The ability of amphiphilic block copolymers to self‐assemble in selective solvents has been widely studied in academia and utilized for various commercial products. The self‐assembled polymer vesicle is at the forefront of this nanotechnological revolution with seemingly endless possible uses, ranging from biomedical to nanometer‐scale enzymatic reactors. This review is focused on the inherent advantages in using polymer vesicles over their small molecule lipid counterparts and the potential applications in biology for both drug delivery and synthetic cellular reactors.magnified image

Amphiphilic diblock copolymers composed of two covalently linked, chemically distinct chains can be considered to be biological mimics of cell membrane-forming lipid molecules, but with typically more than an order of magnitude increase in molecular weight. These macromolecular amphiphiles are known to form a wide range of nanostructures (spheres, worms, vesicles, etc.) in solvents that are selective for one of the blocks. However, such self-assembly is usually limited to dilute copolymer solutions (<1%), which is a significant disadvantage for potential commercial applications such as drug delivery and coatings. In principle, this problem can be circumvented by polymerization-induced block copolymer self-assembly. Here we detail the synthesis and subsequent in situ self-assembly of amphiphilic AB diblock copolymers in a one pot concentrated aqueous dispersion polymerization formulation. We show that spherical micelles, wormlike micelles, and vesicles can be predictably and efficiently obtained (within 2 h of polymerization, >99% monomer conversion) at relatively high solids in purely aqueous solution. Furthermore, careful monitoring of the in situ polymerization by transmission electron microscopy reveals various novel intermediate structures (including branched worms, partially coalesced worms, nascent bilayers, "octopi", "jellyfish", and finally pure vesicles) that provide important mechanistic insights regarding the evolution of the particle morphology during the sphere-to-worm and worm-to-vesicle transitions. This environmentally benign approach (which involves no toxic solvents, is conducted at relatively high solids, and requires no additional processing) is readily amenable to industrial scale-up, since it is based on commercially available starting materials.

Polymerization-induced self-assembly (PISA) of poly(glycerol monomethacrylate)–poly(2-hydroxypropyl methacrylate) (PGMA–PHPMA) diblocks is conducted using a RAFT aqueous dispersion polymerization formulation at 70 °C. Several PGMA macromolecular chain transfer agents (macro-CTAs) are chain-extended using a water-miscible monomer (HPMA): the growing PHPMA block becomes increasingly hydrophobic and hence drives in situ self-assembly. The final copolymer morphology in such PISA syntheses depends on just three parameters: the mean degree of polymerization (DP) of the PGMA stabilizer block, the mean DP of the PHPMA core-forming block, and the total solids concentration. Transmission electron microscopy is used to construct detailed diblock copolymer phase diagrams for PGMA DPs of 47, 78, and 112. For the shortest stabilizer block, there is essentially no concentration dependence: spheres, worms, or vesicles can be obtained even at 10% w/w solids simply by selecting the DP of the PHPMA block that gives the appropriate molecular curvature. For a PGMA DP of 78, the phase diagram is rich: and the copolymer morphology depends strongly on the total solids concentration. There is also a narrow region where spheres, worms, and vesicles coexist, which may be due to the effect of polydispersity. For a PGMA112 macro-CTA, the phase diagram is dominated by spherical morphologies. This is probably because the longer core-forming block DPs required to reduce the molecular curvature are significantly more dehydrated and hence less mobile, which prevents the in situ evolution of morphology from spheres to higher order morphologies. This hypothesis is supported by the observation that addition of ethanol to aqueous PISA syntheses conducted using the longer macro-CTAs allows access to diblock copolymer worms or vesicles, since this cosolvent solvates the core-forming PHPMA chains and hence increases their mobility at 70 °C. Elucidation of such phase diagrams is vital to ensure reproducible targeting of pure phases, rather than mixed phases.

Biocompatible hydrogels have many applications, ranging from contact lenses to tissue engineering scaffolds. In most cases, rigorous sterilization is essential. Herein we show that a biocompatible diblock copolymer forms wormlike micelles via polymerization-induced self-assembly in aqueous solution. At a copolymer concentration of 10.0 w/w %, interworm entanglements lead to the formation of a free-standing physical hydrogel at 21 °C. Gel dissolution occurs on cooling to 4 °C due to an unusual worm-to-sphere order-order transition, as confirmed by rheology, electron microscopy, variable temperature (1)H NMR spectroscopy, and scattering studies. Moreover, this thermo-reversible behavior allows the facile preparation of sterile gels, since ultrafiltration of the diblock copolymer nanoparticles in their low-viscosity spherical form at 4 °C efficiently removes micrometer-sized bacteria; regelation occurs at 21 °C as the copolymer chains regain their wormlike morphology. Biocompatibility tests indicate good cell viabilities for these worm gels, which suggest potential biomedical applications.

