An Oriented Polymer in a Dynamic Microsolution Pierces Molecular Rings: An Approach toward Polyrotaxane Synthesis under Precise Kinetic Control.

In this study, we demonstrated host-guest chemistry under dynamic conditions using a polymer-ring system as a model. We found that a Hagen-Poiseuille flow drives a guest polymer into the cavities of ring hosts repeatedly, in a manner distinct from self-threading under thermodynamic equilibrium. Using poly(ethylene glycol) (PEG) and γ-cyclodextrin (γ-CD) as a representative polymer-ring system, two PEG units were threaded into γ-CD in a head-to-tail fashion, forming extremely long pseudodouble-stranded polyrotaxane (DS-PR) nanofibers. These novel DS-PR structures further assembled hierarchically through facial hydrogen bonding, resulting in micrometer-scale crystalline fibers. We systematically investigated the influence of solution motion on host-guest interactions by varying hydrodynamic parameters (e.g., total flow rate, channel geometry, and channel length) and structural parameters (e.g., polymer length and γ-CD/PEG ratio). Remarkably, the forward end of the PEG chain preferentially pierced the wider rim of the γ-CD. Based on these observations, we devised an active-threading mechanism, wherein the end of the PEG chain pierces multiple rings while satisfying both energetic and steric requirements, with the microflow channel acting as a catalyst to accelerate host-guest interaction. Finally, we explored the possibility of cothreading α- and γ-CD units onto a single PEG chain. Notably, overcoming the classical "lock-and-key" paradigm of host-guest chemistry, the observed "piercing effect" enabled the same PEG chain to thread into two differently sized CDs depending on the α-CD/γ-CD ratio in solution.