A Mild and Efficient Approach to Functional Single-Chain Polymeric Nanoparticles via Photoinduced Diels–Alder Ligation

En Route to Stabilized Compact Conformations of Single-Chain Polymeric Nanoparticles in Complex Media

Precise control over the folding pathways of polypeptides using a combination of noncovalent and covalent interactions has evolved into a wide range of functional proteins with a perfectly defined 3D conformation. Inspired hereby, we develop a series of amphiphilic copolymers designed to form compact, stable, and structured single-chain polymeric nanoparticles (SCPNs) of defined size, even in competitive conditions. The SCPNs are formed through a combination of noncovalent interactions (hydrophobic and hydrogen-bonding interactions) and covalent intramolecular cross-linking using a light-induced [2 + 2] cycloaddition. By comparing different self-assembly pathways of the nanoparticles, we show that, like for proteins in nature, the order of events matters. When covalent cross-links are formed prior to the folding via hydrophobic and supramolecular interactions, larger particles with less structured interiors are formed. In contrast, when the copolymers first fold via hydrophobic and hydrogen-bonding interactions into compact conformations, followed by covalent cross-links, good control over the size of the SCPNs and microstructure of the hydrophobic interior is achieved. Such a structured SCPN can stabilize the solvatochromic dye benzene-1,3,5-tricarboxamide-Nile Red via molecular recognition for short periods of time in complex media, while showing slow exchange dynamics with the surrounding complex media at longer time scales. The SCPNs show good biocompatibility with cells and can carry cargo into the lysosomal compartments of the cells. Our study highlights the importance of control over the folding pathway in the design of stable SCPNs, which is an important step forward in their application as noncovalent drug or catalyst carriers in biological settings.

End-functionalized polymers by controlled/living radical polymerizations: synthesis and applications

This review focuses on end-functionalized polymers synthesized by controlled/living radical polymerizations and the applications in fields including bioconjugate formation, surface modification, topology construction, and self-assembly.

Synthesis and properties of colorless copolyimides derived from 4,4′-diaminodiphenyl ether-based diamines with different substituents

Highly transparent PI films with excellent mechanical strength, high heat-resistance and superior fracture toughness were fabricated.

Delineating synchronized control of dynamic covalent and non-covalent interactions for polymer chain collapse towards cargo localization and delivery

Synergistic control of photo-responsive dynamic covalent and non-covalent interaction over the chain collapse of single chain thermo-responsive polymers towards cargo localization and augmented release.

Photoresponsive chain collapse in a flexo-rigid functional copolymer to modulate the self-healing behaviour

Synthetic systems mimicking the natural self-folding process are attractive to impart multiple structural control over polymer crosslinking and the subsequent alteration of their macroscopic self-healing properties. In that regard, polymers P1-P5 containing pendant photo-crosslinkable moieties were designed and underwent intra- or interchain collapse to form diverse nanostructures. The shape and dimension of the nanostructures could be efficiently controlled by the concentration, solvent compatibility and characteristics of the polymers. Photodimerization of the coumarin moieties transformed the extended coiled chain of the polymer to uniform sized nanoparticles in a dilute condition, while in the crowded macromolecular concentration regime, the polymer folded into nanostructures with polydisperse topologies that were far from a condensed globule or partially swollen globule conformation. Scaling law exponents for polymer chain compaction suggested an interchain collapse with rigid compact segments connected by flexible polymer chains that draws an analogy with elastomers. Such a hardening of the rigid segment as a consequence of photodimerization rendered a significant increase in the glass transition temperature (Tg), which could be reversibly controlled upon decrosslinking. Lastly, the structural variation of this class of polymers over self-healing was explored and the crosslinked polymers showed phototriggered non-autonomic and intrinsic self-healing behaviour under ambient conditions. This is an interesting approach to access a photomodulated self-healing system with low Tg polymers that shows the coexistence of autonomic and nonautonomic self-healing pathways and that may find application in designing smart coatings for photovoltaic devices.

Making the best of it: nitroxide-mediated polymerization of methacrylates via the copolymerization approach with functional styrenics

The addition of 5 mol% of functional styrenics imparts control to the SG1-mediated polymerization of methacrylates and provides access to nanostructured functional methacrylic materials.

Pushing the limits of single chain compaction analysis by observing specific size reductions via high resolution mass spectrometry

Herein, we push the limits of single chain nanoparticle analysis to directly observe the specific compaction of defined single chains dependent on the number of compaction steps.

