Endo and Exo Diels-Alder Adducts: Temperature-Tunable Building Blocks for Selective Chemical Functionalization

Biomass-Derived Degradable Polymers via Alternating Ring-Opening Metathesis Polymerization of Exo-Oxanorbornenes and Cyclic Enol Ethers

Degradable polymers made via ring-opening metathesis polymerization (ROMP) hold tremendous promise as eco-friendly materials. However, most of the ROMP monomers are derived from petroleum resources, which are typically considered less sustainable compared to biomass. Herein, we present a synthetic strategy to degradable polymers by harnessing alternating ROMP of biomass-based cyclic olefin monomers including exo-oxanorbornenes and cyclic enol ethers. A library of well-defined poly(enol ether)s with modular structures, tunable glass transition temperatures, and controlled molecular weights was achieved, demonstrating the versatility of this approach. Most importantly, the resulting copolymers exhibit high degrees of alternation, rendering their backbones fully degradable under acidic conditions.

Synthetically encoded complementary oligomers

Creating the next generation of advanced materials will require controlling molecular architecture to a degree typically achieved only in biopolymers. Sequence-defined polymers take inspiration from biology by using chain length and monomer sequence as handles for tuning structure and function. These sequence-defined polymers can assemble into discrete structures, such as molecular duplexes, via reversible interactions between functional groups. Selectivity can be attained by tuning the monomer sequence, thereby creating the need for chemical platforms that can produce sequence-defined polymers at scale. Developing sequence-defined polymers that are specific for their complementary sequence and achieve their desired binding strengths is critical for producing increasingly complex structures for new functional materials. In this Review Article, we discuss synthetic platforms that produce sequence-defined, duplex-forming oligomers of varying length, strength and association mode, and highlight several analytical techniques used to characterize their hybridization.

Circular Upcycling of Bottlebrush Thermosets

The inability to re-process thermosets hinders their utility and sustainability. An ideal material should combine closed-loop recycling and upcycling capabilities. This trait is realized in polydimethylsiloxane bottlebrush networks using thermoreversible Diels-Alder cycloadditions to enable both reversible disassembly into a polymer melt and on-demand reconfiguration to an elastomer of either lower or higher stiffness. The crosslink density was tuned by loading the functionalized networks with a controlled fraction of dormant crosslinkers and crosslinker scavengers, such as furan-capped bis-maleimide and anthracene, respectively. The resulting modulus variations precisely followed the stoichiometry of activated furan and maleimide moieties, demonstrating the lack of side reactions during reprocessing. The presented circularity concept is independent from the backbone or side chain chemistry, making it potentially applicable to a wide range of brush-like polymers.

Design of Surface-Aligned Main-Chain Liquid-Crystal Networks Prepared under Ambient, Light-Free Conditions Using the Diels-Alder Cycloaddition

Surface-aligned liquid-crystal networks (LCNs) offer a solution for developing functional materials capable of performing a range of tasks, including actuation, shape memory, and surfaces patterning. Here we show that Diels-Alder cycloaddition can be used to prepare the backbone of planar aligned LCNs under mild ambient conditions without the addition of additives or UV irradiation. The mechanical properties of the networks have robust viscoelastic modulus and stiffness with a reversible local free volume change upon physical aging. This study shows new opportunities to design surface-aligned LCNs based on additive free step-growth Diels-Alder polymerization and enables the potential to incorporate a wider range of photochromic materials into LCNs.

Controlled Diels-Alder "Click" Strategy to Access Mechanically Aligned Main-Chain Liquid Crystal Networks

Aligned liquid crystal polymers are materials of interest for electronic, optic, biological and soft robotic applications. The manufacturing and processing of these materials have been widely explored with mechanical alignment establishing itself as a preferred method due to its ease of use and widespread applicability. However, the fundamental chemistry behind the required two-step polymerization for mechanical alignment has limitations in both fabrication and substrate compatibility. In this work we introduce a new protection-deprotection approach utilizing a two-stage Diels-Alder cyclopentadiene-maleimide step-growth polymerization to enable mild yet efficient, fast, controlled, reproducible and user-friendly polymerizations, broadening the scope of liquid crystal systems. Thorough characterization of the films by DSC, DMA, POM and WAXD show the successful synthesis of a uniaxially aligned liquid crystal network with thermomechanical actuation abilities.

