Unravelling Competitive Interactions between Polymer Side Chains and End Groups with ß-Cyclodextrin.

This study investigates unexpected competitive host-guest interactions of ß-cyclodextrin (ß-CD), which can occur with polymers in aqueous solution, using the examples of the two polymers poly(oligo(ethylene glycol) methyl ether methacrylate) and poly(glycerol mono methacrylate). Systematic structural modifications of the polymer provide insight into the host-guest interaction with ß-CD and the competition between side chains and end groups such as hydrophobic end groups remaining from reversible addition fragmentation chain transfer polymerization or intentionally implemented molecular recognition units such as arylazopyrazole photoswitches.

Photocleavable Polymer Cubosomes: Synthesis, Self-Assembly, and Photorelease.

Polymer cubosomes (PCs) are a recent class of self-assembled block copolymer (BCP) microparticles with an accessible periodic channel system. Most reported PCs consist of a polystyrene scaffold, which provides mechanical stability for templating but has a limited intrinsic functionality. Here, we report the synthesis of photocleavable BCPs with compositions suitable for PC formation. We analyze the self-assembly mechanism and study the model release of dyes during irradiation, where the transition of the BCPs from amphiphilic to bishydrophilic causes the rapid disassembly of the PCs. A combination of modeling and experiment shows that the evolution of PCs proceeds first via liquid-liquid phase separation into polymer-rich droplets, followed by microphase separation within this droplet confinement, and finally, membrane reorganization into high internal order. This insight may encourage exploration of alternative preparation strategies to better control the size and homogeneity of PCs.

Water-soluble polyphosphonate-based bottlebrush copolymers via aqueous ring-opening metathesis polymerization.

Ring-opening metathesis polymerization (ROMP) is a versatile method for synthesizing complex macromolecules from various functional monomers. In this work, we report the synthesis of water-soluble and degradable bottlebrush polymers, based on polyphosphoesters (PPEs) via ROMP. First, PPE-macromonomers were synthesized via organocatalytic anionic ring-opening polymerization of 2-ethyl-2-oxo-1,3,2-dioxaphospholane using N-(hydroxyethyl)-cis-5-norbornene-exo-2,3-dicarboximide as the initiator and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the catalyst. The resulting norbornene-based macromonomers had degrees of polymerization (DPn) ranging from 25 to 243 and narrow molar mass dispersity (D ≤ 1.10). Subsequently, these macromonomers were used in ROMP with the Grubbs 3rd-generation bispyridyl complex (Ru-G3) to produce a library of well-defined bottlebrush polymers. The ROMP was carried out either in dioxane or in aqueous conditions, resulting in well-defined and water-soluble bottlebrush PPEs. Furthermore, a two-step protocol was employed to synthesize double hydrophilic diblock bottlebrush copolymers via ROMP in water at neutral pH-values. This general protocol enabled the direct combination of PPEs with ROMP to synthesize well-defined bottlebrush polymers and block copolymers in water. Degradation of the PPE side chains was proven resulting in low molar mass degradation products only. The biocompatible and biodegradable nature of PPEs makes this pathway promising for designing novel biomedical drug carriers or viscosity modifiers, as well as many other potential applications.

Homoleptic and heteroleptic ketodiiminate zinc complexes for the ROP of cyclic l-lactide.

Homo- and heteroleptic ketodiiminate zinc complexes L12Zn2 (1, L1 = [Me2NC2H4NC(Me)CH]2CO), L2(ZnCp)2 (2, L2 = [Me2NC3H6NC(Me)CH]2CO, Cp = C5H5) and L2HZnCp* (3, Cp* = C5Me5) were synthesized and characterized by 1H and 13C NMR and IR spectroscopy as well as by elemental analysis and single crystal X-ray diffraction (sc-XRD, 2, 3). The catalytical activity of heteroleptic complexes 2 and 3 were tested in the ring-opening polymerization (ROP) of l-lactide. Homobimetallic complex 2 showed the highest activity and selectivity for the synthesis of cyclic polylactide (cPLLA; TOF = 17 460 h-1) at 100 °C in toluene solution, while linear polymers are formed with mononuclear complex 3.

