Evaluation of Isoprene Chain Extension from PEO Macromolecular Chain Transfer Agents for the Preparation of Dual, Invertible Block Copolymer Nanoassemblies

Chemical recycling of bromine-terminated polymers synthesized by ATRP

Chemical recycling of polymers is one of the biggest challenges in materials science. Recently, remarkable achievements have been made by utilizing polymers prepared by controlled radical polymerization to trigger low-temperature depolymerization. However, in the case of atom transfer radical polymerization (ATRP), depolymerization has nearly exclusively focused on chlorine-terminated polymers, even though the overwhelming majority of polymeric materials synthesized with this method possess a bromine end-group. Herein, we report an efficient depolymerization strategy for bromine-terminated polymethacrylates which employs an inexpensive and environmentally friendly iron catalyst (FeBr2/L). The effect of various solvents and the concentration of metal salt and ligand on the depolymerization are judiciously explored and optimized, allowing for a depolymerization efficiency of up to 86% to be achieved in just 3 minutes. Notably, the versatility of this depolymerization is exemplified by its compatibility with chlorinated and non-chlorinated solvents, and both Fe(ii) and Fe(iii) salts. This work significantly expands the scope of ATRP materials compatible with depolymerization and creates many future opportunities in applications where the depolymerization of bromine-terminated polymers is desired.

A Dual-Kinetic Control Strategy for Designing Nano-Metamaterials: Novel Class of Metamaterials with Both Characteristic and Whole Sizes of Nanoscale

Increasingly intricate in their multilevel multiscale microarchitecture, metamaterials with unique physical properties are challenging the inherent constraints of natural materials. Their applicability in the nanomedicine field still suffers because nanomedicine requires a maximum size of tens to hundreds of nanometers; however, this size scale has not been achieved in metamaterials. Therefore, "nano-metamaterials," a novel class of metamaterials, are introduced, which are rationally designed materials with multilevel microarchitectures and both characteristic sizes and whole sizes at the nanoscale, investing in themselves remarkably unique and significantly enhanced material properties as compared with conventional nanomaterials. Microarchitectural regulation through conventional thermodynamic strategy is limited since the thermodynamic process relies on the frequency-dependent effective temperature, T_eff (ω), which limits the architectural regulation freedom degree. Here, a novel dual-kinetic control strategy is designed to fabricate nano-metamaterials by freezing a high-free energy state in a T_eff (ω)-constant system, where two independent dynamic processes, non-solvent induced block copolymer (BCP) self-assembly and osmotically driven self-emulsification, are regulated simultaneously. Fe³⁺ -"onion-like core@porous corona" (Fe³⁺ -OCPCs) nanoparticles (the products) have not only architectural complexity, porous corona and an onion-like core but also compositional complexity, Fe³⁺ chelating BCP assemblies. Furthermore, by using Fe³⁺ -OCPCs as a model material, a microstructure-biological performance relationship is manifested in nano-metamaterials.

Degradable polyisoprene by radical ring-opening polymerization and application to polymer prodrug nanoparticles

Radical ring-opening copolymerization of isoprene and dibenzo[c,e]oxepane-5-thione via free-radical and controlled radical polymerizations led to degradable polyisoprene under basic, oxidative and physiological conditions with application to prodrug nanoparticles.

Macrochain Transfer Agents for Catalytic Ring-Opening Metathesis Polymerization

A monosubstituted 1,3-diene derivative attached to a polymer is demonstrated to act as a macrochain transfer agent in catalytic ring-opening metathesis polymerization. PEG- and PLA-based macrochain transfer agents were synthesized in a few steps and were characterized using NMR spectroscopy, size exclusion chromatography (SEC) and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-ToF) mass spectrometry. Poly(l-lactide) based diblock copolymer, poly(ethylene glycol)-based diblock, and triblock (ABA type) copolymers of varied chain lengths were prepared catalytically in a one-pot approach via metathesis polymerization. Block copolymers were characterized by SEC and showed monomodal molecular weight distributions. Moreover, DOSY NMR spectroscopy further proved the block microstructures of the synthesized polymers.