Clickable Polymer Ligand-Functionalized Iron Oxide Nanocubes: A Promising Nanoplatform for 'Local Hot Spots' Magnetically Triggered Drug Release

Exploiting the local heat on the surface of magnetic nanoparticles (MNPs) upon exposure to an alternating magnetic field (AMF) to cleave thermal labile bonds represents an interesting approach in the context of remotely triggered drug delivery. Here, taking advantages of a simple and scalable two-step ligand exchange reaction, we have prepared iron oxide nanocubes (IONCs) functionalized with a novel multifunctional polymer ligand having multiple catechol moieties, furfuryl pendants, and polyethylene glycol (PEG) side chains. Catechol groups ensure a strong binding of the polymer ligands to the IONCs surface, while the PEG chains provide good colloidal stability to the polymer-coated IONCs. More importantly, furfuryl pendants on the polymer enable to click the molecules of interest (either maleimide-fluorescein or maleimide-doxorubicin) via a thermal labile Diels-Alder adduct. The resulting IONCs functionalized with a fluorescein/doxorubicin-conjugated polymer ligand exhibit good colloidal stability in buffer saline and serum solution along with outstanding heating performance in aqueous solution or even in viscous media (81% glycerol/water) when exposed to the AMF of clinical use. The release of conjugated bioactive molecules such as fluorescein and doxorubicin could be boosted by applying AMF conditions of clinical use (16 kAm^-1 and 110 kHz). It is remarkable that the magnetic hyperthermia-mediated release of the dye/drug falls in the concentration range 1.0-5.0 μM at an IONCs dose as low as 0.5 g_Fe/L and at no macroscopical temperature change. This local release effect makes this magnetic nanoplatform a potential tool for drug delivery with remote magnetic hyperthermia actuation and with a dose-independent action of MNPs.

Rational mechanochemical design of Diels-Alder crosslinked biocompatible hydrogels with enhanced properties

An important but often overlooked feature of Diels-Alder (DA) cycloadditions is the ability for DA adducts to undergo mechanically induced cycloreversion when placed under force. Herein, we demonstrate that the commonly employed DA cycloaddition between furan and maleimide to crosslink hydrogels results in slow gelation kinetics and "mechanolabile" crosslinks that relate to reduced material strength. Through rational computational design, "mechanoresistant" DA adducts were identified by constrained geometries simulate external force models and employed to enhance failure strength of crosslinked hydrogels. Additionally, utilization of a cyclopentadiene derivative, spiro[2.4]hepta-4,6-diene, provided mechanoresistant DA adducts and rapid gelation in minutes at room temperature. This study illustrates that strategic molecular-level design of DA crosslinks can provide biocompatible materials with improved processing, mechanical durability, lifetime, and utility.

Mapping the Morphological Landscape of Oligomeric Di-block Peptide-Polymer Amphiphiles

Peptide-polymer amphiphiles (PPAs) are tunable hybrid materials that achieve complex assembly landscapes by combining the sequence-dependent properties of peptides with the structural diversity of polymers. Despite their promise as biomimetic materials, determining how polymer and peptide properties simultaneously affect PPA self-assembly remains challenging. We herein present a systematic study of PPA structure-assembly relationships. PPAs containing oligo(ethyl acrylate) and random-coil peptides were used to determine the role of oligomer molecular weight, dispersity, peptide length, and charge density on self-assembly. We observed that PPAs predominantly formed spheres rather than anisotropic particles. Oligomer molecular weight and peptide hydrophilicity dictated morphology, while dispersity and peptide charge affected particle size. These key benchmarks will facilitate the rational design of PPAs that expand the scope of biomimetic functionality within assembled soft materials.

Photoinduced Organocatalyzed Atom Transfer Radical Polymerization (O-ATRP): Precision Polymer Synthesis Using Organic Photoredox Catalysis

The development of photoinduced organocatalyzed atom transfer radical polymerization (O-ATRP) has received considerable attention since its introduction in 2014. Expanding on many of the advantages of traditional ATRP, O-ATRP allows well-defined polymers to be produced under mild reaction conditions using organic photoredox catalysts. As a result, O-ATRP has opened access to a range of sensitive applications where the use of a metal catalyst could be of concern, such as electronics, certain biological applications, and the polymerization of coordinating monomers. However, key limitations of this method remain and necessitate further investigation to continue the development of this field. As such, this review details the achievements made to-date as well as future research directions that will continue to expand the capabilities and application landscape of O-ATRP.

