Supramolecular polymers based networks

Having discussed self-assembly strategies toward noncovalently functionalized side chain supramolecular polymers as well as studies toward the orthogonahty of using multiple noncovalent interactions in the same system, this section presents some of the potential applications of these systems as reported in the literature. The apphcations based on these systems can be broadly classified into two areas 1) self-assembled functional materials and 2) functionalized reversible network formation. 

Figure 6. Formation of a linear ladder-type supramolecular polymer (14) or a hydrogen-bonded network (15) based on the single hydrogen bond between a pyridine unit and a benzoic acid unit.

The following sections describe the preparation and characterization of supramolecular polymer networks, particularly emphasizing their physical-chemical features with regard to the type and strength of physical chain cross-linking and the resulting macroscopic material properties. Furthermore, recent work on the formation and characterization of supramolecular hydrogels based on synthetic and natural precursors is summarized with a focus on their application and potential in biomedicine. 

Inclusion complexation has developed to becoming another widely exploited supramolecular interaction for the formation of supramolecular polymer networks, mostly in water [197, 198]. Several classes of macrocycles have been developed, including crown ethers [199, 200], porphyrins [201, 202], cyclophanes [203], catenanes [204], cavitands [205, 206], cryptophanes [207], calix[n]arenes [208], and carcerands [209]. Macrocyclic-based supramolecular gels can either be formed from low molecular weight precursors or from macromolecular building blocks. The following discussion focuses on the latter. 

More recently, the importance of introducing supramolecular interactions between macromolecular chains has become evident, and many new options have been introduced. The final step in this development would be to develop polymers based on reversible, noncovalent interactions. Rather than linking the monomers in the desired arrangement via a series of polymerization reactions, the monomers could be designed in such a way that they self-assemble autonomously into the desired stracture. As with covalent polymers, a variety of structures of these so-called supramolecular polymers are possible, with block-copolymers or graft copolymers - as well as polymer networks - being created in this way. 

Clearly, supramolecular materials based on telechelics combine many of the mechanical properties of conventional macromolecules with the low melt viscosity of low molecular weight organic compounds. The reversibility of supramolecular polymers adds new aspects to many of the principles that are known from condensation polymerizations. For example, a mixture of different supramolecular monomers will yield copolymers, but it is extremely simple to adjust the copolymer composition instantaneously by adding an additional monomer. Moreover, the use of unimers with a functionality of three or more, will give rise to network formation. However, in contrast to condensation networks, the self-healing supramolecular network can reassemble to form the thermodynamically most favorable state, thus forming denser networks (Figure 6). 

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