Grabchuk G. Effect of polymethine dyes structure on the methyl methacrylate free radical polymerization in the solution

The mechanism of running of MMA thermoinitiated polymerization, depending on chemical structure, ionic nature of polymethine dyes, solvent nature, concentration, temperature, was determined. It was revealed, that polymethine dyes, depending on their structure, are able to initiate as well as to inhibit MMA thermoinitiated free radical polymerization even when standard initiator is absent. It was shown, that highest occupied molecular orbital (HOMO) of the dye has to have lower energy than analogous orbital of MMA, so as to the free radical was produced. It was determined, that HOMO energy of the cationic polymethines grows in the following cases: the polymethine chain lengthens, electron-donor ability of the terminal groups deviates from the average one, and electron asymmetry increases. That leads to reduction of initiating ability of the cationic dyes under such changes of their chemical structure. HOMO energy of anionic and intraionic merocyanic dyes with positive and negative solvatochromism was shown to be much higher than in the case of cationic dyes and exceeds MMA HOMO energy. Hence they do not initiate MMA free radical polymerization. Intraionic squarylium and borondipyrromethene dyes are weak initiators of MMA polymerization both in presence of the initiator and in its absence, despite the fact that they have higher HOMO energies than MMA. Formation of biradicals due to the electron transfer from the acceptor part of the molecule to the donor one is assumed in this case.