In Situ Visualization of Reversible Diels-Alder Reactions with Self-Reporting Aggregation-Induced Emission Luminogens

The dynamic reversible Diels-Alder (DA) reactions play essential roles in both academic and applied fields. Currently, in situ visualization and direct monitoring of the formation and cleavage of covalent bonds in DA reactions are hampered by finite compatibility and expensive precise instruments, especially limited in solid reactions. We herein report a fluorescence system capable of in situ visualization by naked eyes and monitoring DA/retro-DA reactions. With the fluorescence quenching effect, the synthesized TPEMI could work as an innovative self-indicator for both DA termination and retro-DA occurrence. The fluorescence increases during DA reactions, and the mechanism is investigated to establish qualitative and quantitative relations. Besides rapid screening of reaction conditions and monitoring of DA exchange processes, the TPEMI fluorescence system can visualize heterogeneous and solid-state reactions with the AIE character. The TPEMI platform is expected to offer novel insights into reversible DA processes and dynamic covalent chemistry.

Trends in the Diels-Alder reaction in polymer chemistry

The Diels-Alder (DA) reaction is regarded as quite a useful strategy in organic and macromolecular syntheses. The reversibility of this reaction and the advent of self-repair technology, as well as other applications in controlled macromolecular architectures and crosslinking, have strongly boosted the research activity, which is still attracting a huge interest in both academic and industrial research. The DA reaction is a simple and scalable toolbox. Though it is well-established that furan/maleimide is the most studied diene/dienophile couple, this perspective article reports strategies using other reversible systems with deeper features on other types of diene/dienophile pairs being either petro-sourced (cyclopentadiene, anthracene) or bio-sourced (muconic and sorbic acids, myrcene and farnesene derivatives, eugenol, cardanol). This review is composed of four sections. The first one briefly recalls the background on the DA reactions involving cyclodimerizations, dienes, and dienophiles, parameters affecting the reaction, while the second part deals with the furan/maleimide reaction. The third one deals with petro-sourced and bio-sourced (or products becoming bio-sourced) reactants involved in DA reactions are also listed and discussed. Finally, the authors' opinion is given on the potential future of the crosslinking-decrosslinking reaction, especially regarding the process (e.g., key temperatures of decrosslinking) or possibly monocomponents. It presents both fundamental and applied research on the DA reaction and its applications.

Photomediated core modification of organic photoredox catalysts in radical addition: mechanism and applications

Dihydrophenazines and their analogues have been widely used as strong reducing photoredox catalysts in radical chemistry, such as organocatalyzed atom transfer radical polymerization (O-ATRP). However, when dihydrophenazines were employed as organic photoredox catalysts (OPCs) to mediate O-ATRP, the initiator efficiency was nonquantitative due to cross-coupling between dihydrophenazines and radical species. Here, a new kind of core modification for dihydrophenazines, phenoxazines and phenothiazines was developed through this cross-coupling process. Mechanistic studies suggested that the radical species would be more likely to couple with OPC' radical cations rather than the ground-state OPC. Core modification of OPCs could stabilize the radical ions in an oxidative quenching catalytic cycle. Significantly, core modifications of OPCs could lower the energy of light required for photoexcitation. Compared with their noncore-modified counterparts, all the core-modified dihydrophenazines and phenoxazines exhibited efficient performance in controlling O-ATRP for the synthesis of poly(methyl methacrylate) with higher initiator efficiencies under the irradiation of simulated sunlight.

Self-assembly using a retro Diels-Alder reaction

Despite their great utility in synthetic and materials chemistry, Diels-Alder (DA) and retro Diels-Alder (rDA) reactions have been vastly unexplored in promoting self-assembly processes. Herein we describe the first example of a retro Diels-Alder (rDA) reaction-triggered self-assembly method. Release of the steric bulkiness associated with the bridged bicyclic DA adduct by the rDA reaction allowed generation of two building blocks that spontaneously self-assembled to form a supramolecular polymer. By employing photopolymerizable lipid building blocks, we demonstrated the efficiency of the rDA-based self-assembly strategy. Generation of reactive functional groups (maleimide and furan) that can be used for further modification of the supramolecular polymer is an additional meritorious feature of the rDA-based approach. Advantage was taken of reactive functional groups to fabricate stimulus-responsive selective and tunable colorimetric sensors. The strategy developed in this study should be useful for the design of systems that participate in triggered molecular assembly.

