Search

To advance the applications of direct laser writing (DLW), adaptability of the printed structure is critical, prompting a shift toward printing structures that are comprised of different materials, and/or can be partially or fully erased on demand. However, most structures that contain these features are often printed by complex processes or require harsh developing techniques. Herein, a unique photoresist for DLW is introduced that is capable of printing 3D microstructures that can be erased by exposure to darkness. Specifically, microstructures based on light-stabilized dynamic materials are fabricated that remain stable when continously irradiated with green light, but degrade once the light source is switched off. The degradation and light stabilization properties of the printed materials are analyzed in-depth by time-lapse scanning electron microscopy. It is demonstrated that these resists can be used to impart responsive behavior onto the printed structure, and –critically– as a temporary locking mechanism to control the release of moving structural features.

Donor-Acceptor Stenhouse Adducts (DASAs) are molecular photoswitches that reversibly isomerize from a linear to a cyclized form upon visible light irradiation. In the cyclized form, these molecules selectively react with a thiol in a thiol-Michael addition. Driven by the complexity of the switching mechanism and chemical properties of DASAs, the effect of concentration (ranging from 1 to 5 mM DASA) on the thiol-Michael addition was investigated. We find that the rate limiting step in the reaction is the photochemically induced DASA isomerization, as higher concentrations of thiol do not alter the rate at which the thiol-Michael adduct (TMA) is formed. Moreover, the kinetics of the product formation were found to be more significantly affected by irradiation at higher concentrations. In two distinct scenarios, where either the concentration or the molar equivalence of the reaction partner was kept constant, the TMA was found to be more efficiently formed under irradiation at higher concentrations. These findings indicate that the DASA reactivity towards thiol-Michael reactions appear to be equilibrium-driven at lower concentrations and light-driven at higher concentrations.

There exists a critical need in biomedical molecular imaging and diagnostics for molecular sensors that report on slight changes to their local microenvironment with high spatial fidelity. Herein, a modular fluorescent probe, termed StyPy, is rationally designed which features i) an enormous and tunable Stokes shift based on twisted intramolecular charge transfer (TICT) processes with no overlap, a broad emission in the far-red/near-infrared (NIR) region of light and extraordinary quantum yields of fluorescence, ii) a modular applicability via facile para-fluoro-thiol reaction (PFTR), and iii) a polarity- and viscosity-dependent emission. This renders StyPy as a particularly promising molecular sensor. Based on the thorough characterization on the molecular level, StyPy reports on the viscosity change in all-DNA microspheres and indicates the hydrophilic and hydrophobic compartments of hybrid DNA-based mesostructures consisting of latex beads embedded in DNA microspheres. Moreover, the enormous Stokes shift of StyPy enables one to detect multiple fluorophores, while using only a single laser line for excitation in DNA protocells. The authors anticipate that the presented results for multiplexing information are of direct importance for advanced imaging in complex soft matter and biological systems.

Herein, a concise overview of the use of heavier main group elements in multicomponent reactions and their use in polymer chemistry is provided. Incorporating heavier elements into macromolecular structures via multicomponent reactions allows for the rapid development of materials with unique properties that are not readily achieved using carbon, nitrogen, and/or oxygen. Elements in Group 13, Group 14, Group 15, and Group 16 are specifically covered examining both the familiar and unfamiliar properties of these elements and how they are used in multicomponent chemistry. Furthermore, elements that both take part in the reaction mechanism and remain in the macromolecular structure upon completion are only briefly explored. Some of the state-of-the-art work going into developing these heavier element multicomponent reactions are highlighted and it is hoped to inspire other polymer chemists to explore other parts of the periodic table.

There exists a growing need for smart materials suitable for use in biomedical applications. Herein, a photoresist that can be used to fabricate a biocompatible material entirely degradable by the enzyme chymotrypsin is introduced. The photoresist is based on a crosslinker with a tyramine moiety that is recognized and cleaved by the enzyme chymotrypsin. Macroscopic films as well as microstructures are fabricated via the use of direct laser writing. Multi-material boxing ring microstructures are generated and selectively degraded by the enzyme. Cell biocompatibility studies indicate that cells are able to attach and proliferate over one week on the material. A photoresist that is biocompatible and can be entirely removed by a biocompatible stimulus such as an enzyme can potentially be used as an easily removed tissue engineering scaffold and is especially promising for basic cell biology research.

