Chain Propagation Research Articles

Self‐promoted Controlled Ring‐opening Polymerization via Side Chain‐mediated Proton Transfer for the Synthesis of Tertiary Amine‐pendant Polypeptoids

Proton transfer is essential in virtually all biochemical processes, with enzymes facilitating this transfer by optimizing the proximity and orientation of reactants through site‐specific hydrogen bonds. Proton transfer is also crucial in the rate‐determining step for the ring‐opening polymerization of N‐carboxyanhydrides (NCAs), widely used to prepare various peptidomimetic materials. This study utilizes side chain‐assisted strategy to accelerate the rate of chain propagation by using NCAs with tertiary amine pendants. This moiety enables hydrogen bond formation between the incoming NCA and the polymer amino growing end. The tertiary amine side chain of the NCA forms a proton shuttle, via a less constrained transition state, to facilitate the proton transfer process. Moreover, the tertiary amine side chains enable the precipitation of NCA monomers through in situ protonation during the monomer synthesis. This greatly facilitates the synthesis of these unreported monomers, allowing the direct controlled synthesis of tertiary amine‐pendant polypeptoids. This side chain‐promoted polymerization has rarely been reported. Additionally, the tertiary amine side chains, as widely used functional groups, endow the polymers with unique properties including pH‐ and thermo‐responsiveness, tunable pKas, and siRNA transfection capability. The self‐promoted synthesis, facile monomer preparation, and attractive properties make tertiary amine‐pendant polypeptoids promising materials for various applications.

Self-promoted Controlled Ring-opening Polymerization via Side Chain-mediated Proton Transfer for the Synthesis of Tertiary Amine-pendant Polypeptoids.

Proton transfer is essential in virtually all biochemical processes, with enzymes facilitating this transfer by optimizing the proximity and orientation of reactants through site-specific hydrogen bonds. Proton transfer is also crucial in the rate-determining step for the ring-opening polymerization of N-carboxyanhydrides (NCAs), widely used to prepare various peptidomimetic materials. This study utilizes side chain-assisted strategy to accelerate the rate of chain propagation by using NCAs with tertiary amine pendants. This moiety enables hydrogen bond formation between the incoming NCA and the polymer amino growing end. The tertiary amine side chain of the NCA forms a proton shuttle, via a less constrained transition state, to facilitate the proton transfer process. Moreover, the tertiary amine side chains enable the precipitation of NCA monomers through in situ protonation during the monomer synthesis. This greatly facilitates the synthesis of these unreported monomers, allowing the direct controlled synthesis of tertiary amine-pendant polypeptoids. This side chain-promoted polymerization has rarely been reported. Additionally, the tertiary amine side chains, as widely used functional groups, endow the polymers with unique properties including pH- and thermo-responsiveness, tunable pKas, and siRNA transfection capability. The self-promoted synthesis, facile monomer preparation, and attractive properties make tertiary amine-pendant polypeptoids promising materials for various applications.

Pro-aromatic Natural Terpenes as Unusual "Slingshot" Antioxidants with Promising Ferroptosis Inhibition Activity.

Ferroptosis is a cell death mechanism based on extensive cellular membrane peroxidation, implicated in neurodegenerative and other diseases. The essential oil component γ-terpinene, a natural monoterpene with a unique highly oxidizable pro-aromatic 1,4-cyclohexadiene skeleton, inhibits peroxidation of polyunsaturated lipid in model heterogeneous systems (micelles and liposomes). Upon H-atom abstraction, an unstable γ-terpinene-derived peroxyl radical is formed, that aromatizes to p-cymene generating HOO• radicals. As HOO• are small and hydrophilic radicals, they quickly diffuse outside the lipid core, blocking the radical chain propagation of polyunsaturated lipids. This unprecedented antioxidant "slingshot" mechanism explains why γ-terpinene shows a protective activity against ferroptosis, being effective at submicromolar concentrations in human neuroblastoma (SH-SY5Y) cells.