Janus nanoplates, -bowls, and -cups: controlling size and curvature via terpolymer/homopolymer blending in 3D confinement.

The synthesis and properties of Janus nanoparticles with spherical, cylindrical, and disk-like shapes are nowadays rather well understood. Other topologies such as nanorings and bowl-shaped Janus nanoparticles are believed to show distinctly different solution behavior and interaction with interfaces, but limitations in their synthesis currently prevents a proper investigation of these properties. Especially the combination of shape- and surface-anisotropy of bowl-shaped Janus nanoparticles could result in enhanced selectivity in uptake of cargo and enhanced directional diffusion. We here produce bowl-shaped Janus nanoparticles without noticeable side products through evaporation-induced confinement assembly (EICA) of triblock terpolymers blended with high molecular weight homopolymer. The triblock terpolymer phase separates from the homopolymer into spherical domes, where the terpolymer adopts a hemispherical lamella-lamella morphology (ll). Selective cross-linking, removal of the homopolymer, and disassembly of the microparticles releases the bowl-shaped Janus nanoparticles. The amount of blended homopolymer determines the size of the spherical dome, allowing to control particle curvature into flat Janus nanoplates, hemispherical Janus nanobowls, and deep Janus nanocups. The use of Shirasu Porous Glass (SPG) membranes with pore sizes in the range of dpore = 0.2-2.0 µm further provides control of particle diameter. Size and shape were analyzed with electron microscopy and the Janus character through selective surface decoration. The diffusion behavior of bowl-shaped Janus nanoparticles was investigated depending on particle curvature and anisotropy using angle-dependent dynamic light scattering.

Prismatic Block Copolymer Hexosomes.

Cubosomes and hexosomes are recent solution morphologies with an ordered porous structure and are observed for lipids and amphiphilic block copolymers (BCPs) with high hydrophobic fractions. Whereas lipid hexosomes typically exhibit a prismatic shape, BCP hexosomes have so far only been observed as closed microspheres where inner channels are not connected to the surrounding medium. Here, we describe the formation of flat, prismatic BCP hexosomes with pronounced faceting and a highly ordered lattice of hexagonally packed channels. We assemble polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP or SV) into the hexosome framework using polystyrene-block-poly(4-vinylpyridine)-block-poly(tert-butyl methacrylate) (PS-b-P4VP-b-PT or SVT) as a macromolecular surfactant in low-χ solvents. During solvent exchange, SV-rich domains form through liquid-liquid phase separation, followed by solidification and confined assembly within these domains. Since the final solvent (acetone) has a very low χ parameter toward PS and P4VP (equaling low interfacial tension), solidification of the hexosome occurs under confinement conditions that we term "supersoft". The low interfacial tension allows the stabilization of the hexagonal-prismatic shape, which originates from the hexagonal lattice of channels. Increasing the interfacial tension with polar cosolvents at some point dominates the particle shape, resulting in deformation of prismatic BCP hexosomes into spinning-top structures. The use of low-χ solvents for confined assembly of BCPs may allow the formation of unusual particle shapes simply by tuning the polymer-solvent interaction.

Effect of Surfactant Selectivity on Shape and Inner Morphology of Triblock Terpolymer Microparticles.

The confined assembly of block copolymers (BCPs) has become a useful tool to prepare microparticles with controlled anisotropy and inner structure. While a solid understanding of the behavior of AB diblock copolymers exists, knowledge on the parameters affecting ABC triblock terpolymer assembly is much more limited. In this work, the effect of block-selective surfactants, sodium-4-vinylbenzenesulfonate (VBS) and sodium dodecylsulfate (SDS), is analyzed in the evaporation-induced confined assembly (EICA) of a polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock terpolymer (SBM). Despite using the same terpolymer and emulsification process, SDS results in ellipsoidal microparticles with axially stacked lamellae, while VBS results in spherical microparticles with concentric lamellae or 3D spiral morphology. This change in morphology upon switching the surfactant is further substantiated by molecular simulations and enhances the understanding of terpolymer microphase separation in confinement.

