Supramolecular Nanoparticles via Single-Chain Folding Driven by Ferrous Ions

Trimethylsilanol Cleaves Stable Azaylides As Revealed by Unfolding of Robust "Staudinger" Single-Chain Nanoparticles

Herein, we disclose a unique and selective reagent for the cleavage of stable azaylides prepared by the nonhydrolysis Staudinger reaction, enabling the on-demand unfolding of robust single-chain nanoparticles (SCNPs). SCNPs with promising use in catalysis, nanomedicine, and sensing are obtained through intrachain folding of discrete synthetic polymer chains. The unfolding of SCNPs involving reversible interactions triggered by a variety of external stimuli (e.g., pH, temperature, light, and redox potential) or substances (e.g., competitive reagents, solvents, and anions) is well known. Conversely, methods for the unfolding (i.e., intrachain disassembly) of SCNPs with stronger covalent interactions are scarce. We show that trimethylsilanol (Me3SiOH) triggers the efficient unfolding of robust "Staudinger" SCNPs with stable azaylide (-N=P-) moieties as intrachain cross-linking units showing exceptional stability toward water, air, and CS2, a standard reagent for azaylides. As a consequence, Me3SiOH arises as a rare, exceptional, and valuable reagent for the cleavage of stable azaylides prepared by the nonhydrolysis Staudinger reaction.

Reversible Change in Performances of Polymer Networks via Invertible Architecture-Transformation of Cross-Links

A polymer nanoparticle network using single-chain nanoparticles (SCNPs) as cross-links is designed. The experimental and theoretical study shows that incorporating SCNPs in polymer networks leads to smaller mesh size, faster terminal relaxation time, and reduced fluctuation among cross-links, resulting in a significant increase in shear storage modulus, and enhancement in tensile stress. Notably, the reversible single-chain collapse of SCNPs under thermal stimulation enables the polymer network to undergo coherent changes between two topological states, thereby exhibiting reversible transformations between soft and stiff states. This approach and finding can effectively tailor the mechanical properties of polymer networks, potentially leading to the development of intelligent, responsive materials.

Protein-Inspired Control over Synthetic Polymer Folding for Structured Functional Nanoparticles in Water

The folding of proteins into functional nanoparticles with defined 3D structures has inspired chemists to create simple synthetic systems mimicking protein properties. The folding of polymers into nanoparticles in water proceeds via different strategies, resulting in the global compaction of the polymer chain. Herein, we review the different methods available to control the conformation of synthetic polymers and collapse/fold them into structured, functional nanoparticles, such as hydrophobic collapse, supramolecular self-assembly, and covalent cross-linking. A comparison is made between the design principles of protein folding to synthetic polymer folding and the formation of structured nanocompartments in water, highlighting similarities and differences in design and function. We also focus on the importance of structure for functional stability and diverse applications in complex media and cellular environments.

Sidechain Metallopolymers with Precisely Controlled Structures: Synthesis and Application in Catalysis

Inspired by the cooperative multi-metallic activation in metalloenzyme catalysis, artificial enzymes as multi-metallic catalysts have been developed for improved kinetics and higher selectivity. Previous models about multi-metallic catalysts, such as cross-linked polymer-supported catalysts, failed to precisely control the number and location of their active sites, leading to low activity and selectivity. In recent years, metallopolymers with metals in the sidechain, also named as sidechain metallopolymers (SMPs), have attracted much attention because of their combination of the catalytic, magnetic, and electronic properties of metals with desirable mechanical and processing properties of polymeric backbones. Living and controlled polymerization techniques provide access to SMPs with precisely controlled structures, for example, controlled degree of polymerization (DP) and molecular weight dispersity (Đ), which may have excellent performance as multi-metallic catalysts in a variety of catalytic reactions. This review will cover the recent advances about SMPs, especially on their synthesis and application in catalysis. These tailor-made SMPs with metallic catalytic centers can precisely control the number and location of their active sites, exhibiting high catalytic efficiency.