Single-chain crosslinked polymers via the transesterification of folded polymers: from efficient synthesis to crystallinity control

Herein we report efficient synthetic systems of single-chain crosslinked polymers via the intramolecular transesterification of folded random copolymers in organic media and the unique crystallization behavior of their crosslinked polymers.

Advances in the Phototriggered Synthesis of Single-Chain Polymer Nanoparticles

Clean use of photons from light to activate chemical reactions offers many possibilities in different fields, from chemistry and biology to materials science and medicine. This review article describes the advances carried out in last decades toward the phototriggered synthesis of single-chain polymer nanoparticles (SCNPs) as soft nanomaterials with promising applications in enzyme-mimicking catalysis and nanomedicine, among other different uses. First, we summarize some different strategies developed to synthesize SCNPs based on photoactivated intrachain homocoupling, phototriggered intrachain heterocoupling and photogenerated collapse induced by an external cross-linker. Next, we comprehensively review the emergent topic of photoactivated multifolding applied to SCNP construction. Finally, we conclude by summarizing recent strategies towards phototriggered disassembly of SCNPs.

Supramolecular Single-Chain Polymeric Nanoparticles

Nature has unparalleled control over the conformation and dynamics of its folded macromolecular structures. Nature’s ability to arrange amino acids into a precise spatial organization by way of folding allows proteins to fulfill specific functions in an extremely efficient manner. Chemists and materials scientists have used the delicate structure–function relationships observed in proteins to elucidate nature’s design principles. These insights have led to the development of various revolutionary macromolecular architectures, mimicking the structural features of proteins. In this review, we focus on the folding of single polymer chains into well-defined nanoparticles using supramolecular interactions and their possible use as enzyme mimics.

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.

Controllable Self-Assembly of Amphiphilic Tadpole-Shaped Polymer Single-Chain Nanoparticles Prepared through Intrachain Photo-cross-linking

We report the use of intramolecular cross-linking chemistry as a tool to control the self-assembly of amphiphilic diblock copolymers (di-BCPs). Two amphiphilic di-BCPs of poly( N, N'-dimethylacrylamide)- block-polystyrene (PDMA- b-PS) with photo-cross-linkable cinnamoyl groups in either hydrophobic or hydrophilic blocks were prepared using reversible addition-fragmentation chain transfer polymerization. Intramolecular photo-cross-linking of cinnamoyl groups led to the formation of tadpole-shaped polymer single-chain nanoparticles (SCNPs) consisting of a self-collapsed block as the "head" and an un-cross-linked block as the "tail". When intramolecular photo-cross-linking was carried out in hydrophobic PS blocks, a clear morphological transition from branched cylindrical micelles (for the linear di-BCP) to completely spherical micelles at a dimerization degree of ∼63% was observed. A pattern of morphological transitions from cylindrical micelles to spherical micelles is observed through stepwise downsizing the length of cylindrical micelles when increasing the self-collapse degree of PS blocks, whereas, in case of photo-cross-linking carried out in hydrophilic PDMA blocks, the size of micelles showed a dramatic increase due to the shift of hydrophobic-to-hydrophilic balance. When the cross-linking degree of PDMA blocks reached >60%, tadpole-shaped SCNPs assembled into nonconventional aggregates with a nonsmooth surface. Our results illustrate the impact of chain topologies on the self-assembly outcomes of amphiphilic di-BCPs, which likely opens a door to control the micellar morphologies from just one parent linear di-BCP, rather than resynthesizing BPCs with different volume fractions of the two blocks.

Valuable structure-size relationships for tadpole-shaped single-chain nanoparticles with long and short flexible tails unveiled

Tadpole-shaped single-chain nanoparticles (TSCNPs) are useful soft building blocks for nanotechnology composed of a flexible polymer chain tethered to an intramolecularly folded single-chain nanoparticle.

A novel thermo-responsive multiblock architecture composed of a sequential peptide and an amino acid-derived vinyl polymer: toward protein-mimicking single-chain folding

A novel multiblock hybrid copolymer composed of a β-sheet peptide and a glycine-derived vinyl polymer was developed to achieve single-chain folding into well-defined nanoparticles.

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.

Synthesis and folding behaviour of poly(p-phenylene vinylene)-based β-sheet polychromophores

This contribution describes the design and synthesis of β-sheet-mimicking synthetic polymers comprising distinct poly(p-phenylene vinylene) (PPV) and poly(norbornene) (PNB) backbones with multiple turns.

