A broadly applicable cross-linker for aliphatic polymers containing C-H bonds

Thermogravimetric Analysis on a Resonant Microcantilever

Thermogravimetric analysis (TGA) is a widely applied classic method for material characterization. However, the existing TGA method has reached its technical ceiling in regard to its sensitivity, efficiency, application scope, and extensibility. With temperature-programming and a picogram (10^-12 g) mass resoluble measurement function, an integrated resonant microcantilever is proposed and developed into micro-electromechanical system-based TGA (MEMS TGA) technology to satisfy the significantly higher TGA requirements. With only a nanogram (10^-9 g) level of a loaded sample, the microcantilever can conduct ultrasensitive mass-loss analysis along with ultrafast and controllable heating up to 1200 °C. Experiments have verified that MEMS TGA can improve heating efficiency by at least one order of magnitude compared to conventional TGA while maintaining measurement accuracy. The trace sample requirement also enables MEMS TGA to directly test explosive substances in an air atmosphere, while conventional TGA with a larger amount of sample has difficulty in avoiding explosion-induced equipment damage during heating. The silicon MEMS TGA microchip also exhibits functional combination (even integration) with other analytical techniques such as Raman spectroscopy to realize operando characterization.

Beyond a Linker: The Role of Photochemistry of Crosslinkers in the Direct Optical Patterning of Colloidal Nanocrystals

Surface chemistry mediated direct optical patterning represents an emerging strategy for incorporating colloidal nanocrystals (NCs) in integrated optoelectronic platforms including displays and image sensors. However, the role of photochemistry of crosslinkers and other photoactive species in patterning remains elusive. Here we show the design of nitrene- and carbene-based photocrosslinkers can strongly affect the patterning capabilities and photophysical properties of NCs, especially quantum dots (QDs). Their role beyond physical linkers stems from structure-dictated electronic configuration, energy alignment and associated reaction kinetics and thermodynamics. Patterned QD layers with designed carbene-based crosslinkers fully preserve their photoluminescent and electroluminescent properties. Patterned light emitting diodes (QLEDs) show a maximum external quantum efficiency of ≈12 % and lifetime over 4800 h, among the highest for reported patterned QLEDs. These results would guide the rational design of photoactive species in NC patterning and create new possibilities in the monolithic integration of NCs in high-performance device platforms.

Silver-catalyzed site-selective C(sp3)-H benzylation of ethers with N-triftosylhydrazones

The insertion of carbenes into the α-C-H bonds of ethers represents one of the most powerful approaches to access polysubstituted α-branched ethers. However, intermolecular carbene insertions remain challenging, since current approaches are generally limited to the use of toxic and potentially explosive α-diazocarbonyl compounds. We now report a silver-catalyzed α-C-H benzylation of ethers using bench-stable N-triftosylhydrazones as safe and convenient carbene precursors. This approach is well suited for both inter- and intramolecular insertions to deliver medicinally relevant homobenzylic ethers and 5-8-membered oxacycles in good yields. The synthetic utility of this strategy is demonstrated by its easy scalability, broad scope with valuable functional groups, high regioselectivity, and late-stage functionalization of complex oxygen-containing molecules. The relative reactivities of different types of silver carbenes and C-H bonds were also investigated by experments and DFT calculations.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Photoactive Fe Catalyst for Light-Triggered Alkyd Paint Curing