Driven by recent advances in rapid multiphoton single-focus 3D laser nanoprinting, multifocus variants thereof, and projection-based multiphoton 3D laser nanoprinting, the necessary average total laser powers from femtosecond laser oscillators or even from amplified femtosecond laser systems have exceeded the Watt level. Aiming at ever faster 3D printing, there exist two options: Using yet more powerful lasers or searching for more sensitive photoresists allowing for higher speeds at comparable or lower power levels. Here, altogether more than 70 different photoresists from the literature and a few new candidates are reviewed with regard to effective multiphoton sensitivity. A dimensionless sensitivity figure-of-merit allows to directly compare data taken under sometimes vastly different conditions.

High-performance polymers such as polymethacrylimides have outstanding properties, for example, a unique strength-to-weight ratio and a high thermal stability, usually coupled to a high glass transition temperature. However, the requirement of high processing temperatures caused by these high glass-transition temperatures is often not desired for melt extrusion processes. Herein, a novel and straightforward imidization process of poly(methacrylic anhydrides) is presented with different ratios of ammonia and N-isopropylamine that is induced by thermal treatment. Therefore, polymethacrylimides with a varying degree of N-substitution, and thus a varying number of hydrogen-bond-donating moieties, are synthesized under facile reaction conditions. An in-depth investigation into the structures obtained with this new methodology is undertaken via a combination of nuclear magnetic resonance spectroscopy (NMR), Fourier-transform infrared spectroscopy (FT-IR), and high-resolution electrospray ionization mass spectrometry (ESI-MS). Additionally, thermal properties of the materials are investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses. These latter measurements highlight the key opportunity available with this novel synthesis to tailor the thermal properties of the polymer by providing a clear correlation between hydrogen bond formation, as observed by FT-IR, and the glass transition temperature.

We report a photochemical reaction system which requires activation by two colors of light. Specifically, a dual wavelength gated system is established by fusing the visible light mediated deprotection of a dithioacetal with the UV light activated Diels–Alder reaction of an o-methylbenzaldehyde with N-ethylmaleimide. Critically, both light sources are required to achieve the Diels–Alder adduct, irradiation with visible or UV light alone does not lead to the target product. The introduced dual gated photochemical system is particularly interesting for application in light driven 3D printing, where two color wavelength activated photoresists may become reality.

Light-induced curing of photoresists is a well-established technique to fabricate polymer networks with precise control over the material properties. Herein, control is exerted over network material properties of a single polymer blend through the spatiotemporally controlled incorporation of secondary polymer networks resulting in interpenetrating polymer networks (IPNs). Light gated control over materials properties of IPNs is assessed theoretically by applying the general rule of mixture and experimentally by using a multifunctional polymer resist. The full IPNs are formed through covalent bonds, based on photocuring of the complementary photoreactive groups o-methyl benzaldehyde (UV light) and styrylpyrene (visible light). Applied to a spin coated polymer blend of UV light curable responsive poly(4-vinylpyridine) (P4VP) and visible light curable poly(methyl methacrylate) (PMMA), the developed platform technology not only affords spatial control over the curing of the pH-swellable P4VP component from the polymer blend, but also allows adjustment of the extent of swelling by forming an IPN through subsequent curing of the PMMA component. Gated only by light irradiation of a polymer blend, it is possible to control IPN formation in 2D structures, which precisely adjusts the swelling behavior of the respective materials segments.

The year 2020 marks the 100th anniversary of the first article on polymerization, published by Hermann Staudinger. It is Staudinger who realized that polymers consist of long chains of covalently linked building blocks. Polymers have had a tremendous impact on the society ever since this initial publication. People live in a world that is almost impossible to imagine without synthetic polymers. But what does the future hold for polymer science? In this article, the editors and advisory board of Macromolecular Chemistry and Physics reflect on this question.

An alternative synthetic route towards the widely employed electroactive poly(TEMPO methacrylate) (PTMA) via a thermally robust methoxyamine-protecting group is demonstrated herein. Protection of the radical moiety of hydroxy-TEMPO with a methyl functionality and subsequent esterification with methacrylic anhydride allows the high-yielding formation of the novel monomer methyl-TEMPO methacrylate (MTMA). The polymerization of MTMA to poly(MTMA) (PMTMA) is investigated via free radical polymerization and reversible addition–fragmentation chain-transfer polymerization (RAFT), a reversible-deactivation radical polymerization technique. Cleavage of the temperature-stable methoxyamine functionality by oxidative treatment of PMTMA with meta-chloroperbenzoic acid (mCPBA) releases the electroactive PTMA. The redox activity of PTMA was confirmed by cyclic voltammetry in lithium-ion coin cells.