This contribution describes the design and synthesis of β-sheet-mimicking synthetic polymers comprising distinct poly(p-phenylene vinylene) (PPV) and poly(norbornene) (PNB) backbones with multiple turns. The rod–coil–coil–rod tetrablock copolymers, synthesized using ring-opening metathesis polymerization (ROMP) and featuring orthogonal face-to-face π–π stacking and phenyl/perfluorophenyl interactions, show persistent folding both in bulk and at the single molecule level, irrespective of the number of β-turns. Single molecule polarization studies reveal that the copolymers are more anisotropic than the corresponding homopolymers. Examination of the spectral signatures of the single molecules shows a dominant emissive chromophore in the linked materials compared to the homopolymer. The lack of significant spectral changes of the folded materials along with the existence of a dominant emission spectrum supports the proposed structure of well-aligned, minimally-interacting chromophores. Utilization of this reliably folding, phenyl/perfluorophenyl functionality could provide an extremely useful tool in future functional materials design.

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.

The Effect of Intramolecular Cross-Linking on Polymer Interactions in Solution

The conformation of a polymer in a solvent is typically defined by the solvent quality, which is a consequence of the solvent and macromolecule's chemistry. Yet, additional factors can affect the polymer conformation, such as non-covalent interactions to surfaces or other macromolecules, affecting the amount of polymer-solvent interactions. Herein, chemically folded polymers with protein-like architectures are studied and compared to their unfolded linear precursor in good solvents using rheology measurements. The current research reveals that permanent folding by intramolecular chemical cross-linking limits the chain mobility and therefore causes a reduction in polymer-solvent interactions, making a good solvent become theta. This change not only affects the "solvent quality" but also leads to a change in particle-particle interactions as a function of concentration. These findings provide crucial insight into the effects of intramolecular cross-links on macromolecule solubility and self-assembly, which are critical for mimicking structurally similar biological materials.

Self-reporting and refoldable profluorescent single-chain nanoparticles

We pioneer the formation of self-reporting and refoldable profluorescent single-chain nanoparticles (SCNPs) via the light-induced reaction (λ_max = 320 nm) of nitroxide radicals with a photo-active crosslinker.

We pioneer the formation of self-reporting and refoldable profluorescent single-chain nanoparticles (SCNPs) via the light-induced reaction (λ_max = 320 nm) of nitroxide radicals with a photo-active crosslinker. Whereas the tethered nitroxide moiety in these polymers fully quenches the luminescence (i.e. fluorescence) of the aromatic backbone, nitroxide trapping of a transient C-radical leads to the corresponding closed shell alkoxyamine thereby restoring luminescence of the folded SCNP. Hence, the polymer in the folded state is capable of emitting light, while in the non-folded state the luminescence is silenced. Under oxidative conditions the initially folded SCNPs unfold, resulting in luminescence switch-off and the reestablishment of the initial precursor polymer. Critically, we show that the luminescence can be repeatedly silenced and reactivated. Importantly, the self-reporting character of the SCNPs was followed by size-exclusion chromatography (SEC), dynamic light scattering (DLS), fluorescence, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR) and diffusion ordered NMR spectroscopy (DOSY).

Folding polymer chains with visible light

A simple and versatile tool for generating fluorescent single chain polymer nanoparticles with visible light.

Fluorescent Glyco Single-Chain Nanoparticle-Decorated Nanodiamonds

We introduce the light-induced collapse of single glycopolymer chains in water generating fluorescent glyco single-chain nanoparticles (SCNPs) and their subsequent functionalization onto nanodiamonds. The glycopolymer precursors are prepared by polymerizing an acetylated mannose-based methacrylate monomer followed by a deprotection and postpolymerization functionalization step, introducing profluorescent photoactive tetrazole groups and furan-protected maleimide moieties. Subsequent UV irradiation in highly diluted aqueous solution triggers intramolecular tetrazole-mediated cycloadditions, yielding glyco SCNPs featuring fluorescence as well as lectin binding properties. The obtained SCNPs are coated onto nanodiamonds by adsorption, and the obtained hybrid nanoparticles are in depth characterized in terms of size, functionality, and bioactivity. Different coating densities are achieved by altering the SCNP concentration. The prepared nanoparticles are nontoxic in mouse RAW 264.7 macrophages. Furthermore, the fluorescence of the SCNPs can be exploited to image the SCNP-coated nanodiamonds in macrophage cells via confocal fluorescence microscopy.

