Chemistry Reference
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of the ligand dNbpy, the catalyst, both in its Cu
I
and Cu
II
form, is partially solu-
ble in water under the polymerization conditions. In a mixture of water/butyl
methacrylate, up to 90% of Cu
II
/dNbpy is present in the aqueous phase - the
Cu
I
/dNbpy species being much less water soluble. This indicates that catalyst dif-
fusion through the aqueous phase is possible.
It must be noted that, even with an extremely hydrophobic catalyst, catalyst-free
particles would be formed by radical exit and nucleation of new particles, and
thus uncontrolled polymerization would occur. On the other hand, it is also pro-
posed that with more hydrophilic ligands (such as Me
6
-TREN, or pentamethyl
diethylenetriamine (PMDETA) [221]) all the catalyst is located in the aqueous
phase, and uncontrolled polymerization occurs in the hydrophobic polymer parti-
cles. However, these ligands are also ineffective in controlling the aqueous poly-
merization of
water-soluble
monomers (see above), hinting that ligand dissociation
might also occur in these systems [215].
In addition to a proper choice of ligand hydrophobicity, the surfactant used is also
critical. In general, non-ionic surfactants were found to be efficient for colloidal sta-
bilization of the polymer particles [221]. This is to be expected, as, owing to the ionic
nature of the catalyst, and the corresponding ionic strength, electrostatic stabiliza-
tion is likely to be poor. Adequate hydrophilic/lipophilic balance (HLB) [222] is also
a necessary criterion to ensure latex stability in ATR emulsion polymerization [223].
For the controlled polymerization of MMA, catalyzed by CuBr/dNbpy, an HLB of 17
was found to be optimal, and for the polymerization of butyl methacrylate, stabiliza-
tion was optimal with surfactants Brij98 (H
35
C
18
(OCH
2
CH
2
)
20
OH) (HLB=15) and
Tween80 (polyoxyethylene sorbitan monolaurate) (HLB=15) [220]. Nevertheless,
the colloidal stability of the formed latex is usually poor, even at low solid contents
(15%) and in the presence of large amounts of surfactant (10%). Similar results were
reported by Chambard et al. [224] for MMA polymerization using CuBr/dNbpy. The
polymerization is controlled, as shown by linear evolution of
M
n
versus conversion,
but at 60% conversion a low-molecular weight tail in the GPC trace appears that re-
sults from “dead” polymer formed by termination reactions. Most of these problems
were alleviated through the use of a miniemulsion technique (see Section 7.1.1), as
shown in the ATRP of butyl methacrylate with CuBr/dNbpy [225]. The miniemul-
sion recipe used employed hexadecane (10% in volume relative to monomer), as
dNbpy is not hydrophobic enough to prevent Ostwald ripening, and the mixture
was sheared by means of sonication. Sonication generates droplets with sizes of
down to 300 nm, allowing for the preparation of stable latexes. In addition, the mo-
lecular weight of the polymer increases linearly with conversion, but the initiation
efficiency is moderate. The found molecular weight (
M
n
=3.0
10
4
g mol
-1
,at
90% conversion achieved after 2 h at 70
C) is somewhat higher than the theoretical
10
4
g mol
-1
). The authors hint that escape of Cu
II
species from the
miniemulsion droplets occurs, resulting in a slightly increased termination.
Copper-based ATRP in emulsion has been achieved successfully under reverse
ATRP conditions, where one starts with a conventional free radical initiator and
CuBr
2
/dNbpy as a catalyst [226]. Typical commercial water-soluble free radical ini-
tiators such as potassium peroxodisulfate (in a phosphate buffer, pH=7), 2,2
value (
M
nth
=2.4
-azo-
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