ReviewDynamic covalent gels assembled from small molecules: from discrete gelators to dynamic covalent polymers
Graphical abstract
This article reviews the progress in the research and development of dynamic covalent chemistry in low molecular weight gels and dynamic covalent polymer gels assembled from small molecules.
Introduction
A gel is “a soft, solid or solid-like material consisting of two or more components one of which is a liquid, present in substantial quantity” according to Kramer's definition [1], [2], [3]. Gels consist of three-dimensional cross-linked networks, typically surrounded by a large number of solvent molecules. They maintain shape like solid materials. Solvent is the major component, and the gelator is the minor component. Gels combine the elastic behaviour of solids with the microviscous properties of fluids. Gels are generally classified as chemical and physical gels according to the strength of the driving interactions involved in their formation (Fig. 1). Chemical gels are usually tough and stable, whereas they are brittle, poorly transparent and unable to self-heal. In contrast physical gels are weak and show stimuli-responsibility because non-covalent interactions are susceptible to the external environment [4], [5], [6].
Gelators can be considered as ‘functional’ molecules, polymeric precursors or colloidal particles in solution (Fig. 1). The gelators aggregate via weak interactions to form one-dimensional (1D), 2D and 3D structures. For discrete molecules, 1D structures may be formed via weak interactions. For bridging molecular precursors, oligomeric/polymeric aggregates may be formed. Reversible reactions occur between these nanoscale aggregates. The solubility is important for gelation. If an insoluble polymer/aggregate is formed, amorphous precipitation or crystallization will occur in solution. The precipitation or crystallization can be considered as irreversible sequestration. For example, metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) are 2D and 3D polymeric materials that crystallize from the metal-organic and dynamic covalent systems, respectively. If the initially formed aggregates have a proper solubility in solution, there are two routes for further growing. First, the aggregates grow in one direction to form fibres. Second, the nanoparticles are interconnected via interactions to form a sponge-like porous structure. Either route can lead to gelation.
Physical gels of low molecular weight gelators (LMWGs) are commonly referred as supramolecular gels. Sometimes, supramolecular gels also include the gels that are induced by non-covalently cross-linked polymeric compounds (Fig. 1). In supramolecular gels, the gelators are self-assembled via non-covalent interactions [7], [8]. Self-assembly of the gelators are typically under thermodynamic control, so it is possible to make intricate structures that are difficult to achieve by traditional stepwise synthesis. The interactions between the gelators are reversible, so supramolecular gels show responses to external stimuli. Supramolecular gels usually stay at room temperature and liquefy at high temperatures due to breaking of the intermolecular interactions [9], [10], [11], [12], [13], [14], [15], [16].
In gels, the driving forces (a broad realm of interatomic forces) range from strong chemical bonds to weak intermolecular interactions (Fig. 1). Strong chemical bonds like most covalent bonds are built irreversibly. The resulting chemical gels are robust, tolerant to physical deformation and thermally irreversible. It is difficult to change their structures after preparation. This catalogue of gels is dominated by kinetically controlled chemistry, e.g., covalently crosslinked gels obtained by radical polymerization. Various intermolecular non-covalent interactions are also driving forces for aggregation, such as electrostatic interactions (including hydrogen bonding [17], [18], [19], [20], halogen bonding [21]), π–π interactions [22], [23], [24], [25], van der Waals/hydrophobic forces [26], [27], [28] and host-guest interactions [29], [30], [31], multiple combined interactions and others [32], [33], [34]. Non-covalent interactions are commonly reversible and sensitive to environment. These gels are typically physical gels under thermodynamic control. Their states are readily changed by external stimuli (e.g., temperature, substance).
A number of dynamic bonds are receiving growing intention recently. Their strengths lie between strong chemical bonds and weak intermolecular interactions. They selectively undergo reversible breaking and reformation under equilibrium conditions. They are relatively weak in contrast to covalent bonds, but multiple non-covalent bonds work in concert to profoundly affect the final structure. Multiple non-covalent bonds usually exist between two or more components with functional units via hydrogen bonding, metal-ligand coordination or donor-acceptor interactions [35], [36], [37], [38], [39], [40], [41]. Dynamic covalent bonds are one typical category of dynamic bonds [42], [43], [44], [45], [46], [47], [48]. Dynamic covalent chemistry, which was introduced by Lehn, focuses on reversible reactions of making and breaking of covalent bonds under mild conditions [49]. Different from traditional covalent chemistry that has predominantly focused on the use of kinetically controlled reactions, dynamic covalent chemistry highlights thermodynamic assemblies and products. The relative proportions of the individual assemblies and products are decided by the differences in free energy between the transition states. Energetic differences are small among alternative structures, and product distributions depend on the relative stabilities of the final products. The reversibility introduces “proofreading” and “error checking” and allows the exchange of molecular components at equilibrium. It will assure that kinetically trapped imperfections are corrected and ultimately favour formation of the most stable structures.
