FACILE PREPARATION OF NANOPARTICLES BY INTRAMOLECULAR CROSSLINKING OF ISOCYANATE FUNCTIONALIZED COPOLYMERS

Visible light photoflow synthesis of a Cu(ii) single-chain polymer nanoparticle catalyst

We herein pioneer the visible light (λ max = 410 nm) mediated flow synthesis of catalytically active single-chain nanoparticles (SCNPs). Our design approach is based on a copolymer of poly(ethylene glycol) methyl ether methacrylate and a photocleavable 2-((((2-nitrobenzyl)oxy)carbonyl)amino)ethyl methacrylate monomer which can liberate amine groups upon visible light irradiation, allowing for single-chain collapse via the complexation of Cu(ii) ions. We initially demonstrate the successful applicability of our design approach for the batch photochemical synthesis of Cu(ii) SCNPs and transfer the concept to photoflow conditions, enabling, for the first time, the continuous production of functional SCNPs. Critically, we explore their ability to function as a photocatalyst for the cleavage of carbon-carbon single and double bonds on the examples of xanthene-9-carboxylic acid and oleic acid, demonstrating the advantageous effect SCNPs can provide over analogous small molecule catalysts.

Single Chain Nanoparticles in Catalysis

Over the last six decades folded polymer chains-so-called Single Chain Nanoparticles (SCNPs)-have evolved from the mere concept of intramolecularly crosslinked polymer chains to tailored nanoreactors, underpinned by a plethora of techniques and chemistries to tailor and analyze their morphology and function. These monomolecular polymer entities hold critical promise in a wide range of applications. Herein, we highlight the exciting progress that has been made in the field of catalytically active SCNPs in recent years.

Single-chain polymer nanoparticles in biomedical applications

Single-chain polymer nanoparticles (SCNPs) are a well-defined and uniquely sized class of polymer nanoparticles. The advances in polymer science over the past decades have enabled the development of a variety of intramolecular crosslinking systems, leading to particles in the 5-20 nm size regime. Which is aligned with the size regime of proteins and therefore making SCNPs an interesting class of NPs for biomedical applications. The high modularity of SCNP design and the ease of their functionalization have led to growing research interest. In this review, we describe different crosslinking systems, as well as the preparation of functional SCNPs and the variety of biomedical applications that have been explored.

Tailored Biocompatible Polyurethane-Poly(ethylene glycol) Hydrogels as a Versatile Nonfouling Biomaterial

Polyurethane-based hydrogels are relatively inexpensive and mechanically robust biomaterials with ideal properties for various applications, including drug delivery, prosthetics, implant coatings, soft robotics, and tissue engineering. In this report, a simple method is presented for synthesizing and casting biocompatible polyurethane-poly(ethylene glycol) (PU-PEG) hydrogels with tunable mechanical properties, nonfouling characteristics, and sustained tolerability as an implantable material or coating. The hydrogels are synthesized via a simple one-pot method using commercially available precursors and low toxicity solvents and reagents, yielding a consistent and biocompatible gel platform primed for long-term biomaterial applications. The mechanical and physical properties of the gels are easily controlled by varying the curing concentration, producing networks with complex shear moduli of 0.82-190 kPa, similar to a range of human soft tissues. When evaluated against a mechanically matched poly(dimethylsiloxane) (PDMS) formulation, the PU-PEG hydrogels demonstrated favorable nonfouling characteristics, including comparable adsorption of plasma proteins (albumin and fibrinogen) and significantly reduced cellular adhesion. Moreover, preliminary murine implant studies reveal a mild foreign body response after 41 days. Due to the tunable mechanical properties, excellent biocompatibility, and sustained in vivo tolerability of these hydrogels, it is proposed that this method offers a simplified platform for fabricating soft PU-based biomaterials for a variety of applications.