Mixtures of Acid- and Base-Functionalized Microparticles for One-Pot Cascade Reaction.

This study reports on the synthesis of surface-protected acid and base microparticles via surfactant-free emulsion polymerization. The protective layer allows to form particle mixtures with both colloidal catalysts that show excellent performance in a well-established model cascade reaction involving an acid-catalyzed deacetalization and a subsequent base-catalyzed Knoevenagel condensation. Compared to nanoreactors that contain both catalysts, the colloidal mixture shows comparable reaction kinetics despite the local separation of both catalysts onto separate particles. Since the synthesis of individual microparticles requires less steps, is scalable, and more versatile in terms of catalyst variation, colloidal mixtures are certainly an alternative to multifunctional nanoreactors, making them attractive for practical applications.

Fabrication of diverse multicompartment micelles by redispersion of triblock terpolymer bulk morphologies.

Redispersing block copolymer (BCP) bulk films in selective solvents is a simple and efficient method to prepare BCP micelles and polymersomes. While ABC triblock terpolymers are known to form multicompartment micelles (MCMs) with intricate nanoarchitecture, this is typically done by solvent exchange instead of redispersion of bulk films despite obvious advantages of greatly reduced solvent usage. Here, we provide guidelines on how to form MCMs with defined shapes and inner structure through direct redispersion of terpolymer bulk morphologies in selective plasticizing solvents. For this purpose, we redisperse a series of polystyrene-b-polybutadiene-b-poly(tert-butyl methacrylate) (PS-b-PB-b-PT) triblock terpolymers in acetone/isopropanol mixtures, where PT is always soluble, PB always insoluble, and PS will range from soft (high acetone content) to kinetically frozen (high isopropanol content). We investigate the effect of solvent mixtures, block composition, and thermal annealing on MCM shape and core morphology. Additionally, we performed terpolymer blend experiments to open up a simple route to further diversify the range of accessible MCM morphologies.

Confinement Assembly of Terpolymer-Based Janus Nanoparticles.

While the confinement assembly of block copolymers into functional microparticles has been extensively studied, little is known about the behavior of Janus nanoparticles (JNPs) in spherical confinement. Here, the confinement self-assembly of JNPs in drying emulsion droplets is investigated and their behavior compared to their ABC triblock terpolymer precursors. Emulsions of both materials are prepared using Shirasu Porous Glass membranes leading to narrow size distributions of the microparticles with average hydrodynamic radii in the range of Rh  = 250-500 nm (depending on the membrane pore radius, Rpore ). The internal structure of the microparticles is verified with transmission electron microscopy (TEM) on ultrathin cross sections and compared to the corresponding bulk morphologies. While the confinement assembly of terpolymers results in microparticles with ordered inner morphologies, order for JNPs diminishes when the Janus balance deviates from parity.

Morphology and Degradation of Multicompartment Microparticles Based on Semi-Crystalline Polystyrene-block-Polybutadiene-block-Poly(L-lactide) Triblock Terpolymers.

The confinement assembly of block copolymers shows great potential regarding the formation of functional microparticles with compartmentalized structure. Although a large variety of block chemistries have already been used, less is known about microdomain degradation, which could lead to mesoporous microparticles with particularly complex morphologies for ABC triblock terpolymers. Here, we report on the formation of triblock terpolymer-based, multicompartment microparticles (MMs) and the selective degradation of domains into mesoporous microparticles. A series of polystyrene-block-polybutadiene-block-poly(L-lactide) (PS-b-PB-b-PLLA, SBL) triblock terpolymers was synthesized by a combination of anionic vinyl and ring-opening polymerization, which were transformed into microparticles through evaporation-induced confinement assembly. Despite different block compositions and the presence of a crystallizable PLLA block, we mainly identified hexagonally packed cylinders with a PLLA core and PB shell embedded in a PS matrix. Emulsions were prepared with Shirasu Porous Glass (SPG) membranes leading to a narrow size distribution of the microparticles and control of the average particle diameter, d ≈ 0.4 µm-1.8 µm. The core-shell cylinders lie parallel to the surface for particle diameters d < 0.5 µm and progressively more perpendicular for larger particles d > 0.8 µm as verified with scanning and transmission electron microscopy and particle cross-sections. Finally, the selective degradation of the PLLA cylinders under basic conditions resulted in mesoporous microparticles with a pronounced surface roughness.