Herein, we show that the photoactive complexes [(Cp)Fe(arene)]⁺ (Cp = cyclopentadienyl; arene = C₆H₆, C₆H₅Me) act as latent catalysts that allow for photochemical control over the onset of alkyd paint curing, without the need for antiskinning agents such as the volatile 2-butanone oxime normally used to prevent curing during paint storage. The highly soluble neutral complexes [(Cp)Fe(Ch)] and [(Cp)Fe(Ch')] (Ch = cyclohexadienyl, Ch' = methylcyclohexadienyl) readily convert to the photoactive complexes [(Cp)Fe(arene)]⁺ upon oxidation in alkyd, allowing the latter to be dosed in a wide range of concentrations. Infrared and Raman studies show similar spectral changes of the alkyd paint matrix as have been observed in alkyd curing mediated by well-known, industrially applied cobalt- and manganese-based catalyst Co(neodecanoate)₂ and [(Me₃TACN)₂Mn₂(μ-OOCR)₃](OOCR). The [(Cp)Fe(Ch)]/[(Cp)Fe(arene)]⁺ system performs equally well as these cobalt- and manganese-based catalysts in terms of drying time and outperform the manganese catalyst by showing a hardness development (increase) similar to that of the cobalt-based catalyst. Based on electron paramagnetic resonance and light-activity studies, we propose that photolysis of [(Cp)Fe(arene)]⁺ generates short-lived active Fe^II species, explaining the desired latency. The [(Cp)Fe(Ch)]/[(Cp)Fe(arene)]⁺ alkyd curing systems presented herein are unique examples of intrinsically latent paint curing catalysts that (1) are based on an abundant and harmless transition metal (Fe), (2) do not require any antiskinning agents, and (3) show favorable performance in terms of drying times and hardness development.

Diversification of aliphatic C-H bonds in small molecules and polyolefins through radical chain transfer

The ability to selectively introduce diverse functionality onto hydrocarbons is of substantial value in the synthesis of both small molecules and polymers. Herein, we report an approach to aliphatic carbon-hydrogen bond diversification using radical chain transfer featuring an easily prepared O-alkenylhydroxamate reagent, which upon mild heating facilitates a range of challenging or previously undeveloped aliphatic carbon-hydrogen bond functionalizations of small molecules and polyolefins. This broad reaction platform enabled the functionalization of postconsumer polyolefins in infrastructure used to process plastic waste. Furthermore, the chemoselective placement of ionic functionality onto a branched polyolefin using carbon-hydrogen bond functionalization upcycled the material from a thermoplastic into a tough elastomer with the tensile properties of high-value polyolefin ionomers.

Emerging Applications of Aryl Trifluoromethyl Diazoalkanes and Diazirines in Synthetic Transformations

Aryl trifluoromethyl diazoalkanes and diazirines have become unique as reactants in synthetic methodology. As privileged compounds containing CF₃ groups and ease of synthetic access, aryl trifluoromethyl diazoalkanes and diazirines have been highlighted for their versatility in applications toward a wide range of synthetic transformations. This Perspective highlights the synthetic applications of these reactants as precursors of stabilized metal carbenes, i.e., donor-acceptor-substituted ones.

Electronically optimized diazirine-based polymer crosslinkers

Electronically optimized bis-diazirine crosslinkers allow aliphatic polymers to be crosslinked with up to 10-fold improved efficacy, relative to earlier designs. Activation is achieved using modest temperatures or through UV or visible light.

Covalent functionalization of polypropylene filters with diazirine-photosensitizer conjugates producing visible light driven virus inactivating materials

The SARS-CoV-2 pandemic has highlighted the weaknesses of relying on single-use mask and respirator personal protective equipment (PPE) and the global supply chain that supports this market. There have been no major innovations in filter technology for PPE in the past two decades. Non-woven textiles used for filtering PPE are single-use products in the healthcare environment; use and protection is focused on preventing infection from airborne or aerosolized pathogens such as Influenza A virus or SARS-CoV-2. Recently, C-H bond activation under mild and controllable conditions was reported for crosslinking commodity aliphatic polymers such as polyethylene and polypropylene. Significantly, these are the same types of polymers used in PPE filtration systems. In this report, we take advantage of this C-H insertion method to covalently attach a photosensitizing zinc-porphyrin to the surface of a melt-blow non-woven textile filter material. With the photosensitizer covalently attached to the surface of the textile, illumination with visible light was expected to produce oxidizing 1O2/ROS at the surface of the material that would result in pathogen inactivation. The filter was tested for its ability to inactivate Influenza A virus, an enveloped RNA virus similar to SARS-CoV-2, over a period of four hours with illumination of high intensity visible light. The photosensitizer-functionalized polypropylene filter inactivated our model virus by 99.99% in comparison to a control.

