Compound (1‐carbomethoxyethyl)pentacarbonylmanganese(I), [MnR(CO)5] (R = CHMeCOOMe, 1), was synthesized from K+[Mn(CO)5]– and methyl 2‐bromopropionate and isolated in pure form. Upon thermal activation, the Mn–R bond is homolytically cleaved and the resulting 1‐carbomethoxyethyl radical is able to initiate the polymerization of methyl acrylate (MA). A kinetic study of the decomposition of 1 in the presence of tris(trimethylsilyl)silane, TTMSS (10 equiv., saturation conditions) at 70, 65 and 60 °C yielded the T‐dependent activation rate constant, ka, which allowed the calculation of the activation enthalpy (ΔH‡ = 35.3 ± 2.8 kcal mol–1) and entropy (ΔS‡ = 27.2 ± 8.1 cal mol–1 K–1) through the use of the Eyring relationship. The ΔH‡ value can be taken as an upper limit for the thermodynamic bond dissociation enthalpy, which was estimated as 36.9 kcal mol–1 by DFT calculations. The higher thermal stability of 1 relative to that of simpler R derivatives that form stronger Mn–R bonds can be attributed to more difficult CO dissociation, precluding the decomposition by β‐H elimination.