Despite their great utility in synthetic and materials chemistry, Diels-Alder and retro Diels-Alder reactions have been vastly unexplored in promoting self-assembly processes. Here the authors show the release of steric bulkiness associated with a bridged bicyclic Diels Alder adduct by the retro Diels-Alder reaction that allowed generation of two building blocks that spontaneously self-assembled to form a supramolecular polymer.

Tritium O-Methylation of N-Alkoxy Maleimide Derivatives as Labeling Reagents for Biomolecules

An efficient procedure to access tritium-labeled maleimide derivatives in a high specific activity has been developed. N-Substituted maleimides containing the hydroxy functionality are O-methylated in a three-step synthesis route, including (1) Diels-Alder protection of the maleimide core, (2) O-methylation by the use of commercially available [³H]methyl nosylate, and (3) deprotection by retro-Diels-Alder reaction. With our procedure, N-hydroxyalkyl maleimide derivatives can be labeled in overall radiochemical yields of 13-15% and in >98% radiochemical purity. The major advantage of N-alkoxy maleimides in comparison to N-alkylated maleimides such as N-ethylmaleimide is their lower volatility, which enables safer handling with respect to radiation-safety protection. Tritium-labeled maleimide building blocks allow subsequent Michael-type conjugation reactions of thiol-containing biomolecules for mechanistic in vitro or in vivo studies.

Solvent Effects and Side Reactions in Organocatalyzed Atom Transfer Radical Polymerization for Enabling the Controlled Polymerization of Acrylates Catalyzed by Diaryl Dihydrophenazines

Investigation of the effects of a solvent on the photophysical and redox properties of the photoredox catalyst (PC), N,N-di(2-naphthyl)-5,10-dihydrophenazine (PC 1), revealed the opportunity to use tetrahydrofuran (THF) to modulate the reactivity of PC 1 toward achieving a controlled organocatalyzed atom transfer radial polymerization (O-ATRP) of acrylates. Compared with dimethylacetamide (DMAc), in tetrahydrofuran (THF), PC 1 exhibits a higher quantum yield of intersystem crossing (ΦISC = 0.02 in DMAc, 0.30 in THF), a longer singlet excited-state lifetime (τ Singlet = 3.81 ns in DMAc, 21.5 ns in THF), and a longer triplet excited-state lifetime (τ Triplet = 4.3 μs in DMAc, 15.2 μs in THF). Destabilization of 1 •+, the proposed polymerization deactivator, in THF leads to an increase in the oxidation potential of this species by 120 mV (E 1/2 0 = 0.22 V vs SCE in DMAc, 0.34 V vs SCE in THF). The O-ATRP of n-butyl acrylate (n-BA) catalyzed by PC 1 proceeds in a more controlled fashion in THF than in DMAc, producing P(n-BA) with low dispersity, Đ (Đ < 1.2). Model reactions and spectroscopic experiments revealed that two initiator-derived alkyl radicals add to the core of PC 1 to form an alkyl-substituted photocatalyst (2) during the polymerization. PC 2 accesses a polar CT excited state that is ~40 meV higher in energy than PC 1 and forms a slightly more oxidizing radical cation (E 1/2 0 = 0.22 V for 1 •+ and 0.25 V for 2 •+ in DMAc). A new O-ATRP procedure was developed wherein PC 1 is converted to 2 in situ. The application of this method enabled the O-ATRP of a number of acrylates to proceed with moderate to good control (Đ = 1.15-1.45 and I* = 83-127%).