Herein, various 3D additive manufacturing approaches are reviewed in terms of two important figures of merit: maximum voxel printing rate and minimum voxel size. Voxel sizes from several 100 µm down to the 100 nm scale are covered. Original results on multifocus two-photon printing at around voxel printing rates of 107 voxels s−1 are presented in this context, which significantly surpass previous best values. These advances are illustrated by and applied to the making of microstructured 3D (chiral) mechanical metamaterials that are composed of more than one-hundred-thousand unit cells in three dimensions. Previous best values for unit cells of similar complexity are smaller by two orders of magnitude.

Hydrogels with on-demand tunable mechanical properties within sensitive biological environments are of critical importance for examining cellular responses to cell culture platforms. Herein, the first bio-orthogonal hydrogel that can be formed and subsequently tuned in its mechanical properties by simply switching different wavelengths of visible light (i.e., 455 and 420 nm) is reported. Specifically, both the initial hydrogelation and the tuning of the mechanical properties can be fully decoupled and selectively initiated by different colors of light. Sparing the need for any catalysts, the development of such a dual wavelength selective hydrogel for biological applications spans four levels: First, the development of the until today most redshifted photocycloaddition to allow for the selective initiation of only one photoreaction; second, the investigation of its wavelength-dependent ligation efficiency; third, translation of the ligation chemistry into a hydrogelator, and fourth, establishing a biocompatible hydrogel platform for applications in biomaterials engineering including detachment of fibroblasts from 2D culture areas or primary 3D culture of human mesenchymal stem cells. The introduced platform technology enables the fabrication of a hydrogel of predefined mechanical properties exclusively with visible light.

3D printing of adaptive and dynamic structures, also known as 4D printing, is one of the key challenges in contemporary materials science. The additional dimension refers to the ability of 3D printed structures to change their properties—for example, shape—over time in a controlled fashion as the result of external stimulation. Within the last years, significant efforts have been undertaken in the development of new responsive materials for printing at the macroscale. However, 4D printing at the microscale is still in its early stages. Thus, this progress report will focus on emerging materials for 4D printing at the microscale as well as their challenges and potential applications. Hydrogels and liquid crystalline and composite materials have been identified as the main classes of materials representing the state of the art of the growing field. For each type of material, the challenges and critical barriers in the material design and their performance in 4D microprinting are discussed. Importantly, further necessary strategies are proposed to overcome the limitations of the current approaches and move toward their application in fields such as biomedicine, microrobotics, or optics.

A new strategy for particle synthesis is enabled by utilizing modern synthetic, polymer, and photochemical techniques to facilitate the synthesis of highly narrow–disperse multifunctional microspheres from visible-light induced crosslinking of prepolymers in both a single and dual polymer system. The approach requires no stabilizers, bases, or initiators, and proceeds at ambient temperature to yield microspheres with a tunable size range (0.25–5 µm) in less than 4 h, depending largely on solvent composition, but also polymer concentration (2–10 mg mL−1), ratio, and irradiation intensity (3–20 W). Critically, the visible-light induced dimerization reaction exploited herein enables simple functional particle syntheses via a single polymer system. Underpinned by an in-depth kinetic analysis of the particle formation as well as a detailed small molecule study, the mechanism for particle formation is also elucidated. Importantly, inherent advantages of the system are exploited for surface functionalization of residual acrylate and hydroxyl groups (generating inherently fluorescent particles).

Chemiluminescence (CL) reactions have been widely employed and explored over the past 50 years because they offer unique light emission upon a defined chemical stimulus. In this Minireview, we focus on peroxyoxalate (PO) compounds because they feature very high quantum yields tuneable over the entire visible spectrum, allowing for visible-light detection by the naked eye without the necessity for expensive analytical instruments. Although analytical methods have been extensively described, PO-CL read-out is a strongly emerging field with ample industrial potential. The state-of-the-art PO-CL detection read-out systems for various key analytes is here explored. In particular, structural requirements, recent developments of PO-CL read-out probes and current limitations of selected examples are detailed. Furthermore, innovative approaches and synthetic routes to push the boundaries of PO-CL reactions into biological systems are highlighted. Underpinned by recent contributions, we share perspectives on embedding PO-CL molecules into polymeric materials, which they consider the next step in designing high performance solid-phase read-out systems.

