Chemistry Reference
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(HEMA) [212]). Monomer
42
(MPC) spontaneously polymerizes at room tempera-
ture in aqueous solution and the choice of the slowest catalyst is therefore justi-
fied. Because poly-38 [212] is not soluble in water, the polymerization is run in a
50 : 50 MeOH:water mixture. Consequently, rates are lower than in water (95% con-
version requires 3-4 h reaction time at room temperature). A comparison of poly-
merization rates in aqueous and non-aqueous media reveals strong solvent effects.
Polar solvents have been found to increase the polymerization rate, possibly because
of the combined effect of an increase of rate constant
k
+
[213] and a competitive co-
ordination of the solvent and the ligand in the copper species [214].
CuX/bpy (X=Cl, Br) is not the only catalyst efficient in aqueous ATRP. Pelton et
al. have extensively studied the polymerization of
35
(MADAME) in organic solvents
[209] and aqueous media [215] at room temperature, and found that CuBr/HMTETA
(HMTETA = 1,1,4,7,10,10-hexamethyltriethylenetetraamine) is less effective in en-
suring molecular weight control than CuBr/bpy under similar conditions. Using
methyl bromophenylacetate as an initiator, the conversion in aqueous polymeriza-
tion is similar with both catalyst systems CuX/bpy and CuBr/HMTETA (85% conver-
sion at room temperature), affording, however, monodisperse polymers for the for-
mer (
M
n
=1.28
10
4
g mol
-1
, corresponding to the theoretically calculated
M
nth
;
M
w
/
M
n
=1.2) and polymers with broader molecular weight distribution for the latter
(
M
n
=1.84
10
4
g mol
-1
;
M
w
/
M
n
=1.5). Excellent control in the aqueous polymeriza-
tion of
36
(OEGMA) can be achieved with a copper-based catalyst with ligand
47
[216]. Unexpectedly, the Schiff base ligand is not hydrolyzed during the polymeriza-
tion reaction, and at room temperature a polymer of
M
n
=1.2
10
4
g mol
-1
(
M
w
/
M
n
=1.18) is obtained in 93% yield after a reaction time of only 4 h.
Despite the large numbers of water-soluble monomers that can be polymerized
by ATRP, several problems remain to be solved. For example, Brittain et al. [217]
have shown that the aqueous polymerization of
45
(DMA), catalyzed by CuBr/L
(L=HMTETA, tris[2-(dimethylamino)ethyl]amine (Me
6
-TREN) and 1,4,8,11-tetra-
methyl-1,4,8,11-tetraazacyclotetradecane (Me
4
Cyclam)) is not controlled because of
the slow but irreversible decomposition of the bromo-terminated end-group of the
dormant chains. Aqueous polymerization of acrylic [199] and styrenic monomers
[203] also appears to be difficult to control, as implied by the rarity of known ex-
amples.
Except in the case of polymerization of
38
(HEMA), water-soluble polymers are
formed in all the polymerizations regarded, and in this sense the reactions can be
designated as solution polymerizations.
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