Norbornadiene Chain-End Functional Polymers as Stable, Readily Available Precursors to Cyclopentadiene Derivatives

A novel method for facile postpolymerization functionalization of synthetic polymers using terminal norbornadiene (NBD) building blocks is presented. Incorporation of the NBD functionality streamlines the synthesis of a wide array of block polymers utilizing multistep click chemistry strategies. Previously, the use of NBD-functionalized initiators produced polymers that underwent a cascade of Diels-Alder (DA) reactions to unveil a reactive cyclopentadiene (Cp) chain end. When coupled with a maleimide-bearing counterpart, a highly efficient DA cycloaddition with the terminal Cp can occur. To extend this concept to a range of polyacrylates and commercially available poly(ethylene glycol) systems, we developed a novel NBD acid building block for postpolymerization functionalization. Employing this process, we have demonstrated straightforward access to a library of block polymers that leverage this NBD click platform.

SLIM: A Short-Linked, Highly Redox-Stable Trityl Label for High-Sensitivity In-Cell EPR Distance Measurements

The understanding of biomolecular function is coupled to knowledge about the structure and dynamics of these biomolecules, preferably acquired under native conditions. In this regard, pulsed dipolar EPR spectroscopy (PDS) in conjunction with site-directed spin labeling (SDSL) is an important method in the toolbox of biophysical chemistry. However, the currently available spin labels have diverse deficiencies for in-cell applications, for example, low radical stability or long bioconjugation linkers. In this work, a synthesis strategy is introduced for the derivatization of trityl radicals with a maleimide-functionalized methylene group. The resulting trityl spin label, called SLIM, yields narrow distance distributions, enables highly sensitive distance measurements down to concentrations of 90 nm, and shows high stability against reduction. Using this label, the guanine-nucleotide dissociation inhibitor (GDI) domain of Yersinia outer protein O (YopO) is shown to change its conformation within eukaryotic cells.

Using Polypeptide Bearing Furan Side Chains as a General Platform to Achieve Highly Effective Preparation of Smart Glycopolypeptide Analogue-Based Nano-Prodrugs for Cancer Treatment

Although several synthetic polypeptide-based nano-prodrugs (NPDs) have entered clinical trials for cancer treatment, achieving a highly effective production of the NPDs for clinical translation remains a challenge. Herein, we develop a typical preparation of pH/glutathione (GSH) dual-responsive glycopolypeptide analogue NPDs having a high drug capsulation/loading efficiency of ca. 93% and ca. 27% even based on ring-opening polymerization (ROP) of a novel and general furan-containing N-carboxyanhydride (NCA) monomer, which facilitates the Diels-Alder (D-A) side-chain functionalization by maleimide modified chemotherapy drug without using any reactive additives. High reactivity of the D-A reaction resulting in the high preparation efficiency of the NPDs is confirmed by ¹H NMR and density functional theory (DFT) calculations. The self-assembled properties as well as the dual-responsiveness of the NPDs are systemically studied by particle size and zeta potential assay, transmission electron microscopy and drug-delivery dynamics. The cell uptake mechanism, intracellular drug distribution, in vitro/vivo antitumor activity evaluations and the main organ damages of the NPDs are all investigated. Our work can provide a good solution to solve the inefficient fabrication of the smart synthetic polypeptide-based micelles for cancer treatment by following this general and sophisticated platform.

Tuning the thermoreversible temperature domain of PTMC-based networks with thermosensitive links concentration

In this work, thermoreversible poly(trimethylene carbonate) (PTMC) based networks with different crosslinking densities were obtained by Diels-Alder (DA) reaction between furan-functionalized PTMC precursors and a bismaleimide. Furan-grafted PTMC with various functionalities determined by ¹H-NMR analyses were prepared from telechelic PTMC oligomer, glycerol, 4,4'-methylenebis(cyclohexyl isocyanate) (H₁₂MDI) and furfuryl alcohol. The formation of network structures by DA reaction between furan and maleimide groups were proved by Fourier-transform infrared spectroscopy (FT-IR). Although both exo and endo DA adduct forms exist, the thermally more stable exo form dominates. The thermoreversibility of networks was evidenced by FT-IR, solubility, differential scanning calorimetry (DSC) and rheology experiments at different temperatures. By increasing furan functionality or node concentration, denser and stiffer networks could be formed with higher Young's modulus and true stress at break in tensile tests, as well as higher crossover temperature, which indicates a nominal transition from elastic behavior to viscous state. The disruption of networks was found to occur in high temperature ranges from 130 to 160 °C, depending on their crosslinking density.