The [4+4] photocycloaddition of anthracene is one of most relevant photoreactions and is widely applied in materials science, as it allows to remote-control soft matter material properties by irradiation. However, highly energetic UV irradiation is commonly applied, which limits its application. Herein, the wavelength dependence of the photodimerization of anthracene is assessed for the first time, revealing that the reaction is induced just as effectively with mild visible light (410 nm). To fully establish [4+4] cycloadditions within defined chemical environments, a conceptual framework for the solution kinetics of the photo-dimerization up to long reaction times is established by developing a novel photoreaction rate law that is dependent on individual rate coefficients of the key reaction steps. These coefficients can be determined based on low conversion photochemical experiments. Both differential and integral quantum yields can subsequently be predicted that are strongly time-dependent, highlighting the need for a detailed reaction pathway analysis. The presented approach simplifies a complex photochemical scenario, making the photochemical anthracene dimerization, or potentially any other photochemical dimerization, amenable to a time-dependent understanding at the elementary reaction level.

Flow reactors become more and more automated by enabling on-line reaction monitoring and adjusting the process parameters. On-line monitoring of chemical processes is a valuable tool to steer processes, leading to precise engineering of macromolecular materials. Detailed information about specific product patterns, end-group functionality or material composition can be obtained by coupling a flow reactor to a mass spectrometer (e. g. ESI-MS). In this work, we study the deprotection of maleimides and its subsequent photoenol functionalization to synthesize complex macromolecules using a photo flow reactor coupled to an ESI-MS. Using a trapping agent (TA), furan is efficiently removed from the maleimide Diels−Alder adduct within just minutes at 175 °C and quantitatively converted into an unreactive species that does not interfere with further reactions of the maleimide. The photoenol reaction was likewise shown to be highly effective to proceed in microreactors, reaching quantitative conversion of trifunctional molecules in as little as 2 min.

The two-photon Schwarzschild effect in photoresists suitable for 3D laser lithography is revisited. The study ranges over seven orders of magnitude in exposure time (from 1 µs to 10 s) and investigates a wide variety of different photoresist compositions. For short exposure times (“regime I”), the laser power at the polymerization threshold can scale with the inverse square root of the exposure time, as naively to be expected for two-photon absorption. Substantial deviations occur, however, for low photoinitiator concentrations. For intermediate exposure times (“regime II”), a Schwarzschild-type of behavior is found, as discussed previously. For very long exposure times (“regime III”), an unexpected deviation from regime II is found. By presenting numerical solutions of the coupled 3D reaction–diffusion equations, this behavior is explained in terms of the diffusion of oxygen and photoinitiator molecules, respectively.

The peroxyoxalate chemiluminescence (PO‐CL) reaction is among the most powerful and versatile techniques for the detection of hydrogen peroxide (H2O2) and has been employed in various biological and chemical applications over the past 50 years. However, its two‐component nature (peroxyoxalate and fluorophore) limits its use. This contribution introduces an innovative and versatile photochemical platform technology for the synthesis of inherently fluorescent PO probes by exploiting the nitrile imine‐mediated tetrazole‐ene cycloaddition (NITEC) reaction. In the presence of hydrogen peroxide, the pioneered “2‐in‐1” molecule emits either yellow or blue light, depending on tetrazole (Tz) structure. Even in the absence of base, the emitted light remains visible and H2O2 could be detected in the nanomolar range. Critically, the PO‐Tz can be readily incorporated into polymeric materials. As a first application of this promising material, a tailor‐made PO‐Tz is grafted on poly(divinylbenzene) (PDVB) particles to enable solid‐phase chemiluminescence on microspheres.

We demonstrate how a normally base-induced or transition-metal-catalyzed cycloaddition reaction can be executed without additives by equipping the reactant with a chromophore as a light-harvesting unit, able to exploit light to boost the reaction. Specifically, we generate a highly reactive nitrile oxide upon light irradiation which is able to undergo cycloaddition reactions with a wide range of substrates by introducing a pyrene unit into the oxime precursor. Commercially available LED setups, and even ambient light, were successfully employed to induce the cycloaddition. Our approach demonstrates that the careful design of light-harvesting chromophores can be a powerful tool to facilitate light-induced chemical transformations.

We introduce a protecting-group-free concept for the powerful para-fluoro-thiol reaction (PFTR) employing a source of fluoride ions as base. The reaction is shown to be self-propagating, with under-stoichiometric amounts of base, thus effectively foregoing the need for high base concentrations. Careful tuning of the solvent–thiol combination allows for quantitative conversion, in some cases within a short timeframe, when only minimal amounts of base are used, allowing the PFTR reaction to essentially proceed base-free.

