Chitosan Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 76

CH 3

CH 3

OH

OH

O

O

O

O

COOH

APS

+

HO OH

O

O

CH 3

NH 2

n

n

CH 3

NH C

CH 2

Chitosan m

COOH

Figure 2.31
Graft copolymerization of MAA on hydroxypropyl chitosan.

temperature, and time on graft copolymerization were analyzed by determining graft-
ing percentage and grafting efficiency. After grafting, the chitosan derivatives had
improved water solubility. Similarly, Xie et al. [253] prepared hydroxypropyl chitosan-
grafted MAA by using APS initiator (Figure 2.31), obtaining a derivative that also pre-
sented good solubility in water.
Graft copolymerization of maleic acid sodium onto carboxymethyl chitosan and hydroxy-
propyl chitosan using APS initiator has been reported [248, 294]. Compared with chitosan,
the graft chitosan derivatives were found to have improved scavenging ability against
superoxide anion. Graft chitosan derivatives with hydroxypropyl groups had relatively
higher superoxide anion scavenging ability owing to the incorporation of hydroxyl groups.
Acylation of chitosan with maleic anhydride furnishes carbon-carbon double bonds,
which are available for subsequent polymerization. The copolymerization of the derivative
with acrylamide in water in the presence of APS has been used to obtain three-dimen-
sional cross-linked products [255]. The resulting copolymers swelled highly in water with
a volume increase of 20-150 times.
Graft copolymerization of vinyl monomers onto chitosan can also be carried out using
redox initiator systems, such as CAN and PPS. Poly(vinyl acetate) (PVAc) is known as a
leathery and water-resistant polymer, which may improve the properties of chitosan mate-
rial, and hence the graft polymerization of vinyl acetate onto chitosan by using CAN as an
initiator was reported. Experimental results indicated that chitosan molecules not only took
part in the graft copolymerization but also acted as a surfactant, providing stability of the
dispersed particles. The data also showed that the incorporation of PVAc into chitosan
chains increased the toughness and decreased the water absorption of chitosan. CAN was
also found to be a suitable initiator for grafting N , N -dimethyl- N -methacryloxyethyl- N -(3-
sulfopropyl)ammonium [238], poly(acrylonitrile) (PAN) [232], polyacrylamide, poly(acrylic
acid), and poly(4-vinylpyridine) [295] onto chitosan.
Chitosan was modified with poly(acrylic acid), a well-known hydrogel-forming poly-
mer, using a grafting reaction in a homogeneous phase [229]. Grafting was carried out in
the presence of PPS and FAS as the combined redox initiator system. It was observed that
the level of grafting could be controlled to some extent by varying the amount of ferrous
ion as a cocatalyst in the reaction. Tahlawy and Hudson [261] have discussed the effects
of the reaction conditions and temperature on the grafting efficiency of 2-hydroxyethyl