Metal carbonyl homopolymers (PFpPs) are hydrophobic but able to self-assemble in water into uniform and stable colloids. Hydrodynamic radius (R-h) of the colloids varies from 42 to 77 nm depending on preparation conditions. Light scattering, TEM, and AFM analyses of the colloids indicate that the colloids have a vesicular morphology with membrane thickness of ca. 3.0-4.0 nm. This thin membrane is, however, relatively rigid compared to conventional polymersomes. FT-IR and NMR analyses indicate that water-carbonyl interactions (WCI) is a motif responsible for the self-assembly and colloidal stability. By using nanoprecipitation encapsulation technique, PFpP vesicles can enclose hydrophilic molecules with extremely high encapsulation efficiency (EE) and loading capacity (LC). This work opens up a new research topic on hydration driven self-assembly and resolves a long-standing challenge of low EE and LC for the encapsulation of hydrophilic molecule via nanoprecipitation using amphiphilic molecules.