The development of practical routes that enable the synthesis of unimolecular non-natural macromolecules on multi-gram scales remains a key challenge in polymer chemistry. Herein, we present a continuous flow setup employing the light-induced chemistry based on photocaged dienes as a controlled ligation system to establish a new platform for scalable monodisperse macromolecular synthesis. 800mg of sequence-defined linear chains with 8 repeating units were obtained in a sequential protecting group based 2-step process. The current study demonstrates that photoflow synthesis is a viable approach to generate large quantities of monodisperse oligomers.

Cyclotides are small cyclic polypeptides found in a variety of organisms, ranging from bacteria to plants. Their ring structure endows those polypeptides with specific properties, such as improved stability against enzymatic degradation. Optimal cyclotide activity is often observed only in the presence of intra-ring disulfide bonds. Synthesis of soft nano-objects mimicking the conformation of natural cyclotides remains challenging. Here, a new class of natural cyclotide mimics synthesized by a stepwise folding-activation-collapse process at high dilution starting from simple synthetic precursor polymers is established. The initial folding step is carried out by a photoactivated hetero Diels–Alder (HDA) ring-closing reaction, which is accompanied by chain compaction of the individual precursor polymer chains as determined by size exclusion chromatography (SEC). The subsequent activation step comprises a simple azidation procedure, whereas the final collapse step is driven by CuAAC in the presence of an external cross-linker, providing additional compaction to the final single-ring nanoparticles (SRNPs). The unique structure and compaction degree of the SRNPs is established via a detailed comparison with conventional single-chain nanoparticles (SCNPs) prepared exclusively by chain collapse from the exact same precursor polymer (without the prefolding step). The stepwise folding-activation-collapse approach opens new avenues for the preparation of artificial cyclotide mimetics.

Catalyst-free and bond-forming light-induced reactions have seen an unprecedented renaissance in the realm of soft matter materials science due to their efficiency, spatio-temporal controllability and, sometimes, photoreversible nature. However, many of these reactions rely on the application of high energy UV light that can cause photo-degradation and is inapplicable in biological environments. If up-conversion systems or two-photon processes are to be avoided, strategies for red-shifting catalyst-free ligation technology are critically required. This Concept article introduces the reader to recent methods that lead to efficient, catalyst-free visible-light-induced ligation chemistry based on polyaromatic substituted photoreactive compounds—pyrene and anthracene—and, furthermore, emphasizes the broad and facile applicability of these molecules in polymeric material design. Concomitantly, we highlight that a careful action plot analysis of photochemical reactivity can provide deep insights into reactivity patterns, far beyond those suggested by the absorption spectrum. Indeed, we suggest that an action plot analysis is necessary for the evaluation of any photochemical system and its response to structural chemical changes.

Chemiluminescent (CL) reactions are powerful analytical tools and are present in commercially available everyday objects such as glow sticks. Herein, the photons generated by chemiluminescence are exploited to induce covalent bond breakage and formation, using a chemically generated photonic field at ambient temperature through space as energy transducer. Remarkably, the generated photons enable both the cleavage of species generating radicals as well as the execution of [2 + 2] cycloadditions, demonstrating that disparate types of reactions can be triggered. The herein-presented photochemical concept establishes the field of CL-induced photochemistry, which is poised to enable photochemical transformations in situations where physical light sources, such as lamps, LEDs, and lasers cannot be employed, including in biological environments.

We introduce the convergent synthesis of linear monodisperse sequence-defined oligomers through a unique approach, combining the Passerini three-component reaction (P-3CR) and a Diels–Alder (DA) reaction based on photocaged dienes. A set of oligomers is prepared resting on a Passerini linker unit carrying an isocyano group for chain extension by P-3CR and a maleimide moiety for photoenol conjugation enabling a modular approach for chain growth. Monodisperse oligomers are accessible in a stepwise fashion by switching between both reaction types. Employing sebacic acid as a core unit allows the synthesis of a library of symmetric sequence-defined oligomers. The oligomers consist of alternating P-3CR and photoblocks with molecular weights up to 3532.16 g mol−1, demonstrating the successful switching from P-3CR to photoenol conjugation. In-depth characterization was carried out including size-exclusion chromatography (SEC), high-resolution electrospray ionization mass spectrometry (ESI-MS) and NMR spectroscopy, evidencing the monodisperse nature of the precision oligomers.