Biodegradable supramolecular micelles via host-guest interaction of cyclodextrin-terminated polypeptides and adamantane-terminated polycaprolactones.

Biodegradable supramolecular micelles were prepared exploiting the host-guest interaction of cyclodextrin and adamantane. Cyclodextrin-initiated polypeptides acted as the hydrophilic corona, whereas adamantane-terminated polycaprolactones served as the hydrophobic core.

Size-Controlled Formation of Polymer Janus Discs.

A straightforward method is presented for the preparation of nano- to micrometer-sized Janus discs with controlled shape, size, and aspect ratio. The method relies on cross-linkable ABC triblock terpolymers and involves first the preparation of prolate ellipsoidal microparticles by combining Shirasu porous glass (SPG) membrane emulsification with evaporation-induced confinement assembly (EICA). By varying the pore diameter of the SPG membrane, we produce Janus discs with controlled size distributions centered around hundreds of nanometers to several microns. We further transferred the discs to water by mild sulfonation of PS to polystyrene sulfonic acid (PSS) and verified the Janus character by subsequent labelling with cationic nanoparticles. Finally, we show that the sulfonated Janus discs are amphiphilic and can be used as efficient colloidal stabilizers for oil-in-water (O/W) emulsions.

Soft synthetic microgels as mimics of mycoplasma.

Artificial model colloids are of special interest in the development of advanced sterile filters, as they are able to efficiently separate pleomorphic, highly deformable and infectious bacteria such as mycoplasma, which, until now, has been considered rather challenging and laborious. This study presents a full range of different soft to super soft synthetic polymeric microgels, including two types with similar hydrodynamic mean diameter, i.e., 180 nm, and zeta potential, i.e., -25 ± 10 mV, but different deformability, synthesized by inverse miniemulsion terpolymerization of acrylamide, sodium acrylate and N,N'-methylenebisacrylamide. These microgels were characterized by means of dynamic, electrophoretic and static light scattering techniques. In addition, the deformability of the colloids was investigated by filter cake compressibility studies during ultrafiltration in dead-end mode, analogously to a study of real mycoplasma, i.e., Acholeplasma laidlawii, to allow for a direct comparison. The results indicate that the variation of the synthesis parameters, i.e., crosslinker content, polymeric solid content and content of sodium acrylate, has a significant impact on the swelling behavior of the microgels in aqueous solution as well as on their deformability under filtration conditions. A higher density of chemical crosslinking points results in less swollen and more rigid microgels. Furthermore, these parameters determine electrokinetic properties of the more or less permeable colloids. Overall, it is shown that these soft synthetic microgels can be obtained with tailor-made properties, covering the size of smallest species of and otherwise similar to real mycoplasma. This is a relevant first step towards the future use of synthetic microgels as mimics for mycoplasma.

Frustrated Microparticle Morphologies of a Semicrystalline Triblock Terpolymer in 3D Soft Confinement.

