Chitosan Derivatives - Chitosan Based Hydrogels: Functions and Applications - page 79

living polymer cation, due to the steric hindrance of the functional groups of chitosan with
increasing molecular weight of living polymer. The viscosity of the resulting polymer was
found to increase with increasing percentage of grafting. This grafted polymer was also
found to be soluble in water.

2.11.5 Copolymerization via Oxidative Coupling

With an aim to prepare conductive polymers, polyaniline was grafted onto chitosan by the
method of oxidative coupling (Figure 2.33) [325].

2.11.6 Cyclic Monomer Copolymerization via ring Opening

In general, four groups of cyclic monomers have been mainly used for graft copolymeriza-
tion onto polysaccharides: α-amino acid, N -carboxy anhydrides (NCAs), lactones, oxiranes
(epoxides), and 2-alkyl oxazolines. An NCA ring can undergo nucleophilic attack to open
and polymerize with evolution of CO 2 to yield a polypeptide chain. The free amine of chi-
tosan is believed to initiate graft copolymerization by means of attack upon carbonyl, ulti-
mately creating the grafted chitosan derivative. The advantages of this method are a low
level of homopolymer formation and the possibility of side chain length control through
the regulation of NCA concentration under proper conditions. DP, however, is not usually
higher than 20 [231]. The resulting copolymers are new types of hybrid materials com-
posed of both a polysaccharide and polypeptides.
The novel artificial glycoprotein, chitosan-poly(l-tryptophan) copolymers with two-
dimensional structure side chains, has been synthesized successfully under homogeneous
conditions with ring-opening graft polymerization between l-tryptophan NCA and the
water-soluble chitosan [301] ( cf. Figure 3.34). With increased DP, the conformation of side
chains on glycopeptides starts to convert from β-sheet to α-helix. The copolymers have a
strong fluorescence emission at 360 nm, and emission intensities can be adjusted by DP
and quenched by coordination with copper ions within 30 min. Significantly enhanced
mechanical rigidity has been achieved by graft polymerization, in comparison with that
by chitosan. The solubility of graft copolymers depends strongly on the length and confor-
mation of the poly-l-tryptophan side chains.
Living poly(2-alkyl oxazolines) telechelic polymers have been grafted onto partially
deacetylated chitins in DMSO by Aoi et al. [280] and studied for process optimization and
solubility profile ( cf. Figure 2.35). In this solvent, the water-soluble chitin swells to some
extent; thus the amino group was used to terminate the living polyoxazolines (synthesized
by cationic ring-opening polymerization of the corresponding 2-alkyloxazolines with

OH

OH

O

O

NH 2

O

O

HO

O

O

HO H

S 2 O 8 2- /HCI

NH 2

H

*

n

Polyaniline grafted chitosan

Chitosan

Figure 2.33
Grafting of polyaniline on chitosan.