Biomedical Engineering Reference
In-Depth Information
O
OH
N
O
O
HO
O
O
HO
O
O
NIPAM
N
O
n
O
O
AIBN, DMF, 60°C
H
3
C
CH
3
S
S
C
12
H
25
Chitosan-RAFT agent
S
O
OH
N
O
O
HO
O
O
O
HO
O
O
N
n
O
O
H
3
C
H
3
C
S
S
C
12
H
25
PNIPAAm
S
Chitosan-
g-
PNIPAAm
Figure 6.9
Schematic representation of the modifications and graft polymerization of chitosan by using the RAFT
approach (From Tang, J. et al. 2008.
Int. J. Biol. Macromol.
43: 383-389. With permission.)
Recently, controlled/living radical polymerization methods, such as atom transfer radical
polymerization and reversible addition fragmentation transfer (RAFT), were employed to
synthesize chitosan graft copolymers with controlled topologies and graft-controlled seg-
ments. Zhu and coworkers [47] were the first research team to successfully synthesize
chitosan-g-PNIPAAm and chitosan-g-PAA using the RAFT approach (
cf
. Figure 6.9). They
prepared chitosan-RAFT agents in dry dimethylformamide (DMF) by reacting
N
-phthaloylchitosan with
S
-1-dodecyl-
S
′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate in
the presence of 1,3-dicyclohexylcarbodiimide and 4-(
N
,
N
-dimethylamino)pyridine. Both
graft copolymers were carried out in dry DMF, using azobisisobutyronitrile as initiator.
The results showed that PNIPAAms from the polymerization mixtures had controlled
molecular weights and narrow molecular weight distributions [47]. The “well-defined”
chitosan-g-PNIPAAms with thermally responsive property would enable chitosan to be
used in drug delivery applications [47].
6.3 Chitosan-Based Gel Formation Principles
Irrespective of the kinds of shapes and geometries the chitosan hydrogels are in, they are
formed by physical or chemical cross-links of the macromolecules [4,6,9,10], being appropri-
ately used to give the three-dimensional structures specific mechanical and chemical
characteristics. The cross-link can be formed by covalent or noncovalent interactions. Non-
covalent hydrogels are called physical gels when the networks are cross-linked through
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