Rational Design of Single-Chain Polymeric Nanoparticles That Kill Planktonic and Biofilm Bacteria

Infections caused by multidrug-resistant bacteria are on the rise and, therefore, new antimicrobial agents are required to prevent the onset of a postantibiotic era. In this study, we develop new antimicrobial compounds in the form of single-chain polymeric nanoparticles (SCPNs) that exhibit excellent antimicrobial activity against Gram-negative bacteria (e.g., Pseudomonas aeruginosa) at micromolar concentrations (e.g., 1.4 μM) and remarkably kill ≥99.99% of both planktonic cells and biofilm within an hour. Linear random copolymers, which comprise oligoethylene glycol (OEG), hydrophobic, and amine groups, undergo self-folding in aqueous systems due to intramolecular hydrophobic interactions to yield these SCPNs. By systematically varying the hydrophobicity of the polymer, we can tune the extent of cell membrane wall disruption, which in turn governs the antimicrobial activity and rate of resistance acquisition in bacteria. We also show that the incorporation of OEG groups into the polymer design is essential in preventing complexation with proteins in biological medium, thereby maintaining the antimicrobial efficacy of the compound even in in vivo mimicking conditions. In comparison to the last-resort antibiotic colistin, our lead agents have a higher therapeutic index (by ca. 2-3 times) and hence better biocompatibility. We believe that the SCPNs developed here have potential for clinical applications and the information pertaining to their structure-activity relationship will be valuable toward the general design of synthetic antimicrobial (macro)molecules.

Photochemically Induced Folding of Single Chain Polymer Nanoparticles in Water

We pioneer the synthesis of fluorescent single chain nanoparticles (SCNPs) via UV-light induced folding based on tetrazole chemistry directly in pure water. Water-soluble photoreactive precursor polymers based on poly(acrylic acid) (PAA) bearing tetrazole, alkene and tetraethylene glycol monomethyl ether moieties, (PAA_n(Tet/p-Mal/TEG)), or simply tetrazoles moieties, PAA_n(Tet), were generated via RAFT polymerization. While tetrazole, ene, and acrylic acid containing polymers fold via dual nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) as well as nitrile imine-carboxylic acid ligation (NICAL), tetrazole and acrylic acid only functional prepolymers fold exclusively via NICAL. A detailed study of the underpinning photochemistry of NITEC and NICAL is also included. The resulting water-soluble SCNPs were carefully characterized via analytical techniques such as NMR, UV-vis, and fluorescence spectroscopy, as well as SEC and DLS.

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.

Direct light-induced (co-)grafting of photoactive polymers to graphitic nanodiamonds

We report the light-driven grafting and controlled simultaneous co-grafting of various functional polymers to graphitic nanodiamonds (grNDs).

Advantages and limitations of diisocyanates in intramolecular collapse

A comprehensive examination of the synthesis of single chain polymer nanoparticles (SCPNs) from a copolymer of methyl acrylate (MA) and 2-hydroxyethyl acrylate (HEA) via the intra-chain urethane formation by using hexamethylene diisocyanate (HDI) as a cross-linker is described.

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.

Self-assembly and disassembly of stimuli responsive tadpole-like single chain nanoparticles using a switchable hydrophilic/hydrophobic boronic acid cross-linker

pH/sugar responsive behaviour of tadpole-like single chain nanoparticles based on a switchable hydrophilic/hydrophobic boronic acid cross-linker is described.

High resolution mass spectrometric access to nitroxide containing polymers

We introduce a mass spectrometric access route to nitroxide containing polymers via high resolution electrospray ionization mass spectrometry (HR ESI MS), a polymer class that is – due to the presence of unpaired spins – highly challenging to analyze via NMR techniques.

Degradable fluorescent single-chain nanoparticles based on metathesis polymers

We introduce the facile synthesis of fluorescent single-chain nanoparticles (SCNPs) based on chain-shattering acyclic diene metathesis (ADMET) polymers featuring self-immolative azobenzene motifs.

Photoresponsive liquid crystalline polymer single-chain nanoparticles

Single-chain nanoparticles prepared from a side-chain liquid crystalline polymer bearing azobenzene moieties can be multifunctional.

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.