Effects of water sprays on hydrogen autoignition in heated air

The effects of water mist with different diameters and mass loadings on hydrogen auto-ignition in air with elevated temperatures and pressures are studied. It is shown that smaller droplets with larger mass loading are the most effective in inhibiting the auto-ignition process. Under elevated initial pressures and temperatures, the role of water mist in impacting auto-ignition is minimal, primarily because the ignition delay is substantially shorter than the droplet evaporation duration. As the initial temperature decreases, there exists a temperature value below which water mist markedly extends the ignition delay time, even escalating it by up to two orders of magnitude or causing a complete suppression of ignition. The rate of production for important radicals was analyzed. The data indicates that the evaporation of the water mist lowers the initial temperature, leading to a reduction in the rate of the temperature-sensitive fast chain branching reaction. In contrast, the third body reaction that generates HO2 is not sensitive to temperature. Meanwhile, the introduction of H2O results in an earlier activation of the third body reactions. Consequently, the production of HO2 is enhanced, emphasizing the role of subsequent slow chain propagation reactions that generate OH, thereby extending the ignition delay. By deactivating the mass transfer between two phases and introducing artificial species, it is evident that water vapor dilution and thermal effects predominantly influence the ignition delay, while its role as a direct reactant is minimal.

Highly Tolerant Living/Controlled Anionic Polymerization of Dialkyl Acrylamides Enabled by Zinc Triflate/Phosphine Lewis Pair.

Living polymerizations of polar vinyl monomers have been successful for decades. However, they still suffer the following challenges: fast propagation, air-moisture tolerance, and negligible side reactions even at elevated temperatures. Here, we developed an unprecedented polymerization that overcomes these limitations using a Lewis pair catalyst. The anionic polymerization of dialkyl acrylamides proceeded in a living/controlled matter using Zn(OTf)2/PPh3 within a wide temperature range of 25-100 °C for short times (1-10 min) even under open-air conditions. The recovery and reuse of Zn(OTf)2 without loss of polymerization activity were observed to be possible. The polymerization was retarded by excess Zn(OTf)2, the additive methanol, and water, indicating equilibriums of the propagating species with them. The putative propagating zinc triflate-ate complex was tolerant to the protic additives and significantly selective for the propagation.

Capability of isoprene and butadiene as comonomers under metallocene catalysis: Activating active sites with the addition of 1-hexene and propylene

The copolymerization of ethylene (E) with cyclic diene has been reported often in the literature. However, to evaluate isoprene (IP) and butadiene (BD) as co- and ter-monomers catalyzed by metallocene are comparatively rare. In this work, we performed a set of co- and terpolymerization of IP and BD by using symmetrical metallocene catalysts Cat-I and Cat-II. The E/IP and E/BD copolymers Cat-I catalyzed activity is lower than PE and E/P and significantly increased, reaching 3.1 × 107gpolymer/molCat*h, but unlikely the insertion of IP and BD were not detected. The active sites of Cat-I are not factional for IP and BD, and they are not even activated with the addition of 1-hexene (1H) and P. The 1,4-insertion of these into catalyst active sites produced a stable π-allyl type species that actively produced dormant sites in the consequent propagation chain, and many other reactions that convert to dormant sites occur. In contrast, the Cat-II bearing silicon bridged moiety shows additional activity with IP and BD and an increasing trend with P and IH, demonstrating an activating feature. To conduct an extensive investigation, we performed a systematic comparison of the proportion of catalytically [Zr]/[C*] active sites and polymerization rate constant (kp) for both catalysts to shed light on the Zn-based chain termination and transfer reactions. The [C*]/[Zr] was significantly decreased with IP and BD, but compared to BD, IP shows a higher level of active sites and molecular weight, demonstrating that chain transfer reactions are faster with BD. Adding P and 1H increased the [C*]/[Zr], reactivating the prior inactive sites of the catalyst that were dormant or not functioning during the PE, E/IP, and E/BD polymers and reducing the chain transfer reactions. Compared to Cat-I, Cat-II demonstrates a higher level of [C*]/[Zr]; furthermore, the addition of 1H, a slight change in the [C*]/[Zr], while P with IP overall higher level (92.92 %) of the [C*]/[Zr] showed that P with IP shows a double activation for Cat-II and activates all those active sites that were dormant. BD significantly reduced the level of active sites to 49 % compared to IP and showed poisoning for the active site that was active for ethylene. The kpE of E/BD and E/IP polymers are lower than the kpE of E/BD/1H, E/P/IP, E/BD/1H, and E/IP/P, indicating P and 1H significantly activated the dormant sites of the E, IP, and BD that were dormant during the copolymerization. Cat-I catalyzed E/IP/1Hpolymers, the kp1H of 1H is 15.8 L/mol·s, and the kpP of P is 83.3 L/mol·s, demonstrating that the P and more involved in the activation of active sites, and kpP of P is more increased with Cat-II, indicating that substituents of the Cat-II make the catalyst more active for P.

