Metal-coordination Controlled Degradation Of Chitosan And Utilization Of Low Molecular Weight Chitosan

Because there are a lot of NH2 and OH groups on the skeleton of chitosan, it's easy to chitosan to chelate transition-metal cations and rare-earth metal cations. So, a new degradation method for chitosan - metal-coordination controlled degradation is put forward first time here. Following operations are steps of metal-coordination controlled degradation. Firstly, dilute aqueous solution of metal salt is added to very dilute acetic acid solution of chitosan to prepare chitosan-metal complex. It is hoped to obtain complex whose polymer chain is more easy to be broken at some sites than chitosan, and to control the place and quantity of the sites through controlling the amount of salt added to chitosan solution. Secondly, add some oxidant to the complex to cut its polymer chain at those sites to obtain low molecular weight chitosan(LCTS) whose molecular weight(Mw) distribution is narrow. Controlling amount of the reactants(salts and oxidant) and other reaction conditions, the degradation degree of chitosan and the Mw of the product could be controlled. Finally, eliminate the metal cation on the degraded chitosan and lyophilize to get dry LCTS .Several light transition-metal salts FeSO4.7H2O, Cu(OAc)2.H2O, Ni(OAc)2. 4H2O, Mn(OAc)2.4H2O, Co(OAc)2.4H2O, Zn(OAc)2.4H2O and rare earth metal salt LaCl3.nH2O were added to chitosan's acetic acid solution separately to prepare chitosan-metal complexes. Elemental analysis, UV, IR, XRD, DTA, ESR, metal distribution of the complexes were determined. The results of the determination expressed actually there was coordination between the metal cations and chitosan, and the chain of complexes was easier to be broken than that of chitosan.During degradation, the viscosity of chitosan solution to which salt was added decreased faster than that of chitosan solution no salt was added to, and its decrease velocity was different with the type of the metal. Up to the velocity, there was a sequence as following: Cu2+> Fe2+ > Co2+ > Mn2+ > Ni2+ > La3+ >Zn2+. Two oxidants CHaCOOOH, and H2O2 were used to degrade chitosan-copper complex, and CHsCOOOH expressed better ability to break polymer chain than H2O2. With the increase of salt amount, oxidant amount and temperature, the viscosity of chitosan solution decreased faster. The atmosphere had an influence on thedegradation, and to both chitosan and its copper complex, the velocity of viscosity-decrease became smaller as: N2 > ah- > O2. Material chitosan with different molecular weight or different nitrogen content degraded with different velocity. The higher molecular weight, or the lower nitrogen content, the faster chitosan degraded. Two radical absorbents dimethylformamide(DMF) and dimethylsulfoxi-de(DMSO) were added to the degradation system. The degradation velocity wasslowed obviously by the absorbents, and the amount and add time difference ofabsorbent didn't bring different degradation results. The fact that hydroxyl radical could be produced by chitosan or chitosan-metal complexes with t^Oa was proved through colorimetric method in which salicylic acid was used to catch hydroxyl radical. The result showed hydroxyl radical played an important role in chitosan degradation with J^Oa. All the results showed there were some sites on chitosan-metal complexes more easy to be broken than the same sites on chitosan, and chitosan-copper complex expressed different reaction ability with other complexes. So, it could be concluded that chitosan-copper complex could catalyze the equal departure of HaOa.Gel permeation chromatography(GPC) was used to determine the Mw and Mw distribution of degraded chitosan. The plots of GPC showed chitosan obtained through metal-coordination controlled degradation had lower Mw and narrower Mw distribution than that obtained through direct degrading chitosan at the same conditions. Mass spectrum showed oligomerchitosan whose DP was 2-6 could be obtained through deep degradation of chitosan-metal complex. And investigation results showed chitosan-metal complexes were degraded with weakness deploymer...