Engineered Polymersomes for the Treatment of Fish Odor Syndrome: A First Randomized Double Blind Olfactory Study

Trimethylamine (TMA) is a metabolite overtly present in patients suffering from trimethylaminuria (TMAU), a rare genetic disorder characterized by a strong "fishy" body odor. To date, no approved pharmacological treatment to sequester excess TMA on the skin of patients exists. Here, transmembrane pH gradient poly(isoprene)-block-poly(ethylene glycol) (PI-b-PEG) polymersomes are investigated for the topical removal of TMA. PI-b-PEG amphiphiles of varying chain length are synthesized and evaluated for their ability to form vesicular structures in aqueous media. The optimization of the PI/PEG ratio of transmembrane pH gradient polymersomes allows for the rapid and efficient capture of TMA both in solution and after incorporation into a topical hydrogel matrix at the pH of the skin. A subsequent double blind olfactory study reveals a significant decrease in perceived odor intensity after application of the polymersome-based formulation on artificial skin substrates that has been incubated in TMA-containing medium. This simple and novel approach has the potential to ease the burden of people suffering from TMAU.

End-functional polyolefins for block copolymer synthesis

Polyolefins that contain polar functionalities are highly desired because they could extend the range of applications of these low production cost materials by modifying surface and other interfacial properties. Block copolymers containing polyolefin and polar segments are among the most sought-after architectures because of their ability to span the phase boundaries. This review focusses on the end-functionalisation of polyolefins by catalytic olefin polymerisation processes, almost invariably by metal-catalysed routes, followed by the growth polar blocks by various polymerisation techniques.

Self-Assembly of Block and Graft Copolymers in Organic Solvents: An Overview of Recent Advances

This review is an attempt to update the recent advances in the self-assembly of amphiphilic block and graft copolymers. Their micellization behavior is highlighted for linear AB, ABC triblock terpolymers, and graft structures in non-aqueous selective polar and non-polar solvents, including solvent mixtures and ionic liquids. The micellar characteristics, such as particle size, aggregation number, and morphology, are examined as a function of the copolymers' architecture and molecular characteristics.

A Useful Method for Preparing Mixed Brush Polymer Grafted Nanoparticles by Polymerizing Block Copolymers from Surfaces with Reversed Monomer Addition Sequence

The preparation of well-defined block copolymers using controlled radical polymerization depends on the proper order of monomer addition. The reversed order of monomer addition results in a mixture of block copolymer and homopolymer and thus has typically been avoided. In this paper, the low blocking efficiency of reversed monomer addition order is utilized in combination with surface initiated reversible addition-fragmentation chain-transfer polymerization to establish a facile procedure toward mixed polymer brush grafted nanoparticles SiO₂ -g-(PS (polystyrene), PS-b-PMAA (polymethacrylic acid)). The SiO₂ -g-(PS, PS-b-PMAA) nanoparticles are analyzed by gel permeation chromatography deconvolution, and the fraction of each polymer component is calculated. Additionally, the SiO₂ -g-(PS, PS-b-PMAA) are amphiphilic in nature and show unique self-assembly behavior in water.

Star-Shaped Copolymers Based on Poly(N-vinylcaprolactam) and their Use as Nanocarriers of Methotrexate

Star-shaped poly(N-vinylcaprolactam)-block-poly(ethylhexylacrylate)-block-polyethylene glycol (PNVCL-b-PEHA-b-PEG) triblock copolymers and star-shaped poly(N-vinylcaprolactam)-block-polyethylene glycol (PNVCL-b-PEG) diblock copolymers were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. The resulting star block copolymers were characterized using 1H NMR and UV-vis spectroscopy, gel permeation chromatography, and dynamic light scattering. The star-shaped PNVCL-b-PEG and PNVCL-b-PEHA-b-PEG block copolymers self-assemble spontaneously into aggregates in water. The aggregates formed ranged from ~17 to 135 nm in diameter and were used to encapsulate methotrexate (MTX). It was observed that the aggregates from PNVCL-b-PEHA-b-PEG copolymers exhibited a higher drug loading and a lower release of MTX (19 wt-% and 54 %) as compared with star copolymers without PEHA (5 wt-% and 81 %) after 24 h at a temperature below their lower critical solution temperature values.

