Disulfide Metathesis Research Articles

Additive manufacturing of self-healing elastomers

Nature excels in both self-healing and 3D shaping; for example, self-healable human organs feature functional geometries and microstructures. However, tailoring man-made self-healing materials into complex structures faces substantial challenges. Here, we report a paradigm of photopolymerization-based additive manufacturing of self-healable elastomer structures with free-form architectures. The paradigm relies on a molecularly designed photoelastomer ink with both thiol and disulfide groups, where the former facilitates a thiol-ene photopolymerization during the additive manufacturing process and the latter enables a disulfide metathesis reaction during the self-healing process. We find that the competition between the thiol and disulfide groups governs the photocuring rate and self-healing efficiency of the photoelastomer. The self-healing behavior of the photoelastomer is understood with a theoretical model that agrees well with the experimental results. With projection microstereolithography systems, we demonstrate rapid additive manufacturing of single- and multimaterial self-healable structures for 3D soft actuators, multiphase composites, and architected electronics. Compatible with various photopolymerization-based additive manufacturing systems, the photoelastomer is expected to open promising avenues for fabricating structures where free-form architectures and efficient self-healing are both desirable.

Dual Cross-Linked Self-Healing and Recyclable Epoxidized Natural Rubber Based on Multiple Reversible Effects

Generally, self-healing research based on commercial rubber is of great significance in sustainable development by extending the lifetime of materials. However, it is still a great challenge so far to prepare recyclable rubber that combine excellent self-healing properties with good mechanical strength and is also recyclable. Herein, we report the use of epoxidized natural rubber (ENR), a reactive polymer presenting dual functional groups (unsaturated double bonds and epoxy sites) available for cross-linking, to prepare a dual cross-linked self-healing ENR based on dynamic disulfide metathesis and thermoreversible hydrogen bonding. Specifically, different structures of aromatic disulfide compounds are introduced into the same system to promote the disulfide metathesis and thus improving the self-healing efficiency of the material. As a result, the dual cross-linked ENR shows high mechanical strength (9.3 ± 0.3 MPa), high self-healing efficiency (up to 98%), and ideal recyclability. In addition, cyclic fatigue tensile test shows that the self-healing properties of the present material are not affected by the damage forms, whether it is complete fracture or cyclic fatigue damage. These outcomes are expected to promote the development of self-healing technology in the sustainable application of cross-linked rubber materials.

Photo-crosslinkable, self-healable and reprocessable rubbers

As a proof of concept, this study reports a facile method to prepare photo-crosslinkable, self-healable and reprocessable rubbers, which are rapidly crosslinked and patterned variform products via thiol-ene photoclick polymerization between the double bonds in polybutadiene and thiols in polysulfide. Disulfide linkages in polysulfide can dynamically rearrange the crosslinked networks through UV irradiation activated disulfide metathesis. The model small molecule experiments prove that disulfide metathesis is efficiently promoted by UV light and inclines to second-order reaction. The stress of photo-crosslinked rubbers is almost completely relaxed under UV irradiation at higher than 25 mW/cm2. More interestingly, stress relaxation can be controlled by sequentially turning on/off UV light. The self-adhesion assisted self-healing property is revealed by repeated tensile shear tests. The content of disulfide bonds, transmittance, irradiation intensity and time have a synergistic effect on the self-healing ability. The healed samples can regain higher than 90% shear strength, and the efficiency is not significantly influenced by repeated healing. Due to the dynamic reversibility of disulfide metathesis, the molded rubbers can be transformed into other shapes under UV illumination. Moreover, the pulverized rubber can also be compressed into new items by means of UV induced disulfide metathesis. This study thus illustrates a feasible approach to endow photo-cured rubbers with UV triggered self-healing and reprocessing capacities.

Self-Healable Antifouling Zwitterionic Hydrogel Based on Synergistic Phototriggered Dynamic Disulfide Metathesis Reaction and Ionic Interaction.

