Switchable Macromolecular Systems – From Dynamic Chemistry to Self‐Healing (2012)

Our general perception of chemical bonds tends to be a static one. As synthetic chemists we like to place covalent connections into place that make a molecule durable and retain its configuration so that it can perform its function in a given application over a long time span. Of course, there are exceptions, for example if a polymeric material is designed to degrade under a certain stimulus (for example an increase in pH value or in the presence of water), yet these applications tend to be – certainly in terms of bulk materials – the minority. Increasingly, however, polymer chemists have come to realize that macro-molecular systems can be designed in such a way that the formed bonds holding two polymer strands, or even an entire network, together can be broken to impart reversibly changing properties on the material. Such a novel polymer material class possessing reversible (covalent) cross-links has the capacity to adapt to an externally applied stimulus. These networks represent a trend towards the rational design of structural and smart materials showing dynamic characteristics for specialty applications. The chemistry governing such systems can be described with the term “dynamic covalent chemistry” and has recently been the focus of much attention. However, the switching of macromolecular systems encompasses much more than the reversible formation of covalent bonds. For example, self-assembled systems, nano- and microparticles, and vesicles made of polymeric chains can be switched by an external trigger signal such as light, pH, or heat to alter their shape and/or their chemical properties. Not surprisingly, many efforts at designing switchable macro-molecular systems are driven by a desire to employ them as or incorporated them in self-healing systems. In this special issue, we attempt to provide a representative cross-section of switchable macromolecular systems and to highlight some of their applications in the context of self-healing materials. Because the topic of self-healing materials is complex and the term “self-healing” is employed variably depending on the context, we felt that it was important to begin the special issue with an overview that highlights the current trends and selected examples from the field of self-healing polymer systems. It is evident that a series of approaches are being taken to generate intrinsic and triggered self-healing system and considerable progress has been made in the past few years. The trend article reflects a series of possible approaches to switchable macromolecular systems, and it is thus not surprising that this diversity is also reflected in the variety of articles included in the special issue. One set of studies investigates the field of reversible covalent networks. Landfester, Froimowicz, and co-workers review the progress of reversible photo-cycloadditions as a tool for creating nanomaterials, while Broekhuis and co-workers report on the reversible properties of networks based on furan–maleiimde couples. Staying with the theme of Diels–Alder chemistry, Sanyal and his team use Diels–Alder chemistry to reversibly write onto surfaces via microcontact printing. Diels–Alder chemistry, which remains the firm favorite for designing self-healing systems, is also shown to be applicable to enable healing processes on surfaces in solution. Palmese and co-workers explore the diffusion and reaction phenomena that occur during solution-based self-healing processes on solid sub-strates. Another interesting approach is reported by Wenz and co-workers, who combine reversible network formation based on [2 + 4] cycloadditions with a cellulose as a naturally occurring resource. The versatility of thermally induced [2 + 4] cycloadditions in the context of a shape-memory system is explored by Jérǒme and co-workers, who explore the effect of the adduct stability on the shape memory effects of the obtained materials. Volumetric shrinkage can be a very significant problem in such dynamically changing systems and Leung and Bowman explore a strategy based on reversible addition fragmentation chain transfer chemistry to overcome this problem. Also contained in the current special issue are strategies that do not rely on Diels–Alder chemistry. For example, Binder and his team employ copper catalyzed [2 + 3] dipolar cycloadditions as a potential avenue to self-healing polymers. Employing a completely different approach, Voit and co-workers describe reversibly switchable core/shell nanogels that can be switched via a change in pH or temperature and in which the core or shell structure can be independently addressed. Ritter and colleagues explore the photoswitchable behavior of azobenzenes complexed with cyclodextrins on the surface of nanoparticles, while Bosman and colleagues take an industrially inspired look at supramolecular binding concepts for the design of self-healing poly mer systems. Finally, in a second contribution, Ritter and co-workers employ cyclodextrin-based binding motifs that can undergo reversible supramolecular association and be employed to obtain reversibly cross-linkable materials. Given the wide variety of the addressed topics, we hope that this special issue on “Switchable Macro-molecular Systems” will stimulate the field even further and inspire more polymer chemists to actively design dynamic macromolecular systems.