Temperature-mediated molecular ladder self-assembly employing Diels–Alder cycloaddition

Thermal annealing of sequence-defined, maleimide- and furan-bearing oligomers enables sequence-selective hybridization to afford molecular ladders incorporating Diels–Alder adduct-based rungs.

Thermoreversible cross-linking of ethylene/propylene copolymers based on Diels–Alder chemistry: the cross-linking reaction kinetics

The kinetics of the cross-linking reaction of ethylene/propylene rubbers based on Diels–Alder chemistry was detected by the rheological method.

Stereoisomeric furan/maleimide adducts as latent monomers for one-shot sequence-controlled polymerization

Two stereoisomeric latent monomers were used for one-shot sequence-controlled polymerization to create diverse sequence structures.

Structural and theoretical study of four novel norcantharidine derivatives: two new cases of conditional isomorphism

Structural and theoretical studies of four novel 5,6-dehydronorcantharidine (DNCA)/norcantharidine (NCA) derivatives, namely (3aR,4S,7R,7aS)-2-phenyl-3a,4,7,7a-tetrahydro-4,7-epoxy-1H-isoindole-1,3(2H)-dione, C₁₄H₁₁NO₃ (DNCA-A), (3aR,4S,7R,7aS)-2-(4-nitrophenyl)-3a,4,7,7a-tetrahydro-4,7-epoxy-1H-isoindole-1,3(2H)-dione, C₁₄H₁₀N₂O₅ (DNCA-NA), (3aR,4S,7R,7aS)-2-(4-nitrophenyl)-3a,4,5,6,7,7a-hexahydro-1H-4,7-epoxyisoindole-1,3(2H)-dione, C₁₄H₁₂N₂O₅ (NCA-NA), and (3aR,4S,7R,7aS)-2-(2-hydroxyethyl)-3a,4,5,6,7,7a-hexahydro-1H-4,7-epoxyisoindole-1,3(2H)-dione, C₁₀H₁₃NO₄ (NCA-AE), are reported. The supramolecular interactions and single-crystal structural characteristics of these molecules, together with the crystal structures of four other similar molecules, i.e. NCA-A (the 4-phenyl derivative of NCA-NA), DNCA-AE (the 5,6-unsaturated derivative of NCA-AE), DNCA and NCA, were analysed. Surprisingly, DNCA-A and NCA-A, as well as DNCA-NA and NCA-NA, proved to be isomorphic, while DNCA-AE and NCA-AE, as well as DNCA and NCA, have very different crystal structures. These are very rare isostructural examples between unsaturated and saturated oxanorbornene/oxanorbornane derivatives. To further explore how noncovalent interactions (NCIs) affect the degree of isomorphism in this particular series of rigid molecules where there is a fairly limited conformational degree of freedom, all four pairs of crystal structures were analyzed in parallel. The differentiation in NCIs which entails the packing mode of similar molecules is supported by energy calculations based on real or exchanged crystal structures. Our results show that minor structural differences may result in very different supramolecular interactions, and so lead to altered packing modes in the crystalline solids. Even if isostructurality sometimes occurs, the possibility of various molecular packing types cannot be ruled out. On the other hand, isomorphism may just be the result of kinetic possibilities instead of relative thermodynamic stabilities. Though crystal structure prediction is formidable, the comparison method based on existing crystal structures and quantum calculations can be used to predict the probability of isomorphism. This understanding will help us to design new norbornene derivatives with specified structures.

Norbornadienes: Robust and Scalable Building Blocks for Cascade "Click" Coupling of High Molecular Weight Polymers

Herein, we report the development of a scalable and synthetically robust building block based on norbornadiene (NBD) that can be broadly incorporated into a variety of macromolecular architectures using traditional living polymerization techniques. By taking advantage of a selective and rapid deprotection with tetrazine, highly reactive "masked" cyclopentadiene (Cp) functionalities can be introduced into synthetic polymers as chain-end groups in a quantitative and efficient manner. The orthogonality of this platform further enables a cascade "click" process where the "unmasked" Cp can rapidly react with dienophiles, such as maleimides, through a conventional Diels-Alder reaction. Coupling proceeds with quantitative conversions allowing high molecular weight star and dendritic block copolymers to be prepared in a single step under ambient conditions.

