Designing Hydrogels by ATRP - In-Situ Gelling Polymers: For Biomedical Applications - page 80

surfactants with hydrophilic lipophilic balance (HLB) values close to 14, such as
commercially available polyoxyethylene(20) oleyl ether (Brij 98) were success-
fully applied to a controlled ATRP in aqueous dispersed media. Okubo et al. also
reported the use of Tween 80 [ 130 ] and poly(vinyl alcohol) (PVA) [ 131 ] as non-
ionic surfactants for ATRP. Cationic surfactants, such as cetyltrimethylammonium
bromide (CTAB), have also been used successfully for a controlled ATRP as well
as preparation of stable latex particles [ 132 ]. The chemical structures of these sur-
factants are also illustrated in Scheme 4 .
Another critical issue related to a controlled ATRP in aqueous dispersed media,
in addition to the selection of proper ligands and surfactants, is the initiation tech-
nique. The biphasic nature of heterogeneous media precludes freeze-pump-thaw
or vigorous N 2 -bubbling to completely remove the oxygen from the reaction
medium. As a result, any ATRP formulation with an initial addition of air-sensitive
Cu(I) species could lose part of the Cu activator before initiating polymerization
[ 133 ]. This issue is more pronounced in miniemulsion because it is very difficult
to prevent oxygen diffusing into the samples during the ultrasonication proce-
dure that is necessary for preparation of the submicron monomer droplets prior to
polymerization [ 134 ]. Thus, ATRP techniques that start from air-stable Cu(II) spe-
cies in the formula are desirable for conducting ATRP in aqueous dispersed sys-
tems. For instance, conducting a reverse ATRP of n -butyl methacrylate ( n BMA) in
an aqueous dispersed system using a water-soluble initiator (V-50), a nonionic sur-
factant (Brij 98), and a hydrophobic ligand (dNbpy) provided a controlled minie-
mulsion polymerization, as indicated by a linear increase of molecular weight
with monomer conversion and a narrow MWD [ 133 ]. In another study, Simms and
Cunningham reported a successful reverse ATRP of n BMA in miniemulsion using
a cationic emulsifier, CTAB, at 90 °C with EHA 6 TREN as ligand and VA-044 as
thermal initiator [ 132 ]. A loading of CTAB as low as 1 wt%, relative to monomer,
provided sufficient colloidal stability, i.e., a concentration considerably lower than
that required when a nonionic surfactant, Brij 98, was used.
The successful development of SR&NI [ 49 , 135 , 136 ] ATRP, AGET [ 36 , 137 -
142 ] ATRP and ARGET ATRP [ 143 , 144 , 145 ] significantly expanded the number
of ATRP procedures that can be used in aqueous dispersed systems. The recent
development of super-hydrophobic ligands with longer alkyl chains significantly
decreased the partitioning of Cu(II) complexes in water and allowed the amount
of catalyst to be as low as 50 ppm Cu while maintaining control in miniemulsion
ATRP [ 144 , 145 ]. In addition to the synthesis of linear and branched polymers,
crosslinked latex particles containing degradable disulfide crosslinks were also
prepared in miniemulsion by using SR&NI ATRP [ 146 ]. The efficient degrada-
tion of the latex particles into homopolymers, upon the addition of tri( n -butyl)
phosphine reducing agent, was monitored by dynamic light scattering (DLS)
measurements. When reactive surfactants with dual reactive sites were used, the
polymerization provided direct methods to introduce surface and core functionali-
ties into the nanoparticles for advanced applications [ 147 , 148 ].
Most of the early reports of successful ATRP in aqueous dispersed media
employed miniemulsion systems, since an ideal miniemulsion system has no mass