The process of radical thermo-initiated polymerization of DMF (dimethylformamide) MMA (methylmethacrylate) solutions is investigated kinetically. It is shown that the reaction occurs in the presence of symmetric cation polymethine dyes containing terminal groups of strong, moderate, and weak electron-donating abilities. It is established that, with growing the terminal group electron-donation, an increase of the monomer conversion occurs at the same reaction times. Using DFPH (diphenylpicrylhydrazyl), it is shown that the process is to be considered as a radical one.