Ring-chain equilibria of dynamic macrocycles with a bis(hindered amino)disulfide linker

Abstract

Entropy-driven ring-opening polymerization (ED-ROP) of macrocyclic monomers (MMs) facilitates simple and green processes derived from ring-chain equilibria; however, the preparation of MMs typically requires highly diluted conditions and catalysts, preventing a detailed investigation of equilibrium reactions and application potentials based on the dynamic nature of equilibrium reactions. We have previously reported simple ROP of macrocycles containing one bis(2,2,6,6-tetramethylpiperidin-1-yl)disulfide (BiTEMPS) structure as a dynamic unit. In this report, we investigate the effect of chain length on ring-chain equilibria to both reveal their dynamic equilibrium reactions and expand the availability of BiTEMPS-based MMs. Equilibrium reactions based on BiTEMPS are ideal for studying the dynamic nature of cyclic topology because (1) the radicals generated by BiTEMPS upon heating exhibit high tolerance to oxygen-based moieties and olefins and high reactivity in exchange reactions, and (2) catalysts and/or additives are not required to induce the dynamic state. We prepared linear precursors (LPs) with various chain lengths (N) between each BiTEMPS unit and characterized the cyclodepolymerization, i.e., ring-chain equilibria. We found that the yield of MMs clearly depends on N and the presence of distortions in some ring structures. This study provides important insights into ring-chain equilibria for entropy-driven ROP systems because our MMs are characterized by a well-defined structure, good stability at room temperature, high reactivity in high-temperature exchange reactions, and reversibility.