In this Perspective, we discuss the
recent development of polymerization-induced
self-assembly mediated by reversible addition–fragmentation
chain transfer (RAFT) aqueous dispersion polymerization. This
approach has quickly become a powerful and versatile technique for
the synthesis of a wide range of bespoke organic diblock copolymer
nano-objects of controllable size, morphology, and surface functionality.
Given its potential scalability, such environmentally-friendly formulations
are expected to offer many potential applications, such as novel Pickering
emulsifiers, efficient microencapsulation vehicles, and
sterilizable thermo-responsive hydrogels for the cost-effective
long-term storage of mammalian cells.
A poly(ethylene
glycol) (PEG) macromolecular chain transfer agent
(macro-CTA) is prepared in high yield (>95%) with 97% dithiobenzoate
chain-end functionality in a three-step synthesis starting from a
monohydroxy PEG113 precursor. This PEG113-dithiobenzoate
is then used for the reversible addition–fragmentation chain
transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl
methacrylate (HPMA). Polymerizations conducted under optimized conditions
at 50 °C led to high conversions as judged by 1H NMR
spectroscopy and relatively low diblock copolymer polydispersities
(Mw/Mn <
1.25) as judged by GPC. The latter technique also indicated good blocking
efficiencies, since there was minimal PEG113 macro-CTA
contamination. Systematic variation of the mean degree of polymerization
of the core-forming PHPMA block allowed PEG113-PHPMAx diblock copolymer spheres, worms, or vesicles
to be prepared at up to 17.5% w/w solids, as judged by dynamic light
scattering and transmission electron microscopy studies. Small-angle X-ray scattering (SAXS) analysis revealed that more exotic
oligolamellar vesicles were observed at 20% w/w solids when targeting
highly asymmetric diblock compositions. Detailed analysis of SAXS
curves indicated that the mean number of membranes per oligolamellar
vesicle is approximately three. A PEG113-PHPMAx phase diagram was constructed to enable the reproducible
targeting of pure phases, as opposed to mixed morphologies (e.g.,
spheres plus worms or worms plus vesicles). This new RAFT PISA formulation
is expected to be important for the rational and efficient synthesis
of a wide range of biocompatible, thermo-responsive PEGylated diblock
copolymer nano-objects for various biomedical applications.
Specular neutron reflection and surface tension have been used to investigate the composition and
structure of the surfactant−polymer mixture of sodium dodecyl sulfate, SDS, and the cationic polymer
poly(dimethyldiallylammonium chloride) at the air−water interface. The variation of surface tension with
SDS concentration shows a complex behavior, with a marked increase between the concentrations normally
associated with the critical aggregation concentration and the critical micellar concentration. The neutron
reflectivity measurements show that this change in surface tension is associated with changes in the
amount of SDS and polymer at the interface. The changes are attributed to the competition between the
formation of surface and solution surfactant−polymer complexes.
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