Preparation of metallosupramolecular single-chain polymeric nanoparticles and their characterization by Taylor dispersion

Polymers with pendant ligands furnish single-chain polymeric nanoparticles upon intramolecular metal–ligand complex formation with different metal-ions and Taylor dispersion analysis is employed to reliably characterize the dispersed particles.

Downsizing feature of microphase-separated structures via intramolecular crosslinking of block copolymers

A novel strategy for downsizing the feature of microphase-separated structures was developed via the intramolecular crosslinking reaction of block copolymers (BCPs) without changing the molecular weight. A series of BCPs consisting of poly[styrene-st-(p-3-butenyl styrene)] and poly(rac-lactide) (SBS-LA) was subjected to Ru-catalyzed olefin metathesis under highly diluted conditions to produce intramolecularly crosslinked BCPs (SBS(cl)-LAs). Small-angle X-ray scattering measurement and transmission electron microscopy observation of the SBS(cl)-LAs revealed feature size reduction in lamellar (LAM) and hexagonally close-packed cylinder (HEX) structures in the bulk state, which was surely due to the restricted chain dimensions of the intramolecularly crosslinked SBS block. Notably, the degree of size reduction was controllable by varying the crosslink density, with a maximum decrease of 22% in the LAM spacing. In addition, we successfully observed the downsizing of the HEX structure in the thin film state using atomic force microscopy, indicating the applicability of the present methodology to next-generation lithography technology.

Strengths and limitations of size exclusion chromatography for investigating single chain folding – current status and future perspectives

Synthetic approaches for Single-Chain Nanoparticles (SCNPs) developed rapidly during the last decade, opening a multitude of avenues for the design of functional macromolecular chains able to collapse into defined nanoparticles. However, the analytical evaluation of the SCNP formation process still requires critical improvements.

Effect of Intra- versus Intermolecular Cross-Linking on the Supramolecular Folding of a Polymer Chain

Anfinsen's famous experiment showed that the restoration of catalytic activity of a completely unfolded ribonuclease A is only possible when the correct order of events is followed during the refolding process. Inspired by this work, the effect of structural constraints induced by covalent cross-links on the folding of a synthetic polymer chain via hydrogen-bonding interactions is investigated. Hereto, methacrylate-based monomers comprising either benzene-1,3,5-tricarboxamide (BTA)-based or coumarin-based pendants are copolymerized with n-butyl methacrylate in various ratios via reversible addition-fragmentation chain-transfer (RAFT) polymerization. To assess whether the folding and single-chain polymeric nanoparticle (SCPN) formation depend on the order of events, we compare two folding pathways. In the one case, we first covalently cross-link the coumarin pendants within the polymers in a solvent that prevents hydrogen bonding, after which hydrogen bonding is activated, inducing folding of the polymer. In the other case, we induce hydrogen-bonding interactions between tethered BTAs prior to covalent cross-linking of the coumarin pendants. A combination of circular dichroism (CD) spectroscopy, UV-vis spectroscopy, size-exclusion chromatography (SEC), and dynamic light scattering (DLS) is employed to understand the effect of the structural constraints on the folding behavior of these synthetic polymers. The results show that like in ribonuclease A, the order of events matters greatly and determines the outcome. Importantly, a hydrogen-bond-promoting solvent prevents the formation of SCPNs upon covalent cross-linking and results in multichain aggregates. In contrast, covalently cross-linking the polymer when no hydrogen bonds are present followed by inducing hydrogen bonding favors the formation of SCPNs above the UCST of the methacrylate-based polymer. To our surprise, the two systems show a fundamentally different response to changes in temperature, indicating that also in synthetic polymers differences in the folding pathway induce differences in the properties of the resultant nanostructures.

