Mechanistic studies were performed on the sulfinyl and the Gilch routes, both p-quinodimethane-based polymerizations, toward OC1C10-PPV. The influence of a combination of additives was investigated to verify the nature of their polymerization mechanisms. In contrast to the sulfinyl route, the Gilch route was irreproducible when performed in THF. Therefore, a reproducible Gilch procedure was developed in dioxane at room temperature. The results of the additives were evaluated by size exclusion chromatography (SEC). All observed effects of both the sulfinyl and Gilch routes are consistent with a radical polymerization mechanism.