A self-healable antifouling hydrogel based on zwitterionic block copolymer was prepared via reversible addition-fragmentation chain transfer polymerization and Diels-Alder "click" chemistry. The hydrogel consists of a core-cross-linked zwitterionic block copolymer having poly(furfuryl methacrylate) as core and poly(dimethyl-[3-(2-methyl-acryloylamino)-propyl]-(3-sulfopropyl)ammonium) (poly(sulfobetaine)) as shell. The core was cross-linked with dithiobismaleimidoethane. The block copolymers were characterized by dynamic light scattering, field emission scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy (AFM), differential scanning calorimetry, water contact angle, and small-angle X-ray scattering analyses. This zwitterionic hydrogel showed self-healing activity via combined effect of phototriggered dynamic disulfide metathesis reaction and zwitterionic interaction, which was monitored by optical microscopy and AFM depth profilometry. The mechanical properties of the hydrogel before and after self-healing were studied using depth-sensing nanoindentation method. It was observed that the prepared zwitterionic hydrogel could reduce the formation of biofilm, which was established by studying the bovine serum albumin (model protein) adsorption over the coating. This multifunctional hydrogel can pave a new direction in antifouling self-healable gel coating applications.

Dynamic Covalent Chemistry under High-Pressure:A New Route to Disulfide Metathesis.

This work describes, for the first time, the application of combined pressure and temperature stimuli in disulfide metathesis reactions. In the system studied, above a pressure of 0.2 GPa, equimolar amounts of symmetric disulfides bis 4-chlorophenyl disulfide [(4-ClPhS)2 ] and bis 2-nitrophenyl disulfide [(2-NO2 PhS)2 ] react to give the heterodimeric product 4-Cl-PhSSPh-2-NO2 . In contrast to experiments conducted in solution at atmospheric pressure or in mechanochemical experiments under ball-mill grinding conditions, there is no necessity to use a base or thiolate anion as a catalyst for the exchange reaction under investigated conditions. Single-crystal and powder X-ray diffraction revealed also that, despite the high-pressure conditions of this reaction, the heterodimeric-disulfide product unexpectedly crystallizes into the low-density polymorph A. This counterintuitive result contrasts with the high-pressure stability of the higher-density polymorph B, confirmed by its compression up to 2.8 GPa with no signs of a phase transition.

Evaluation of disulfide chain extender effect on the mechanical properties of unsaturated polyurethane‐urea networks

ABSTRACT4‐Aminophenyl disulfide and bis(4‐aminophenyl)methane chain extenders containing hydroxyl‐terminated polybutadiene‐based polyurethane‐ureas are prepared one‐shot to explore the effect of the chain extender structure on the elastomers mechanical properties. However, the results revealed that the participation of the disulfide chain extender in side reactions like thiol‐ene and proton abstraction prevented disulfide metathesis reaction due to decomposing chain extender in the polyurethane‐urea matrix. Also, these side reactions improved the phase mixing via chemical crosslinking between polyurethane‐ureas soft and hard segments, too. Tensile test results showed higher stress strength of the elastomers in the presence of the disulfide chain extender in comparison with the nondisulfide bond containing elastomers. This result was in agreement with the observed result in dynamical mechanical analysis. Dynamic mechanical analysis results established that the absence of the disulfide bond in the polyurethane‐urea matrix led to the higher viscous modulus. The swelling test revealed chemical crosslinking increased in the presence of the disulfide bond. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46309.

Entropy-Driven Ring-Opening Disulfide Metathesis Polymerization for the Synthesis of Functional Poly(disulfide)s.

Metal-free entropy-driven disulfide metathesis polymerization of unsaturated l-cystine based macrocycles produces high-molar-mass heterofunctional poly(disulfide)s, i.e., poly(ester-disulfide-alkene) and poly(amide-disulfide-alkene); Mwapp = 44-60 kDa, Ð > 1.7. The polymerization is fast and reaches equilibrium within 1-5 minutes (monomer conversion 70-90%) in polar aprotic solvents such as N,N-dimethylacetamide, dimethylsulfoxide, or γ-valerolactone. Thiol-terminated polymers are stable in bulk or when dissolved in weakly polar solvents, but rapidly depolymerize in dilute polar solution.

Superior Toughness and Fast Self-Healing at Room Temperature Engineered by Transparent Elastomers.

