Thioacetate-Based Initiators for the Synthesis of Thiol-End-Functionalized Poly(2-oxazoline)s.

New functional initiators for the cationic ring-opening polymerization of 2-alkyl-2-oxazolines are described to introduce a thiol moiety at the α terminus. Both tosylate and nosylate initiators carrying a thioacetate group are obtained in multigram scale, from commercial reagents in two steps, including a phototriggered thiol-ene radical addition. The nosylate derivative gives access to a satisfying control over the cationic ring-opening polymerization of 2-ethyl-2-oxazoline, with dispersity values lower than 1.1 during the entire course of the polymerization, until full conversion. Cleavage of the thioacetate end group is rapidly achieved using triazabicyclodecene, thereby leading to a mercapto terminus. The latter gives access to a new subgeneration of α-functional poly(2-oxazoline)s (butyl ester, N-hydroxysuccinimidyl ester, furan) by Michael addition with commercial (meth)acrylates. The amenability of the mercapto-poly(2-ethyl-2-oxazoline) for covalent surface patterning onto acrylated surfaces is demonstrated in a microchannel cantilever spotting (µCS) experiment, characterized by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectrometry (ToF-SIMS).

Hetero-Diels-Alder Cycloaddition with RAFT Polymers as Bioconjugation Platform.

We introduce the bioconjugation of polymers synthesized by RAFT polymerization, bearing no specific functional end group, by means of hetero-Diels-Alder cycloaddition through their inherent terminal thiocarbonylthio moiety with a diene-modified model protein. Quantitative conjugation occurs over the course of a few hours, at ambient temperature and neutral pH, and in the absence of any catalyst. Our technology platform affords thermoresponsive bioconjugates, whose aggregation is solely controlled by the polymer chains.

Reactive and Functional Nanoobjects by Polymerization-Induced Self-Assembly.

Polymerization-induced self-assembly (PISA) is becoming a standard technique for generating core-shell polymeric nanoparticles of various morphologies ranging from classic spheres to rods/fibers ("worm-like") and vesicles. After the initial quest for polymerization control in dispersed media, the focus of PISA research has drastically shifted, first to morphological control and now to the introduction of reactivity and functionality in order to generate useful materials. The present review is dedicated to the latter aspect. Reactivity is distinguished from functionality such as complexing, templating, and catalyzing. Approaches for either shell or core functionalization are also detailed separately.

Heparinized Polyurethane Surface Via a One-Step Photografting Method.

Traditional methods using coupling chemistry for surface grafting of heparin onto polyurethane (PU) are disadvantageous due to their generally low efficiency. In order to overcome this problem, a quick one-step photografting method is proposed here. Three heparin derivatives incorporating 0.21, 0.58, and 0.88 wt% pendant aryl azide groups were immobilized onto PU surfaces, leading to similar grafting densities of 1.07, 1.17, and 1.13 µg/cm², respectively, yet with increasing densities of anchoring points. The most negatively charged surface and the maximum binding ability towards antithrombin III were found for the heparinized PU with the lowest amount of aryl azide/anchor sites. Furthermore, decreasing the density of anchoring points was found to inhibit platelet adhesion to a larger extent and to prolong plasma recalcification time, prothrombin time, thrombin time, and activated partial thromboplastin time to a larger extent. This was also found to enhance the bioactivity of immobilized heparin from 22.9% for raw heparin to 36.9%. This could be explained by the enhanced molecular mobility of immobilized heparin when it is more loosely anchored to the PU surface, as well as a higher surface charge.

Ultra-Fast Synthesis of Multivalent Radical Nanoparticles by Ring-Opening Metathesis Polymerization-Induced Self-Assembly.

We report the straightforward, time-efficient synthesis of radical core-shell nanoparticles (NPs) by polymerization-induced self-assembly. A nitroxide-containing hydrophilic macromolecular precursor was prepared by ring-opening metathesis copolymerization of norbornenyl derivatives of TEMPO and oligoethylene glycol and was chain-extended in situ with norbornene in ethanolic solution, leading to simultaneous amphiphilic block copolymer formation and self-assembly. Without any intermediate purification from the monomers to the block copolymers, radical NPs with tunable diameters ranging from 10 to 110 nm are obtained within minutes at room temperature. The high activity of the radical NPs as chemoselective and homogeneous, yet readily recyclable catalysts is demonstrated through oxidation of a variety of alcohols and recovery by simple centrifugation. Furthermore, the NPs show biocompatibility and antioxidant activity in vitro.

