www.asiachem.news December 2021 | 21 Towards a Sustainable Future WHY ARE TRADITIONAL polymeric materials so pervasive? Because the constituent monomer units that make up polymeric materials are covalently linked, these materials are chemically and mechanically tough, benefiting many applications, but significantly slowing their breakdown. If the constituent monomers were connected by noncovalent bonds, the resulting polymers could be readily depolymerized back to the monomers and recycled. Such polymers are called “supramolecular polymers” and were invented three decades ago. Because of their intrinsically dynamic nature, supramolecular polymers are self-repairable and reconfigurable, meaning that the monomer sequence in a copolymer may in principle be transformable into a different sequence, for example by applying external energy. One can then anticipate that such materials are adaptive, exhibiting intelligent structural transformations in response to changes in the surrounding environment. For supramolecular polymerization, a diverse range of monomers such as biomolecular machines that are not amenable to conventional polymerization can be used (Fig. 2e). Supramolecular polymer materials have long been considered fragile, weak, and unsuitable as structure-forming materials due to the dynamic nature of noncovalent bonds that connect their constituent monomer units. However, as these concerns are addressed, they may gradually replace traditional plastics to a certain extent in various applications. The reconfigurable and self-repairable nature of supramolecular polymers and their ability to readily be recycled to the monomer state will result in extended product lifespans, reducing energy and raw materials demands in manufacturing, and also reductions in the generation of the pervasive plastic and microplastic wastes that currently harm the environment. In 1988, 68 years after polymer science was established as a new research field, Aida published, as the first author, a short paper featuring a polymer-like one-dimensional assembly as a prototype of supramolecular polymers, in which the constituent monomer units are connected by a van der Waals interaction (Fig. 1b).2 Before this work, he had been involved in a project for developing precision covalent polymerization using specially designed metal complexes as polymerization initiators. During the course of this study, he developed “immortal polymerization”, a catalytic version of living polymerization, in which polymers with uniformmolecular weights are catalytically produced when protonic compounds as chain transfer reagents are present in combination with his special initiator for the living polymerization. In immortal polymerization, polymer chains are formed quantitatively from protonic compounds. Hence, he envisioned the incorporation of a polymer chain of predetermined length into protonic groups in functional compounds and succeeded in the synthesis of an amphiphilic porphyrin with four water-soluble polyether side chains using tetrakis(p-hydroxyphenyl)porphyrin as a protonic compound (Fig. 1a). Due to the water-solubility of the side chains, the amphiphilic compounds with long side chains appeared to be soluble in water. However, those with shorter side chains turned out to stack in a face-to-face manner via the core porphyrin, forming a 1D polymeric assembly. One month after this work was published, Aida noticed that the lid of the flask containing an aqueous solution of this supramolecular polymer had come off. Examining the flask, he was surprised to find the appearance of numerous cracks, which were not actually cracks but rather thin fibers of the supramolecular polymer generated by the evaporation of water from its aqueous solution. Although fascinated by the beauty of the large number of uniform fibers with a metallic luster, he never published this interesting observation or related papers, as he did not have an opportunity to follow up until he became an independent PI. In the meantime, the concept of supramolecular polymers began to take hold. Many early works focused on the assembly of molecules with By Takuzo Aida and Kiyoshi Morishita https://doi.org/10.51167/acm00019
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