The most important properties of self-healing polymers are efficient recovery at room temperature and prolonged durability. However, these two characteristics are contradictory, making it difficult to optimize them simultaneously. Herein, a transparent and easily processable thermoplastic polyurethane (TPU) with the highest reported tensile strength and toughness (6.8 MPa and 26.9 MJ m-3 , respectively) is prepared. This TPU is superior to reported contemporary room-temperature self-healable materials and conveniently heals within 2 h through facile aromatic disulfide metathesis engineered by hard segment embedded aromatic disulfides. After the TPU film is cut in half and respliced, the mechanical properties recover to more than 75% of those of the virgin sample within 2 h. Hard segments with an asymmetric alicyclic structure are more effective than those with symmetric alicyclic, linear aliphatic, and aromatic structures. An asymmetric structure provides the optimal metathesis efficiency for the embedded aromatic disulfide while preserving the remarkable mechanical properties of TPU, as indicated by rheological and surface investigations. The demonstration of a scratch-detecting electrical sensor coated on a tough TPU film capable of auto-repair at room temperature suggests that this film has potential applications in the wearable electronics industry.

Self‐healing polyurethane based on disulfide bond and hydrogen bond

Tough and transparent polyurethane networks with self‐healing capability at mild temperature conditions were successfully prepared in a 1‐pot procedure. The self‐healing ability of synthesized polyurethane comes from the covalent disulfide metathesis and non‐covalent H‐bonding. The mechanical testing indicates that disulfide metathesis reforms the covalent bonds on a longer time scale, while H‐bonding gives rise to a healing efficiency of around 46% in the early healing processing. The compromise between mechanical performance and healing capability is reached by tailoring the concentration of disulfide. The tensile strength of the sample with 100% self‐heal efficiency can get to 5.01 MPa, which can be explained by higher mobility of polymer chain under ambient temperature from creep testing.

N-Heterocyclic Carbene Catalyzed Disulfide Metathesis and Phosphorylation

N-Heterocyclic Carbene Catalyzed Disulfide Metathesis and Phosphorylation

Dynamic covalent chemistry in aqueous solution by photoinduced radical disulfide metathesis

Photoinduced radical disulfide metathesis (PRDM) is a dynamic covalent reaction that requires UV light to induce the homolytic cleavage of the disulfide bond, thus offering the opportunity to construct dynamic covalent systems that are dormant and can be photo-activated on demand. In this work, we showcase how PRDM can be utilized in aqueous solution and demonstrate its potential by generating a UV responsive hydrogel from an asymmetrical disulfide precursor.

Self-healing, Reshaping, and Recycling of Vulcanized Chloroprene Rubber: A Case Study of Multitask Cyclic Utilization of Cross-linked Polymer

To develop a new technology for sustainable application of sulfur cross-linked rubbers, a novel latent catalyst, organic complex copper(II) methacrylate (MA-Cu), is explored to trigger disulfide metathesis at elevated temperature. When it is incorporated into vulcanized chloroprene rubber fabricated following industrial recipe, reshuffling of inherent sulfur cross-links is enabled typically at 120 °C, while microstructure of the material remains unchanged at lower temperature (typically lower than 100 °C). Such a latent character of MA-Cu is believed to ensure operation stability of the rubber under ambient conditions, and its organic nature helps to improve dispersion homogeneity. By taking advantage of the activated cross-linked networks, the sulfur bonds can be reversibly exchanged inside the vulcanized chloroprene rubber as well as across the surfaces at 120 °C. As a result, the rubber is coupled with the capabilities of repeated reshaping, self-healing, and recycling without need of changing its macr...

