From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 30

Ph

H

CH 2

nPh

SnCl 4

H 2
C

H 2
C

Chitin-CH 2 + Sn - Cl 4 I Chitin-CH 2 I Chitin

H n

Figure 1.24
Ion induced graft polymerization

but only trace homopolymers of styrene can be formed. Percentage grafting is remarkably
increased when the solvent is 30% methanol-20% water. It is thus clear that the graft reac-
tion changes with the physical structure of chitosan. In the same condition, ethenyl etha-
noate cannot graft chitin or chitosan, acrylamide forms gel products that are hard to
separate, and methyl methylacrylate reacts with chitosan with 94.2% grafting degree and
84% grafting efficiency. The largest grafting degree of chitosan and hydroxylethyl acrylate
appears when the ratio of water to methanol is the best [97].
The UV method completes the reaction in a short time and is suitable for preparing a
large amount of samples. Methyl methylacrylate grafts chitin or chitin oxide when they are
irradiated by a low-pressure mercury lamp [98]. Chitin oxide is better for such a reaction.
The possible cause is carbonyl. 10% DMF obviously increases grafting degree.
Moreover, grafting degrees of UV-irradiated chitin iodide at room temperature [99] and
polymerization of chitin thiol and styrene in DMSO [100] are 97%.
Research on ion-induced graft polymerization is limited; however, one method was
introduced with the following steps (Figure 1.24): dispersing chitin iodide in nitroben-
zene, and adding Lewis acid SnCl 4 to the mixture for grafting styrene via cation graft
polymerization [101].
Another method for N -carboxylic anhydride graft polymerization of chitosan and non-
allyl monomer comprises the following steps [102]: dispersing chitosan powder in DMSO,
adding N -carboxylic anhydride to the solution, stirring at room temperature for about 1
week to form gel swelled by water and DMSO, diluting with water, repeatedly centrifug-
ing the mixture to remove polypeptide, and depositing from acetone to form a new graft
polymer that is a polysaccharide peptide. The grafted groups are aminos and the polypep-
tide for grafting has about six amino acid residues.

1.5.8 Cross-linking

Chitin and chitosan can cross-link by aldehydes or anhydrides with two functional groups.
Such a reaction forms stable products that are insoluble and difficult to swell. The products
can be used as carriers for chromatographic fractionation or immobilized enzymes.
Chitosan needs to be cross-linked on various occasions.
Mostly, cross-linking occurs between molecules. However, intramolecular cross-linking
exists as well. Primarily, the cross-linking reaction forms Schiff base by the amino of chi-
tosan and aldehyde. The reaction between aldehyde and hydroxyl does not occur so fre-
quently. Common cross-linking agents include glutaraldehyde, formaldehyde, and glyoxal,
and reaction can occur in both water and heterogeneous medium at room temperature.
The reaction rate is high and the pH range is wide.
Additionally, quite a few methods can introduce active groups during cross-linking. For
example, when using epoxy chloropropane to cross-link chitosan powder in dilute alkali
solution [103,104], hydroxyls are generated between crossbonds. Cyanuric chloride [105],
toluene diisocyanate [106], and chloromethyl thiirane [107,108] are also available.