Intrinsically Fluorescent, Stealth Polypyrazoline Nanoparticles with Large Stokes Shift for In Vivo Imaging.

Recent advances in super-resolution microscopy and fluorescence bioimaging allow exploring previously inaccessible biological processes. To this end, there is a need for novel fluorescent probes with specific features in size, photophysical properties, colloidal and optical stabilities, as well as biocompatibility and ability to evade the reticuloendothelial system. Herein, novel fluorescent nanoparticles are introduced based on an inherently fluorescent polypyrazoline (PPy) core and a polyethylene glycol (PEG) shell, which address all aforementioned challenges. Synthesis of the PPy-PEG amphiphilic block copolymer by phototriggered step-growth polymerization is investigated by NMR spectroscopy, size-exclusion chromatography, and mass spectrometry. The corresponding nanoparticles are characterized for their luminescent properties and hydrodynamic size in various aqueous environments (e.g., cell culture media). PPy nanoparticles particularly exhibit a large Stokes shift (Δλ = 160 nm or Δν > 7000 cm-1 ) with visible light excitation and strong colloidal stability. While clearance by macrophages and endothelial cells is minimal, PPy displays good biocompatibility. Finally, PPy nanoparticles prove to be long circulating when injected in zebrafish embryos, as observed by in vivo time-lapse fluorescence microscopy. In summary, PPy nanoparticles are highly promising to be further developed as fluorescent nanodelivery systems with low toxicity and exquisite retention in the blood stream.

Impact of Polymer Bioconjugation on Protein Stability and Activity Investigated with Discrete Conjugates: Alternatives to PEGylation.

Covalent attachment of synthetic polymers to proteins, known as protein-polymer conjugation, is currently one of the main approaches for improving the physicochemical properties of these biomolecules. The most commonly employed polymer is polyethylene glycol (PEG), as evidenced by extensive research and clinical track records for its use in biopharmaceuticals. However, the occurrence of allergic reactions or hypersensitivity and the discovery of PEG antibodies, on the one hand, and the rise of controlled polymerization techniques and novel monomers, on the other hand, have been driving the search for alternative polymers for bioconjugation. The present study describes the synthesis, purification, and properties of conjugates of lysozyme with poly( N-acryloylmorpholine) (PNAM) and poly(oligoethylene glycol methyl ether methacrylate) (POEGMA). Particularly, conjugate species with distinct conjugation degrees are investigated for their residual activity, aggregation behavior, and solubility, by using a high-throughput screening approach. Our study showcases the importance of evaluating conjugates obtained by nonsite-specific modification through isolated species with discrete degrees of conjugation rather than on the batch level. Monovalent conjugates with relatively low molar mass polymers displayed equal or even higher activity than the native protein, while all conjugates showed an improved protein solubility. To achieve a comparable effect on solubility as with PEG, PNAM and POEGMA of higher molar masses were required.

Nanostructured Thin Films of Moderately Functionalized PMMA-b-PS.

A library of poly(methyl methacrylate)-block-polystyrene (PMMA-b-PS) block copolymers (BCPs) bearing small amounts (<10 mol%) of functional comonomer in either one or both blocks is investigated for their phase separation behavior in bulk and in thin films. Particularly, functionalities typically involved in modern postpolymerization modifications are considered, e.g., azide, pentafluorophenyl, furfuryl. Small-angle X-ray scattering and atomic force microscopy are employed to determine the characteristic dimensional features of lamellae-forming BCPs, which differ essentially in the functional groups. It is shown that the presence of the reactive moieties does not perturb the ability to phase separate in bulk, yet has an impact on the dimensions of the domains. Using a classic two-step procedure involving surface neutralization with a statistical PMMA-co-PS copolymer, it is observed that some functional copolymers are not able to form homogeneous thin films. Solvent stability and crosslinking ability of the films are then briefly assessed as a first step to establishing the functional films as nanoresolved molecular immobilization platform with feature sizes of 20 nm and below.

