Novel amphiphilic polypeptoid-polyester diblock copolymers based on poly(sarcosine) (PSar) and poly(epsilon-caprolactone) (PCL) are synthesized by a one-pot glovebox-free approach. In this method, sarcosine N-carboxy anhydride (Sar-NCA) is firstly polymerized in the presence of benzylamine under N-2 flow, then the resulting poly(sarcosine) is used in situ as the macro-initiator for the ring-opening polymerization (ROP) of epsilon-caprolactone using tin(II) octanoate as a catalyst. The degree of polymerization of each block is controlled by various feed ratios of monomer/initiator. The diblock copolymers with controlled molecular weight and narrow molecular weight distributions (D-M < 1.2) are characterized by H-1 NMR, C-13 NMR, and size-exclusion chromatography. The self-assembly behavior of PSar-b-PCL in water is investigated by dynamic light scattering (DLS) and transmission electron microscopy. DLS results reveal that the diblock copolymers associate into nanoparticles with average hydrodynamic diameters (D-H) around 100 nm in water, which may be used as drug delivery carriers.