Remarkable Reactivity of Boron-Substituted Furans in the Diels-Alder Reactions with Maleic Anhydride

The reactivity of boron-substituted furans as dienes in the Diels-Alder reaction with maleic anhydride has been investigated. Gratifyingly, the furans with boryl substituents at C-3 gave the exo cycloadduct exclusively with excellent yields. In particular, the potassium trifluoroborate exhibited outstanding reactivity at room temperature. Theoretical calculations suggested that the trifluoroborate group is highly activating and also that the thermodynamics is the main factor that determines whether the products can be obtained efficiently or not.

Photoinduced Organocatalyzed Atom Transfer Radical Polymerization Using Low ppm Catalyst Loading

Photoinduced organocatalyzed atom-transfer radical polymerization (O-ATRP) is a controlled radical polymerization methodology that can be mediated by organic photoredox catalysts under the influence of light. However, typical O-ATRP systems require relatively high catalyst loadings (1000 ppm) to achieve control over the polymerization. Here, new core-extended diaryl dihydrophenazine photoredox catalysts were developed for O-ATRP and demonstrated to efficiently operate at low catalyst loadings of 5-50 ppm to produce polymers with excellent molecular weight control and low dispersity, while achieving near-quantitative initiator efficiency. Photophysical and electrochemical properties of the catalysts were computationally predicted and experimentally measured to correlate these properties with improved catalytic performance. Furthermore, these catalysts were utilized to synthesize materials with complex architectures, such as triblock copolymers and star polymers. To demonstrate their broad utility, polymerizations employing these catalysts were successfully scaled up to 5 g and revealed to efficiently operate under air.

Tunable Orthogonal Reversible Covalent (TORC) Bonds: Dynamic Chemical Control over Molecular Assembly

Dynamic assembly of macromolecules in biological systems is one of the fundamental processes that facilitates life. Although such assembly most commonly uses noncovalent interactions, a set of dynamic reactions involving reversible covalent bonding is actively being exploited for the design of functional materials, bottom-up assembly, and molecular machines. This Minireview highlights recent implementations and advancements in the area of tunable orthogonal reversible covalent (TORC) bonds for these purposes, and provides an outlook for their expansion, including the development of synthetically encoded polynucleotide mimics.

A photo-selective chain-end modification of polyacrylate-iodide and its application in patterned polymer brush synthesis

A photo-selective chain-end modification of polyacrylate-iodide (polymer-I) was developed. The method was used to generate chain-end patterned polymer brushes.

Phenothiazines, Dihydrophenazines, and Phenoxazines: Sustainable Alternatives to Precious-Metal-Based Photoredox Catalysts

The application of photoredox catalysis to atom-transfer radical polymerization (ATRP) has resulted in the development of strongly reducing organic photoredox catalysts (PCs) that are some of the most reducing catalysts known. The objectives of this review are to highlight these PCs with regard to their development and applications in polymer and organic synthesis, as well illuminate aspects of these PCs that remain to be studied further.

Multi-stimuli responsive trigger for temporally controlled depolymerization of self-immolative polymers

The design and development of a multi-stimuli trigger enables temporal control over trigger cleavage and subsequent depolymerization of self-immolative polymer.

Organocatalyzed Atom Transfer Radical Polymerization Catalyzed by Core Modified N-Aryl Phenoxazines Performed under Air

Organocatalyzed atom transfer radical polymerization (O-ATRP) was performed under air using core modified N-aryl phenoxazines as photoredox catalysts (PCs) to synthesize poly(methyl methacrylate) in a controlled fashion with initiator efficiency (I* = M n,theo/M n × 100) ranging from 84 to 99% and dispersity being ∼1.2-1.3. Reduction of the reaction vial headspace was key for enabling the polymerization to proceed in a controlled fashion, as has been observed in Cu catalyzed controlled radical polymerizations. The ability to synthesize block copolymers and turn the polymerization on and off via manipulation of the light source was demonstrated. Six core modified N-aryl phenoxazines were able to catalyze O-ATRP under air, albeit with most PCs achieving I*s ∼ 5% lower under air compared to when the reaction was performed under nitrogen.

Photo-selective chain end transformation of polyacrylate-iodide using cysteamine and its application to facile single-step preparation of patterned polymer brushes

A novel photo-selective reaction for converting polymer-I to polymer-SH and polymer-H and its application to patterned functional polymer brushes.