The mechanism of running of the MMA thermoinitiated polymerization depending on the chemical structure, ionic nature of polymethine dyes, solvent nature, concentration, temperature, is determined. It is revealed that polymethine dyes are able to initiate, as well as to inhibit, the MMA thermoinitiated free radical polymerization depending on their structure even when the standard initiator is absent. Using quantum chemical calculations, it is found that the initiating ability of polymethine dyes depends on their HOMO energy compared with the same value for the MMA.