The precision synthesis of poly(ionic liquid)s (PILs) in water is achieved for the first time by the cobalt-mediated radical polymerization (CMRP) of N-vinyl-3-alkylimidazolium-type monomers following two distinct protocols. The first involves the CMRP of various 1-vinyl-3-alkylimidazolium bromides conducted in water in the presence of an alkyl−cobalt(III) complex acting as a monocomponent initiator and mediating agent. Excellent control over molar mass and dispersity is achieved at 30°C. Polymerizations are complete in a few hours, and PIL chain-end fidelity is demonstrated up to high monomer conversions. The second route uses the commercially available bis(acetylacetonato)cobalt(II) (Co-(acac) 2 ) in conjunction with a simple hydroperoxide initiator (tertbutyl hydroperoxide) at 30, 40, and 50°C in water, facilitating the scaling-up of the technology. Both routes prove robust and straightforward, opening new perspectives onto the tailored synthesis of PILs under mild experimental conditions in water. P oly(ionic liquid)s (PILs) have emerged as a special class of polyelectrolyte materials, featuring tunable solubility, high ionic conductivity, and a broad range of glass transition temperatures. 1 Due to their specific properties emanating from the ionic liquid (IL) units and their intrinsic polymeric nature, PILs find potential applications in various areas, such as analytical chemistry, biotechnology, gas separation, dispersants, solid ionic conductors for energy, catalysis, etc. The main synthetic strategies for PILs include direct chain-growth polymerizationwhether controlled 2 or noncontrolled 3 of ionic liquid (IL) monomers, step-growth polymerization, 4 and postpolymerization modification of uncharged polymers by nitrogen quaternization. 5 Exchange of the counterion (anion or cation) is typically efficient, providing an additional method for the variation of PIL properties.In recent years, controlled radical polymerization (CRP) techniques have been applied to the synthesis of structurally well-defined PILs, with control attained over molar mass, dispersity, and end-group fidelity. Control of end-group functionality enables the precision engineering of IL-based block copolymers that possess unique self-assembly modes in solution 6 or in the bulk. 7 Confinement of PIL blocks within lamellar nanodomains of block copolymers has, for instance, led to polymer films with enhanced ionic conductivity. 7 The design of novel PIL structures is therefore necessary for the establishment of important structure/property relationships, required to expand the scope and realize the full potential of ILbased materials.The majority of studies has focused on the direct CRP of styrenic-or (meth)acrylic monomers containing pendant imidazolium groups, featuring various counteranions (e.g., Br − , BF 4 − , PF 6 − , and (CF 3 SO 2 ) 2 N − ). 2,8 In contrast, processes for CRP of N-vinyl-3-alkylimidazolium salts (VImX) are relatively underdeveloped due to difficulties in controlling the highly reactive growing polymer chains. Whil...