Density-driven reentrant polymer transitions via saturable bridging crowders

Reentrant coil-globule-coil transitions, in which a polymer collapses and then reexpands as a single parameter is varied, have been observed across diverse soft matter systems, yet the minimal ingredients required to produce them remain unclear. Using molecular dynamics simulations of coarse-grained polymers interacting with a single species of attractive crowder, we show that crowder volume fraction $\phi_c$ alone is sufficient to drive a complete reentrant transition. At low $\phi_c$, crowders bridge distant monomers and drive cooperative collapse; at high $\phi_c$, saturation of monomer binding sites suppresses bridging connectivity and produces reentrant expansion. This density-driven transition is absent with purely repulsive crowders, which produce only monotonic compaction while preserving self-avoiding walk (SAW) chain statistics. In contrast, bridging breaks SAW universality: the rescaled size distributions no longer collapse onto a universal curve, and the conformational distributions trace the full coil-globule-coil trajectory as $\phi_c$ is varied. For charged polymers with explicit counterions, electrostatics amplifies rather than suppresses reentrance: bridging crowders displace counterions from the chain, and upon saturation the unscreened backbone charges drive expansion well beyond the original chain size. Saturable geometric bridging thus emerges as a minimal mechanism linking reentrant phenomena across neutral and charged polymers in crowded environments.