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Abstract

Crosslinking in polymer networks leads to intrinsic structural inhomogeneities that result in brittle materials. Replacing fixed covalent crosslinks with mobile ones results in mechanically interlocked polymers (MIPs) such as slide-ring networks (SRNs), where polymer chains threaded through rings form interlocked crosslinks that act as frequency-dependent pulleys to relax internal stresses, leading to tougher, more robust materials. SRNs are typically made by crosslinking rings on different polyrotaxanes, interlocked architectures comprised of a polymer chain threaded with rings and capped with stoppers to prevent dethreading, to form figure-of-eight crosslinks and an uncontrolled amount of singly-threaded (uncrosslinked) rings. MIP networks containing doubly-threaded rings are less explored because of the difficulty of synthesizing rings big enough to thread two polymer chains and the challenges in preventing slippage of the larger rings. This work explores different click chemistries to develop a new way to access doubly-threaded SRNs from a simple mixture of tetra-functional pseudo[3]rotaxane (P3R) crosslinker, difunctional monomer, and large stoppers to keep rings in the network. Ditopic rings comprised of two 2,6-bis(N-alkyl-benzimidazolyl)pyridine (BIP) ligands were threaded with two BIP threads using metal-templating to access the doubly-threaded P3R complex. SRNs were synthesized by incorporating the P3R crosslinker into a catalyst-free nitrile-oxide/alkyne click polymerization of PEG-based networks with varying amounts of interlocked crosslinks, where covalent crosslinks act as stoppers to prevent slippage of the rings. 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 doubly-threaded rings at longer timescales. After remetallation, this transition disappears, and SRNs show similar trends as the original metallated network, proving ring sliding can be turned off in the network by adding metal ions. The synthetic design of these doubly-threaded MIPs allows for systematic control over the number and type of rings, and other components and polymers, that can be added to the click polymerization. The versatility of the macrocycle synthesis used in this work provides a basis for controlling the rings’ size, conformation, and valency to tune MIP material properties. To this end, monotopic rings were synthesized to assemble pseudo[2]rotaxanes that polymerize under catalyst-free conditions. The resulting pseudopolyrotaxanes can be incorporated into the network synthesis to form doubly-threaded SRNs embedded with singly-threaded rings. This straightforward, catalyst-free click approach is a promising platform for understanding the complex structure-property relationships in doubly-threaded MIPs.

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