22 | December 2021 www.facs.website complementary H-bonding moieties. Onedimensional H-bonded chains were shown to form in the solid state by the assembly of a dipyridone with non-self-complimentary H-bonding units by Wuest,3 melamine and cyanuric acid derivatives by Whitesides,4 and triaminopyrimidine and barbituric acid derivatives by Lehn.5 Lehn also extended supramolecular polymers to the liquid crystalline state with a triple H-bonded design of monomers that assemble into chiral fiber-like structures.6 Ghadiri expanded on the scope of monomers by introducing the H-bond directed stacking of cyclic peptides.7 Percec reported on the columnar supramolecular polymerization of branched oligomers as monomers.8 Finally, Meijer reported his seminal 1997 paper on a supramolecular polymer consisting of a quadruple hydrogen-bonding interaction9 and pushed the research field to the forefront of chemistry by disclosing for the first time the viscoelastic properties of supramolecular polymers in solution and under dry conditions. Aida reinitiated the project on supramolecular polymerization nearly 10 years after his pioneering paper in 1988 and reported the first self-sorting phenomenon in the stereochemical copolymerization of two enantiomers of a chiral monomer in 2002.10 In 2004, he reported one of his seminal works featuring the nanotubular supramolecular polymerization of an amphiphilic molecular graphene to obtain electronically conductive nanotubes with a very high structural integrity (Fig. 2b).11 Despite the prevalence of supramolecular polymers with H-bonding interactions, analogous to his original work published in 1988, he employed only a van der Waals interaction to connect the monomer units, and further obtained radial and linear supramolecular block copolymers in 200612 and 2011,13 respectively. In 2020, Aida had the opportunity to jointly write a review article with Bert Meijer on supramolecular polymers for a special issue of the Israel Journal of Chemistry in celebration of the 100-year anniversary of polymer science.14f The joint review article daringly included the special subtitle, “We’ve Come Full Circle”, as a reference to the historical background of polymer science. Before polymer science was launched as a new field of research in 1920, there had been a long-term debate between two groups, pushing the “macromolecular theory” by Staudinger and the “colloidal theory” by van’t Hoff, Fischer, Wieland, et al. After Staudinger experimentally substantiated the existence of long and gigantic molecules in 1920,15 the “colloid theory” declined. However, it is meaningful to consider the idea of supramolecular polymerization, which started in 1988, to be a modernized version of the “colloid theory” with a flavor of physical organic chemistry. Over the past 30 years, supramolecular polymerization has grown into a very hot research field,14 in which our group has enjoyed contributing to its progress and conceptual expansion. Our major achievements include nanotubular supramolecular polymerization (Fig. 2b),11-13,16,17 living chain-growth supramolecular polymerization (Fig. 2f ),18 supramolecular block copolymerization,13,19 stereoselective supramolecular polymerization,16a and thermally bisignate supramolecular polymerization.20 These concepts are integral to conventional polymer science and our contributions filled in the critical gap between supramolecular and conventional polymerizations. Furthermore, we extended the basic concept of supramolecular polymerization to the development of a variety of innovative soft materials such as bucky gels (Fig. 2a),21 carbon nanotubes noncovalently crosslinked by ionic liquids, and their use in the first metal-free stretchable electronics22 and battery-driven dry actuators,23 aquamaterials (Fig. 2c),24 robust and dynamic materials consisting mostly of water, with small amounts of additives, ATP-responsive nanotubular DDS carriers using the biological machine GroEL as the monomer (Fig. 2e),25 and stimuli-responsive columnar liquid crystalline materials (Figs. 2d and 2g).26 In 2018, our group also reported a major advancement towards mechanically robust materials with self-repairable features.27a We showed that low molecular weight ether thiourea oligomers (Fig. 3a), which can quickly and easily self-heal under ambient temperatures by swapping H-bonded thiourea pairs at fractured edges, display high mechanical robustness due to a dense nonlinear array of H-bonds (Fig. 3b). This work swept away the preconceptions that mechanically robust polymers necessarily have ultra-high molecular weights and are unable to self-heal due to the sluggish diffusion of such long polymer chains. We recently reported an updated version of this material in which copolymerization with dicyclohexylmethane thiourea units imparts humidity resistance to the material, expanding the potential range of usage conditions (Figs. 3c and 3d).27b Considering the short polymer chains employed in these reports, the next target is to further reduce their molecular weights into a range of ordinary monomers, which may unlock the full potential of supramolecular polymer materials in terms of complete reconfigurability and recyclability. Lastly, we highlight our new achievement, “solvent-free autocatalytic supramolecular polymerization (SF-ASP)” (Fig. 4),19 its significance in the field and relevance to the emergent environmental issues caused by plastic waste introduced above. Historically, supramolecular polymerization has been extensively studied in solution, with its mechanical interpretations being greatly elaborated in the last decade. Nevertheless, much work remains when considering the practical applications of supramolecular polymers. One common concern is that the structure of a supramolecular polymer in solution is not guaranteed to be the same as that in the dry state. This concern would be avoided if supramolecular polymers were synthesized under solvent-free conditions. Note also that solvent-free chemical manufacturing is an awaited green technology for the realization of a sustainable society, because of its reductions in raw materials usage, pollution and CO2 emission. However, there exists a preconception that supramolecular polymerization under solvent-free conditions may proceed heterogeneously, Figure 1. (a) Synthetic scheme for the synthesis of an amphiphilic porphyrin with PEG side chains of well-defined chain lengths. (b) 1D polymeric assembly of an amphiphilic porphyrin in water as a prototype of supramolecular polymerization.2
RkJQdWJsaXNoZXIy NDU2MA==