Nitrilotriacetic Amine-Functionalized Polymeric Core-Shell Nanoparticles as Enzyme Immobilization Supports.

Nitrilotriacetic amine (NTA)-functionalized nanoparticles obtained by aqueous polymerization-induced self-assembly (PISA) are introduced as immobilization supports for polyhistidine-functionalized (His-tagged) enzymes. A novel initiator for nitroxide-mediated polymerization based on the nitroxide SG1 and carrying a protected NTA moiety was first synthesized. Size-exclusion chromatography (SEC) and electrospray ionization mass spectrometry (ESI-MS) proved the ability of this initiator to produce well-defined end-functional vinyl polymers. Subsequently, oligo(ethylene glycol) methacrylate-based macroinitiators were synthesized and chain-extended to form amphiphilic block copolymer nanoparticles, either by nanoprecipitation or by PISA. The latter method yielded spherical nanoparticles with a higher definition, as demonstrated by dynamic light scattering (DLS). Deprotection of the NTA moiety and complexation with nickel ions were assessed by DLS and inductively coupled plasma optical emission spectroscopy/mass spectrometry (ICP-OES/MS). Finally, immobilization of His-tagged horseradish peroxidase and ester hydrolase were successfully carried out, leading to catalytically active nanobiocatalysts, as shown by UV-vis measurements.

Stepwise Light-Induced Dual Compaction of Single-Chain Nanoparticles.

A photochemical strategy for the sequential dual compaction of single polymer chains is introduced. Two photoreactive methacrylates, with side chains bearing either a phenacyl sulfide (PS) or an α-methylbenzaldehyde (photoenol, PE) moiety, are selectively incorporated by one-pot iterative reversible-addition fragmentation chain transfer copolymerization into the outer blocks of a well-defined poly(methyl methacrylate) based ABC triblock copolymer possessing a nonfunctional spacer block (Mn = 23 400 g mol-1 , D = 1.2; ≈15 units of each photoreactive moieties of each type) as well as in model statistical copolymers bearing only one type of photoreactive unit. Upon UVA irradiation, PS and PE lead to highly reactive thioaldehydes and o-quinodimethanes, which rapidly react with dithiol and diacrylate linkers, respectively. The monomerfunctional copolymers are employed to establish the conditions for controlled intramolecular photo-crosslinking, which are subsequently applied to the bifunctional triblock copolymer. All compaction/folding experiments are monitored by size-exclusion chromatography and dynamic light scattering. The dual compaction consists of two events of dissimilar amplitude: the first folding step reveals a large reduction in hydrodynamic diameters, while the second compaction lead to a far less pronounced reduction of the single-chain nanoparticles size, consistent with the reduced degrees of freedom available after the first covalent compaction step.

3D Laser Micro- and Nanoprinting: Challenges for Chemistry.

3D printing is a powerful emerging technology for the tailored fabrication of advanced functional materials. This Review summarizes the state-of-the art with regard to 3D laser micro- and nanoprinting and explores the chemical challenges limiting its full exploitation: from the development of advanced functional materials for applications in cell biology and electronics to the chemical barriers that need to be overcome to enable fast writing velocities with resolution below the diffraction limit. We further explore chemical means to enable direct laser writing of multiple materials in one resist by highly wavelength selective (λ-orthogonal) photochemical processes. Finally, chemical processes to construct adaptive 3D written structures that are able to respond to external stimuli, such as light, heat, pH value, or specific molecules, are highlighted, and advanced concepts for degradable scaffolds are explored.

Single-Molecule Encapsulation: A Straightforward Route to Highly Stable and Printable Enzymes.

A mild, fast, and sequence-independent method for controlled enzyme immobilization is presented. This novel approach involves the encapsulation of single-enzyme molecules and the covalent attachment of these nanobiocatalysts onto surfaces. Fast and mild immobilization conditions, combined with low nonspecific adsorption on hydrophobic substrates, enables well-defined surface patterns via microcontact printing. The biohybrid materials show enhanced activity in organic solvents.

