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...

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.

Vitrimers are covalently cross-linked polymeric materials that can be thermally processed in a liquid state without losing their network integrity. In this work, novel vitrimers based on the dynamic amine exchange reaction of vinylogous urea moieties are introduced. Following a systematic exploration of different vinylogous acyl compounds (urethanes, amides and urea), vinylogous urea clearly emerge as showing the fastest intrinsic exchange kinetics. The combination of these networks with a simple acid catalyst (0.5 mol% pTsOH) resulted in materials with good mechanical properties (Tg ~ 110°C, E ~ 2.2 GPa) and remarkably short relaxation times above Tg, in the order of a few seconds when heated. This attractive combination of properties made it possible to prepare vinylogous urea based composites as enduring prepregs. We demonstrate that the dynamic material properties give fully cured composites that still allow an efficient thermal fusion of multiple layers as well as thermoforming. Finally, we also demonstrate fiber-recycling by a simple chemical treatment.

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