This study demonstrates that the gradual and slow production of initiating radicals (i.e., hydroxyl radicals here) is the key point for the synthesis of ultra-high molecular weight (UHMW) polymers via controlled radical polymerization. Hydrogen peroxide (H2O2) and ferrous iron (Fe2+) react via Fenton redox chemistry to initiate RAFT polymerization. This work presents two enzymatic-mediated (i.e., Bio-Fenton-RAFT and Semi Bio-Fenton-RAFT) and one syringe pump-driven Fenton-RAFT polymerization processes in which the initiating radicals are carefully and gradually dosed into the reaction solution. The "livingness" of the synthesized UHMW polymers is demonstrated by chain extension and aminolysis experiments. Zimm plots obtained from static light scattering (SLS) technique are used to characterize the UHMW polymers. This Fenton-RAFT polymerization provides access to polymers of unprecedented UHMW (M-w similar to 20 x 10(6) g mol(-1)) with potential in diverse applications. The UHMW polymers made via the controlled Fenton-RAFT polymerization by using a syringe pump shows that it is possible to produce such materials through an easy-to-set up and scalable process. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1922-1930