Structure-function relationships in aryl diazirines reveal optimal design features to maximize C-H insertion

Diazirine reagents allow for the ready generation of carbenes upon photochemical, thermal, or electrical stimulation. Because carbenes formed in this way can undergo rapid insertion into any nearby C-H, O-H or N-H bond, molecules that encode diazirine functions have emerged as privileged tools in applications ranging from biological target identification and proteomics through to polymer crosslinking and adhesion. Here we use a combination of experimental and computational methods to complete the first comprehensive survey of diazirine structure-function relationships, with a particular focus on thermal activation methods. We reveal a striking ability to vary the activation energy and activation temperature of aryl diazirines through the rational manipulation of electronic properties. Significantly, we show that electron-rich diazirines have greatly enhanced efficacy toward C-H insertion, under both thermal and photochemical activation conditions. We expect these results to lead to significant improvements in diazirine-based chemical probes and polymer crosslinkers.

An Efficient Diazirine-Based Four-Armed Cross-linker for Photo-patterning of Polymeric Semiconductors

A diazirine-based four-armed cross-linker (4CNN) with a tetrahedron geometry is presented for efficient patterning of polymeric semiconductors by photo-induced carbene insertion. After blending of 4CNN with no more than 3 % (w/w), photo-patterning of p-, n-, and ambipolar semiconducting polymers with side alkyl chains was achieved; regular patterns with size as small as 5 μm were prepared with appropriate photomasks after 365 nm irradiation for just 40 s. The interchain packing order and the thin film morphology were nearly unaltered after the cross-linking and the semiconducting properties of the patterned thin films were mostly retained. A complementary-like inverter with a gain value of 112 was constructed easily by two steps of photo-patterning of the p-type and n-type semiconducting polymers. The results show that 4CNN is a new generation of cross-linker for the photo-patterning of polymeric semiconductors for all-solution-processible flexible electronic devices.

New Trends in Diaziridine Formation and Transformation (a Review)

This review focuses on diaziridine, a high strained three-membered heterocycle with two nitrogen atoms that plays an important role as one of the most important precursors of diazirine photoaffinity probes, as well as their formation and transformation. Recent research trends can be grouped into three categories, based on whether they have examined non-substituted, N-monosubstituted, or N,N-disubstituted diaziridines. The discussion expands on the conventional methods for recent applications, the current spread of studies, and the unconventional synthesis approaches arising over the last decade of publications.

Photochemical Cyclopropenation of Alkynes with Diazirines as Carbene Precursors in Continuous Flow

An efficient synthesis of 3-trifluoromethyl-3-aryl-cyclopropenes via the cyclopropenation reaction of alkynes with photolytically generated carbenes from diazirine compounds is described. This reaction is performed in continuous flow using readily available LEDs under mild reaction conditions. This new and efficient method describes the synthesis of 25 examples of 3-trifluoromethyl-3-aryl-cyclopropenes with yields up to 97%, achieved in continuous flow with a 5 min residence time. Control experiments highlighted that diazirines are more efficient than diazo compounds for this transformation.

Ruthenium-Catalyzed, Chemoselective and Regioselective Oxidation of Polyisobutene

Polyolefins are important commodity plastics, yet their lack of functional groups limits their applications. The functionalization of C-H bonds holds promise for incorporating functionalities into polymers of ethylene and linear α-olefins. However, the selective functionalization of polyolefins derived from branched alkenes, even monobranched, 1,1-substituted alkenes, has not been achieved. These polymers are less reactive, due to steric effects, and they are prone to chain scission that degrades the polymer. We report the chemoselective and regioselective oxidation of a commercially important polymer of a branched olefin, polyisobutene. A polyfluorinated ruthenium-porphyrin catalyst incorporates ketone units into polyisobutene at methylene positions without chain cleavage. The oxidized polymer is thermally stable, yet it undergoes programmed reactions and possesses enhanced wetting properties.

