Biomedical Engineering Reference
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CH
2
OH
CH
2
OH
O
O
O
O
+ Ce
4+
OH
OH
*
*
*
*
Ce
4+
NH
2
NH
2
n
n
CH
2
OH
CH
2
OH
O
O
O
O
O
H
OH
*
*
*
*
H
NH
2
H
NH
2
n
n
(I)
(II)
Figure 1.21
Oxidation-reduction initiating reaction mechanism by cerium ions.
A key factor of free-radical-induced graft copolymerization is the generation of free radi-
cals. There are four initiators for generating free radicals in free-radical-induced graft
copolymerization of chitin and chitosan.
The oxidation-reduction initiator is frequently used in graft polymerization. Cerium
ion is a common initiator for promoting allyl monomers such as acrylic acid, acrylate,
acrylamide, methyl methylacrylate, and styrene to graft with residues of chitin or chito-
san [84-88]. The reaction is carried out by adding cerium nitride or cerium ammonium
sulfate to a mixture of chitin or chitosan, water, and allyl monomer, and heating them in
heterogeneous phase. Grafting degree increases with monomer concentration, but does
not necessarily lead to increased percentage grafting. The initiator is available over a cer-
tain concentration because chitin or chitosan is a reductive polysaccharide whose reduc-
ing end groups consume a certain amount of initiator; thus graft polymerization occurs
only when the initiator is beyond that amount [89]. In 1993, Li et al. [86] put forward the
complete mechanism of cerium ions as a graft polymerization initiator. The mechanism
can be described as follows: Ce
4+
ions react with C2-NH
2
and C3-OH of residue to form
a complex, -CH=NH (I) and -CH (OH)(II) free radicals are formed by disproportionating,
cerium ions oxidize II into carbonyl free radicals at temperatures higher than 40°C for
initiating graft polymerization, the reaction continues and I is hydrolyzed into aldehyde
and ammonium ions at temperatures higher than 90°C, and aldehyde is further oxidized
into carbonyl free radicals for initiating polymerization. The reactions are described in
Figure 1.21.
Persulfate used as the initiator forms an oxidation-reduction system with sodium
bisulfite or ferrous sulfate [90-91]. The system is cheap and convenient to operate, and does
not remain in the graft polymer. Wei et al. [92] believe that the reaction mechanism is as
shown in
Figure 1.22.
The reactions are chain propagation and chain termination.
Except for the reactions mentioned above, the oxidization-reduction reaction of persul-
fate and aminos of the chitosan may also occur to form macromolecular free radicals. The
H
2
O
2
-Fe system is another common oxidation-reduction initiator that initiates the reaction
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