These files contain data supporting all results reported in Jacky et. al. Controlled Anionic Polymerization Mediated by Carbon Dioxide. In Jacky et. al. Controlled Anionic Polymerization Mediated by Carbon Dioxide we found: Anionic polymerizations of vinyl monomers are a powerful synthetic platform to make well-defined materials. However, these reactions are extremely sensitive to moisture and oxygen, require the use of highly purified reagents, must be run at low temperatures, and use hazardous and difficult-to-handle alkyl lithium initiators. Together, these drawbacks limit the practicality of these polymerizations and impede their widespread usage. On this basis, the development of a user-friendly anionic polymerization process for methacrylates is a grand challenge. Herein, we report an anionic polymerization of methacrylates mediated by CO2 that can be run at elevated temperatures and allows the use of an easy-to-handle solid initiator. The reversible addition of CO2 to the enolate chain-end efficiently tempers the reactivity of the anion, giving polymers with narrow molar mass distributions and excellent molecular weight targeting at elevated temperatures. Our scalable and more user-friendly CO2-mediated method improves the accessibility and safety of anionic polymerizations and facilitates the production of a variety of polymeric materials.