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.

"Bitter-Sweet" Polymeric Micelles Formed by Block Copolymers from Glucosamine and Cholic Acid

Natural compounds glucosamine and cholic acid have been used to make acrylic monomers which are subsequently used to prepare amphiphilic block copolymers by reversible addition-fragmentation chain transfer (RAFT) polymerization. Despite the striking difference in polarity and solubility, three diblock copolymers consisting of glucosamine and cholic acid pendants with different hydrophilic and hydrophobic chain lengths have been synthesized without the use of protecting groups. They are shown to self-assemble into polymeric micelles with a "bitter" bile acid core and "sweet" sugar shell in aqueous solutions, as evidenced by dynamic light scattering and transmission electron microscopy. The critical micelle concentration varies with the hydrophobic/hydrophilic ratio, ranging from 0.62 to 1.31 mg/L. Longer chains of polymers induced the formation of larger micelles in range of 50-70 nm. These micelles can solubilize hydrophobic compounds such as Nile Red in aqueous solutions. Their loading capacity mainly depends upon the hydrophobic/hydrophilic ratio of the polymers, and may be also related to the length of the hydrophilic block. These polymeric micelles allowed for a 10-fold increase in the aqueous solubility of paclitaxel and showed no cytotoxicity below the concentration of 500 mg/L. Such properties make these polymeric micelles interesting reservoirs for hydrophobic molecules and drugs for biomedical applications.

Catalyst-Free Photoinduced End-Group Removal of Thiocarbonylthio Functionality

An initiator- and catalyst-free method for polymer end-group modification has been designed. Under long-wave ultraviolet irradiation, polymers with thiocarbonylthio end groups undergo photolytic cleavage to reveal an active macroradical capable of irreversible termination with a suitable hydrogen source. This straightforward method was successfully demonstrated by the removal of a range of end groups that commonly result from reversible addition-fragmentation chain transfer or photoiniferter polymerizations, including trithiocarbonate, dithiobenzoate, xanthate, and dithiocarbamate mediating agents. This strategy proved efficient for polymers derived from acrylamido, acrylic, methacrylic, styrenic, and vinylpyrrolidone monomers.

Dual-pathway chain-end modification of RAFT polymers using visible light and metal-free conditions

We report a metal-free strategy for the chain-end modification of RAFT polymers utilizing visible light. By turning the light source on or off, the reaction pathway in one pot can be switched between either complete desulfurization (hydrogen chain-end) or simple cleavage (thiol chain-end), respectively. The versatility of this process is exemplified by application to a wide range of polymer backbones under mild, quantitative conditions using commercial reagents.

H2O2 Enables Convenient Removal of RAFT End-Groups from Block Copolymer Nano-Objects Prepared via Polymerization-Induced Self-Assembly in Water

RAFT-synthesized polymers are typically colored and malodorous due to the presence of the sulfur-based RAFT end-group(s). In principle, RAFT end-groups can be removed by treating molecularly dissolved copolymer chains with excess free radical initiators, amines, or oxidants. Herein we report a convenient method for the removal of RAFT end-groups from aqueous dispersions of diblock copolymer nano-objects using H2O2. This oxidant is relatively cheap, has minimal impact on the copolymer morphology, and produces benign side products that can be readily removed via dialysis. We investigate the efficiency of end-group removal for various diblock copolymer nano-objects prepared with either dithiobenzoate- or trithiocarbonate-based RAFT chain transfer agents. The advantage of using UV GPC rather than UV spectroscopy is demonstrated for assessing both the kinetics and extent of end-group removal.

Trehalose Glycopolymer Enhances Both Solution Stability and Pharmacokinetics of a Therapeutic Protein

Biocompatible polymers such as poly(ethylene glycol) (PEG) have been successfully conjugated to therapeutic proteins to enhance their pharmacokinetics. However, many of these polymers, including PEG, only improve the in vivo lifetimes and do not protect proteins against inactivation during storage and transportation. Herein, we report a polymer with trehalose side chains (PolyProtek) that is capable of improving both the external stability and the in vivo plasma half-life of a therapeutic protein. Insulin was employed as a model biologic, and high performance liquid chromatography and dynamic light scattering confirmed that addition of trehalose glycopolymer as an excipient or covalent conjugation prevented thermal or agitation-induced aggregation of insulin. The insulin-trehalose glycopolymer conjugate also showed significantly prolonged plasma circulation time in mice, similar to the analogous insulin-PEG conjugate. The insulin-trehalose glycopolymer conjugate was active as tested by insulin tolerance tests in mice and retained bioactivity even after exposure to high temperatures. The trehalose glycopolymer was shown to be nontoxic to mice up to at least 1.6 mg/kg dosage. These results together suggest that the trehalose glycopolymer should be further explored as an alternative to PEG for long circulating protein therapeutics.