Flexible polyfluorinated bis-diazirines as molecular adhesives

Motivated by a desire to develop flexible covalent adhesives that afford some of the same malleability in the adhesive layer as traditional polymer-based adhesives, we designed and synthesized two flexible, highly fluorinated bis-diazirines. Both molecules are shown to function as effective crosslinkers for polymer materials, and to act as strong adhesives when painted between two polymer objects of low surface energy, prior to thermal activation. Data obtained from lap-shear experiments suggests that greater molecular flexibility is correlated with improved mechanical compliance in the adhesive layer.

Multifunctional Polydopamine Particles as a Thermal Stability Modifier to Prepare Antifouling Melt Blend Composite Membranes

This study reports a novel, multifunctional, and easily obtained modifier to support the rapid advancements in the field of filtration. Polydopamine (PDA) particles (PDAPs) have been reported as a filler for constructing polymer composites, but because of their poor thermal stability, the use of PDAPs in high-temperature blend melt systems to construct antifouling membranes was rare. In this paper, high-thermal-stability methoxy polyethylene glycol amine (mPEG-NH₂)-functionalized PDA nanoparticles (mPDAPs) were first used as a modifier in high-temperature blend melt polymer composites to construct antifouling composite membranes. First, high-thermal-stability mPDAPs with an average diameter of about 390 nm were prepared by immobilized mPEG-NH₂ on the PDAP surface, then melt blend mPDAPs with ultrahigh-molecular-weight polyethylene/liquid paraffin (LP) solution and thermally reduced phase separation (TIPS) to construct antifouling membranes. A combination of properties including mechanical properties, filtration efficiency, and antifouling properties of hybrid composite membranes was investigated and demonstrated that mPDAPs were an efficient modifier for high-temperature melt blending systems. The aim of this study was to provide an effective approach to improve the membrane filtration performance by bulk hybrid modification of multifunctional nanoparticles.

Rapid Processing of Bottlebrush Coatings through UV-Induced Cross-Linking

Bottlebrush polymers can be used to introduce novel surface properties including hydrophilicity, stimuli-responsiveness, and reduced friction forces. However, simple, general, and efficient approaches to cross-linking bottlebrush polymer films and coatings are limited. Here, we report that bottlebrush polymers with an unsaturated polynorbornene backbone and thiol-terminated side chains can be cross-linked on demand by UV irradiation to produce uniform and insoluble bottlebrush polymer coatings. To quantify the kinetics and efficiency of cross-linking by UV exposure (254 nm), we measured the normalized residual thickness (NRT) of bottlebrush and linear polymer films after UV exposure and solvent washing. For bottlebrush polymers with thiol-terminated polystyrene (PS) side chains, the NRT exceeded 60% for a UV dose of 1.0 J/cm², while unfunctionalized linear PS required a dose of 7.9 J/cm² to achieve similar NRT values. Rapid UV-induced cross-linking of the bottlebrush PS was attributed to the thiol-ene coupling of the thiol-terminated side chains with the unsaturated polynorbornene backbones, as demonstrated through FTIR measurements and control studies involving bottlebrush polymers with saturated backbones. To establish the broader applicability of this approach, UV-induced cross-linking was demonstrated for thin films of bottlebrush polymers with thiol-terminated poly(methyl acrylate) (BB-PMMA-SH) side chains and those with poly(ethylene glycol) (BB-PEG) and poly(lactic acid) (BB-PLA) side chains which do not contain thiol end groups. UV-induced cross-linking of BB-PEG and BB-PLA films required the use of a multifunctional thiol additive. Finally, we demonstrated that bottlebrush polymer multilayers can be fabricated through sequential deposition and UV-induced cross-linking of different bottlebrush polymer chemistries. The cross-linking process outlined in this work is simple, general, and efficient and produces solvent-resistant coatings that preserve the unique properties and functions of bottlebrush polymers.

The post-modification of polyolefins with emerging synthetic methods

This Perspective highlights the present state of polyolefin post-modification research, especially concerning recently developed C–H functionalization chemistry. Remaining challenges and emerging strategies within the field have also been discussed.