Molecular structure is a vital determinant of the physical properties and functions of molecules and materials. However, several medium- and macromolecules with unique structures (topology) are still challenging to synthesize. Dr. Yamago focused on medium molecules and polymers that have characteristic topology but are difficult to synthesize, and developed a new and practical synthetic method. He also elucidated the physical properties and functions derived from the topology of the synthesized molecules. The following is an overview of his major accomplishments.

1. Development of Synthetic Methods for Cyclic π-Conjugated Oligomers and Elucidation of Their Physical Properties
While fullerenes and carbon nanotubes (CNTs) are interesting π-conjugated molecules with cyclic topology, they are only available as mixtures by physical synthetic methods. Thus, the impact would be enormous if such cyclic π-conjugated molecules could be synthesized by bottom-up organic synthesis with a controlled structure. Since π-conjugated molecules are composed of sp² carbons with planar structures, they must be "bent" to synthesize the cyclic form. Dr. Yamago has achieved this by developing a novel synthetic method by focusing on the cis-coordination of platinum complexes. For example, he succeeded in synthesizing [8]cycloparaphenylene (CPP) by accumulating a biphenyl molecule into a cyclic tetramer by a platinum complex followed by reductive elimination of platinum. (The number in [ ] is the number of paraphenylene units constituting CPP.) By further developing this method, he succeeded in synthesizing CPPs of different sizes from [5]CPP to [21]CPP, various CPP derivatives, and π-conjugated molecules with cage-like structures and Möbius topology. Similar reactions are possible with nickel and palladium but, in practice, are not feasible with these metals due to the fast reductive elimination reactions. Instead, the reductive elimination reaction of the platinum complex is slow, allowing time for the formation of the ring structure. This platinum-based method is already widely used by researchers around the world and is recognized as one of the most representative synthetic methods for cyclic π-conjugated molecules.
Dr. Yamago has also made remarkable achievements in elucidating the characteristic properties and functions of CPPs derived from their ring structure and size. For example, he theoretically predicted that CPP differs from ordinary π-conjugated molecules in that the smaller the conjugation length, the narrower the band gap. He demonstrated this by photophysical and electrochemical measurements. He further elucidated spin and charge delocalization and in-plane aromaticity based on the isolation of ionic species of CPP produced by oxidation. In addition, the size dependence of the dynamics of excited states in neutral and ionized CPPs and the host function of CPPs were also elucidated for the first time.

2. Synthesis and Characterization of Hyperbranched Polymers (HBPs) with Dendritic Structure
HBPs are key molecules for fabricating functional polymer materials because they possess many characteristic properties compared to linear polymers. However, the lack of practical methods to synthesize HBPs by controlling their molecular weight and branching structure has severely limited their use. Dr. Yamago developed a new monomer that selectively induces branching in organotellurium-mediated radical polymerization (TERP), and succeeded in developing a synthetic method of HBPs that satisfies practicality and structural control. The key to molecular design with branch-inducing monomers is that branching does not occur until after the monomer moiety has reacted. The advantage of this method includes the control of the branch structure by the ratio of TERP chain transfer agent and branch-inducing monomers and the synthesis of block copolymers with different topologies, linear and branched, by taking advantage of the living character of the polymerization. Furthermore, physical properties of the HBPs, such as intrinsic viscosity, can be systematically controlled by branch structure and topology. The branch structure can also systematically control the lower critical solution temperature of polyisopropylacrylamide dissolved in water.

3. Elucidation of the Termination Mechanism of Radical Polymerization
　Dr. Yamago has made significant contributions not only to the control of radical polymerization but also to the elucidation of the termination mechanism of conventional radical polymerization. He proposed a new method to elucidate the termination mechanism of radical polymerization by using polymers with controlled molecular weight and analyzing the structures of the products. For this purpose, he used a structure-controlled polymer synthesized by TERP as a precursor for polymer-end radicals and photochemically activated the resulting polymer. As a result, contrary to conventional understanding, disproportionation occurs selectively in acrylate monomers, which were thought to terminate by recombination because they are monosubstituted monomers. He also clarified that the termination mechanism is dynamic; the mechanism in styrene polymerization is completely reversed from recombination to disproportionation by viscosity. The same viscosity effect was also observed for methacrylate monomers, which are disubstituted monomers, and selective disproportionation occurs in highly viscous media. In addition, the selectivity of acrylonitrile varies greatly with solvent polarity and viscosity. These findings provide a breakthrough in the understanding and quantification of the termination mechanism, as well as important insights into the design of polymer materials by conventional radical polymerization.

As described above, Dr. Yamago has successfully developed practical synthetic methods based on new ideas for medium- and macromolecules with cyclic and branched topology, which are considered challenging to synthesize. He has also clarified many unique properties and functions derived from topology. With these discoveries, Dr. Yamago has opened up new frontiers ranging from synthetic chemistry, physical organic chemistry, and polymer chemistry to materials science. All of these results, while based on fundamental chemistry, have the potential to evolve into applied and practical research. In addition, these research results are original and groundbreaking, completely unprecedented in the world, and highly recognized both domestically and internationally. Thus, his work was recognized as worthy of the Chemical Society of Japan Award.