CHEMICAL MODIFICATION OF BIOPOLYMERS MECHANISM OF MODEL GRAFT-COPOLYMERIZATION OF CHITOSAN

Mechanism of graft copolymerization of vinyl monomers onto chitosan in itiated by Ce(IV) ion, one of the important ways of chemical modificat ion of chitosan, has been investigated by means of kinetics measuremen t and polymer chain structure analysis. It is found that when Ce(IV) i ons reacts with adjacent hydroxylamine structure in chitosan, a chelat e complex is first formed and then it disproportionates to radical for initiation of graft copolymerization. There exist two ways for such i nitiation depending upon the reaction temperature. For a reaction temp erature lower than 40-degrees-C, a -CHO group and a -C=NH group are in troduced, and the aldehyde group reacts with Ce(IV) ions to form an ac yl radical which initiates a graft polymer chain in this saccharide un it. For temperatures higher than 90-degrees-C, the -C=NH group hydroly zes to form an amine and an aldehyde group which also reacts with Ce(I V) ions. In this case there should be two aldehyde groups, i.e. two in itiation sites, in one adjacent hydroxyl-amine structure. That means t hat the initial radical in the chitosan/Ce(IV) system is similar to th at in the cellulose/Ce(IV) one, but the former usually gives a lower g rafting reactivity than the latter due to the higher stability of the chelate.