Synthetic Strategy for Mechanically Interlocked Cyclic Polymers via the Ring-Expansion Polymerization of Macrocycles with a Bis(hindered amino)disulfide Linker

Mechanically interlocked polymers that contain rotaxane and catenane structures have attracted much attention on account of their unique properties arising from the restricted mobility of their interlocked structure and their robustness which are comparable to those of covalent bonds. Among these polymers, mechanically interlocked cyclic polymers (MICPs) exhibit great potential as a novel type of polymer with a large movable area of the interlocked structure. However, synthetic routes to MICPs are not well developed, and it is still challenging to create MICPs. The present study has resulted in an effective method for the synthesis of MICPs from the combination of the ring-expansion polymerization (REP) of cyclic disulfide monomers with supramolecular interactions. Macrocyclic monomers (MMs) that consist of a bis(hindered amino)disulfide (BiTEMPS) linker and a supramolecular moiety, such as naphthalenediimide (NDI) and dialkoxynaphthalene (DAN), capable of forming a strong 1/1 charge-transfer complex, were synthesized as the monomers for the subsequent REP. These MMs were used in a heat-induced REP in the bulk state, which led to their swift polymerization via an intermolecular exchange reaction of BiTEMPS. The change in mechanical properties during the polymerization was monitored by rheological measurements of the increase of the storage modulus, G'. Importantly, the bulk copolymerization of the MMs containing NDI and DAN increased the hydrodynamic volume of the resulting copolymers, which is due to the spatial entanglement of the polymer chains. The change in the physical properties of the resulting polymers stands in sharp contrast to that observed in polymers with a linear topology made from the same monomers, thus supporting the formation of MICPs. The results provide guidelines for the successful design of MICPs, that is, a combination of the dynamic nature of the MMs and supramolecular interactions. Given that the present method is highly versatile, it can be expected to be applicable to various molecular skeletons and supramolecular systems.

Automated summary

This study presents an effective method for the synthesis of MICPs through a combination of REP and supramolecular interactions of MMs. The copolymerization of MMs containing NDI and DAN increases the hydrodynamic volume of the resulting copolymers and leads to changes in physical properties, supporting the formation of MICPs. The results provide guidelines for the successful design of MICPs by combining the dynamic nature of MMs and supramolecular interactions.