Cobalt-mediated radical polymerization (CMRP) is a versatile technique for controlling the polymerization of vinyl monomers via reversible termination using Co^II complexes as persistent radical deactivators. Here, we report a facile approach for the in situ generation of Co–H as a discrete initiator and mediator for CMRP of acrylate and acrylamide monomers, overcoming the limitations of existing initiation strategies. In situ oxidation of a Co^II complex followed by transmetalation with silane generates a Co-H species, which initiates polymerization via hydrometalation of the monomer. This method precludes an induction period with excellent control over targeted molecular weight and dispersity. Strikingly, our approach allows complete polymerization when the induction period ends for conventional CMRP. A broad scope of monomers is amenable to this protocol, including acrylates and acrylamides. Tunable catalyst electronics afford tailored dispersity while maintaining agreement in molecular weight in stark contrast to conventional methods. Elimination of this induction period imbues polymerization behavior entirely to the catalyst electronic effects on reversible deactivation/activation rates.