Self-assembly of block copolymers (BCPs) in three-dimensional (3D) confinement of emulsion droplets has emerged as a versatile route for the formation of functional micro- and nanoparticles. While the self-assembly of amorphous coil-coil BCPs is fairly well documented, less is known about the behavior of crystalline-coil BCPs. Here, we demonstrate that confining a linear ABC triblock terpolymer with a crystallizable middle block in oil-in-water (O/W) emulsions results in a range of microparticles with frustrated inner structure originating from the conflict between crystallization and curved interfaces. Polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA, S32E36M3293) in toluene droplets was subjected to different preparation protocols. If evaporation was performed well above the bulk crystallization temperature of the PE block (Tevap > Tc), S32E36M3293 first microphase-separated into microparticles with lamella morphology followed by crystallization into a variety of frustrated morphologies (e.g., bud-like, double staircase, spherocone). By evaporating at significantly lower temperatures that allow the PE block to crystallize from solution (Tevap < Tc), S32E36M3293 underwent crystallization-driven self-assembly into patchy crystalline-core micelles, followed by confinement assembly into lenticular microparticles with compartmentalized hexagonal cylinder lattices. The frequency of these frustrated morphologies depends on polymer concentration and the evaporation protocol. These results provide a preliminary understanding of the morphological behavior of semicrystalline block copolymers in 3D soft confinement and may provide alternative routes to structure multicompartment microparticles from a broader range of polymer properties.

Scalable and Recyclable All-Organic Colloidal Cascade Catalysts.

We report on the synthesis of core-shell microparticles (CSMs) with an acid catalyst in the core and a base catalyst in the shell by surfactant-free emulsion polymerization (SFEP). The organocatalytic monomers were separately copolymerized in three synthetic steps allowing the spatial separation of incompatible acid and base catalysts within the CSMs. Importantly, a protected and thermo-decomposable sulfonate monomer was used as acid source to circumvent the neutralization of the base catalyst during shell formation, which was key to obtain stable, catalytically active CSMs. The catalysts showed excellent performance in an established one-pot model cascade reaction in various solvents (including water), which involved an acid-catalyzed deacetalization followed by a base-catalyzed Knoevenagel condensation. The CSMs are easily recycled, modified, and their synthesis is scalable, making them promising candidates for organocatalytic applications.

Recent Advances in the Synthesis and Application of Polymer Compartments for Catalysis.

Catalysis is one of the most important processes in nature, science, and technology, that enables the energy efficient synthesis of essential organic compounds, pharmaceutically active substances, and molecular energy sources. In nature, catalytic reactions typically occur in aqueous environments involving multiple catalytic sites. To prevent the deactivation of catalysts in water or avoid unwanted cross-reactions, catalysts are often site-isolated in nanopockets or separately stored in compartments. These concepts have inspired the design of a range of synthetic nanoreactors that allow otherwise unfeasible catalytic reactions in aqueous environments. Since the field of nanoreactors is evolving rapidly, we here summarize-from a personal perspective-prominent and recent examples for polymer nanoreactors with emphasis on their synthesis and their ability to catalyze reactions in dispersion. Examples comprise the incorporation of catalytic sites into hydrophobic nanodomains of single chain polymer nanoparticles, molecular polymer nanoparticles, and block copolymer micelles and vesicles. We focus on catalytic reactions mediated by transition metal and organocatalysts, and the separate storage of multiple catalysts for one-pot cascade reactions. Efforts devoted to the field of nanoreactors are relevant for catalytic chemistry and nanotechnology, as well as the synthesis of pharmaceutical and natural compounds. Optimized nanoreactors will aid in the development of more potent catalytic systems for green and fast reaction sequences contributing to sustainable chemistry by reducing waste of solvents, reagents, and energy.

Active Ga-catalysts for the ring opening homo- and copolymerization of cyclic esters, and copolymerization of epoxide and anhydrides.

