Doubly Threaded Slide-Ring Polycatenane Networks

Crosslinking in polymer networks leads to intrinsic structural inhomogeneities that result in brittle materials. Replacing fixed covalent crosslinks with mobile ones in mechanically interlocked polymers (MIPs), such as in slide-ring networks (SRNs) in which interlocked crosslinks are formed when polymer chains are threaded through crosslinked rings, can lead to tougher, more robust networks. An alternative class of MIPs is the polycatenane network (PCN), in which the covalent crosslinks are replaced with interlocked rings that introduce the unusual catenane's mobility elements (elongation, rotation, and twisting) as connections between polymer chains. A slide-ring polycatenane network (SR-PCN), with doubly threaded rings embedded as crosslinks in a covalent network, combines the mobility features of both the SRNs and PCNs, where the catenated ring crosslinks can slide along the polymer backbone between the two limits of network bonding (covalent and interlocked). This work explores using a metal ion-templated doubly threaded pseudo[3]rotaxane (P3R) crosslinker, combined with a covalent crosslinker and a chain extender, to access such networks. A catalyst-free nitrile-oxide/alkyne cycloaddition polymerization was used to vary the ratio of P3R and covalent crosslinker to yield a series of SR-PCNs that vary in the amount of interlocked crosslinking units. Studies on their mechanical properties show that metal ions fix the rings in the network, leading to similar behavior as the covalent PEG gels. Removal of the metal ion frees the rings resulting in a high-frequency transition attributed to the additional relaxation of polymer chains through the catenated rings while also increasing the rate of poroelastic draining at longer timescales.