Controlling the Processability and Stability of Supramolecular Polymers Using the Interplay of Intra- and Intermolecular Interactions

Polymer networks crosslinked via non-covalent interactions afford interesting materials for a wide range of applications due to their self-healing capability, recyclability, and tunable material properties. However, when strong non-covalent binding motifs in combination with high crosslink density are used, processing of the materials becomes troublesome because of high viscosities and the formation of insoluble gels. Here, we present an approach to control the processability of grafted polymers containing strong non-covalent interactions by balancing the interplay of intra- and intermolecular hydrogen bonding. A library of copolymers with different degrees of polymerization and content of protected ureido-pyrimidinone-urea (UPy-urea) grafts was prepared. Photo-deprotection in a good solvent like tetrahydrofuran (THF) at low concentrations (≤1 mg mL-1) created intramolecularly assembled nanoparticles. Remarkably, the intrinsic viscosity of these nanoparticle solutions was an order of magnitude lower compared to solutions of the intermolecularly assembled analogues, highlighting the crucial role of intra- versus intermolecular interactions. Due to the strong hydrogen bonds between UPy dimers, the intramolecularly assembled structures were kinetically trapped. As a result, the polymer nanoparticles were readily processed into a bulk material, without causing major changes in the morphology as verified by atomic force microscopy. Subsequent intermolecular crosslinking of the nanoparticle film, by heating to temperatures where the hydrogen-bond exchange becomes fast, resulted in a crosslinked network. The reversibility of the hereby obtained polymer networks was shown by retrieving the intramolecularly assembled nanoparticles via redissolution and sonication of the intermolecularly crosslinked film in THF with a small amount of acid. Our results highlight that the stability and processability of highly supramolecularly crosslinked polymers can be controlled both in solution and in bulk by using the interplay of intra- and intermolecular non-covalent interactions in grafted polymers.

Reprocessable and Recyclable Chain-Growth Polymer Networks Based on Dynamic Hindered Urea Bonds

Conventional cross-linked polymers cannot be reprocessed because of the presence of permanent covalent cross-links, preventing reuse and recycling. Covalent adaptable networks (CANs) employ dynamic covalent bonds that undergo dynamic reactions under external stimulus, allowing recyclability of these network materials. Hindered urea chemistry is one of the recently discovered dissociative dynamic chemistries. While hindered urea bonds have traditionally been exploited in the synthesis of step-growth type CANs, the use of hindered urea bonds in the synthesis of chain-growth-type dynamic networks has only been narrowly explored. Here, we present a simple, catalyst-free, fast method to synthesize a hindered-urea-based dynamic cross-linker that can undergo a free radical polymerization with vinyl-type monomers or polymers to form reprocessable CANs. Using this cross-linker, we developed dynamic polymethacrylate networks that can be (re)processed at 80 °C. These dynamic covalent networks exhibit full recovery of cross-link density after multiple recycling steps; they are only the second chain-growth network synthesized directly and exclusively from carbon-carbon double bond monomers to demonstrate such recovery. Unlike other dissociative dynamic polymer networks, polymethacrylate networks that contain dissociative dynamic hindered urea bonds do not flow and maintain their network structure even at high temperature (300 °C). Despite its relatively fast reprocessability, the network showed delayed and extremely slow stress relaxation at the processing temperature. This work offers a simple approach to obtain reprocessable addition-type networks based on hindered urea bonds while revealing the limitations of stress relaxation experiments in relationship to the processability of some dynamic polymer networks.

Rapid Synthesis of Polyester Based Single-Chain Polymeric Nanoparticles via Intra-Molecular Aza Michael Addition Reaction

An alternative strategy was developed to prepare intramolecularly crosslinked soft nanoparticles using aza Michael addition reaction on a linear polyester (PCA) containing in-chain reactive triple bonds. The linear precursor was...

Advances in the Multi-Orthogonal Folding of Single Polymer Chains into Single-Chain Nanoparticles

The folding of certain proteins (e.g., enzymes) into perfectly defined 3D conformations via multi-orthogonal interactions is critical to their function. Concerning synthetic polymers chains, the "folding" of individual polymer chains at high dilution via intra-chain interactions leads to so-called single-chain nanoparticles (SCNPs). This review article describes the advances carried out in recent years in the folding of single polymer chains into discrete SCNPs via multi-orthogonal interactions using different reactive chemical species where intra-chain bonding only occurs between groups of the same species. First, we summarize results from computer simulations of multi-orthogonally folded SCNPs. Next, we comprehensively review multi-orthogonally folded SCNPs synthesized via either non-covalent bonds or covalent interactions. Finally, we conclude by summarizing recent research about multi-orthogonally folded SCNPs prepared through both reversible (dynamic) and permanent bonds.

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.

Photocycloreversions within single polymer chains

Reversible photocycloadditions hold great potential to control formation and fissure of bonds. However, the wavelength selective addressability of cycloaddition and -reversion was found to be lost in the confined environment of single polymer chains.