End group modification of poly(acrylates) obtained via ATRP: a user guide

The versatile and high yielding end-functionalization with a varienty of functional groups is presented for poly(acrylates) obtained by ATRP.

Polymers with acyl-protected perthiol chain termini as convenient building blocks for doubly responsive H2S-donating nanoparticles

H₂S-releasing polymers with an acyl-protected perthiol chain terminus were prepared using a simple, high yielding end-group modification process.

Diels–Alder cycloaddition and RAFT chain end functionality: an elegant route to fullerene end-capped polymers with control over molecular mass and architecture

Fullerene C60 end-capped polymers are synthesised using RAFT chain end functionality and Diels–Alder reaction with excellent yield and composition control.

Amphiphilic acrylamide block copolymer: RAFT block copolymerization and monolayer behaviour

Amphiphilic acrylamide block copolymer, synthesized by RAFT polymerization, takes a stable monolayer formation with phase-separated structures at the air–water interface.

RAFT polymerization to form stimuli-responsive polymers

Stimuli-responsive polymers respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals. This paper is concerned with the process of forming such polymers by RAFT polymerization.

Novel alkoxyamines for the successful controlled polymerization of styrene and methacrylates

The design of alkoxyamine/nitroxide species capable of mediating the polymerization of both styrene and methacrylic monomers has been notoriously difficult.

Light-responsive polymer brushes: active topographic cues for cell culture applications

Azopolymer brushes were patterned using interference lithography and erased using ultrasonication. This substrate was able to induce cell orientation.

An amphiphilic poly(vinylidene fluoride)-b-poly(vinyl alcohol) block copolymer: synthesis and self-assembly in water

This study is the first report of the synthesis and self-assembly in water of an amphiphilic PVDF-b-PVA block copolymer.

Study of the RAFT homopolymerization and copolymerization of N-[3-(dimethylamino)propyl]methacrylamide hydrochloride and evaluation of the cytotoxicity of the resulting homo- and copolymers

Well-defined p(DMAPMA·HCl) homopolymers with good chain extension ability were obtained by the RAFT in acidic conditions and precipitation in acetone.

A detailed mechanistic study of bulk MADIX of styrene and its chain extension

By combining experimental and modeling tools, a detailed characterization study of MADIX properties becomes possible.

Synthesis of ultra-high molecular weight ABA triblock copolymers via aqueous RAFT-mediated gel polymerisation, end group modification and chain coupling

Novel ultra-high molecular weight ABA triblock copolymers were synthesised using aqueous RAFT polymerisation, end-group modification and chain coupling.

Efficient DNA-Polymer Coupling in Organic Solvents: A Survey of Amide Coupling, Thiol-Ene and Tetrazine-Norbornene Chemistries Applied to Conjugation of Poly(N-Isopropylacrylamide)

A range of chemistries were explored for the efficient covalent conjugation of DNA to poly(N-isopropylacrylamide) (poly(NIPAM)) in organic solvents. Amide coupling and thiol–ene Michael addition were found to be ineffective for the synthesis of the desired products. However, the inverse electron-demand Diels–Alder (DA_inv) reaction between tetrazine (Tz) and norbornene (Nb) was found to give DNA–polymer conjugates in good yields (up to 40%) in organic solvents (N,N-dimethylformamide, N,N-dimethylacetamide and N-methyl-2-pyrrolidone), and without the need for a catalyst. Methods for the synthesis of Tz-and Nb- functionalised DNA were developed, along with a post-polymerisation functionalisation strategy for the production of Tz-functionalised polymers.

Computational Investigation of the Aminolysis of RAFT Macromolecules

This work presents a detailed computational study and kinetic analysis of the aminolysis of dithioates, dithiobenzoates, trithiocarbonates, xanthates, and thiocarbamates, which are frequently used as chain-transfer agents for reversible addition-fragmentation chain-transfer (RAFT) polymerization. Rate coefficients were obtained from ab initio calculations, taking into account a diffusional contribution according to the encounter pair model. A kinetic model was constructed and reveals a reaction mechanism of four elementary steps: (i) formation of a zwitterionic intermediate, (ii) formation of a complex intermediate in which an assisting amine molecule takes over the proton from the zwitterionic intermediate, (iii) breakdown of the complex into a neutral tetrahedral intermediate with release of the assisting amine molecule, and (iv) amine-assisted breakdown of the neutral intermediate to the products. Furthermore, a comparative analysis indicates that the alkanedithioates and dithiobenzoates react the fastest, followed, respectively, by xanthates and trithiocarbonates, which react almost equally fast, and dithiocarbamates, which are not reactive at typical experimentally used conditions.