This work pioneers the design of visible (415 nm) and UV-B light (300 nm) reactive nanoparticles via radical polymerization in aqueous heterogeneous media based on methyl methacrylate (MMA) and unique acrylates bearing tetrazole functionalities in a simple and straightforward two step reaction. Stable colloidal nanoparticles with an average diameter of 150 nm and inherent tetrazole functionality (varying from 2.5 to 10 wt% relative to MMA) are prepared via one-pot miniemulsion polymerization. In a subsequent step, fluorescent pyrazoline moieties serving as linkage points are generated on the nanoparticles by either photoinduced nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) or nitrile imine carboxylic acid ligation (NICAL) in water, thus enabling the particles as fluorescent tracers. Through in-depth molecular surface analysis, it is demonstrated that the photoreactive nanoparticles undergo ligation to a variety of substrates bearing functionalities including maleimides, acrylates, or carboxylic acids, illustrating the versatility of the particle modification process. Critically, the unique ability of the photoreactive nanoparticles to be activated with visible light allows for their decoration with UV light–sensitive molecules. Herein, the ligation of folic acid—a vitamin prone to degradation under UV light—to the photoreactive nanoparticles using visible light is exemplified, demonstrating the synthetic power of our photoreactive fluorescent nanoparticle platform technology.

The controlled incorporation of phenanthroline moieties into polymers is introduced, demonstrating their application as metal-ion complexing ligands for the construction of advanced macromolecular structures. Specifically, two phenanthroline-containing monomers based on acrylate and styrene functionalities, were synthesized. Each monomer was readily copolymerized with either N,N-dimethylacrylamide or styrene via nitroxide-mediated polymerization, resulting in narrowly distributed polar or non-polar copolymers. To demonstrate the versatility of the established polymer systems, the polar polymer was employed for transition metal induced single-chain nanoparticle formation, verified by diffusion-ordered NMR and UV/Vis spectroscopy. Furthermore, the non-polar polymer allows facile incorporation of lanthanide ions, creating luminescent metallo-polymers, in-depth characterized by advanced photophysical experiments and 2D NMR measurements.

The selective removal of structural elements plays a decisive role in 3D printing applications enabling complex geometries. To date, the fabrication of complex structures on the microscale is severely limited by multistep processes. Herein, a subtractive photoresist platform technology that is transferable from microscopic 3D printing via direct laser writing to macroscopic structures via stereolithography is reported. All resist components are readily accessible and exchangeable, offering fast adaptation of the resist's property profile. The micro‐ and macroprinted structures can be removed in a facile fashion, without affecting objects based on standard photoresists. The cleavage is analyzed by time‐lapse optical microscopy as well as via in‐depth spectroscopic assessment. The mechanical properties of the printed materials are investigated by nanoindentation. Critically, the power of the subtractive resist platform is demonstrated by constructing complex 3D objects with flying features on the microscale.

Invited for the cover of this issue are the groups of Andreas‐Neil Unterreiner and Peter W. Roesky at the Karlsruhe Institute of Technology (KIT) and Christopher Barner‐Kowollik at the Queensland University of Technology (QUT). The image depicts the incorporation of phenanthroline ligands for fabricating metal‐ion‐containing functional polymeric materials. Read the full text of the article at 10.1002/chem.201803692.

A substrate‐independent and versatile coating platform for (spatially resolved) surface functionalization, based on nitroxide radical coupling (NRC) reactions and the formation of thermo‐labile alkoxyamine functional groups, was introduced. Nitroxide‐decorated poly(glycidyl methacrylate) (PGMA) microspheres, obtained through bioinspired copolymer surface deposition using dopamine and a nitroxide functional dopamine derivative as monomers, were conjugated with small functional groups in a rewritable process. Reversible coding of the nitroxide functional microspheres by NRC and decoding through thermal alkoxyamine fission were monitored and characterized by electron paramagnetic resonance (EPR) spectroscopy and X‐ray photoelectron spectroscopy (XPS). In addition, this nitroxide coating system was exploited in “grafting‐to” polymer surface ligations of poly(methyl methacrylate) (PMMA) and poly(2,2,2‐trifluoroethyl methacrylate) (PTFEMA) in spatially confined areas. Polymer strands terminated with an Irgacure 2959 (2‐hydroxy‐4′‐(2‐hydroxyethoxy)‐2‐methylpropiophenone) photoinitiator were obtained through chain‐transfer polymerization, and subsequently coupled to nitroxide‐immobilized poly(dopamine) (PDA)‐coated silicon substrates by using rapid photoclick NRC reactions. Light‐driven polymer surface coding was visualized by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and XPS imaging.

