The ascorbic acid (AA) and hydroxyl peroxide (H2O2) redox pair induced free radical grafting reaction is a promising approach to conjugate phenolic groups with chitosan (CS). In order to reveal the exact mechanisms of the AA/H2O2 redox pair induced grafting reaction, free radicals generated in the AA/H2O2 redox system were compared with hydroxyl radical (•OH) produced in the Fe2+/H2O2 redox system. Moreover, the structural and physicochemical properties of caffeic acid grafted CS (CA-g-CS) synthesized in these two redox systems were compared. Results showed that only ascorbate radical (Asc•-) was produced in the AA/H2O2 system. The reaction between Asc•- and CS produced novel carbon-centered radicals, whereas no new free radicals were detected when •OH reacted with CS. Thin layer chromatography, UV-vis, Fourier transform infrared, and nuclear magnetic resonance spectroscopic analyses all confirmed that CA was successfully grafted onto CS through Asc•-. However, CA could be hardly grafted onto CS via •OH. CA-g-CS synthesized through Asc•- exhibited lower thermal stability and crystallinity than the reaction product obtained through •OH. For the first time, our results demonstrated that the synthesis of CA-g-CS in the AA/H2O2 redox system was mediated by Asc•- rather than •OH.
Keywords: chitosan; grafted copolymerization; physicochemical properties; reaction mechanisms; structural characterization.