High-Capacity Drug Carriers from Common Polymer Amphiphiles

We report herein a dual-purpose role for polyacidic domains in an aqueous-phase polymer amphiphile assembly. In addition to their typical role as ionized water-solubilizing and self-repulsive motifs, we find that polycarboxylic acid domains uniquely enable high levels of hydrophobic drug encapsulation. By attenuated total reflectance infrared spectroscopy, we find significant differences in the carbonyl stretching region of the nanoparticles formed by polyacidic amphiphiles relative to those in soluble, single-domain poly(acrylic acid), suggesting that stabilization may be derived from limited ionization of the carboxylate groups upon assembly. Acidic-hydrophobic diblock polyacrylates were prepared and coassembled with up to 60 wt % camptothecin (CPT) into nanoparticles, the highest loading reported to date. Controlled release of bioactive CPT from polymer nanoparticles is observed, as well as protection from human serum albumin-induced hydrolysis. Surface protection with PEG limits uptake of the CPT-loaded nanoparticles by MCF-7 breast cancer cells, as expected. Acidic-hydrophobic polymer amphiphiles thus have the hallmarks of a useful and general drug delivery platform and are readily accessible from living radical polymerization of cheap, commercially available monomers. We highlight here the potential utility of this common polymer design in high-capacity, controlled-release polymer nanoparticle systems.

Synthetic Polymer Hybridization with DNA and RNA Directs Nanoparticle Loading, Silencing Delivery, and Aptamer Function

We report herein discrete triplex hybridization of DNA and RNA with polyacrylates. Length-monodisperse triazine-derivatized polymers were prepared on gram-scale by reversible addition-fragmentation chain-transfer polymerization. Despite stereoregio backbone heterogeneity, the triazine polymers bind T/U-rich DNA or RNA with nanomolar affinity upon mixing in a 1:1 ratio, as judged by thermal melts, circular dichroism, gel-shift assays, and fluorescence quenching. We call these polyacrylates "bifacial polymer nucleic acids" (bPoNAs). Nucleic acid hybridization with bPoNA enables DNA loading onto polymer nanoparticles, siRNA silencing delivery, and can further serve as an allosteric trigger of RNA aptamer function. Thus, bPoNAs can serve as tools for both non-covalent bioconjugation and structure-function nucleation. It is anticipated that bPoNAs will have utility in both bio- and nanotechnology.

Asymmetric neutral, cationic and anionic PEO-based double-hydrophilic block copolymers (DHBCs): synthesis and reversible micellization triggered by temperature or pH

The syntheses of three DHBCs (thermosensitive or ionizable) are described. To act as structure directing agents in mesoporous silica synthesis, their ability to undergo micellization under appropriate conditions was checked.

Fluorinated amphiphilic block copolymers via RAFT polymerization and their application as surf-RAFT agent in miniemulsion polymerization

Preparation of an amphiphilic block copolymer (Am-BCP) based on poly(ethylene glycol) methyl ether methacrylate (PEGMA) and heptafluorobutyl acrylate (HFBA) via RAFT polymerization and application of this Am-BCP as surf-RAFT agent for polymerization of styrene.

Polymerization-induced self-assembly of block copolymer nano-objects via RAFT aqueous dispersion polymerization

In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells.

RAFT aqueous dispersion polymerization yields poly(ethylene glycol)-based diblock copolymer nano-objects with predictable single phase morphologies

A poly(ethylene glycol) (PEG) macromolecular chain transfer agent (macro-CTA) is prepared in high yield (>95%) with 97% dithiobenzoate chain-end functionality in a three-step synthesis starting from a monohydroxy PEG₁₁₃ precursor. This PEG₁₁₃-dithiobenzoate is then used for the reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). Polymerizations conducted under optimized conditions at 50 °C led to high conversions as judged by ¹H NMR spectroscopy and relatively low diblock copolymer polydispersities (M_w/M_n < 1.25) as judged by GPC. The latter technique also indicated good blocking efficiencies, since there was minimal PEG₁₁₃ macro-CTA contamination. Systematic variation of the mean degree of polymerization of the core-forming PHPMA block allowed PEG₁₁₃-PHPMA_x diblock copolymer spheres, worms, or vesicles to be prepared at up to 17.5% w/w solids, as judged by dynamic light scattering and transmission electron microscopy studies. Small-angle X-ray scattering (SAXS) analysis revealed that more exotic oligolamellar vesicles were observed at 20% w/w solids when targeting highly asymmetric diblock compositions. Detailed analysis of SAXS curves indicated that the mean number of membranes per oligolamellar vesicle is approximately three. A PEG₁₁₃-PHPMA_x phase diagram was constructed to enable the reproducible targeting of pure phases, as opposed to mixed morphologies (e.g., spheres plus worms or worms plus vesicles). This new RAFT PISA formulation is expected to be important for the rational and efficient synthesis of a wide range of biocompatible, thermo-responsive PEGylated diblock copolymer nano-objects for various biomedical applications.