Pentafluorophenyl-based single-chain polymer nanoparticles as a versatile platform towards protein mimicry

Pentafluorophenyl-single chain polymer nanoparticles are readily conjugated with functional amines enabling facile SCNP modification, adjustment of physicochemical properties, and even protein mimicry.

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.

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.

What happens in the dark? Assessing the temporal control of photo-mediated controlled radical polymerizations

A signature of photo-mediated controlled polymerizations is the ability to modulate the rate of polymerization by turning the light source 'on' and 'off.' However, in many reported systems, growth can be reproducibly observed during dark periods. In this study, emerging photo-mediated controlled radical polymerizations are evaluated with in situ 1H NMR monitoring to assess their behavior in the dark. Interestingly, it is observed that Cu-mediated systems undergo long-lived, linear growth during dark periods in organic media.

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.

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.

Controlled synthesis of diverse single-chain polymeric nanoparticles using polymers bearing furan-protected maleimide moieties

This study is devoted to the control fabrication of SCNPs from the same precursor and exploring the surface properties of SCNP-made films.

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.

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.

Synthesis and functionalization of dextran-based single-chain nanoparticles in aqueous media

Water-dispersible dextran-based single-chain polymer nanoparticles (SCPNs) were prepared in aqueous media and under mild conditions.

Poly(ethylene glycol)-graft-poly(vinyl acetate) single-chain nanoparticles for the encapsulation of small molecules

Amphiphilic poly(ethylene glycol)-graft-poly(vinyl acetate) copolymers with a low degree of grafting undergo self-folding in water driven by hydrophobic interactions, resulting in single-chain nanoparticles (SCNPs) possessing a hydrodynamic radius of about 10 nm.

Photoresponsive liquid crystalline polymer single-chain nanoparticles

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

A New Methodology for Assessing Macromolecular Click Reactions and Its Application to Amine--Tertiary Isocyanate Coupling for Polymer Ligation

Click reactions have provided access to an array of remarkably complex polymer architectures. However, the term "click" is often applied inaccurately to polymer ligation reactions that fail to respect the criteria that typify a true "click" reaction. With the purpose of providing a universal way to benchmark polymer-polymer coupling efficiency at equimolarity and thus evaluate the fulfilment of click criteria, we report a simple one-pot methodology involving the homodicoupling of α-end-functionalized polymers using a small-molecule bifunctional linker. A combination of SEC analysis and chromatogram deconvolution enables straightforward quantification of the coupling efficiency. We subsequently employ this methodology to evaluate an overlooked candidate for the click reaction family: the addition of primary amines to α-tertiary isocyanates (α-(t)NCO). Using our bifunctional linker coupling strategy, we show that the amine-(t)NCO reaction fulfills the criteria for a polymer-polymer click reaction, achieving rapid, chemoselective, and quantitative coupling at room temperature without generating any byproducts. We demonstrate that amine-(t)NCO coupling is faster and more efficient than the more common amine-tertiary active ester coupling under equivalent conditions. Additionally, we show that the α-(t)NCO end group is unprecedentedly stable in aqueous media. Thus, we propose that the amine-(t)NCO ligation is a powerful new click reaction for efficient macromolecular coupling.

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.

Stimuli-responsive single-chain polymeric nanoparticles towards the development of efficient drug delivery systems

Novel dynamic single-chain polymeric nanoparticles not only significantly improve drug transport efficiency in vitro but can also reside stably and facilitate precisely triggered drug-release in tumor-like microenvironments.

Supramolecular Assembly Mediates the Formation of Single-Chain Polymeric Nanoparticles

A breakthrough innovation in water-based polymeric nanoparticles has enabled significant progress in mimicking the folding of natural proteins by generating novel "single-chain polymeric nanoparticles" (SCPNs) via supramolecular interactions. In this study, a practical approach to the living polymerization of functionalized oligo(ethylene glycol) methacrylate monomers allows the incorporation of self-constituted multiple hydrogen-bonded groups into physically cross-linked polymer networks, which enables the formation of highly functionalized SCPNs in an aqueous environment. The newly developed materials are particularly attractive from a practical point of view since they have a very low critical micellization concentration and uniform particle diameters of ca. 25 nm, making them extremely stable under dilute conditions. Concentration-dependent experiments showed that SCPNs formed at polymer concentrations up to 40 mg/mL with no significant change in morphology observed. Moreover, the formed SCPNs had a very high stability in an aqueous solution containing surfactant, suggesting potential for a wide variety of applications as a promising candidate nanocarrier for bioimaging, controlled release, and drug delivery systems.