Cross-linking reaction kinetics of cubic hydrogen silsesquioxane radicals: A theoretical study

In this work, high-level density functional theory calculations were employed to investigate the cross-linking reactions of hydrogen silsesquioxane (HSQ) radicals. We provide a comprehensive elucidation of the whole reaction process, spanning from hydroxylation of hydrogen silsesquioxane radical by water to dehydration condensation of hydroxylated HSQ molecules, by the characterization of key species, energy barriers, and reaction energies. Furthermore, the potential effect of ambient humidity on the cross-linking process was assessed by involving an extra water molecule in both the hydroxylation and dehydration condensation reaction steps. We found that the participation of an extra water molecule can reduce the transition-state energies of both reactions relative to the corresponding reactants by hydrogen-bonds. By incorporating the water concentration at a specific humidity into the kinetic calculations, we demonstrated that the extra water molecules do not catalyze the hydroxylation of HSQ radicals; however, the water molecules facilitate the dehydration condensation reaction, potentially increasing the rate constants by up to a factor of three under high-humidity conditions, as compared to the anhydrous reaction. This study provides a theoretical explanation for that the presence of water promotes HSQ cross-linking, suggesting that a sufficiently high humidity could be beneficial in practical production processes.