We report the preparation of tetrazole-containing step-growth microparticles and the subsequent use of photoinduced nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) reactions on the particles with spatiotemporal control. Microparticles with an average diameter of 4.1 µm and with inherent tetrazole-ene dual functionality are prepared by a one-pot off-stoichiometric thiol-Michael addition dispersion polymerization. The NITEC reaction is performed efficiently in the solid phase by UV irradiation, leading to the formation of fluorescent pyrozoline adducts, with an estimated quantum yield of 0.7. Particle concentration-independent reaction kinetics are observed and full conversion is reached within 10 min of UV exposure at an intensity of 8 mW cm−2. Temporal control is demonstrated with either UV or rooftop sunlight irradiation of variable duration. By using two-photon writing with a laser centered around 700 nm wavelength, spatial control is demonstrated with micrometer-scale resolution via surface patterning of the microparticles. Further, microparticles with exclusive tetrazole functionality are prepared by a one-pot, two-step thiol-Michael addition dispersion polymerization. The NITEC reaction between tetrazole-functional particles and acrylates in solution is examined at various tetrazole/alkene molar ratios, and a 10:1 excess of alkenes in solution is found necessary for efficient functionalization.

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 alpha-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 (M-n = 23 400 g mol(-1), D = 1.2; approximate to 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.

Free to read on publisher website

The importance of the development of kinetic modeling tools to mechanistically understand and design bulk and solution reversible addition fragmentation chain transfer (RAFT) polymerization is highlighted. Both deterministic and stochastic kinetic modeling methods are covered, considering a detailed reaction scheme and accounting for the impact of diffusional limitations on the reaction rates. A novel strategy is introduced to fundamentally calculate the diffusional contributions for the apparent RAFT addition and fragmentation rate coefficients. Next to literature examples, case studies are included to demonstrate that detailed theoretical studies are indispensable to completely map the effect of the polymerization conditions and RAFT agent reactivity on the control over microstructural properties and the overall polymerization time. Guidelines for future kinetic modeling activities are formulated to enhance joined theoretical and experimental research.

Oligonucleotides containing photo-caged dienes were prepared and shown to react quantitatively in a light-induced Diels–Alder cycloaddition with functional maleimides in aqueous solution within minutes. Due to its high yield and fast rate, the reaction was exploited for DNA surface patterning with sub-micrometer resolution employing direct laser writing (DLW). Functional DNA arrays were written by direct laser writing (DLW) in variable patterns, which were further encoded with fluorophores and proteins through DNA directed immobilization. This mild and efficient light-driven platform technology holds promise for the fabrication of complex bioarrays with sub-micron resolution.

Propagation rate coefficient values, kp, reported by several groups for radical polymerization of bulk vinyl acetate are critically evaluated. All data are obtained by the combination of pulsed-laser polymerization and subsequent polymer analysis by size exclusion chromatography, as recommended by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Although a small (≈15%) increase in kp is observed as laser pulse repetition rate is increased from low (25–100 Hz) to high (300–500 Hz) values, all of the data fulfill the required consistency criteria and thus are combined into a benchmark set covering the temperature range of 5–70°C. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 1.35 × 107 L mol−1 s−1 and an activation energy of 20.4 kJ mol−1, with 95% confidence ellipsoids for the parameters also presented.

The majority of photoreactions reported in the literature rely on wavelengths within the UV and—to a lesser extent—visible regime for activation, either by directly triggering the reaction or by photocatalysis. However, photoreactions induced by near-infrared (NIR) light are highly attractive for biomedical applications as NIR light can penetrate deeper into tissue and causes less damage to photosensitive biological systems. Recently, upconverting nanoparticles (UCNPs) have been employed by us and others to assist NIR photoinduced reactions. UCNPs convert NIR light to UV or visible light, which can trigger photoreactions of conventional UV or visible-light-sensitive compounds. In the current Concept article, we introduce the fundamentals of UCNP-assisted photochemistry and highlight selected future applications and innovative potential for the field.

A rationale for predicting photochemical tetrazole activation in the popular nitrile imine mediated tetrazole–ene cycloaddition (NITEC) reaction, based on a combined theoretical and experimental study, is reported. DFT calculations of three tetrazole derivatives reveal that the efficiency of intersystem crossing is determined by the chromophore moiety. This species is critical in dictating the reactivity of the tetrazoles towards nitrile imine formation, yet mere inspection of the chromophore moiety's absorptivity is insufficient for predicting the tetrazole's photochemical reactivity.

