Chemistry Reference
In-Depth Information
7.3.3
Polymerization of Water-Insoluble Monomers by ATRP
The first example of the ATRP of methyl methacrylate (MMA) in an aqueous sys-
tem was reported in 1996 by Granel et al.
3
-
N,C,N
-1,3-
(CH
2
NMe
2
)
2
C
6
H
3
}] as a catalyst precursor and 2-ethylbromoisobutyrate as an ini-
tiator, suspension polymerization of MMA was carried out in water at 80
[164]. Using [Ni
II
Br{
C. The
polymerization proceeded to high conversion, but the polymer molecular weight
was twice as high as the predicted value (
M
n
=6
10
4
g mol
-1
;
M
w
/
M
n
=1.7). It is
possible to infer that the polymerization was somewhat controlled, but that very
poor initiation occurred, possibly due to initiator hydrolysis. Lecomte et al. [170]
have shown that MMA polymerization can also be carried out conveniently in
aqueous suspension with Pd(OAc)
2
/PPh
3
and CCl
4
as an initiator. In the absence
of surfactants, bimodal distributions have been obtained due to diffusion limita-
tions at high conversion. This problem has been overcome by adding a nonionic
surfactant (Tween 80) that facilitates the dispersion of the polymer particles, re-
sulting in a relatively narrow molecular weight distribution (
M
n
=3.25
10
4
g mol
-
1
;
M
w
/
M
n
=1.55) at high conversion.
Sawamoto has described the controlled radical polymerization of MMA using
[RuCl
2
(PPh
3
)
3
] as a catalyst [218]. In organic solvents, this ruthenium catalyst
[197] necessitates the use of an aluminum-containing co-catalyst, usually
Al(O
i
Pr)
3
, thus affording a very efficient control of the polymerization of
methacrylic monomers. As the aluminum compound can not be used in neat
water, polymerizations were initially carried out in a biphasic toluene :water 1 : 1
mixture. Polymerizations in this aqueous system were found to be somewhat fast-
er than in neat toluene. In view of these results, the authors have demonstrated
that aqueous polymerizations do not necessitate the use of any aluminum co-cata-
lyst. Thus, in the absence of an aluminum salt, suspension polymerization of
MMA in water at 80
C was found to be efficient and controlled (83% conversion;
10
3
g mol
-1
;
M
w
/
M
n
=1.42).
ATRP in aqueous emulsion or suspension with copper-based catalysts has been
the object of extensive research. Makino et al. [219] were the first to report MMA
emulsion polymerization using CuBr/bpy as a catalyst and SDS as a surfactant.
No living characteristics were observed, but it has later been shown that SDS inter-
acts with the copper complexes to form a characteristic blue copper sulfate complex
[185]. Early experiments by Matyjaszewski et al. [185] demonstrated that butyl meth-
acrylate can be polymerized in emulsion using dNbpy and dAbpy ligands in combi-
nation with CuBr. The polymerization was found to be controlled, as evidenced by
linear evolution of molecular weights with conversion, and by narrow molecular
weight distributions. For example, 89% conversion was achieved after 75 min, yield-
ing a polymer of
M
n
=2.82
M
n
=9.6
10
4
g mol
-1
).
Utilization of other ligands, such as Me
6
-TREN, afforded ill-defined polymers.
Careful elucidation of the polymerization mechanism by Matyjaszewski et al. [220]
revealed that the partitioning of CuBr
2
/2dNbpy and CuBr/2dNbpy between the
apolar organic phase and the aqueous phase is crucial. Despite the hydrophobicity
10
4
g mol
-1
;
M
w
/
M
n
=1.26 (
M
nth
=2.52
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