High-level ab initio molecular orbital theory calculations have been used to calculate the rate coefficients and Arrhenius parameters for dimer models of the propagation steps for the free-radical homopolymerization of methyl acrylate and vinyl acetate at 298.15 K. Gas-phase Arrhenius parameters, as calculated under the hindered rotor model, showed large deviations from the corresponding solution-phase experimental data, because the stabilizing effect of hydrogen bonding in the transition Structures of these reactions is much less significant in solution compared with the gas phase. This also leads to qualitative differences in the preferred transition state conformations in the respective phases, and it is therefore necessary to base all calculations (including the conformational analysis) on the solution-phase free energies. We find that chemically accurate values call be obtained via a thermodynamic cycle in which accurate G3(MP2)-RAD calculations in the gas phase are corrected to the solution phase using free energies of solvation, as computed by the COSMO-RS method. However, the use of DFT methods for the gas-phase energies and/or simple continuum models for the solvation energies call lead to errors of several orders of magnitude and should therefore be avoided for these types of reaction.