We report the first mass spectrometric analysis of poly(ionic liquid)s (PILs) containing weakly coordinating anions introduced by a fast, simple, and quantitative postmodification method on the example of the hydrophilic, well-defined poly(vinylbenzylpyridinium chloride) p([VBPy]Cl) species, analyzed with an in-source collision induced dissociation-Orbitrap mass spectrometry (MS) protocol. Using the MS approach allows for the precise structural elucidation of ion-exchanged p([VBPy]Cl) utilizing AgX (X = NO3-, CF3CO2-, BF4-) salts. The anion exchange is shown to be quantitative - without observing residual chlorinated PIL - on rapid time scales, using only filtration as a standard procedure during sample preparation. In addition, the influence of weakly coordinating anions on the ionization behavior of PILs is studied in detail.

Here, the development of an adhesive is reported – generated via free radical polymerization – which can be degraded upon thermal impact within minutes. The degradation is based on a stimuli responsive moiety (SRM) that is incorporated into the network. The selected SRM is a hetero Diels-Alder (HDA) moiety that features three key properties. First, the adhesive can be degraded at relatively low temperatures (≈80 °C), second the degradation occurs very rapidly (less than 3 min), and third, the degradation of the network can readily be analyzed and quantified due to its self-reporting nature. The new reversible self-reporting adhesion system is characterized in detail starting from molecular studies of the retro HDA reaction. Moreover, the mechanical properties of the network, as well as the adhesion forces, are investigated in detail and compared to common methacrylate-based systems, demonstrating a significant decrease in mechanic stability at elevated temperatures. The current study thus represents a significant advance of the current state of the art for debonding on demand adhesives, making the system interesting for several fields of application including dental adhesives.

Detailed knowledge of the polymerization mechanisms and kinetics of academically and industrially relevant monomers is mandatory for the precision synthesis of tailor-made polymers. The IUPAC-recommended pulsed-laser polymerization-size exclusion chromatography (PLP-SEC) approach is the method of choice for the determination of propagation rate coefficients and the associated Arrhenius parameters for free radical polymerization processes. With regard to specific monomer classes - such as acrylate-type monomers, which are very important from a materials point of view - high laser frequencies of up to 500 Hz are mandatory to prevent the formation of mid-chain radicals and the occurrence of chain-breaking events by chain transfer, if industrially relevant temperatures are to be reached and wide temperature ranges are to be explored (up to 70 C). Herein the progress and state-of-the-art of high-frequency PLP-SEC with pulse repetition rates of 500 Hz is reported, with a critical collection of to-date investigated 500 Hz data as well as future perspectives for the field. The pulsed laser polymerization-size exclusion chromatography (PLP-SEC) approach is the method of choice for the investigation of propagation rate coefficients and Arrhenius parameters in free radical polymerization. An overview of the state-of-the-art of high-frequency PLP and monomers studied via 500 Hz PLP is provided and observed trends and family-type behavior are critically described, jointly with a future perspective for the field. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Free to read on publisher website Under the validity of the degenerative transfer mechanism, the activation/deactivation process in reversible addition-fragmentation chain transfer (RAFT) polymerization can be formally quantified by transfer coefficients, depending on the chemical structure of the participating radicals and dormant species. In the present work, the different literature methods to experimentally determine these RAFT transfer coefficients are reviewed and theoretically re-evaluated. The accuracy of each method is mapped for a broad range of reaction conditions and RAFT transfer reactivities. General guidelines on when which method should be applied are formulated. (Figure presented.) . © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Free to read on publisher website The current contribution serves as a critical update to a previous feature article from us (Macromol. Rapid Commun. 2012, 33, 958-971), and highlights the latest advances in the preparation of single chain polymeric nanoparticles and initial - yet promising - attempts towards mimicking the structure of natural biomacromolecules via single-chain folding of well-defined linear polymers via so-called single chain selective point folding and repeat unit folding. The contribution covers selected examples from the literature published up to ca. September 2015. Our aim is not to provide an exhaustive review but rather highlight a selection of new and exciting examples for single-chain folding based on advanced macromolecular precision chemistry. Initially, the discussion focuses on the synthesis and characterization of single-chain folded structures via selective point folding. The second part of the feature article addresses the folding of well-defined single-chain polymers by means of repeat unit folding. The current state of the art in the field of single-chain folding indicates that repeat unit folding-driven nanoparticle preparation is well-advanced, while initial encouraging steps towards building selective point folding systems have been taken. In addition, a summary of the - in our view - open key questions is provided that may guide future biomimetic design efforts. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Free to read on publisher website (published version)