Reversible addition-fragmentation chain transfer polymerization has been utilized to polymerize 2-hydroxypropyl methacrylate (HPMA) using a water-soluble macromolecular chain transfer agent based on poly(2-(methacryloyloxy)ethylphosphorylcholine) (PMPC). A detailed phase diagram has been elucidated for this aqueous dispersion polymerization formulation that reliably predicts the precise block compositions associated with well-defined particle morphologies (i.e., pure phases). Unlike the ad hoc approaches described in the literature, this strategy enables the facile, efficient, and reproducible preparation of diblock copolymer spheres, worms, or vesicles directly in concentrated aqueous solution. Chain extension of the highly hydrated zwitterionic PMPC block with HPMA in water at 70 °C produces a hydrophobic poly(2-hydroxypropyl methacrylate) (PHPMA) block, which drives in situ self-assembly to form well-defined diblock copolymer spheres, worms, or vesicles. The final particle morphology obtained at full monomer conversion is dictated by (i) the target degree of polymerization of the PHPMA block and (ii) the total solids concentration at which the HPMA polymerization is conducted. Moreover, if the targeted diblock copolymer composition corresponds to vesicle phase space at full monomer conversion, the in situ particle morphology evolves from spheres to worms to vesicles during the in situ polymerization of HPMA. In the case of PMPC(25)-PHPMA(400) particles, this systematic approach allows the direct, reproducible, and highly efficient preparation of either block copolymer vesicles at up to 25% solids or well-defined worms at 16-25% solids in aqueous solution.

We report the synthesis of anionic sterically stabilized diblock copolymer nanoparticles via polymerization-induced self-assembly using a RAFT aqueous dispersion polymerization formulation. The anionic steric stabilizer is a macromolecular chain-transfer agent (macro-CTA) based on poly(potassium 3-sulfopropyl methacrylate) (PKSPMA), and the hydrophobic core-forming block is based on poly(2-hydroxypropyl methacrylate) (PHPMA). The effect of varying synthesis parameters such as the salt concentration, solids content, relative block composition, and anionic charge density has been studied. In the absence of salt, self-assembly is problematic when using a PKSPMA stabilizer because of lateral repulsion between highly charged anionic chains. However, in the presence of added salt this problem can be overcome by reducing the charge density within the coronal stabilizer layer by either (i) statistically copolymerizing the KSPMA monomer with a nonionic comonomer (2-hydroxyethyl methacrylate, HEMA) or (ii) using a binary mixture of a PKSPMA macro-CTA and a poly(glycerol monomethacrylate) (PGMA) macro-CTA. These diblock copolymer nanoparticles were analyzed by (1)H NMR spectroscopy, gel permeation chromatography (GPC), dynamic light scattering (DLS), transmission electron microscopy (TEM), and aqueous electrophoresis. NMR studies suggest that the HPMA polymerization is complete within 2 h at 70 °C, and DMF GPC analysis confirms that the resulting diblock copolymers have relatively low polydispersities (M(w)/M(n) < 1.30). NMR also suggests a significant degree of hydration for the core-forming PHPMA chains. Depending on the specific reaction conditions, a series of spherical nanoparticles with mean diameters ranging from 50 to 200 nm with tunable anionic surface charge can be prepared. If a binary mixture of anionic and nonionic macro-CTAs is utilized, then it is also possible to access a vesicular morphology.

Synthesis of diblock copolymer nano-objects: alcohol is a good idea! RAFT dispersion polymerization of benzyl methacrylate in alcohol using weak polyelectrolyte-based chain transfer agents allows the facile synthesis of sterically stabilized diblock copolymer nano-objects with very high monomer conversions. Such syntheses are usually problematic when conducted in water due to electrostatic repulsion between highly charged stabilizer chains, which impedes in situ self-assembly. Construction of a detailed phase diagram facilitates reproducible syntheses of well-defined diblock copolymer spheres, worms or vesicles, since it allows mixed phase regions to be avoided. Aqueous electrophoresis studies confirm that these nano-objects can acquire substantial surface charge when transferred to aqueous solution due to ionization (or protonation) of the polyacid (or polybase) stabilizer chains.

Cell cytosol and the different subcellular organelles house the most important biochemical processes that control cell functions. Effective delivery of bioactive agents within cells is expected to have an enormous impact on both gene therapy and the future development of new therapeutic and/or diagnostic strategies based on single-cell-bioactive-agent interactions. Herein a biomimetic nanovector is reported that is able to enter cells, escape from the complex endocytic pathway, and efficiently deliver actives within clinically relevant cells without perturbing their metabolic activity. This nanovector is based on the pH-controlled self-assembly of amphiphilic copolymers into nanometer-sized vesicles (or polymersomes). The cellular-uptake kinetics can be regulated by controlling the surface chemistry, the polymersome size, and the polymersome surface topology. The latter is controlled by the extent of polymer-polymer phase separation within the external envelope of the polymersome.