Understanding the Effect of Side Reactions on the Recyclability of Furan-Maleimide Resins Based on Thermoreversible Diels-Alder Network

We studied the effect of side reactions on the reversibility of epoxy with thermoreversible Diels-Alder (DA) cycloadducts based on furan and maleimide chemistry. The most common side reaction is the maleimide homopolymerization which introduces irreversible crosslinking in the network adversely affecting the recyclability. The main challenge is that the temperatures at which maleimide homopolymerization can occur are approximately the same as the temperatures at which retro-DA (rDA) reactions depolymerize the networks. Here we conducted detailed studies on three different strategies to minimize the effect of the side reaction. First, we controlled the ratio of maleimide to furan to reduce the concentration of maleimide groups which diminishes the effects of the side reaction. Second, we applied a radical-reaction inhibitor. Inclusion of hydroquinone, a known free radical scavenger, is found to retard the onset of the side reaction both in the temperature sweep and isothermal measurements. Finally, we employed a new trismaleimide precursor that has a lower maleimide concentration and reduces the rate of the side reaction. Our results provide insights into how to minimize formation of irreversible crosslinking by side reactions in reversible DA materials using maleimides, which is important for their application as novel self-healing, recyclable, and 3D-printable materials.

Trends in the Diels-Alder reaction in polymer chemistry

The Diels-Alder (DA) reaction is regarded as quite a useful strategy in organic and macromolecular syntheses. The reversibility of this reaction and the advent of self-repair technology, as well as other applications in controlled macromolecular architectures and crosslinking, have strongly boosted the research activity, which is still attracting a huge interest in both academic and industrial research. The DA reaction is a simple and scalable toolbox. Though it is well-established that furan/maleimide is the most studied diene/dienophile couple, this perspective article reports strategies using other reversible systems with deeper features on other types of diene/dienophile pairs being either petro-sourced (cyclopentadiene, anthracene) or bio-sourced (muconic and sorbic acids, myrcene and farnesene derivatives, eugenol, cardanol). This review is composed of four sections. The first one briefly recalls the background on the DA reactions involving cyclodimerizations, dienes, and dienophiles, parameters affecting the reaction, while the second part deals with the furan/maleimide reaction. The third one deals with petro-sourced and bio-sourced (or products becoming bio-sourced) reactants involved in DA reactions are also listed and discussed. Finally, the authors' opinion is given on the potential future of the crosslinking-decrosslinking reaction, especially regarding the process (e.g., key temperatures of decrosslinking) or possibly monocomponents. It presents both fundamental and applied research on the DA reaction and its applications.

Chain-Length-Dependent Photolysis of ortho-Nitrobenzyl-Centered Polymers

Herein, we demonstrate that the photochemical cleavage of linear polymers containing a midchain photocleavable moiety strongly depends on the chain length. Based on an ortho-nitrobenzyl (oNB) difunctional reversible addition-fragmentation chain-transfer agent, well-defined poly(methyl acrylate)s (M_n = 1.59-67.6 kg mol^-1, Đ = 1.3-1.4) were synthesized following a core-first approach. Photolysis at λ_max = 350 nm of the ortho-nitrobenzyl moiety led to the generation of equally sized polymer segments. The rate of oNB-driven polymer fragmentation, which can be well described by first-order kinetics, strongly increases with increasing molecular weight in a nonlinear fashion, potentially caused by entropic considerations and is compared to the ideal chain model. The current study thus demonstrates that polymer photolysis is dependent on the polymer chain length, with critical implications for photocleavable network design.

Advances in injectable self-healing biomedical hydrogels

In recent years, implantable biomaterials have attracted significant interest owing to their potentials for use in the therapy of physical defects and traumas. Among the implantable biomaterials, hydrogels have received increasing attention for their tunable structures and good rheological behavior. However, the mechanical failures of traditional gel materials during normal operation remain a serious issue. To overcome this problem, hydrogel materials with self-healing and injectable abilities have been developed, with their potential for autonomous self-recovery and minimally invasive implantation. In this paper, the progress of injectable self-healing hydrogels is presented by combining developments in the fundamental knowledge of polymer designs and discussions on the practical biomedical applications of the materials. The mechanisms of different types of self-healing hydrogels are introduced first and their performances are then discussed, followed by a review of the self-healing hydrogels with injectability. The applications of the injectable self-healing hydrogels are discussed in the final section. STATEMENT OF SIGNIFICANCE: This paper provides an overview of the progress of a smart material, injectable self-healing hydrogel, during the past ten years and mainly focuses on its recent development. This paper presents developments in the fundamental knowledge in polymer designs and discussions on the practical biomedical application of the materials, which sheds more light on the advancement of injectable self-healing hydrogels. This paper should be of interest to the readers who are curious about the advances of injectable self-healing hydrogels.

Design of a thermally controlled sequence of triazolinedione-based click and transclick reactions

The reaction of triazolinediones (TADs) and indoles is of particular interest for polymer chemistry applications, as it is a very fast and irreversible additive-free process at room temperature, but can be turned into a dynamic covalent bond forming process at elevated temperatures, giving a reliable bond exchange or 'transclick' reaction. In this paper, we report an in-depth study aimed at controlling the TAD-indole reversible click reactions through rational design of modified indole reaction partners. This has resulted in the identification of a novel class of easily accessible indole derivatives that give dynamic TAD-adduct formation at significantly lower temperatures. We further demonstrate that these new substrates can be used to design a directed cascade of click reactions of a functionalized TAD moiety from an initial indole reaction partner to a second indole, and finally to an irreversible reaction partner. This controlled sequence of click and transclick reactions of a single TAD reagent between three different substrates has been demonstrated both on small molecule and macromolecular level, and the factors that control the reversibility profiles have been rationalized and guided by mechanistic considerations supported by theoretical calculations.

Tunable temperature responsive liquid chromatography through thiolactone-based immobilization of poly(N-isopropylacrylamide)

A straightforward and efficient functionalization of aminopropylsilica with polymeric structures is described for the development of temperature responsive stationary phases applicable in purely aqueous liquid chromatography. The immobilization of the thermoresponsive polymers involves a thiolactone-based ring opening using the primary amines in aminopropylsilica, with a simultaneous one-pot, thiol-ene functionalization with an acrylate of choice. This mild, straightforward and modular grafting process results in high polymer coupling yields. By variation of the acrylate for the thiol-ene reaction, different stationary phases can be readily obtained. Two stationary phases as a result of the modular modification of aminopropylsilica were evaluated with test mixtures of hydrophobic analytes and a mixture of di- and tripeptides. Analyses using the 5μm material packed in 10cm×4.6mm columns revealed high hydrophobic retention, which proved adaptable as a function of the temperature in aqueous mobile phases. High versus low retention were obtained at temperatures above and below the lower critical solution temperature of the polymer, respectively. Moreover, the columns depict potential for diastereoisomeric peptide separation. Finally, the lower retention, observed when using PEGylated silica, illustrates the potential of the approach for modular stationary phase tuning.

Self-healing gels based on constitutional dynamic chemistry and their potential applications

This review presents recent developments and potential applications of physical and chemical self-healing gels based on constitutional dynamic chemistry.