Dynamic covalent bonds usually have higher bond strengths and more controllable reversibility than other non-covalent interactions. They have generally slower kinetics of bond cleavage and formation. The slow kinetics of most dynamic reactions typically results in microcrystalline powders and no efficient error-correction is allowed for the growth of single crystals in COFs [9], [10], [50], [51], [52], [53], [54], [55]. Dynamic covalent bonds reversibly form and break under certain conditions (e.g., in the presence of catalyst), while they can be strong and permanent under different conditions. Dynamic covalent bonds can be further transformed to kinetically fix the dynamic exchange. For example, the C
N bonds in imines and acylhydrazones are reduced to form non-reversible covalent C
N bonds. So far some dynamic covalent reactions are able to be performed at ambient temperature with efficient error-proof construction under thermodynamic control, ability to self-repair and adaptability (response to external stimuli) like non-covalent interactions. Meanwhile these reactions have advantages over non-covalent interactions. Compared with metal-organic coordination, dynamic covalent bonds are more directional interactions. Dynamic covalent structures are more stable in aqueous environments than hydrogen-bonded structures. These features of dynamic covalent bonds allow for a great degree of control over the gelation process and thus greatly broaden the scope of gels.
A number of review articles have been devoted to supramolecular gels [9], [10], [50], [51], [52], [53], [54], [55] and polymer gels [56], [57], [58], however, a comprehensive analysis of dynamic covalent bonding in gels is not available. The main purpose of this review is to discuss the role of dynamic covalent bonding in gels assembled from small molecules. Dynamic covalent reactions used in gels are reviewed first in order to understand the dynamic covalent bonding. The survey of the literature is classified into two catalogues according to the nature of gelators and the interactions between gelators: gelation by discrete molecules and gelation by dynamic covalent polymers. We aim to illustrate the structure–property relationships of various dynamic covalent gels and the link between primary interactions and bulk material properties. We hope to shed some light on the future work and inspire continuous endeavors in this area.
Section snippets
Dynamic covalent reactions
For a dynamic covalent reaction, rapid equilibration is critical to reaching the thermodynamic minima in a reasonable time frame, and mild reaction conditions allow broader substrate scope [59]. The dynamic reactions that have been applied for gels include formation and exchange of imines, hydrazones, acylhydrazones, Diels-Alder cycloaddition, olefin metathesis, boronic esters, disulfides and so on (Scheme 1). In some of dynamic covalent reactions (e.g., formation of imines and boronic esters),
Design strategies for dynamic covalent gels
Gels may be designed according to the nature of gelators and the interactions between gelators. In this review, design strategies for the gels that are formed from low molecular weight molecules are discussed. Low molecular weight molecules with functional groups may react via dynamic covalent bonding to form discrete molecular gelators or dynamic covalent polymer gelators. The gels are classified into two catalogues, gelation by discrete molecules and gelation by dynamic covalent polymers (
Gelation by discrete molecules
Molecular gels are assembled from low molecular weight gelators. LMWGs assemble into entangled three-dimensional networks through weak intermolecular forces such as hydrogen bonding, π–π stacking, and van der Waals interactions. Such gels can be readily transformed into fluids by external stimuli (heating, sonicating etc.). Two classes of gels have been investigated according to the role of molecular precursors. One class requires all the added components to access the gel (i.e., no component
Gelation by dynamic covalent polymers
There are two methods to obtain dynamic covalent polymer gels from small molecules, post-modification of molecular gels (Fig. 2b) and direct gelation by dynamic covalent polymers (Fig. 2c). The methods allow formation of dynamic covalent polymers from relatively simple precursors. Dynamic covalent polymers exhibit dynamic properties such as self-healing, shape memory and environmental adaptation. Compared to noncovalent interactions, dynamic bonds are more robust with generally slower kinetics
Conclusions and outlooks
Dynamic covalent chemistry has attracted particular research interest due to their potential applications in stimuli-responsive materials. It combines the robustness of covalent bonds with the flexibility typical for non-covalent interactions. In this review, we have summarized the advances in the gels with dynamic covalent bonding. The progress in suparmolecular and polymer gels with dynamic covalent bonding is described. Their structure–property relationship is analyzed. Rationally designed
Acknowledgment
We acknowledge the NSFC (Nos. 51573216 and 21273007), the Program for New Century Excellent Talents in University (No. NCET-13-0615) and the FRF for the Central Universities (No. 16lgjc66) for support.
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