“Waking sleeping beauty”: Regeneration of vulcanized rubber by triggering dynamicity of the inherent sulfur network

society, which causes severe impact on environment in the meantime. This is because there are no proper disposals of the scraps, which are full of infusible and insoluble crosslinked networks. So far, methods have been developed to convert waste rubbers into small molecular weight fragments with commercial value, or granules as fillers of thermoplastics and asphalt. In some cases, the used rubber products are simply burned and buried. The bad resource management leads to significant pollution accordingly (DOI: 10.3144/expresspolymlett.2016.17). When reviewing vulcanized rubber, we can find that the key technique of fabrication, i.e. vulcanization, remains nearly unchanged since the invention of Goodyear about two hundred years ago. The cross linked networks contain tremendous disulfide and polysulfide bonds. Recent investigation indicates that metathesis of disulfide enables self-healing of polymers. In this context, so long as disulfide metathesis among different disulfide and polysulfide bonds can be triggered in sulfur crosslinked networks, the vulcanized rubber would acquire self-healability and recyclability. In a patented invention, tiny amount of catalysts (CuCl2 and copper(II) methacrylate) are incorporated into polybutadiene and chloroprene rubber, which are then compounded with the usual fillers and curatives following industrial formulation. The preliminary results (DOI: 10.1039/c5gc00754b and DOI: 10.1021/ acssuschemeng.6b00224) show that the catalysts can effectively initiate metathesis of disulfide bonds, so that the inherent sulfur crosslinks of the vulcanized rubbers are allowed to be dynamically rearranged and reshuffled at 110~120°C. It means that the sulfur crosslinks are as stable as the version excluding the catalysts under conventional circumstances without fear of loosing stability during service at lower temperature (typically below 110°C). More importantly, by taking advantage of the activated cross linked networks, the vulcanized rubbers can be repeatedly selfhealed, reshaped, and recycled in solid phase at 110~120°C without environmentally unfriendly decrosslinking. Additionally, mechanical properties of the healed and recycled materials approach to those of the original version. The catalysts do not react with the other additives in the matrix and do not accelerate aging of the rubbers. The availability of self-healing, reshaping and reprocessing of vulcanized rubber offers new possibilities of cyclic utilization. Different circulation loops can be selected by various combinations of the smart functionalities according to actual demand. Life cycle of rubbers would thus be extended, while waste of resources is reduced. There maybe other ‘sleeping beauties’ like the above in everyday life, which are waiting for being waked.

Dynamic disulfide metathesis induced by ultrasound.

The reversible metathesis of disulfide bonds is generally induced by a combination of a reducing agent and base or by irradiation with ultraviolet light. Here we report that ultrasound irradiation is suitable for generating clean equilibrium mixtures of disulfides within one hour or one day, depending on the sonication source. Preliminary mechanistic investigations suggest that the solvent plays an active role in producing initiator radicals.

Room-Temperature Self-Healable and Remoldable Cross-linked Polymer Based on the Dynamic Exchange of Disulfide Bonds

Tri-n-butylphosphine (TBP) has been shown to effectively catalyze an air-insensitive disulfide metathesis reaction under alkaline conditions at room temperature. A cross-linked polysulfide containing the phosphine exhibited repeated autonomous self-healing resulting in restoration of tensile strength as a result of the dynamic exchange of disulfide bonds. Interestingly, the cross-linked polysulfide can also be reshaped and reprocessed at room temperature via the TBP-mediated reshuffling of the macromolecular networks. The mechanical properties and self-healing ability of polymeric specimens made from chopped samples remain surprisingly constant. In sharp contrast, control specimens without the phosphine catalyst or S–S bonds are neither self-healable nor reprocessable.

Catalyst-free room-temperature self-healing elastomers based on aromatic disulfide metathesis

Aromatic disulfide metathesis has been reported as one of the very few dynamic covalent chemistries undergone at room-temperature. Here, bis(4-aminophenyl) disulfide is effectively used as a dynamic crosslinker for the design of self-healing poly(urea–urethane) elastomers, which show quantitative healing efficiency at room-temperature, without the need for any catalyst or external intervention.

Phosphine-mediated disulfide metathesis in aqueous media

Dynamic carbohydrate systems have been efficiently generated through phosphine-mediated disulfide metathesis in aqueous media. The protein compatibility and binding features of the dynamic systems were demonstrated in situ using (1)H STD NMR.

A dynamic covalent polymer driven by disulfidemetathesis under photoirradiation

A disulfide-containing polyester indicates a dynamic nature in a bulk state based on a disulfide metathesis reaction driven by photoirradiation.

Phosphine-catalyzed disulfide metathesis

The reaction between disulfides and phosphines generates a reversible disulfide metathesis process.