Synthesis and radical polymerization properties of thermal radical initiators based on o-imino-isourea: The effect of the alkyl side chain on the radical initiation temperature

Four types of thermal radical initiators (TRIs) that are based on o-imino-isourea with cyclohexyl and isopropyl groups were successfully synthesized, namely, C-HexDCC, DiiprDCC, C-HexDIC, and DiiprDIC. The free radical polymerization and thermal properties of those synthesized TRIs were determined via differential scanning calorimetry (DSC) (using n-butyl acrylate) and thermogravimetric analysis (TGA), respectively. The TRI derivatives showed peak temperatures (T_max) from 89 to 97 °C in n-butyl acrylate, and DiiprDIC, with isopropyl groups on both sides of the NO group, showed the lowest peak temperatures. The rates of NO bond homolysis (k_d) of all the TRIs were calculated from their half-lives determined using real-time nuclear magnetic resonance (NMR) spectroscopy, and their theoretical bond dissociation energies (BDEs) were calculated using density functional theory (DFT) calculations. The free radical polymerization of n-butyl acrylate using each TRI was efficiently determined from T_peak of the DSC curves; conversions depending on polymerization temperature (80, 90, and 100 °C) were monitored to observe kinetic information of TRIs during polymerization. Furthermore, to investigate the use of TRIs in curing, we applied them to an automotive clear coating system and monitored the real-time evolution of the elastic modulus (G′) during thermal curing using a rheometer for representative DiiprDIC.