Mechanical Flipping of [1]Rotaxanes Toughens Polymer Networks

Effectively tuning mechanical properties is essential to advance the utility of polymeric materials. Rotaxanes have emerged as promising candidates for this purpose, such as [2]rotaxanes and polyrotaxanes. However, [1]rotaxanes, which are self-interlocked by a single component, remain largely underexplored in this area. Herein, we report the mechanochemical behaviors of two crown ether-based [1]rotaxanes and their impact on bulk mechanical properties when serving as crosslinkers. These two [1]rotaxanes share identical entangled architectures but differ solely in their flipping barriers, which originate from the steric bulkiness of the macrocycle. The DB24C8-based [1]rotaxane requires a substantially higher energy input for flipping, whereas the B24C8-based [1]rotaxane flips freely once the hydrogen bonding is disrupted, as confirmed by solution-phase studies and DFT calculations. When incorporated into polymer networks at a loading as low as 2.0 mol %, the DB24C8-based [1]rotaxane network exhibits a threefold increase in toughness and a twofold enhancement in stretchability compared to the B24C8-based [1]rotaxane network and the corresponding covalent networks. These results demonstrate that properly barriered molecular flipping can effectively dissipate energy and enhance the macroscopic mechanical performance of polymer networks, highlighting the potential of [1]rotaxanes as useful motifs for tuning network mechanics.