Chemistry Reference
In-Depth Information
ganic solvents with this catalyst system, the catalyst is homogeneously dissolved
in a 50 : 50 mixture of water and HEA-monomer. High conversions (87%) are
achieved after 12 h, and the resulting polymer has
M
n
=1.47
10
4
g mol
-1
and
M
w
/
M
n
=1.34.
Armes et al. have intensively studied the aqueous ATRP of various monomers.
Using CuBr/bpy as a catalyst, methacrylic acid polymerization has been shown to
be possible in aqueous media at pH values between 6 and 9 [200]. The polymer-
ization occurs very slowly (80% conversion after 21 h at 90
C, [mono-
mer] : [initiator] : [catalyst] =28 : 1 : 1) yielding polymers with low molecular weight
(
M
n
=2.9
10
3
g mol
-1
) and a polydispersity of
M
w
/
M
n
=1.3. This is probably due
to a loss of catalytic species occurring from the competitive coordination of car-
boxylic acids to the copper centers, as mentioned for the case of acrylic acid. The
choice of pH is important: at pH <6, protonation of the bipyridyl ligand occurs,
resulting in loss of control. The choice of initiator is equally important: the poly-
merization is only controlled when the methoxy-capped macroinitiator
46
is used.
C,
36
(OEG-
MA) undergoes quantitative ATRP in 30 min at room temperature, using the system
CuCl/bpy [201, 202]. Very high ATRP rates seem to be a general observation for a
range of water-soluble monomers: indeed, quantitative and controlled polymeriza-
tions are observed for
44
(NaVBA) [203] (pH=11),
42
(MPC) [204] (pH=7),
43
(MEMA) [205] (pH=7),
41
(CBMA) (pH=7) [206],
37
(SEM) [207],
39
(AEMA)
[207], and
35
(MADAME) [208, 209]. In all cases, CuBr/bpy or CuCl/bpy was used
as a catalyst, in combination with a 1-ketobromo compound. The motivations under-
lying the choice of CuBr versus CuCl are not presented: a study by Matyjaszewski
[210] et al., supported by a recent study by others [211], indicates that, as CuCl
has a lower oxidation potential than CuBr, the rate of activation
k
+
is higher. (
k
-
,
which together with
k
+
determines the radical concentration, is usually close to dif-
fusion limited.) Consequently, polymerization rates are expected to be faster with
CuCl/bpy systems. Indeed, for methacrylic monomers, the CuCl/bpy system was
used preferentially by Armes (with the exception of
42
(MPC)
Contrary to methacrylic acid, which is only polymerized slowly at 90
[204] and
38
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