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

https://pubs.rsc.org/en/content/articlepdf/2020/SC/D0SC01069C

Vitrimers: directing chemical reactivity to control material properties

Recent development of repairable, malleable and recyclable thermosetting polymers through dynamic transesterification

Catalyst-free vitrimer elastomers based on a dimer acid: robust mechanical performance, adaptability and hydrothermal recyclability

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

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

Conventional rubber products, such as tires, seals, tubing, and damping systems are manufactured via a vulcanization process, which forms covalently crosslinked network structures and ensures mechanical robustness, thermal stability and chemical resistance. However, the covalent networks are permanent and these products cannot be reprocessed or reshaped, which makes vulcanised rubbers one of the major challenges facing waste management and the circular economy. To reduce waste pollution for products such as tires, conventional vulcanised rubbers must be replaced with reversibly crosslinked structures which are able to achieve mechanical robustness and chemical stability, whilst also being able to be reprocessed, reshaped, reused and recycled. State-of-the-art developments in supramolecular chemistry have shed light on a new generation of reprocessable elastomers and rubbers, which have the potential to tackle the long-standing issue of waste tire pollution. The introduction of dynamic covalent bonds or supramolecular interactions in traditional elastomers can produce reversibly crosslinked structures, where the synergy between the dynamic bonds in the network are carefully optimised to balance the ease of processing, mechanical properties, and structural stability. Furthermore, dynamic covalent bonds and supramolecular interactions can provide ‘living’ functions to elastomers, such as self-healing and stimuli-responsiveness. These properties can be further enhanced by the addition of nanofillers with tailored surface chemistry to provide a dual role as a dynamic crosslinker and reinforcing element. To create reprocessable and recyclable elastomers, the coupling of multiple dynamic interactions provides unlimited possibilities to optimise the structure and properties of recyclable rubbers. Here we critically overview the applications of dynamic chemistry in rubbers, with a focus on macromolecular design and strategies to balance the mechanical, functional (e.g. self-healing) and reprocessing properties.

Dynamic crosslinked rubbers for a green future: A material perspective

Covalently cross-linked rubbers are indispensable in many important fields due to their unique entropic elasticity. For rubbers cross-linking, the addition of toxic curing agents, release of toxic ...

Mechanically Robust and Recyclable EPDM Rubber Composites by a Green Cross-Linking Strategy

Currently adopted cross-linking methods in rubber industry are suffering from variable persistent issues, including the utilization of toxic curing packages, release of volatile organic compounds (VOCs) and difficulties in the recycling of end-of-life materials. It is of great importance to explore a green cross-linking strategy in the area. Herein, we report a new “green” strategy based on hydrolyzable ester cross-links for cross-linking diene-typed elastomers. As a proof of concept, a commercial carboxylated nitrile rubber (XNBR) is efficiently cross-linked by a bio-based agent, epoxidized soybean oil (ESO), without any toxic additives. ESO exhibits an excellent plasticization effect and excellent scorch safety for XNBR. The cross-linking density and mechanical properties of the ESO-cured XNBR can be manipulated in a wide range by changing simply varying the content of ESO. In addition, zinc oxide (ZnO) performs as a catalyst to accelerate the epoxide opening reaction and improve the cross-linking efficiency, serving as reinforcement points to enhance the overall mechanical properties of the ESO-cured XNBR. Furthermore, the end-of-life elastomer materials demonstrate a closed-loop recovery by selectively cleaving the ester bonds, resulting in very high recovery of the mechanical performance of the recycled composites. This strategy provides an unprecedented green avenue to cross-link diene elastomers and a cost-effective approach to further recycle the obtained cross-linked elastomers at high efficiency.

Design of next-generation cross-linking structure for elastomers toward green process and a real recycling loop

Catalytic oxidation conversion of biomass-derived resource to high value-added products has evoked considerable interest. Herein, we report a Pt atomic clusters catalyst (size: 1.3 nm) supported on rehydrated layered double hydroxides (denoted as Pt/re-Mg4Al-LDHs), which exhibits great catalytic behavior towards selective oxidation reaction of furfural to furoic acid (conversion: 99 %; yield: 97 %; reaction rate: 676.57 mmol gPt−1 h−1). This result stands at the highest standard compared with reported Pt-based catalysts. A combination research confirms the formation of surface Ptδ− species, due to the local electron transfer from support to Pt atomic clusters. LDHs provide a confined effect to achieve tunable size and highly-stabilized metal clusters, which imposes influence on catalytic performance via metal-support interactions. Both experimental methods (In situ DRIFTS, Raman, EPR and Isotope Labelling MS) and theoretical calculations reveal that the negatively-charged Pt active site plays a crucial role in determining catalytic behavior: C O bond undergoes activation adsorption on Ptδ− site, followed by reaction with H2O (Aldehyde-Water Shift); subsequently, oxygen is activated to generate reactive oxide species (O2−) that serves as a hydrogen acceptor (Oxidative Dehydrogenation) to produce furoic acid. This work displays a valuable paradigm for preparation of noble metal atomic clusters catalysts based on memory effect of LDHs support, which would pave a way for the development of heterogeneous catalysts toward selective oxidation reactions.

Pt atomic clusters catalysts with local charge transfer towards selective oxidation of furfural

Glycerol (GLY) aerobic oxidation in an aqueous solution is one of the most prospective pathways in biomass transformation, where the supported catalysts based on noble metals (mainly Au, Pd, Pt) are most commonly employed. Herein, Pt nanoparticles supported on rehydrated MgxAl1-hydrotalcite (denoted as re-MgxAl1-LDH-Pt) were prepared via impregnation-reduction method followed by an in situ rehydration process, which showed high activity and selectivity towards GLY oxidation to produce glyceric acid (GLYA) at room temperature. The metal-support interfacial structure and catalyst basicity were modulated by changing the Mg/Al molar ratio of the hydrotalcite precursor, and the optimal performance was achieved on re-Mg6Al1-LDH-Pt with a GLY conversion of 87.6% and a GLYA yield of 58.6%, which exceeded the traditional activated carbon and oxide supports. A combinative study on structural characterizations (XANES, CO-FTIR spectra, and benzoic acid titration) proves that a higher Mg/Al molar ratio promotes the formation of positively charged Ptδ+ species at metal-support interface, which accelerates bond cleavage of α-C–H and improves catalytic activity. Moreover, a higher Mg/Al molar ratio provides a stronger basicity of support that contributes to the oxidation of terminal-hydroxyl and thus enhances the selectivity of GLYA. This catalyst with tunable metal-support interaction shows prospective applications toward transformation of biomass-based polyols.

Xiaolin Li

Papers

Mechanically Robust and Recyclable EPDM Rubber Composites by a Green Cross-Linking Strategy

Design of next-generation cross-linking structure for elastomers toward green process and a real recycling loop

Pt atomic clusters catalysts with local charge transfer towards selective oxidation of furfural

Glycerol aerobic oxidation to glyceric acid over Pt/hydrotalcite catalysts at room temperature

Geometric effect promoted hydrotalcites catalysts towards aldol condensation reaction