Five S-alkyl di-tert-butylphosphorylated carbodithioates, Z–C(S)SR, were studied as chain transfer agents (CTAs) in the RAFT polymerization of vinyl monomers. These phosphorus-containing CTAs involve new PIII and PV moieties in the Z group (phosphine-borane or free phosphine). Two of them, with an S-bonded 1-methoxycarbonylethyl group as leaving group, have not previously been described. The CTAs with a PV Z group provide good control over the polymerization of styrene (St) and n-butyl acrylate (nBA). Molar masses are close to theoretical values, and dispersities are consistently low during polymerization. The monitoring of polymerization by 31P NMR spectroscopy supports the controlled character and the integrity of the polymer chain ends. Conversely, they inhibit the polymerization of the less-activated monomer vinyl acetate (VAc). The reactivity of the CTAs with a PIII Z group mainly depends on the reaction medium, and their performance for St, nBA, and VAc was surprising and unexpected. Theoretical calculations were carried out to rationalize their behavior. Finally, an innovative strategy was developed to obtain well-controlled polymers with free phosphine ω-chain ends.