Supramolecular chemistry has developed over the last forty years as chemistry beyond the molecule. Starting with the investigation of the basis of molecular recognition, it has explored the implementation of molecular information in the programming of chemical systems towards self-organisation processes, that may occur either on the basis of design or with selection of their components. Supramolecular entities are by nature constitutionally dynamic by virtue of the lability of non-covalent interactions. Importing such features into molecular chemistry, through the introduction of reversible bonds into molecules, leads to the emergence of a constitutional dynamic chemistry, covering both the molecular and supramolecular levels. It considers chemical objects and systems capable of responding to external solicitations by modification of their constitution through component exchange or reorganisation. It thus opens the way towards an adaptive and evolutive chemistry, a further step towards the chemistry of complex matter.

Importing self-repair or self-healing features into inert materials is of great relevance to material scientists, since it is expected to eliminate the necessity of replenishing a damaged material. Be it material chemistry or more specifically polymer chemistry, such materials have attracted the imagination of both material scientists and chemists. A stroll down the memory lane 70 years back, this might have sounded utopian. However with the current progress in supramolecular chemistry and the emergence of dynamic covalent and non-covalent chemistries, novel perspectives have been opened up to materials science towards the development of dynamic materials (DYNAMATS) and in particular dynamic polymers (DYNAMERS), with the ability to produce such species by custom made designs. Chemistry took giant strides to gain control over the structure and features of materials and, besides basic progress, to apply it for tailor-making matter for applications in our daily life. In that applied perspective, materials science plays a paramount role in shaping our present and in contributing to a sustainable future. The goal is to develop materials, which would be dynamic enough to carry out certain functions as effectively as in biological systems with, however, the freedom to recruit the powers of chemistry on a wider scale, without the limitation imposed by life. Material scientists and in particular polymer chemists may build on chemistry, physics and biology for bridging the gap to develop dynamic materials presenting a wide range of novel functionalities and to convert dreams into reality. In this current review we will focus on developments in the area of dynamic polymers, as a class of dynamic materials presenting self-healing features and, more generally, the ability to undergo adaptation under the effect of physical and/or chemical agents, and thus function as adaptive polymers or ADAPTAMERS.

The guanosine hydrazide 1 yields a stable supramolecular hydrogel based on the formation of a guanine quartet (G-quartet) in presence of metal cations. The effect of various parameters (concentration, nature of metal ion, and temperature) on the properties of this gel has been studied. Proton NMR spectroscopy is shown to allow a molecular characterization of the gelation process. Hydrazide 1 and its assemblies can be reversibly decorated by acylhydrazone formation with various aldehydes, resulting in formation of highly viscous dynamic hydrogels. When a mixture of aldehydes is used, the dynamic system selects the aldehyde that leads to the most stable gel. Mixing hydrazides 1, 9 and aldehydes 6, 8 in 1:1:1:1 ratio generated a constitutional dynamic library containing the four acylhydrazone derivatives A, B, C, and D. The library constitution displayed preferential formation of the acylhydrazone B that yields the strongest gel. Thus, gelation redirects the acylhydrazone distribution in the dynamic library as guanosine hydrazide 1 scavenges preferentially aldehyde 8, under the pressure of gelation because of the collective interactions in the assemblies of G-quartets B, despite the strong preference of the competing hydrazide 9 for 8. Gel formation and component selection are thermoreversible. The process amounts to gelation-driven self-organization with component selection and amplification in constitutional dynamic hydrogels based on G-quartet formation and reversible covalent connections. The observed self-organization and component selection occur by means of a multilevel self-assembly involving three dynamic processes, two of supramolecular and one of reversible covalent nature. They extend constitutional dynamic chemistry to phase-organization and phase-transition events.

Component exchange in reversible polymers allows the generation of dynamic constitutional diversity. The polycondensation of dihydrazides with dialdehydes generates polyacylhydrazones, to which the acylhydrazone functionality formed confers both hydrogen-bonding and reversibility features through the amide and imine groups, respectively. Polyacylhydrazones are thus dynamic polyamides. They are able to reversibly exchange either one or both of their repeating monomer units in the presence of different monomers, thus presenting constitutional dynamic diversity. The polymers subjected to monomer exchange/interchange may be brought to exhibit physical properties vastly different from those of the original polymer. The principle may be extended to other important classes of polymers, giving access, for instance, to dynamic polyureas or polycarbamates. These reversible polymers are therefore able to incorporate, decorporate, or reshuffle their constituting monomers, namely in response to environmental physical or chemical factors, an adaptability feature central to constitutional dynamic chemistry.