These files contain data supporting all results reported in Jacky, P.E.; Neukirch, M.A.; Fors, B.P. Anionic Reversible Addition-Fragmentation Chain-Transfer (RAFT) Polymerization of Methacrylates. Anionic polymerizations of methacrylates are controlled processes that give well-defined materials; however, these polymerizations require reactive and pyrophoric organolithium reagents for initiation and must be run at low temperatures to maintain control. The ability to run these reactions closer to room temperature and limit the amount of pyrophoric reagents necessary to carry out these polymerizations would increase the practicality, safety, and scalability of these reactions. Herein, we present an anionic reversible addition–fragmentation chain-transfer (RAFT) polymerization that reduces the amount of reactive alkyl lithium required for initiation and is performed at elevated temperatures compared to traditional anionic processes. By using ethyl 2-formyl-2-phenylbutanoate as a chain-transfer agent, we leverage reversible aldol reactions with the propagating enolate chain ends to achieve chain-transfer and subsequent control over the polymerization. Through the proposed anionic RAFT mechanism, a variety of methacrylates were polymerized in a controlled manner. The resulting polymers had stable, isolable aldehyde chain ends, which could be reinitiated to form block polymers.