Reaction mechanism of chitosan/acrylamides dimer: A DFT study

Environment-sensitive hydrogel is a sensitive hydrogel that responses to outside stimuli. A new temperature- and pH-sensitive hydrogel is able to been prepared by radiation polymerization or emulsion polymerization method combining with the excellent thermosensitive poly(N-isopropyl acrylamide) (PNIPAM), and the pH-sensitive chitosan (CS). However, it is different to understand the reaction mechanisms thoroughly unless we can obtain more details at the molecular level. In this work, the reaction mechanism of chitosan and acrylamides (AMs, that is AM or NIPAM) dimer was shed light on by using the density functional theory (DFT), to investigate all possible reaction channels and reveal the underlying nature of polymerization at the micro level. The geometric structures of all stationary points in the reaction pathways were optimized. The reaction takes place via 3 possible reaction channels, which are RCSN+AMsRCSN/AMs, RCSO3+AMsRCSO3/AMs, and RCSO5+AMsRCSO5/AMs. Comparing the energy barriers of these reaction pathways at a higher level, RCSO5+AMsRCSO5/AMs is the main pathway for forming RCS/AMs. According to the variations of bond-lengths and the corresponding vibration peaks, it is confirmed that there are intramolecular hydrogen bond interactions in RCSO3/AMs dimer and RCSO5/AMs dimer.