Reversible addition–fragmentation chain transfer (RAFT) polymerization has become a straightforward approach to block copolymers using a wide variety of functional vinyl monomers. Polyphosphoester (PPE) macroinitiators from ring-opening polymerization (ROP) of their corresponding cyclic phosphoesters have been previously prepared for atom transfer radical polymerization; however, to date, these biodegradable macroinitiators for RAFT polymerization have not been reported. Herein, a macromolecular RAFT-chain transfer agent (CTA) based on poly(ethyl ethylene phosphonate) was prepared by the organocatalytic ROP of 2-ethyl-2-oxo-1,3,2-dioxaphospholane using 2-cyano-5-hydroxypentan-2-yl dodecyl trithiocarbonate as the initiator and 1,8-diazabycyclo[5.4.0]undec-7-ene as the catalyst. Precise macro-CTAs of degrees of polymerization (DPn) from 34 to 70 with Đ ≤ 1.10 were prepared and used in the dioxane solution RAFT polymerization of acrylamide, acrylates, methacrylates, and 2-vinylpyridine to yield a library of well-defined block copolymers. Additionally, the PPE-based macro RAFT-CTA was used as a nonionic surfactant in a typical aqueous emulsion polymerization of styrene to produce well-defined nanoparticles with the hydrophilic PPEs on their surface as the stabilizing agent. This general protocol allowed the combination of polyphosphoesters with RAFT polymerization.