Controlled synthesis of β-sheet polymers based on side-chain amyloidogenic short peptide segments via RAFT polymerization

A strategy was developed for the controlled synthesis of side-chain peptide containing pH-responsive polymers with an antiparallel β-sheet motif, which was independent of solvent polarity, PEGylation of homopolymers, the block length of PEG or peptidic segments in the block copolymer and temperature.

Side-chain amino-acid-based pH-responsive self-assembled block copolymers for drug delivery and gene transfer

Developing safe and effective nanocarriers for multitype of delivery system is advantageous for several kinds of successful biomedicinal therapy with the same carrier. In the present study, we have designed amino acid biomolecules derived hybrid block copolymers which can act as a promising vehicle for both drug delivery and gene transfer. Two representative natural chiral amino acid-containing (l-phenylalanine and l-alanine) vinyl monomers were polymerized via reversible addition-fragmentation chain transfer (RAFT) process in the presence of monomethoxy poly(ethylene glycol) based macro-chain transfer agents (mPEGn-CTA) for the synthesis of well-defined side-chain amino-acid-based amphiphilic block copolymers, monomethoxy poly(ethylene glycol)-b-poly(Boc-amino acid methacryloyloxyethyl ester) (mPEGn-b-P(Boc-AA-EMA)). The self-assembled micellar aggregation of these amphiphilic block copolymers were studied by fluorescence spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Potential applications of these hybrid polymers as drug carrier have been demonstrated in vitro by encapsulation of nile red dye or doxorubicin drug into the core of the micellar nanoaggregates. Deprotection of side-chain Boc- groups in the amphiphilic block copolymers subsequently transformed them into double hydrophilic pH-responsive cationic block copolymers having primary amino groups in the side-chain terminal. The DNA binding ability of these cationic block copolymers were further investigated by using agarose gel retardation assay and AFM. The in vitro cytotoxicity assay demonstrated their biocompatible nature and these polymers can serve as "smart" materials for promising bioapplications.

Z-Group ketone chain transfer agents for RAFT polymer nanoparticle modification via hydrazone conjugation

A ketal-containing trithiocarbonyl compound has been synthesized and characterized as a chain transfer agent (CTA) in Reversible Addition Fragmentation Transfer (RAFT) polymerization. The ketal functionality does not interfere with RAFT polymerization of acrylate monomers, which proceeds as previously reported to yield macro-CTA polymers and block co-polymers. Post-polymerization ketal cleavage revealed ketone functionality at the polar terminus of an amphiphilic block co-polymer. Hydrazone-formation was facile in both organic solution as well as in aqueous buffer where polymer nanoparticle assemblies were formed, indicating a conjugation/end-functionalization yield of 40-50%. Conjugation was verified with fluorescein, biotin and Gd-DOTA derivatives, and though the trithiocarbonate linkage is hydrolytically labile, we observed stable conjugation for several days at pH 7.4. and 37°C. As expected, streptavidin binding to biotinylated polymer micelles was observed, and size-change based relaxivity increases were observed when Gd-DOTA hydrazide was conjugated to polymer micelles. Cell-uptake of fluorescently labeled polymer micelles was also readily tracked by FACS and fluorescence microscopy. These polymer derivatives demonstrate a range of potential theranostic/biotechnological applications for this conveniently accessible keto-CTA, which include ligand-based nanoparticle targeting and fluorescent/MR nanoparticle contrast agents.

Functional Polyether-based Amphiphilic Block Copolymers Synthesized by Atom-transfer Radical Polymerization

This review deals with the synthesis, physical properties, and applications of amphiphilic block copolymers based on hydrophilic poly(ethylene oxide) (PEO) or hydrophobic poly(propylene oxide) (PPO). Oligomeric PEO and PPO are frequently functionalized by converting their OH end groups into macroinitiators for atom-transfer radical polymerization. They are then used to generate additional blocks as part of complex copolymer architectures. Adding hydrophobic and hydrophilic blocks, respectively, leads to polymers with amphiphilic character in water. They are surface active and form micelles above a critical micellization concentration. Together with recent developments in post-polymerization techniques through quantitative coupling reactions (‘click’ chemistry) a broad variety of tailored functionalities can be introduced to the amphiphilic block copolymers. Examples are outlined including stimuli responsiveness, membrane penetrating ability, formation of multi-compartmentalized micelles, etc.