Biomedical Engineering Reference
In-Depth Information
4.2 ATRP in a Heterogeneous Oil-in-Water System
The interest in applying ATRP to aqueous dispersed media arose soon after the
invention of ATRP. Initial attempts at conducting an ATRP in aqueous dispersed
media, in particular, emulsion media were reported in early 1998 [
123
]. CuBr/bpy
was used as catalyst and sodium dodecyl sulfate (SDS) was used as surfactant
for ATRP of MMA at 60-80 °C. In spite of the relatively high polymer yield, the
MWD was broad, indicating a poorly controlled polymerization. These initial
failures were due to the problems associated with the partitioning and poisoning
of the catalysts in the aqueous phase but stimulated extensive research on deter-
mining the feasibility and criteria required for a well-controlled ATRP in aqueous
dispersed media [
124
-
129
]. In other words, the multiphase nature of aqueous dis-
persed media added new requirements to identify appropriate ligands, surfactants
and even initiation methods to conduct a well-controlled ATRP. For instance, the
chosen ligands should be highly hydrophobic since the Cu(II) species are usually
more soluble and less stable than Cu(I) species in water. A hydrophobic ligand
can complex with Cu(II) and minimize migration of deactivators into the aqueous
phase, which was the main reason of the loss of control and excess termination in
the initial experiments. A further consideration is that all the Cu/ligand complexes
must be fully soluble in the monomer, unlike bulk or solution processes where het-
erogeneous catalysts are commonly used. The chemical structures of hydrophobic
ligands reported to be successfully applied in ATRP in aqueous dispersed systems
are shown in Scheme
4
.
The choice of surfactants for ATRP in aqueous dispersed system is also criti-
cal. A good surfactant for a controlled ATRP in aqueous dispersed media should
not only provide a stable dispersed system throughout the polymerization, but also
have minimal interference in the equilibrium between the radicals and the dormant
species. Therefore, anionic surfactants showed little success in ATRP since they
interact with ATRP catalysts, especially the Cu(II) complexes. To date, non-ionic
Ligands
R'
C
4
H
9
C
4
H
9
N
R'
C
4
H
9
C
4
H
9
C
18
H
37
R'
=
CH
2
CH
2
COOR
R
=
C
4
H
9
C
4
H
9
C
4
H
9
C
4
H
9
C
4
H
9
N
R'
N
N
N
C
4
H
9
N
N
N
R'
R'
R'
N
N
N
N
BPMODA
dNbpy
tNtpy
EHA
6
TREN
Surfactants
*
(OCH
2
CH
2
)
n
OH
(OCH
2
CH
2
)
o
OH
(
OCHCH
2
)
p
O
O
m
(OH
2
CH
2
C)OH
Br
*
C
18
H
35
O
OH
20
OH
N
Brij 98
CTAB
C
17
H
33
PVA
Tween 80
(
m+n+o+p=20
)
Scheme 4
Chemical structures of commonly used ligands and surfactants for ATRP in aqueous
dispersed media
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