Supramolecular Polymerizations

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512 A. Ciferri dimensional growth is possible. [1] While the assembly produced by supramolecular polymerization is a supramolecular polymer, [2] the reverse is not true. Several systems have been reported and defined as supramolecular polymers that do not conform to the growth mechanisms and theory of supramolecular polymerization. A supramolecular polymer (SP) can be defined broadly as a system characterized by non-bonded interactions among repeating units. Such a broad definition includes all systems that have been described as SPs although, due to its general nature, it does not suggest immediately the structural features or potential applications of this exciting class of new materials. In fact, even a molecular polymer, based on covalently bonded repeating units, displays high order structural organization controlled by supramolecular interactions. The occurrence of supramolecular interaction is so widespread that even an organic crystal is described as a supramolecular assembly. [3] Attempts to restrict the definition of SPs, for instance to systems displaying polymer-like properties in dilute solution, have been made. [4] The problem is not a simple one since several variables control the degree of supramolecular polymerization (DPÞ. Moreover, the term supramolecular has great appeal since it invites the use of unifying concepts that cut across the traditional boundaries between colloid, polymer and solid state science. Rather than attempting to further clarify the definition of SP, it is useful to present a classification of the various systems that have been reported. A possible classification, based on assembling mechanisms, is schematized in Figure 1. It offers a glimpse of the impressive growth that has occurred over the past decade and contextually highlights the SPs produced by supramolecular polymerization that form the core of the present review. The reference model of the classical covalent chain resulting from molecular polymerization of small bifunctional monomers is schematized at the top of Figure 1. The selfassembling chain is an open one, meaning that, in principle, it can grow to a distribution of large DPs, irreversible in solution and under a wide range of external variables. Figure 1. Classification of supramolecular polymers. Class A (reversible polymers obtained by supramolecular polymerization) is the main topic of this article. Class A. The major components of this class are equilibrium polymers based on processes that can appropriately be regarded as supramolecular polymerizations. [1, 2] The linear chains are self-assembled, open, growing to a distribution of DPs, and in a state of thermodynamic equilibrium sensitive to solvent type, concentration and external variables. The geometrical shapes in the scheme of class A (Figure 1) remind that unimers in supramolecular polymerization can be of several forms and sizes. In particular, the unimer can be a large supramolecular aggregate, a supermolecule, [2] a covalent polymer (e.g., a globular protein) in which case the SP will actually be a polymer of polymers. In class A we may also include SPs based on unimers with functionality A2, when a variety of multidimensional assemblies (helical, planar, 3D) becomes possible. Examples of linear systems are hydrogen-bonded polymers, [5–17] coordination polymers [18–20] and also micelles. [21–24] Examples of more complex geo- Alberto Ciferri is Chemistry Professor at the University of Genoa, as well as Visiting Professor at Duke University, Durham, North Carolina (1975-present). He has authored about 200 original papers, books and patents mostly in the areas of rubber elasticity, biological and synthetic fibers, interactions between salts and macromolecules, liquid crystals, and supramolecular assemblies. He received his D.Sc. degree in physical chemistry from the University of Rome, and held positions as Scientist at Monsanto Co. and Director of Research at the National Research Council. He currently is President of the Jepa-Limmat Foundation, supporting advanced education in developing countries.
<strong>Supramolecular</strong> <strong>Polymerizations</strong> 513 metries are helical, columnar, tubular soluble [25–44] or fibrous proteins, [45] S-layers, [46] composite systems such as block copolymers [47–49] and the tobacco mosaic virus (TMV). [50] Random networks and blends stabilized by multifunctional supramolecular linkages have also been reported. [51–55] Class B. This class includes self-assembled structures formed by supramolecular binding of monofunctional unimers. Such unimers cannot undergo open supramolecular polymerization, but can nevertheless form closed assemblies involving both low- and high-molecularweight species. Classical host/guest complexes, [56] base pairing of simple nucleoside [57] and supermolecules are low-molecular-weight examples. Polymeric examples described as SPs include side-chain binding of a monofunctional unimer to a covalent chain. For instance, Kato and FrØchet first reported the binding of the monofunctional mesogen stilbazole to the side chains of a nonmesogenic polymer functionalized with pendant benzoic acid groups. [58] Additional examples are double-, and triple-chain assemblies, and globular structures unable to grow further when complementary monofunctional sites are internally saturated. Class C. A number of SPs displaying novel supramolecular features were obtained by superimposition of covalent and supramolecular bonds. These systems are selfassembling but show irreversible DPs. The supramolecular organization may either precede, be simultaneous to, or follow the formation of covalent bonds. Examples of the first type include the rotaxane and catenane polymers described by Stoddart and coworkers, [59–61] the growth of dendrimers though successive generations, [62] and other attempts to stabilize a supramolecular assembly by the subsequent formation of covalent bonds. [63, 64] The final covalent system may retain specific supramolecular features, or the precursor supramolecular organization may just be a step of a supramolecularly assisted synthesis of a complex structure. Examples in which the supramolecular and the molecular order are simultaneously formed are the synthesis of dendrons possessing polymerizable functionality at their focal points, as reported by Percec and Schlüter. [65, 66] These assemblies display most interesting composite architectures such as columns of disks hosting the dendrons, with the main covalent chain running in the center of each column. [67] Cases in which the covalent structure occurs before the supramolecular one include the dendronization of a covalent polymer, reported for instance by Tomalia and coworkers, [68] and the self-assembled monolayers (SAMs) regarded as supramolecular assemblies of short hydrocarbon chains covalently grafted to a gold surface. [69] Class D. The class of engineered assemblies includes systems that do not form spontaneously ordered structures under normal conditions. Their ordered structurization is based on controlled methods of deposition or synthesis. Their classification as SPs can be justified since elements of supramolecular interaction still assist the final organization. Examples are the layered assembly of complementary polyelectrolytes obtained by step-wise deposition under kinetic control, [70] and polymer brushes prepared by grafting a polymer chain over a selfassembled monolayer of an initiator. [71] Both approaches allow a fine tuning of surface properties, complemented by patterning possibilities. Tailored performance in applications, such as biocompatibility, biocatalysis, integrated optics and electronics, are possible. [70, 71] The following sections detail concepts and results relevant to supramolecular polymerization and SPs in class A systems. In particular, Section 4 summarizes the theoretical framework of polymerization mechanisms [1] forming the basis for the critical analysis of the experimental data to be presented in Section 5. Some aspects already described in preceding reviews/analyses by the author [1] (focusing on mechanisms), by Lehn, [2] Zimmermann and coworkers, [5] Meijer and coworkers [4] (focusing on chemical features) are briefly summarized, placing more emphasis on recent data and concepts. 2 The Bond. Site and Shape Recognition The cement holding well organized supramolecular structures requires the description of: (i) interaction between specific sites, (ii) site distribution and (iii) shape complementarity of the unimers (cf. ref. [1] for a detailed discussion). The relevant interactions are schematized in Figure Figure 2. Forces assisting supramolecular organization.
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Supramolecular Polymerizations

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