Metal-Free Removal of Polymer Chain Ends Using Light

A light-mediated method for the facile removal of polymer end groups that are common to controlled radical polymerization techniques is presented. This metal-free strategy is general, being effective for chlorine, bromine, and thiocarbonylthio moieties as well as a number of different polymer families (styrenic, acrylic, and methacrylic). In addition to solution reactions, this process is readily translated to thin films, where light mediation allows the straightforward fabrication of hierarchically patterned polymer brushes.

Retaining individualities: the photodynamics of self-ordering porphyrin assemblies

Retained photochemical properties – a simple porphyrin–polyDMA conjugate with the ability to self assemble into large (∼1 μm) vesicles in water. The photodynamics are remarkably preserved despite the extensive aggregation.

The retention of photochemical properties of individual chromophores is a key feature of biological light harvesting complexes. This is achieved despite extensive aggregation of the chromophores, which in synthetic chromophore assemblies often yields a change in spectral characteristics. As an alternative approach towards mimicking biological light harvesting complexes, we report the synthesis of porphyrin assemblies which retained the photochemical properties of the individual chromophore units despite their substantial aggregation. These new materials highlight a new bottom-up approach towards the design and understanding of more complex biomimetic and naturally occurring biological systems.

One-Pot Modular Synthesis of Functionalized RAFT Agents Derived from a Single Thiolactone Precursor

In this paper, the straightforward preparation of a range of functionalized trithiocarbonates as RAFT chain transfer agents (CTAs) is presented. The crucial step in the one-pot, three-step reaction sequence is the aminolysis of a thiolactone precursor as it introduces the desired functional handle (double bond, hydroxyl, furan, protected amine, ...) and generates the corresponding thiol in situ, facilitating further elaboration of the CTA. Furthermore, the newly synthesized trithiocarbonates were positively evaluated as mediators in the RAFT polymerization of styrene, isobornyl acrylate, and N-isopropylacrylamide, while the presence of the end groups in the heterotelechic polymers was confirmed by NMR and UV-vis analysis.

A Robust Cross-Linking Strategy for Block Copolymer Worms Prepared via Polymerization-Induced Self-Assembly

A poly(glycerol monomethacrylate) (PGMA) chain transfer agent is chain-extended by reversible addition-fragmentation chain transfer (RAFT) statistical copolymerization of 2-hydroxypropyl methacrylate (HPMA) with glycidyl methacrylate (GlyMA) in concentrated aqueous solution via polymerization-induced self-assembly (PISA). A series of five free-standing worm gels is prepared by fixing the overall degree of polymerization of the core-forming block at 144 while varying its GlyMA content from 0 to 20 mol %. 1H NMR kinetics indicated that GlyMA is consumed much faster than HPMA, producing a GlyMA-rich sequence close to the PGMA stabilizer block. Temperature-dependent oscillatory rheological studies indicate that increasing the GlyMA content leads to progressively less thermoresponsive worm gels, with no degelation on cooling being observed for worms containing 20 mol % GlyMA. The epoxy groups in the GlyMA residues can be ring-opened using 3-aminopropyltriethoxysilane (APTES) in order to prepare core cross-linked worms via hydrolysis-condensation with the siloxane groups and/or hydroxyl groups on the HPMA residues. Perhaps surprisingly, 1H NMR analysis indicates that the epoxy-amine reaction and the intermolecular cross-linking occur on similar time scales. Cross-linking leads to stiffer worm gels that do not undergo degelation upon cooling. Dynamic light scattering studies and TEM analyses conducted on linear worms exposed to either methanol (a good solvent for both blocks) or anionic surfactant result in immediate worm dissociation. In contrast, cross-linked worms remain intact under such conditions, provided that the worm cores comprise at least 10 mol % GlyMA.

