exhibit faster stress relaxation and realize extrusion reprocessing. This work provides an ingenious and simple strategy of construction of CANs combining rapid network rearrangement and excellent comprehensive performance, which is beneficial for the application of CANs.

Dynamic Covalent Structure for the Design of Recyclable Polyurethane Based on the Diketone Chemistry

Herein, the rational design of an enaminone compound is reported which can be easily and conveniently designed and obtained via the reaction of amine and diketone. The dynamic enaminoneamide structure is formed via the reaction between isocyanate and enaminone in fabricating a novel dynamic crosslinked polyurethane. The new kind of polyurethane can be efficiently recycled via the shifting between crosslinked structure and oligomer for the reversible dissociation of the enaminoneamide structure. Besides remolding itself via the thermal treatment, oligomers liberated from recycled-polyurethane can directly take part in constructing a new polyurethane without further purification. Therefore, the recyclability of this polyurethane shows high-value characteristics. The ease with which polyurethane can be produced, used, recycled, and reused without losing value offers a new green solution in designing sustainable polymer materials with a high economic value and a minimal environmental burden.

Remolding and Deconstruction of Industrial Thermosets via Carboxylic Acid-Catalyzed Bifunctional Silyl Ether Exchange

Convenient strategies for the deconstruction and reprocessing of thermosets could improve the circularity of these materials, but most approaches developed to date do not involve established, high-performance engineering materials. Here, we show that bifunctional silyl ether, i.e., R'O-SiR₂-OR'', (BSE)-based comonomers generate covalent adaptable network analogues of the industrial thermoset polydicyclopentadiene (pDCPD) through a novel BSE exchange process facilitated by the low-cost food-safe catalyst octanoic acid. Experimental studies and density functional theory calculations suggest an exchange mechanism involving silyl ester intermediates with formation rates that strongly depend on the Si-R₂ substituents. As a result, pDCPD thermosets manufactured with BSE comonomers display temperature- and time-dependent stress relaxation as a function of their substituents. Moreover, bulk remolding of pDCPD thermosets is enabled for the first time. Altogether, this work presents a new approach toward the installation of exchangeable bonds into commercial thermosets and establishes acid-catalyzed BSE exchange as a versatile addition to the toolbox of dynamic covalent chemistry.

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.

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.

Fast-reprocessing, Post-adjustable, Self-healing Covalent Adaptable Networks with Schiff base and Diels-Alder Adduct

Covalent adaptable networks (CANs) are a new type of polymers which possess excellent performance of thermosets and reprocessability of thermoplastics. Nevertheless, it is still a challenge to realize rapid reprocessing and post-adjusting (adjust properties after preparation). Herein, for the first time, a method of combining Schiff base and Diels-Alder adduct in one network was developed to achieve rapid reprocessing and post-adjusting. Through the dissociation of the Diels-Alder adduct at high temperatures, the cross-link densities of the networks were reduced, thereby accelerating the rearrangement of the networks and realizing the rapid reprocessing and self-healing. Moreover, the reconnecting degree of network after dissociation of Diels-Alder adduct could be easily controlled by annealing, as a result, the properties of the obtained CANs were post-adjustable. This work provides a simple and promising approach of achieving excellent reprocessing and post-adjusting for CANs via the synergism of an associative dynamic chemistry with a dissociative dynamic chemistry. This article is protected by copyright. All rights reserved.

Dynamic Covalent Polyurethane Network Materials: Synthesis and Self-Healability

Polyurethane (PU) has not only been widely used in the daily lives, but also extensively explored as an important class of the essential polymers for various applications. In recent years, significant efforts have been made on the development of self-healable PU materials that possess high performance, extended lifetime, great reliability, and recyclability. A promising approach is the incorporation of covalent dynamic bonds into the design of PU covalently crosslinked polymers and thermoplastic elastomers that can dissociate and reform indefinitely in response to external stimuli or autonomously. This review summarizes various strategies to synthesize self-healable, reprocessable, and recyclable PU materials integrated with dynamic (reversible) Diels-Alder cycloadduct, disulfide, diselenide, imine, boronic ester, and hindered urea bond. Furthermore, various approaches utilizing the combination of dynamic covalent chemistries with nanofiller surface chemistries are described for the fabrication of dynamic heterogeneous PU composites.

Recent developments towards performance-enhancing lignin-based polymers

This review examines recent strategies, challenges, and future opportunities in preparing high-performance polymeric materials from lignin and its derivable compounds.