Biomedical Engineering Reference
In-Depth Information
segments. In spite of being widely used, the application of CLRP techniques to the modifi-
cation of chitosan is seldom used.
El Tahlawy and Hudson [312] prepared chitosan macroinitiators by acetylation of chito-
san with 2-bromo-isobutyryl bromide in the presence of pyridine as a base. The chitosan
macroinitiator was used to polymerize a methoxy-poly(ethylene glycol)methacrylate
(MeO(PEG)MA) monomer using the Cu(I)Br-bipyridyl complex under heterogeneous
aqueous conditions at 25°C. The kinetics studies revealed a first-order polymerization and
polydispersities around 1.5. Using a similar approach, Li et al. [313] proposed the con-
trolled synthesis of chitosan beads grafted with polyacrylamide via surface-initiated ATRP
(SI-ATRP). The bromide end groups were immobilized on the surfaces of chitosan beads
through reaction of -NH
2
or -OH groups of 2-bromo-isobutryl bromide using triethylam-
ine as the trapping agent in dry tetrahydrofuran (THF)
(Figure 2.41).
Tang et al. [314] proposed the graft copolymerization of poly(methyl methacrylate)
(PMMA) and amphiphilic block copolymer PMMA-b-poly(ethylene glycol) methyl ether
methacrylate on the surface of chitosan nanospheres, The proposed method used iron(III)-
mediated ATRP with activators generated by electron transfer (AGET). This work explores
the use of an iron(III)-mediated catalytic system, instead of copper(I)/copper(II) salts, due
to the possible toxic effects of copper on human health, when the materials are intended to
be used for
in vivo
biomedical applications. The use of the AGET initiating method can
overcome an important drawback of ATRP, because transition metal compound in lower
oxidation can be easily oxidized leading to uncontrolled polymerization.
Regarding the other widely used CLRP methods RAFT and nitroxide-mediated
polymerization, to the best of our knowledge there are only three reports available in the
literature, all from the same laboratory. As for the RAFT approach, Zhu and coworkers
proposed the synthesis of chitosan-g-PNIPAM [315] and chitosan-g-PAA [316] using
S
-1-
dodecyl-
S
′-(α,α′-dimethyl-α″-acetic acid) trithiocarbonate as the RAFT agent. Chitosan-
RAFT agents were synthesized in dry DMF by reacting
N
-phthaloylchitosan with
S
-1-dodecyl-
S
′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate in the presence of 1,3-dicy-
clohexylcarbodiimide and 4-(
N
,
N
-dimethylamino)pyridine. Both graft copolymerizations
(chitosan-g-PINPAM and chitosan-g-PAA) were carried out in dry DMF, using AIBN as
the initiator and reaction temperatures ranging from 60°C to 80°C [315], leading to poly-
mers with living features.
It is expected that in the next few years, improvement in the graft copolymerization of
natural polymers techniques based on CLRP will take place, leading to the preparation of
O
O
Br
CH
2
CH
Br
O
O
OH
R
Chitosan
bead
Chitosan
bead
Chitosan
bead
O
O
CH
2
CH
Br
Br
NH
2
O
NH
NH
R
H
2
C
Br
(ATRP)
Br
R
Catalyst/ligand
solvent
Et
3
N, dry THF
(or other solvents)
O
O
O
O
Br
Br
Br
CH
CH
2
CH
2
CH
Br
OH
NH
2
O
HN
O
HN
R
R
Chitosan film
Chitosan film
Chitosan film
Figure 2.41
General scheme used to prepare chitosan macroinitiator for ATRP.
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