Carbon-13 kinetic isotope effects (KIEs) have been determined for free-radical and copper-mediated living radical polymerizations of methyl methacrylate at 60 °C. While free-radical polymerization shows only one primary ¹³C KIE, on the least-substituted double bond carbon (k₁₂/k₁₃ = 1.045), two significant KIEs are observed, one on each double bond carbon, for copper-mediated polymerization (k₁₂/k₁₃(H₂C) = 1.050, k₁₂/k₁₃(C<) = 1.010), showing that copper-mediated living radical polymerization does not propagate via a simple free radical process.