The gas phase enthalpies of reaction and activation for the initiation and the first propagation steps of the radical and anionic polymerizations of ethene and formaldehyde were theoretically determined by high-level ab initio calculations, using respectively H-. and H- as model initiators. To correctly describe the behavior of the anionic reactions, the hydride anion must be described by a larger basis set than the hydrogen atom. It is found that the initiation step of the radical polymerization of ethene is favored, but, conversely, the anionic initiation step is favored in the case of formaldehyde. These results agree with the experimental evidence. They also demonstrate the capacity of theoretical chemistry to describe the behavior of these reactions when a high-level methodology is used.