Cycloadditions in modern polymer chemistry.

Synthetic polymer chemistry has undergone two major developments in the last two decades. About 20 years ago, reversible-deactivation radical polymerization processes started to give access to a wide range of polymeric architectures made from an almost infinite reservoir of functional building blocks. A few years later, the concept of click chemistry revolutionized the way polymer chemists approached synthetic routes. Among the few reactions that could qualify as click, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) initially stood out. Soon, many old and new reactions, including cycloadditions, would further enrich the synthetic macromolecular chemistry toolbox. Whether click or not, cycloadditions are in any case powerful tools for designing polymeric materials in a modular fashion, with a high level of functionality and, sometimes, responsiveness. Here, we wish to describe cycloaddition methodologies that have been reported in the last 10 years in the context of macromolecular engineering, with a focus on those developed in our laboratories. The overarching structure of this Account is based on the three most commonly encountered cycloaddition subclasses in organic and macromolecular chemistry: 1,3-dipolar cycloadditions, (hetero-)Diels-Alder cycloadditions ((H)DAC), and [2+2] cycloadditions. Our goal is to briefly describe the relevant reaction conditions, the advantages and disadvantages, and the realized polymer applications. Furthermore, the orthogonality of most of these reactions is highlighted because it has proven highly beneficial for generating unique, multifunctional polymers in a one-pot reaction. The overview on 1,3-dipolar cycloadditions is mostly centered on the application of CuAAC as the most travelled route, by far. Besides illustrating the capacity of CuAAC to generate complex polymeric architectures, alternative 1,3-dipolar cycloadditions operating without the need for a catalyst are described. In the area of (H)DA cycloadditions, beyond the popular maleimide/furan couple, we present chemistries based on more reactive species, such as cyclopentadienyl or thiocarbonylthio moieties, particularly stressing the reversibility of these systems. In these two greater families, as well as in the last section on [2+2] cycloadditions, we highlight phototriggered chemistries as a powerful tool for spatially and temporally controlled materials synthesis. Clearly, cycloaddition chemistry already has and will continue to transform the field of polymer chemistry in the years to come. Applying this chemistry enables better control over polymer composition, the development of more complicated polymer architectures, the simplification of polymer library production, and the discovery of novel applications for all of these new polymers.

On the Orthogonality of Two Thiol-Based Modular Ligations.

The chemoselectivity of two thiol-based modular ligations operating under mild conditions is assessed. For this purpose, a macromolecular scaffold possessing allyl and pentafluorophenyl groups in two distinct parts is employed, which enables facile characterization by NMR spectroscopy ((1)H and (19)F) and size-exclusion chromatography. By using appropriate triggers (introduction of a base or light irradiation), it is possible to direct thiols to an arbitrarily chosen part of the scaffold, without any change to the other part and with no involvement of protecting group chemistry. Dual functionalization experiments are achieved by applying these triggers consecutively with no consideration of the reaction sequence order, evidencing full bidirectionality. A set of one-pot, purification-free procedures that enable near-quantitative to full dual functionalization in (very) short reaction times (17-180 min) is also presented.

Simultaneous Dual Encoding of Three-Dimensional Structures by Light-Induced Modular Ligation.

A highly efficient strategy for the simultaneous dual surface encoding of 2D and 3D microscaffolds is reported. The combination of an oligo(ethylene glycol)-based network with two novel and readily synthesized monomers with photoreactive side chains yields two new photoresists, which can be used for the fabrication of microstructures (by two-photon polymerization) that exhibit a dual-photoreactive surface. By combining both functional photoresists into one scaffold, a dual functionalization pattern can be obtained by a single irradiation step in the presence of adequate reaction partners based on a self-sorting mechanism. The versatility of the approach is shown by the dual patterning of halogenated and fluorescent markers as well as proteins. Furthermore, we introduce a new ToF-SIMS mode ("delayed extraction") for the characterization of the obtained microstructures that combines high mass resolution with improved lateral resolution.

Efficient Photochemical Approaches for Spatially Resolved Surface Functionalization.

