A Comprehensive Depiction of the Furan-Maleimide Coupling via Kinetic and Thermodynamic Investigations of the Diels-Alder Reaction of Poly(styrene- co -2-vinylfuran) with Maleimides

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

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

Chitosan Hydrogels Based on the Diels-Alder Click Reaction: Rheological and Kinetic Study

The Diels–Alder reaction is recognized to generate highly selective and regiospecific cycloadducts. In this study, we carried out a rheological and kinetic study of N-furfuryl chitosan hydrogels based on the Diels–Alder click reaction with different poly(ethylene)glycol-maleimide derivatives in dilute aqueous acidic solutions. It was possible to prepare clear and transparent hydrogels with excellent mechanical properties. Applying the Winter and Chambon criterion the gel times were estimated at different temperatures, and the activation energy was calculated. The higher the temperature of gelation, the higher the reaction rate. The crosslinking density and the elastic properties seem to be controlled by the diffusion of the polymer segments, rather than by the kinetics of the reaction. An increase in the concentration of any of the two functional groups is accompanied by a higher crosslinking density regardless maleimide:furan molar ratio. The hydrogel showed an improvement in their mechanical properties as the temperature increases up to 70 °C. Above that, there is a drop in G’ values indicating that there is a process opposing to the Diels–Alder reaction, most likely the retro-Diels–Alder.

Designing End-of-life Recyclable Polymers via Diels-Alder Chemistry: A Review on the Kinetics of Reversible Reactions

The purpose of this review is to critically assess the kinetic behaviour of the furan/maleimide Diels-Alder click reaction. The popularity of this reaction is evident and still continues to grow, which is likely attributed to its reversibility at temperatures above 100°C, and due to its bio-based "roots" in terms of raw materials. This chemistry has been used to form thermo-reversible crosslinks in polymer networks, and thus allows the polymer field to design strong, but also end-of-life recyclable thermosets and rubbers. In this context, the rate at which the forward reaction (Diels-Alder for crosslinking) and its reverse (retro Diels-Alder for de-crosslinking) proceed as function of temperature is of crucial importance in assessing the feasibility of the design in real-life products. Differences in kinetics based from various studies are not well understood, but are potentially caused by chemical side groups, mass transfer limitations, and on the analysis methods being employed. In this work we attempt to place all the relevant studies in perspective with respect to each other, and thereby offer a general guide on how to assess their recycling kinetics. This review sheds light on the kinetics on the furan/maleimide Diels-Alder reaction. This popular reaction opens up a path to develop end-of-life recyclable polymer networks with self-healing properties. The factors affecting reaction kinetics are discussed, and the importance of accurate reaction kinetics in the context of polymer reprocessing is highlighted. This article is protected by copyright. All rights reserved.

NIR and glutathione trigger the surface release of methotrexate linked by Diels-Alder adducts to anisotropic gold nanoparticles

The administration and controlled release of drugs over time remains one of the greatest challenges of science today. In the nanomaterials field, anisotropic gold nanoparticles (AuNPs) with plasmon bands centered at the near-infrared region (NIR), such as gold nanorods (AuNRs) and gold nanoprisms (AuNPrs), under laser irradiation, locally increase the temperature, allowing the release of drugs. In this sense, temporally controlled drug delivery could be promoted by external stimuli using thermo-reversible chemical reactions, such as Diels-Alder cycloadditions from a diene and a dienophile fragment (compound a). In this study, an antitumor drug (methotrexate, MTX) was linked to plasmonic AuNPs by a Diels-Alder adduct (compound c), which after NIR suffers a retro-Diels-Alder reaction, producing release of the drug (compound b). We obtained two nanosystems based on AuNRs and AuNPrs. Both nanoconstructs were coated with BSA-r₈ (Bovine Serum Albumin functionalized with Arg₈, all-D octa arginine) in order to increase the colloidal stability and promote internalization of the nanosystems on HeLa and SK-BR-3 cells. In addition, the presence of BSA allows protecting the cargo from being released on the extracellular environment and promotes the photothermal release of the drug in the presence of glutathione (GSH). The nanosystems' drug release profile was evaluated after NIR irradiation in the presence and absence of glutathione (GSH), showing a considerable increase of drug release when NIR light and glutathione were combined. This work broadens the range of possibilities of using two complementary strategies for the controlled release of an antitumor drug from AuNRs and AuNPrs: the photothermal cleavage of a thermolabile adduct controlled by an external stimulus (laser irradiation), complemented with the use of the intracellular metabolite GSH.

Structural and solvent control over activation parameters for a pair of retro Diels-Alder reactions

We report the temperature dependent NMR of two Diels-Alder adducts of furan: one formed with maleic anhydride and the other with N-methylmaleimide. These adducts are the products of so-called 'click' reactions, widely valued for providing simple, reliable, and robust reactivity. Under our experimental conditions, these adducts undergo a retro Diels-Alder reaction and we use our temperature dependent NMR to determine the rates of these reactions at multiple temperatures-ultimately providing estimates of the activation parameters for the reversion. We repeat these measurements in three solvents. We find that, in all solvents, the barrier to reversion is larger for the adduct formed with N-methylmaleimide. The barrier to reversion for this adduct is relatively insensitive to changes in solvent while the adduct formed with maleic anhydride responds more strongly to changes in solvent polarity. The differences in reaction barrier and solvent dependence arises because the adduct formed with N-methylmalemide is more stable-leading to a larger barrier to reversion-while the adduct formed with maleic anhydride experiences a larger change in dipole during the reaction-leading to a larger solvent dependence.

The influence of stereochemistry on the reactivity of the Diels–Alder cycloaddition and the implications for reversible network polymerization

The differences in reactivity and thermal stability of the stereoisomers define the thermal properties and responsiveness of the reversible polymer network.

Experimental model design: exploration and optimization of customized polymerization conditions for the preparation of targeted smart materials by the Diels Alder click reaction

The experimental model design proposed herein has proved to be an indispensable tool to rapidly and easily elucidate the optimal polymerization conditions in the preparation of tailor-made responsive materials for biomedical applications.