Abstract
Over the past two decades, block copolymer vesicles have been widely used by many research groups to encapsulate small molecule drugs, genetic material, nanoparticles or enzymes. They have also been used to design examples of autonomous self-propelled nanoparticles. Traditionally, such vesicles are prepared via post-polymerization processing using a water-miscible co-solvent such as DMF or THF. However, such protocols are invariably conducted in dilute solution, which is a significant disadvantage. In addition, the vesicle size distribution is often quite broad, whereas aqueous dispersions of relatively small vesicles with narrow size distributions are highly desirable for potential biomedical applications. Alternatively, concentrated dispersions of block copolymer vesicles can be directly prepared via polymerization-induced self-assembly (PISA). Moreover, using a binary mixture of a relatively long and a relatively short steric stabilizer block enables the convenient PISA synthesis of relatively small vesicles with reasonably narrow size distributions in alcoholic media (C. Gonzato et al., JACS, 2014, 136, 11100-11106). Unfortunately, this approach has not yet been demonstrated for aqueous media, which would be much more attractive for commercial applications. Herein we show that this important technical objective can be achieved by judicious use of two chemically distinct, enthalpically incompatible steric stabilizer blocks, which ensures the desired microphase separation across the vesicle membrane. This leads to the formation of well-defined vesicles of around 200 nm diameter (size polydispersity = 13-16%) in aqueous media at 10% w/w solids as judged by transmission electron microscopy, dynamic light scattering and small-angle X-ray scattering.
Highlights
This design rule can be explained in terms of the geometric packing parameter P introduced by Israelachvili and co-workers to account for surfactant self-assembly,[89] which has been subsequently validated for the self-assembly of amphiphilic diblock copolymers.[90]
Informed by this design rule, Sugihara and co-workers reported that targeting PMPC25–PHPMA400 via aqueous polymerizationinduced self-assembly (PISA) at 25% w/w solids produced a rather polydisperse vesicular morphology at 70 C.34
This was a deliberate choice because this ionizable end-group is known to in uence the electrophoretic behavior of block copolymer nano-objects,[92,93] which was expected to aid discrimination between the three types of vesicles shown in Scheme 1
Summary
Over the past decade or so, polymerization-induced selfassembly (PISA) has become widely recognized as a highly efficient and versatile technique for the rational synthesis of a wide range of block copolymer nano-objects in concentrated solution.[1,2] Systematic variation of the relative volume fractions of the solvophilic and solvophobic blocks allows convenient access to sterically-stabilized spheres, worms and vesicles using many different monomers.[3,4,5,6,7,8,9,10,11] In principle, various types of controlled/living polymerization techniques can be used for such PISA syntheses but reversible addition–fragmentationRAFT-mediated PISA can be conducted in a wide range of solvents.[13,14,15,16,17,18,19,20] In practice, water is the most cost-effective, environmentally-friendly and is best suited for potential biomedical applications. To model spherical micelles comprising a binary mixture of PEG113 and PMPC28 blocks, the Rg was calculated based on their relative volume fractions using an approximate radius of gyration for each pure block [RgPEG 1⁄4 2.6 nm and RgPMPC 1⁄4 1.4 nm].
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