New circular polymeric materials for a carbon-neutral future must be designed to operate within a circular economy framework. Traditionally, polymers have relied on irreversible covalent bonds between repeating monomer units, which limits their recyclability. Recently, however, increasing attention has been directed toward dynamic polymers constructed from reversible covalent or non-covalent bonds, enabling reuse and recycling.
Dynamic polymers contain reversible linkages that can be formed from monomers and cleaved back into monomeric units by external stimuli such as heat or light. Among these, 2π+2π cycloaddition reactions represent a particularly promising class of reversible reactions. Only a limited number of molecular systems, including cinnamate, stilbene, thymine, coumarin, and styrylpyrene derivatives, are known to undergo such reactions.
In the first part of this study, we systematically validate the potential of these reversible reactions through topochemically controlled monomer-to-polymer transformations based on 2π+2π cycloaddition. These approaches enable the creation of functional circular polymers, such as self-healing materials and reusable adhesives.
In the second part, we report the synthesis of novel circular polymers derived from lignin. Glycerol-based polyfunctional monomers bearing cinnamate groups were prepared via a sustainable process using vanillin and syringaldehyde, which are readily obtained from lignin oxidation. These monomers were employed to fabricate bio-based, photo-crosslinkable and decrosslinkable circular polymers. Circular polymers derived from other bio-based compounds, such as soybean oil, are also presented.
Finally, we introduce newly developed environmental assessment tools to evaluate the environmental performance of these materials. In particular, we have extended the conventional E-factor concept to explicitly incorporate product circularity, enabling a more comprehensive and practical assessment of material sustainability. In addition, we briefly introduce the activities of our Circular Materials Centre, which focuses on close collaboration with industrial partners in Japan to accelerate the practical implementation of circular polymer technologies.