Biocompatible and proteolysis-resistant poly-bpeptides have broad applications and are dominantly synthesized via the harsh and water-sensitive ring-opening polymerization of b-lactams in a glovebox or using a Schlenk line, catalyzed by the strong base LiN(SiMe 3 ) 2 . We have developed a controllable and water-insensitive ring-opening polymerization of b-amino acid N-thiocarboxyanhydrides (b-NTAs) that can be operated in open vessels to prepare poly-b-peptides in high yields, with diverse functional groups, variable chain length, narrow dispersity and defined architecture. These merits imply wide applications of b-NTA polymerization and resulting poly-b-peptides, which is validated by the finding of a HDP-mimicking poly-b-peptide with potent antimicrobial activities. The living b-NTA polymerization enables the controllable synthesis of random, block copolymers and easy tuning of both terminal groups of polypeptides, which facilitated the unravelling of the antibacterial mechanism using the fluorophore-labelled poly-b-peptide.Unnatural polypeptides are structurally similar to natural peptides and have prominent advantages in applications owing to their biocompatibility and resistance to proteolysis. [1] Among them, poly-b-peptides have attracted broad interest [1a-c, 2] and have demonstrated great potential in proteinprotein interaction, [3] antimicrobial, [4] glycopeptide mimicking, [5] tissue engineering, [6] constructing molecular brushes, [7] and surface functional coating. [8] In contrast to the polymerization of b-amino acid N-carboxyanhydrides (b-NCAs) for which few follow-up has been reported, [9] ring-opening polymerization (ROP) of b-lactam is the most commonly used method for poly-b-peptides synthesis reported in the literature. [4e,f,h, 5, 6, 10] The advances of LiN(SiMe 3 ) 2 -catalyzed ROP of sidechain-functionalized b-lactams have boosted the functionality and application of poly-b-peptides. [4e,f,h, 5, 6, 10a,d, 11] Nevertheless, this strong basic polymerization condition is not compatible with base-labile groups, such as esters, and has limited the structural diversity and application of poly-bpeptides. In addition, the ROP of b-lactams is extremely sensitive to water and is normally operated in the glovebox using dry solvents even though it is still challenging to prepare long poly-b-peptides over 100 repeating units. [4e,h, 6a, 10a,e] Moreover, the C-terminal modification of sidechain-functionalized poly-b-peptides is notoriously difficult though modification on nonfunctionalized poly-b-peptides was reported previously. [10e,f] These long-lasting shortcomings of poly-b-peptide synthesis urgently call for solutions.Here, we report a convenient synthesis of poly-b-peptides via controllable ROP of b-amino acid N-thiocarboxyanhydrides (b-NTAs), which is insensitive to water and oxygen, and is able to address all aforementioned shortcomings of the currently dominating methods for poly-b-peptide synthesis from b-lactams (Figure 1 a). The ROP of b-NTAs proceeded smoothly under mild condition w...