Vitrimers possess the unique property that they are malleable while being permanently cross-linked. This mini-review highlights the existing vitrimer systems in the period 2011–2015 with the main focus on their chemical origin.

wileyonlinelibrary.comstiffness. Due to their molecular architecture, thermosets cannot be reshaped, processed, or recycled after full curing. In contrast, thermoplastics can fl ow upon heating, enabling multiple and easy processing, such as by extrusion, [ 3 ] as well as recycling in many cases. [ 4 ] Designing polymer systems that combine the benefi ts of traditional thermosets with the "plastic" properties to facilitate processing is therefore a challenge that has recently attracted great interest.A way to make the combination of these properties is offered by the introduction of exchangeable chemical bonds into a polymer network, leading to dynamic cross-links. These bonds should be able to rearrange themselves in a reversible manner, providing on a molecular level a mechanism for macroscopic fl ow without risking structural damage. Polymer networks containing such exchangeable bonds, also known as covalent adaptable networks or CANs, [ 5 ] may be further classifi ed into those relying on respectively dissociative and associative exchange reactions. [ 6 ] The most common "dissociative" group of CANs relies on a reversible covalent bond formation between two groups attached to the polymer chains. By triggering the reversed bond forming step (bond dissociation), the material can achieve topology rearrangements (stress relaxation and fl ow), simply because of a decrease in connectivity during the temporary depolymerization, resulting in a strong and sudden viscosity drop and a loss of dimensional stability. Such systems will always present a sol/ gel transition and can thus be solubilized in the presence of solvent. A representative example of a thermally triggered dissociative CAN relies on the well-known reversible Diels-Alder reaction between furans and maleimides. [ 7 ] A less common type of CAN makes use of associative bond exchanges between polymer chains, [ 8 ] in which the original cross-links between polymer chains are only broken once a bond to another (part of the) polymer chain has been formed. As a result, such systems can change their topology with no loss of connectivity during the dynamic reorganization process, making such networks effectively permanent and insoluble, even at (very) high temperatures. Interestingly, as with all chemical reactions, the rate of this associative exchange increases with the temperature, leading to an Arrhenius-like viscosity dependence, rather than a sudden and marked viscosity drop at the sol/gel transition. Thermally triggered associative CANs have been coined vitrimers, [ 9 ] because of their unique combination of insolubility and gradual thermal viscosity behavior, Vitrimers are a new class of polymeric materials with very attractive properties, since they can be reworked to any shape while being at the same time permanently cross-linked. As an alternative to the use of transesterifi cation chemistry, we explore catalyst-free transamination of vinylogous urethanes as an exchange reaction for vitrimers. First, a kinetic study on model compounds reveals the occu...

A selection of dynamic chemistries is highlighted, with a focus on the reaction mechanisms of molecular network rearrangements, and on how mechanistic profiles can be related to the mechanical and physicochemical properties of polymer materials.

The “click” trick: Many reactions are classified as click reactions even though some are limited to certain applications. Thus, there is danger that the term “click” will become meaningless over time and simply a synonym for “successful”. To prevent this, the original click criteria are evaluated in this Essay specifically for the synthetic polymer field and a set of criteria are specified that distinguishes click from other efficient reactions.

Vinylogous urethane based vitrimers are polymer networks that have the intrinsic property to undergo network rearrangements, stress relaxation and viscoelastic flow, mediated by rapid addition/elimination reactions of free chain end amines. Here we show that the covalent exchange kinetics significantly can be influenced by combination with various simple additives. As anticipated, the exchange reactions on network level can be further accelerated using either Brønsted or Lewis acid additives. Remarkably, however, a strong inhibitory effect is observed when a base is added to the polymer matrix. These effects have been mechanistically rationalized, guided by low-molecular weight kinetic model experiments. Thus, vitrimer elastomer materials can be rationally designed to display a wide range of viscoelastic properties.

With its focus on synthetic reactions that are highly specific and reliable, 'click' chemistry has become a valuable tool for many scientific research areas and applications. Combining the modular, covalently bonded nature of click-chemistry linkages with an ability to reverse these linkages and reuse the constituent reactants in another click reaction, however, is a feature that is not found in most click reactions. Here we show that triazolinedione compounds can be used in click-chemistry applications. We present examples of simple and ultrafast macromolecular functionalization, polymer-polymer linking and polymer crosslinking under ambient conditions without the need for a catalyst. Moreover, when triazolinediones are combined with indole reaction partners, the reverse reaction can also be induced at elevated temperatures, and the triazolinedione reacted with a different reaction partner, reversibly or irreversibly dependent on its exact nature. We have used this 'transclick' reaction to introduce thermoreversible links into polyurethane and polymethacrylate materials, which allows dynamic polymer-network healing, reshaping and recycling.

In this minireview, we survey recent advances in the development of vitrimer materials. Focus on how to chemically control their material properties is used to highlight challenges for boosting the potential of this emerging class of polymer materials.

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