The amino acid cysteine possesses a unique role in nature due to its ability to reversibly cross-link proteins. To transfer this feature to polypeptides and control the process of disulfide formation, a protective group needs to provide stability against amines during synthesis, combined with chemoselective reactivity toward thiols. A protective group providing these unique balance of stability and reactivity toward different nucleophiles is the S-alkylsulfonyl group. In this work we report the polymerization of S-ethylsulfonyk-cysteine N-carboxyanhydride and kinetic evaluations with respect to temperature (-10, 0, and +10 degrees C) and monomer concentration. The polymerization degree of poly(S-ethylsulfonyl-L-cysteines) can be controlled within a range of 10-30, yielding well-defined polymers with molecular weights of 6900-12300 g/mol with dispersity indices of 1.12-1.19 as determined by GPC and MALDI-TOF analysis. The limitation of chain length is, however, not related to side reactions during ring-opening polymerization, but to physical termination during beta-sheet assembly. In the case of poly(S-ethylsulfonyl-L-cysteines), circular dichroism as well as FT-IR experiments confirm an antiparallel beta-sheet conformation. The reaction of poly(S-ethylsulfonyl-L-cysteines) with thiols is completed in less than a minute, leading quantitatively to asymmetric disulfide bond formation in the absence of side reactions. Therefore, poly(S-ethylsulfonyl cysteines) are currently the only reactive cysteine derivative applicable to NCA synthesis and polymerization, which allows efficient and chemoselective disulfide formation in synthetic polypeptides, bypassing additional protective group cleavage steps.