Single-chain polymer nanoparticles in controlled drug delivery and targeted imaging

As a relatively new class of materials, single-chain polymer nanoparticles (SCNPs) just entered the field of (biomedical) applications, with recent advances in polymer science enabling the formation of bio-inspired nanosized architectures. Exclusive intramolecular collapse of individual polymer chains results in individual nanoparticles. With sizes an order of magnitude smaller than conventional polymer nanoparticles, SCNPs are in the size regime of many proteins and viruses (1-20 nm). Multifaceted syntheses and design strategies give access to a wide set of highly modular SCNP materials. This review describes how SCNPs have been rendered water-soluble and highlights ongoing research efforts towards biocompatible SCNPs with tunable properties for controlled drug delivery, targeted imaging and protein mimicry.

Advances in Single-Chain Nanoparticles for Catalysis Applications

Enzymes are the most efficient catalysts known for working in an aqueous environment near room temperature. The folding of individual polymer chains to functional single-chain nanoparticles (SCNPs) offers many opportunities for the development of artificial enzyme-mimic catalysts showing both high catalytic activity and specificity. In this review, we highlight recent results obtained in the use of SCNPs as bioinspired, highly-efficient nanoreactors (3-30 nm) for the synthesis of a variety of nanomaterials (inorganic nanoparticles, quantum dots, carbon nanodots), polymers, and chemical compounds, as well as nanocontainers for CO₂ capture and release.

Single Chain Polymeric Nanoparticles to Promote Selective Hydroxylation Reactions of Phenol Catalyzed by Copper

Metal-containing single chain polymeric nanoparticles (SCPNs) can be used as synthetic mimics of metalloenzymes. Currently, the role of the folded polymer backbones on the activity and selectivity of metal sites is not clear. Herein, we report our findings on how polymeric frameworks modulate the coordination of Cu sites and the catalytic activity/selectivity of Cu-containing SCPNs mimicking monophenol hydroxylation reactions. Imidazole-functionalized copolymers of poly(methyl methacrylate-co-3-imidazolyl-2-hydroxy propyl methacrylate) were used for intramolecular Cu-imidazole binding that triggered the self-folding of polymers. Polymer chains imposed steric hindrance which yielded unsaturated Cu sites with an average coordination number of 3.3. Cu-containing SCPNs showed a high selectivity for the hydroxylation reaction of phenol to catechol, >80%, with a turnover frequency of >870 h^-1 at 60 °C. The selectivity was largely influenced by the flexibility of the folded polymer backbone where a more flexible polymer backbone allows the cooperative catalysis of two Cu sites. The second coordination sphere provided by the folded polymer that has been less studied is therefore critical in the design of active mimics of metalloenzymes.

Exploring structural effects in single-chain “folding” mediated by intramolecular thermal Diels–Alder chemistry

We describe a method to fold single polymer chains into nanoparticles using simple thermal Diels–Alder (DA) chemistry.

Hybrid single-chain nanoparticles via the metal induced crosslinking of N-donor functionalized polymer chains

A set of single-chain nanoparticles was prepared via the intramolecular crosslinking of functionalized copolymers with various metal salts.

Compartmentalization of an ABC triblock copolymer single-chain nanoparticle via coordination-driven orthogonal self-assembly

We present coordination-driven intramolecular orthogonal self-assembly of ABC triblock copolymer into protein-like compartmentalized SCNP, whose sub-10 nm ultrafine subdomains are discrete and can respond to aqueous surroundings individually.

Characterizing single chain nanoparticles (SCNPs): a critical survey

We provide the results of a critical literature survey on the reported sizes of single chain polymer nanoparticles (SCNPs) employing different techniques.

Voltage-responsive single-chain polymer nanoparticles via host–guest interaction

Poly(N-(2-hydroxyethyl)acrylamide) with pendant β-cyclodextrin is synthesized and intramolecularly crosslinked with bridged bis(ferrocene). This supramolecular nanoparticle can be changed reversibly between a coil and a nanoparticle upon external voltage stimuli.