Chitosan Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 75

Chitosan has two types of reactive groups that can be grafted. First, the free amino
groups on deacetylated units and, second, the hydroxyl groups on the C-3 and C-6 carbons
on acetylated or deacetylated units. Grafting of chitosan allows the formation of functional
derivatives by covalent binding of a molecule, the graft, onto the chitosan backbone.
Recently, researchers have also shown that after primary deviation followed by graft mod-
ification, chitosan would obtain much improved water solubility and bioactivities such as
antibacterial and antioxidant properties [290]. Grafting chitosan is a common way to
improve chitosan properties such as increasing chelating [291] or complexation properties
[219], bacteriostatic effect, or enhancing adsorption properties [292]. Although the grafting
of chitosan modifies its properties, it is possible to maintain some interesting characteris-
tics such as mucoadhesivity, biocompatibility, and biodegradability. Many investigations
have been carried out on the graft copolymerization of chitosan in view of preparing poly-
saccharide-based advanced materials with unique bioactivities and thus widening their
applications in biomedicine and environmental fields.
2.11.1 graft Copolymerization by radical generation
Polyvinylic and polyacrylic synthetic materials are the most frequently grafted polymers
on polysaccharides. These copolymers are frequently prepared by radical polymerization
wherein free radicals are generated first on the biopolymer backbone and then these radi-
cals serve as macroinitiators for the vinyl or acrylic monomer. The generation of radicals
can be achieved by chemical or radiation initiation. Among the variety of chemical reagents
reported for initiating graft copolymerization onto chitin/chitosan, CAN, potassium or
ammonium persulfate, and Fenton's reagent are the most important redox systems. Grafting
parameters such as grafting percentage and grafting efficiency are greatly influenced by
type and concentration of the initiator, monomer concentration, reaction temperature, and
time. The obtained grafts were studied for all or some of the parameters such as optimiza-
tion of reaction conditions, and properties such as solubility, water absorption, swelling,
thermal property, pH dependence, adsorption capacity, and so on. Researchers have also
performed graft copolymerization of chitosan after its primary derivatization. The differ-
ent predesignated chitosans subjected to graft copolymerization include carboxymethyl,
N -carboxyethyl, maleoyl and hydroxypropyl trimethyl chitosan, and so on. This simple
method of grafting is plagued with difficulties such as radical-induced depolymerization
and degradation of the polysaccharide itself, lack of a well-defined initiating site and struc-
tures of the resulting copolymers as well as homopolymerization.
N -Isopropylacrylamide (NIPAAm) has a lower critical solution temperature (LCST) of
around 32°C, and this is an advantage in the design of drug delivery systems due to its
ability to form hydrogels with liquid-gel transition occurring at temperatures that are
similar to that of the human body. Grafting NIPPAm onto chitosan provides an increase in
water content on exposure to aqueous media and improvement in mechanical properties
and temperature-responsive properties [293]. Kim and coworkers synthesized a chitosan-
g-NIPAAm copolymer using Ce(IV) ammonium nitrate as the initiator. The copolymer
was then cross-linked with glutaraldehyde. The efficiency and percentage of copolymer-
ization increased as the monomer concentration (NIPAAm) increased. The resulting copo-
lymer exhibited pH-responsive behavior and temperature-responsive behavior, with
swelling ratios higher at pH 4 than at pH 7. At 35°C, above the LCST (32°C), the equilibrium
water content was lower in comparison with the one at 25°C [228].
Sun et al. [252] prepared carboxymethyl chitosan-grafted methacrylic acid (MAA)
by using APS as an initiator in aqueous solution. The effects of APS, MAA, reaction