Superparamagnetic Fe3O4 Nanoparticles: Synthesis by Thermal Decomposition of Iron(III) Glucuronate and Application in Magnetic Resonance Imaging

Monodisperse superparamagnetic Fe3O4 nanoparticles coated with oleic acid were prepared by thermal decomposition of Fe(III) glucuronate. The shape, size, and particle size distribution were controlled by varying the reaction parameters, such as the reaction temperature, concentration of the stabilizer, and type of high-boiling-point solvents. Magnetite particles were characterized by transmission electron microscopy (TEM), as well as electron diffraction (SAED), X-ray diffraction (XRD), dynamic light scattering (DLS), and magnetometer measurements. The particle coating was analyzed by atomic absorption spectroscopy (AAS) and attenuated total reflection (ATR) Fourier transform infrared spectroscopy (FTIR) spectroscopy. To make the Fe3O4 nanoparticles dispersible in water, the particle surface was modified with α-carboxyl-ω-bis(ethane-2,1-diyl)phosphonic acid-terminated poly(3-O-methacryloyl-α-D-glucopyranose) (PMG-P). For future practical biomedical applications, nontoxicity plays a key role, and the PMG-P&Fe3O4 nanoparticles were tested on rat mesenchymal stem cells to determine the particle toxicity and their ability to label the cells. MR relaxometry confirmed that the PMG-P&Fe3O4 nanoparticles had high relaxivity but rather low cellular uptake. Nevertheless, the labeled cells still provided visible contrast enhancement in the magnetic resonance image. In addition, the cell viability was not compromised by the nanoparticles. Therefore, the PMG-P&Fe3O4 nanoparticles have the potential to be used in biomedical applications, especially as contrast agents for magnetic resonance imaging.

A Critical Appraisal of RAFT-Mediated Polymerization-Induced Self-Assembly

Recently, polymerization-induced self-assembly (PISA) has become widely recognized as a robust and efficient route to produce block copolymer nanoparticles of controlled size, morphology, and surface chemistry. Several reviews of this field have been published since 2012, but a substantial number of new papers have been published in the last three years. In this Perspective, we provide a critical appraisal of the various advantages offered by this approach, while also pointing out some of its current drawbacks. Promising future research directions as well as remaining technical challenges and unresolved problems are briefly highlighted.

The pH-responsive behaviour of poly(acrylic acid) in aqueous solution is dependent on molar mass

Fluorescence spectroscopy on a series of aqueous solutions of poly(acrylic acid) containing a luminescent label showed that polymers with molar mass, Mn < 16.5 kDa did not exhibit a pH responsive conformational change, which is typical of higher molar mass poly(acrylic acid). Below this molar mass, polymers remained in an extended conformation, regardless of pH. Above this molar mass, a pH-dependent conformational change was observed. Diffusion-ordered nuclear magnetic resonance spectroscopy confirmed that low molar mass polymers did not undergo a conformational transition, although large molar mass polymers did exhibit pH-dependent diffusion.

Synthesis of well-defined alkyne terminated poly(N-vinyl caprolactam) with stringent control over the LCST by RAFT

The reversible addition-fragmentation chain transfer (RAFT) of N-vinyl caprolactam (NVCL) using two new xanthates with alkyne functionalities is reported. The kinetic data obtained for polymerization of this non-activated monomer using a protected alkyne-terminated RAFT agent (PAT-X1) revealed a linear increase of the polymer molecular weight with the monomer conversion as well as low dispersity (Đ) during the entire course of the polymerization. The system reported here allowed us to enhance the final conversion, diminish Đ and reduce the polymerization temperature compared to the typical values reported in the scarce literature available for the RAFT polymerization of NVCL. The resulting PNVCL was fully characterized using 1H nuclear magnetic resonance (1H NMR), matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), Fourier-transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC) techniques. The temperature-responsive features of PNVCL in aqueous solutions were fully investigated under different conditions using turbidimetry. The presented strategy allows the synthesis of well-defined PNVCL with sharp and reversible phase transition temperatures around 37 °C. By manipulating the polymer molecular weight, or the solution properties, it is possible to tune the PNVCL phase transition. As a proof-of concept, the alkyne functionalized PNVCL was used to afford new linear block copolymers, by reacting with an azide-terminated poly(ethylene glycol) (N3-PEG) through the copper catalyzed azide-alkyne [3+2] dipolar cycloaddition (CuAAC) reaction. The results presented establish a robust system to afford the synthesis of PNCVL with fine tuned characteristics that will enable more efficient exploration of the remarkable potential of this polymer in biomedical applications.

Well-Defined Macromolecules Using Horseradish Peroxidase as a RAFT Initiase

Enzymatic catalysis and control over macromolecular architectures from reversible addition-fragmentation chain transfer polymerization (RAFT) are combined to give a new method of making polymers. Horseradish peroxidase (HRP) is used to catalytically generate radicals using hydrogen peroxide and acetylacetone as a mediator. RAFT is used to control the polymer structure. HRP catalyzed RAFT polymerization gives acrylate and acrylamide polymers with relatively narrow molecular weight distributions. The polymerization is rapid, typically exceeding 90% monomer conversion in 30 min. Complex macromolecular architectures including a block copolymer and a protein-polymer conjugate are synthesized using HRP to catalytically initiate RAFT polymerization.