Interpolyelectrolyte complexes (IPECs) are typically formed when two polyelectrolytes of opposite charge are mixed together in solution. We present an overview of different strategies for the preparation of micellar IPECs, i.e., structures where such IPEC domains form the core or the shell of micelles. In addition, vesicular architectures are considered, where the IPEC domain forms a membrane layer. One intriguing feature of IPECs is that their formation can be directed, their stability towards changes in pH or ionic strength can (to a certain extent) be predicted, and their size can be controlled. Especially the use of ionic/non-ionic block copolymers offers unique potential for the preparation of well-defined and sophisticated nanostructured materials. We also discuss possible applications, especially in the field of life sciences, including biocompatibility, the controlled uptake/release of guest substances, the immobilization of enzymes, or the controlled formation of inorganic/organic hybrid materials.

Throughout the last decades, magnetic nanoparticles (MNP) have gained tremendous interest in different fields of applications like biomedicine (e.g., magnetic resonance imaging (MRI), drug delivery, hyperthermia), but also more technical applications (e.g., catalysis, waste water treatment) have been pursued. Different surfactants and polymers are extensively used for surface coating of MNP to passivate the surface and avoid or decrease agglomeration, decrease or modulate biomolecule absorption, and in most cases increase dispersion stability. For this purpose, electrostatic or steric repulsion can be exploited and, in that regard, surface charge is the most important (hybrid) particle property. Therefore, polyelectrolytes are of great interest for nanoparticle coating, as they are able to stabilize the particles in dispersion by electrostatic repulsion due to their high charge densities. In this review article, we focus on polyzwitterions as a subclass of polyelectrolytes and their use as coating materials for MNP. In the context of biomedical applications, polyzwitterions are widely used as they exhibit antifouling properties and thus can lead to minimized protein adsorption and also long circulation times.

A plethora of stimuli-responsive micellar aggregates with a compartmentalized shell can be formed in aqueous solution from ABC triblock terpolymers with tunable hydrophilicity. Polybutadiene-block-poly(tert-butyl methacrylate)-block-poly(2-(dimethylamino)ethyl methacrylate) (PB-b-PtBMA-b-PDMAEMA) and, after modifications by hydrolysis to poly(methacrylic acid) (PMAA) or quaternization to PDMAEMAq, PB-b-PMAA-b-PDMAEMAq terpolymers self-assemble in water, depending on pH and temperature. We demonstrate control over micellar shape, size, and charge via three different preparation pathways. Even more, the micelles are capable of undergoing rearrangements in both the shell and the corona in response to external stimuli like pH or salinity. In that way, different structures such as multicompartment, core–shell–corona or flower-like micelles were identified and characterized viacryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS). The presence of two oppositely charged polyelectrolyte blocks within the structures leads to the formation of intramicellar interpolyelectrolyte complexes (im-IPECs) in the shell of the particles. Surprisingly, the im-IPEC formed between PMAA and PDMAEMAq can be redissolved by changes in pH, even in the absence of additional salt.

Three-armed star polymers with a cyclodextrin core and polyacrylamide arms were prepared via inclusion complex formation. A novel adamantyl-functionalized chain transfer agent was utilized in the reversible addition–fragmentation chain transfer (RAFT) polymerization of N,N-dimethylacrylamide and N,N-diethylacrylamide to afford guest-functionalized polymers. In combination with a three-pronged β-cyclodextrin core, supramolecular self-assembly was exploited for the formation of star-shaped architectures. This was demonstrated via a combination of dynamic light scattering (DLS) and 2D rotating frame nuclear Overhauser effect spectroscopy (ROESY) in D₂O. Furthermore, turbidity measurements in the case of PDEAAm showed a rise in the LCST of the system. The complex formation was shown to be reversible, as DLS at elevated temperatures (70 °C) indicated a complete disassembly of the star polymers.

Within the last few years, crystallisation-induced self-assembly of block copolymers (BCs) in solution has become more and more attractive for the production of well-defined cylindrical crystalline-core micelles (cCCMs). The “livingness” of this process has been shown for a number of crystallisable blocks and allows fine tuning of the length as well as the length distribution of such structures. This unprecedented control is hardly achievable in the self-assembly of purely amorphous BCs. Furthermore, in an analogy to living/controlled polymerisation methods, the epitaxial growth of different BCs onto preformed cCCMs allows for the preparation of complex micellar architectures, e.g., cylindrical block co-micelles. This highlight tries to provide an overview over recent developments in crystallisation-induced self-assembly of BCs with a particular focus on one-dimensional (1D) micellar nanostructures.