The present contribution reports a novel controlled/living ring-opening polymerization (ROP) method of ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL) using ethyl diphenylphosphinite (EDPP) as an initiator and diphenyl phosphate (DPP) as a dual-role organocatalyst, which involves the activated monomer mechanism (AMM) and the reversible chain end deactivation process through a unique equilibrium reaction between the active phosphinite center and its protonated dormant species. To achieve controlled polymerization, the medium acidity of DPP is found to be significantly crucial because it not only promotes the monomer activation, but also helps in controlling the contents of the active and dormant species by establishing the above equilibrium reaction. In contrast, the use of either the much weaker benzoic acid or the much stronger triflimide cannot lead to a controlled ROP. The monomer-insertion reaction toward the P–O bond has been proven through polymer structure analyses using ¹H NMR and MALDI-TOF MS spectra, from which the residual ethoxy group of EDPP is affirmed to bond at the initiating end of the polymer product. The unique ROP mechanism is verified based on the analyses of elementary reactions, polymerization kinetics, and polymer structures. The living nature of the current ROP is confirmed by polymerization kinetics and further by chain-extension experiments, which are employed to synthesize poly(ε-caprolactone)s (PCLs) and poly(δ-valerolactone)s (PVLs) with various controlled molar masses (PCL: 4300–41 900 g mol⁻¹ and PVL: 2860–18 900 g mol⁻¹) and moderate dispersities (M_w/M_n: 1.15–1.31), and also their block copolymers. Given the amount of an antioxidant agent (usually 0.05–0.5 wt%) practically used in commercial polymer products, these phosphinite end-functionalized polyesters are believed to have sufficient self-antioxidant properties.