Biomedical Engineering Reference
In-Depth Information
CN
CN
CN
-N
2
C
CH
3
C
NN
C
CH
3
2
CH
3
CH
3
CH
3
CH
3
AIBN
O
O
O
C
O
O
C
2
CO
BPO
Fig. 10.2
Structures of two common initiators, namely 2,2
0
-azobis(isobutyronitrile) (AIBN for
brief) and dibenzoyl peroxide (BPO for brief) along with the chemical reactions which occur during
their thermal scission. It can be seen that each initiator molecule produces two radicals
R
1
H
n
C
C
H
R
1
R
1
R
1
R
2
H
H
H
C
H
I
C
C
I
C
C
C
H
H
R
2
R
2
R
2
n
Fig. 10.3
The chemical reactions which occur in free-radical copolymerization process. The two
monomers are ethylene derivatives in which the first carbon atom does not have substituents
copolymerization process. The two monomers are ethylene derivatives in which the
first carbon atom does not have substituents. The second carbon atom has one or
two substituents, R1 and R2. For styrene, R1
D
HandR2
D
phenyl ring. On
the other hand, for methylmethacrylate, R1
D
methyl and R2
D
methacrylate.
An initiator, I
, is present in the mixture. Similar to a catalysts, it starts the reac-
tion. However, the initiator acts in a different manner since it takes part in the
reaction. Figure
10.2
reports the structures of two common initiators, namely 2,2
0
-
azobis(isobutyronitrile) (AIBN for brief) and dibenzoyl peroxide (BPO for brief). It
also reports the chemical reactions that occur during their thermal scission. It can
be seen that each initiator molecule produces two radicals. When photo-scission is
required, other initiators are used, e.g., 2,2
0
-azobis(cyclohexane-l-carbonitrile). As
the reaction goes on, the monomer is transformed into polymer. Figure
10.3
also re-
ports a scheme of the polymerization reaction. In the initiation process, the initiator
attacks one of the two monomers to form a initiator-monomer entity (see figure). In
the propagation process, the latter reacts
n
times with a monomer to yield a growing
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