For the covalent delivery of unmodified probes, guanidinium-containing propagators (e.g., 1–4) are polymerized on thiolated substrates (e.g., 5–7) and terminated with iodoacetamides (e.g., 8, 9). After uptake, cell-penetrating poly(disulfide)s are degraded by reductive depolymerization to eliminate toxicity and release the unmodified substrate.