A series of gallium complexes L12Ga4Me8 (1), L22Ga4Me8 (2), and L32Ga4Me8 (3) was synthesized by reaction of GaMe3 with Schiff base ligands L1-3H2 (L1H2 = 2,4-di-tert-butyl-6-{[(3-hydroxypropyl)imino]methyl}phenol; L2H2 = 2,4-dichloro-6-{[(3-hydroxypropyl)imino]methyl}phenol; L3H2 = 4-tert-butyl-2-{[(3-hydroxypropyl)imino]methyl}phenol) and characterized by 1H, 13C NMR, IR spectroscopy, elemental analysis and single crystal X-ray analysis (1, 2), proving their tetranuclear structure in the solid state. Complexes 1-3 showed good catalytic activity in the ring opening homopolymerization (ROP) and ring opening copolymerization (ROcoP) of lactide (LA) and ε-caprolactone (ε-CL) in the presence of benzyl alcohol (BnOH) in toluene at 100 °C, yielding polymers with the expected average molecular weights (Mn) and narrow molecular weight distributions (MWD), as well as a high isoselectivity for the ROP of rac-lactide (rac-LA), yielding isotactic-enriched PLAs with Pm values up to 0.78. Kinetic studies with complex 1 proved the first order dependence on monomer concentration, while mechanistic studies confirmed the coordination insertion mechanistic (CIM) pathway. Sequential addition of monomers gave well defined diblock copolymers of PCL-b-PLLA and PLLA-b-PCL, proving the living character of the polymerization reactions. The catalysts also showed perfect selectivity for the copolymerization of cyclohexene oxide (CHO) with both succinic anhydride (SA) and maleic anhydride (MA) in the presence of BnOH and produced >99% alternating block copolymers.

Terpolymer Multicompartment Nanofibers as Templates for Hybrid Pt Double Helices.

Hybrid inorganic/block copolymer (BCP) materials have become increasingly relevant for application in heterogeneous catalysis, microelectronics, and nanomedicine. While block copolymer templates are widely used for the formation of inorganic nanostructures, multicompartment templates could give access to more complex shapes and inner structures that are challenging to obtain with traditional processes. Here, we report the formation and characterization of hybrid platinum/polymer helices using multicompartment nanofibers (MCNFs) of polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate) (PS-b-PB-b-PT) triblock terpolymers as templates. Cross-linking of a PS-b-PB-b-PT helix-on-cylinder morphology resulted in uniform nanofibers with a diameter of 90 nm and a length of several micrometers, as well as an inner PB double helix (diameter 35 nm, pitch 25 nm, core 12 nm). The PB double helix served as template for the sol-gel reaction of H2PtCl6 into hybrid Pt double helices (Pt@MCNFs) as verified by STEM, electron tomography, AFM, and SEM. Carbonization of the Pt hybrids into Pt decorated carbon nanofibers (Pt@C) was followed in situ on a TEM heating state. Gradual heating from 25 to 1000 °C induced fusion of amorphous Pt NPs into larger crystalline Pt NP, which sheds light on the aging of Pt NPs in BCP scaffolds under high temperature conditions. The Pt@MCNFs were further sulfonated and incorporated into a filter to catalyze a model compound in a continuous flow process.

pH-Controlled Hierarchical Assembly/Disassembly of Multicompartment Micelles in Water.

Multicompartment micelles (MCMs) have become attractive drug delivery systems as they allow the separate storage of two or more incompatible cargos in their core compartments (e.g., drugs and dyes for imaging). A recent hierarchical self-assembly process for hydrophobic terpolymers in organic solvents showed the ability to form very homogeneous MCM populations, yet the transfer of this process into water requires a better understanding of the formation mechanism and influence of chain mobility during assembly. Here, the synthesis of a linear poly(oligo(ethylene glycol) methacrylate)-block-poly(benzyl acrylate)-block-poly(4-vinylpyridine) (POEGMA-b-PBzA-b-P4VP) triblock terpolymer by reversible addition-fragmentation chain transfer (RAFT) polymerization is reported as well as its step-wise assembly into MCMs in water with POEGMA corona, PBzA patches, and P4VP core. Reversible assembly/disassembly of the MCMs is investigated through protonation/deprotonation of the P4VP core. Interestingly, the low glass transition temperature (Tg ) of the hydrophobic PBzA middle block causes MCMs to directly disassemble into molecularly dissolved chains instead of patchy micelles due to mechanical stress from electrosteric repulsion of the protonated P4VP corona chains. In addition, pH resistant MCMs are created by core-crosslinking and fluorescent properties are added by covalent anchoring of fluorescent dyes via straightforward click chemistry.