Materials interfaces--with a gas, a liquid, or another solid--are highly important for advanced applications. Besides their topological design, controlling interactions at these interfaces is typically realized by tuning the chemical composition of the materials surface. In areas such as nanoscience or biology, it is, however, highly desirable to impart heterogeneously distributed properties. Photopatterning, more than micro- and nanoprinting methods, is often the method of choice for precise functionalization, especially in terms of versatility. Recently, a range of new or rediscovered photochemistry approaches have been applied to precision surface functionalization, with the common aim of increasing efficiency and resolution while concomitantly lowering the amount of required energy. A survey of such methods is presented in this Review, with a focus on those we have explored.

Direct access to dithiobenzoate RAFT agent fragmentation rate coefficients by ESR spin-trapping.

The ß-scission rate coefficient of tert-butyl radicals fragmenting off the intermediate resulting from their addition to tert-butyl dithiobenzoate-a reversible addition-fragmentation chain transfer (RAFT) agent-is estimated via the recently introduced electron spin resonance (ESR)-trapping methodology as a function of temperature. The newly introduced ESR-trapping methodology is critically evaluated and found to be reliable. At 20 °C, a fragmentation rate coefficient of close to 0.042 s(-1) is observed, whereas the activation parameters for the fragmentation reaction-determined for the first time-read EA = 82 ± 13.3 kJ mol(-1) and A = (1.4 ± 0.25) × 10(13) s(-1) . The ESR spin-trapping methodology thus efficiently probes the stability of the RAFT adduct radical under conditions relevant for the pre-equilibrium of the RAFT process. It particularly indicates that stable RAFT adduct radicals are indeed formed in early stages of the RAFT poly-merization, at least when dithiobenzoates are employed as controlling agents as stipulated by the so-called slow fragmentation theory. By design of the methodology, the obtained fragmentation rate coefficients represent an upper limit. The ESR spin-trapping methodology is thus seen as a suitable tool for evaluating the fragmentation rate coefficients of a wide range of RAFT adduct radicals.

Light-induced modular ligation of conventional RAFT polymers.

Making light work of RAFT conjugation: a non-activated RAFT agent at the end of RAFT polymers can readily be coupled with ortho-quinodimethanes (photoenols) in a photo-triggered Diels-Alder reaction under mild conditions without catalyst. The method is universal and opens the door for the conjugation of a large number of RAFT-prepared polymers with photoenol-functionalized (macro)molecules. (RAFT=reversible addition-fragmentation chain transfer.).

(Ultra)fast catalyst-free macromolecular conjugation in aqueous environment at ambient temperature.

Tailor-made water-soluble macromolecules, including a glycopolymer, obtained by living/controlled RAFT-mediated polymerization are demonstrated to react in water with diene-functionalized poly(ethylene glycol)s without pre- or post-functionalization steps or the need for a catalyst at ambient temperature. As previously observed in organic solvents, hetero-Diels-Alder (HDA) conjugations reached quantitative conversion within minutes when cyclopentadienyl moieties were involved. However, while catalysts and elevated temperatures were previously necessary for open-chain diene conjugation, additive-free HDA cycloadditions occur in water within a few hours at ambient temperature. Experimental evidence for efficient conjugations is provided via unambiguous ESI-MS, UV/vis, NMR, and SEC data.

Thioketone-mediated polymerization with dithiobenzoates: proof for the existence of stable radical intermediates in RAFT polymerization.

A novel dithioester control agent [dimethyltetrathioterephtalate (DMTTT)] is presented for the thioketone-mediated radical polymerization (TKMP) of n-butyl acrylate. The rate of polymerization is significantly decreased in the presence of DMTTT indicating formation of dormant radical species. During polymerization, molar masses increase linearly with monomer conversion with reasonably narrow initial molar mass distributions (PDI between 1.3 and 1.8), whereas the dispersity increases during the course of the polymerization due to irreversible termination of both propagating and dormant radicals. The present results thus highlight the possibility of a mixed mechanism operating in RAFT polymerization, which combines slow fragmentation (long-lived intermediates) and intermediate radical termination.