Supramolecular Polymers and Assemblies. Andreas Winter
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far from equilibrium, unique possibilities have been suggested: inherent dynamic nature, adaptivity, and spatiotemporal controllability [142–144]. Noteworthy, many natural self‐assembled systems operate in such states and fulfill crucial functions [145]. Kinetic control of a supramolecular polymerization can be achieved utilizing strong non‐covalent interactions; thus, materials in non‐dissipative, nonequilibrium states can be formed (Figure 1.15). In such processes, the experimental details are of significant importance since nonlinear phenomena (e.g. nucleation or multiple competitive growth) are known to contribute to the outcome of the supramolecular self‐assembly (e.g. the nanoscale morphology of the obtained materials) [40]. Remarkable progress has been made to understand the kinetics and pathway complexity, and, today, the selection of a specific self‐assembly pathway is possible by cautious optimization of the experimental details. As a result, one can generate supramolecular materials with different functional properties, even from a single monomer. The fourth case, which is the dissipative nonequilibrium state, is known from living systems. In these supramolecular assemblies, energy is continuously consumed to account for their stability. In Nature, nucleobase triphosphates represent the main energy resources, which can control the nonequilibrium self‐assembly process in a highly spatiotemporal fashion – a prerequisite for cells to perform their complex functions (e.g. cell division, motility, and intracellular transport) [146]. The evolution of this field of research, an in‐depth discussion of the underlying mechanisms, and selection criteria to assemble supramolecular polymers in nonequilibrium states can be found in various recent reviews [40, 145]. In a recent example, Xu and coworkers demonstrated that only a photo‐reduced homoditopic viologen monomer gave a supramolecular polymer due to the formation of host–guest complexes with cucurbit[8]uril; whereas, under ambient conditions, depolymerization occurred due to a reoxidative process (see Chapter 7) [147].
Figure 1.15 Illustration of the various thermodynamic states in supramolecular polymerizations on Gibbs free energy landscape.
Source: Sorrenti et al. [40]. Licenced under CC BY 3.0.
1.5 Concluding Remarks
In polymer science, two types of materials are in the focus of research: conventional polymers and, with increasing interest, supramolecular polymers. Whereas the former ones are based on covalent bonds, the latter are formed via assembly of smaller entities by specific directional secondary interactions. Supramolecular polymers exhibit properties that are comparable to those of well‐known traditional macromolecules; however, reversibility of the secondary interaction represents an additional feature that gives rise to new applications: supramolecular polymers typically represent species in their thermodynamic equilibrium and their properties can be adjusted by applying external stimuli (e.g. changes in temperature, concentration, or solvent). Moreover, supramolecular polymers in non‐dissipative and/or nonequilibrium states also have to be considered and might be important in the future for the fine‐tuning of, e.g. shape, molecular organization, chirality, and/or dispersity of supramolecular polymers [40]. These aspects have been proposed to be crucial for utilitarian applications, as in energy conversion or biomedicine areas [148].
The broad range of supramolecular polymers that has been published so far can be classified by mainly two approaches: the type of the secondary interaction involved (as noted in the following chapters) or the mechanism by which they have been formed (in accordance with Carothers' classification from the 1930s [2]). From the three main mechanisms discussed in this chapter, particularly the isodesmic and ring‐chain‐mediated supramolecular polymerizations are now well understood; however, various effects (e.g. hysteresis and heterogeneous nucleation) make cooperative supramolecular polymerizations much more difficult to understand. Insight into the kinetics and thermodynamics of supramolecular polymers can be gained by comparing these artificial systems to the well‐documented protein aggregation. Moreover, one may compare these three mechanisms for supramolecular polymerizations to the three classes of covalent polymerization: step‐growth, chain‐growth, and ring‐opening polymerizations. For covalent polymers, the field of application often dictates the way by which mechanism the polymer may be prepared. The same also holds basically true for the supramolecular polymers.
In recent years, new concepts have evolved addressing the issue on how to control supramolecular polymerizations regarding the molecular structure and even the dispersity of the self‐assembled materials. From these, the so‐called living supramolecular polymerization represents a highly promising approach to prepare novel, designer, supramolecular materials via control over, e.g. their shape, size, and dispersity.
Going beyond a rather theoretical discussion, various types of supramolecular polymers will be introduced in the subsequent chapters – differentiated by the nature of their underlying non‐covalent/supramolecular interactions. Nonetheless, a detailed knowledge of the kinetic and thermodynamic driving forces for the formation of these materials remains a fundamental requirement.
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