DFT Modeling of Coordination Polymerization of Polar Olefin Monomers by Molecular Metal Complexes

Introducing polar functional groups into polyolefin chains through polar olefin monomer coordination (co)polymerization can directly and significantly improve the surface properties of polymer materials and expand their application range. Therefore, the related research has always received considerable attention from both academia and industry. Many experimental studies have been reported in this field, and molecular metal complexes have shown high catalytic activity and selectivity in polar olefin monomer polymerizations. Although considerable DFT calculations have also been conducted for better understanding of the (co)polymerization performance, the factors governing the activity, selectivity, and molecular weight of resulting polymers are still ambiguous. This review mainly focuses on the DFT studies of polar olefin monomer coordination (co)polymerization catalyzed by molecular metal complexes in recent years, discussing the chain initiation and propagation, the origin of polymerization activity and selectivity, and the specific role of additives in the (co)polymerization reactions.

A Study on the Influence of Radial Spoiler Arrangement on the Combustion Process of Wankel Rotor Engines

Wankel rotor engines are widely used in various fields due to their high power density and simple structure. This paper presents the optimisation of the Wankel rotor engine by simple modifications of the structure. We propose a radial spoiler arrangement scheme that can affect the flame propagation speed and reaction severity by altering the flow field distribution and pre-reaction distribution in the cylinder. By comparing the effects of four layout schemes on flame propagation speed and reaction intensity, including no spoilers, radial spoilers deflected at an angle of 10° in the negative direction of the Z-axis, radial spoilers deflected at an angle of 10° in the positive direction of the Z-axis, and radial spoilers arranged at the centre of the rotor combustion chamber, the benefits of different layout schemes were evaluated. We conducted a study on the influence of the arrangement of radial spoilers on the combustion process of a Wankel rotary engine through theoretical calculations. This helps to reduce engine vibration, improve engine operation stability, and enhance engine performance.

Et3Al and Et2Zn cooperatively enhanced Ti catalyzed ethylene coordinative chain transfer polymerization

Developing olefin polymerization catalytic systems for both high activity and high chain transfer efficiency by using coordinative chain transfer polymerization (CCTP) is critical for the controlled synthesis of various novel high-value polyolefin materials. While the limited choice of chain transfer agents (CTAs) makes it a challenge to achieve highly efficient CCTP as well as chain propagation due to their competition during the polymerization. Here we report using the combination of R3Al and R2Zn in equal amount for tetradentate amine-bis(phenolate) Ti complex and tridentate ketoiminate Ti complex catalyzed ethylene polymerization and observed significantly enhanced CCTP catalytic performance in terms of activity, chains/cat, fill ratio, and kct/kp vs. using R3Al as CTA. The role of R3Al and R2Zn during ethylene polymerization were probed by preliminary control experiments studies and a possible mechanistic scenario was proposed.

Selective aerobic oxidation of cumene to acetophenone via N-hydroxymaleimide fabricated by a sequential copolymerization and imidization

Selective oxidation of cumene to value-added acetophenone is of great importance and environmental friendliness. N-hydroxymaleimide (NHMI) was immobilized on a polymer substrate via co-polymerization followed by an imidization in this work, and the catalyst was characterized and tested for oxidation of cumene to acetophenone. The catalyst was found active and selective for the transformation from cumene to acetophenone. The optimized catalyst exhibited a cumene conversion of 93.3 % and a selectivity to acetophenone of 79.2 %. The cumene oxidation catalyzed by the immobilized NHMI copolymers may proceed in the presence of the cocatalyst Co2+ by a radical way based on the key intermediates or free radicals for the chain propagation of the auto-oxidation reaction observed. Accordingly, a reaction network and mechanism were proposed. The synthesized NHMI catalysts and the fabrication method may shed some light on the development of immobilized catalysts for the activation and selective oxidation of benzylic C−H bonds.

Unraveling the propagation dynamics of passenger congestion in public transportation systems using big smart card data

We propose a comprehensive framework to explore propagation of passenger crowdedness in public transportation systems using 74.16 million trip chains. A deep learning method HSTGCNT is adopted which firstly models normal spatio-temporal dependencies in time series data, and assigns an anomaly score to new data based on its prediction error. The derived discrete anomaly events are further aggregated into groups based on their spatio-temporal proximity by using a clustering algorithm that binds shared nearest neighbors together based on pairwise correlations. The clustering results uncover several key stages located in a propagation chain, such as a core event, adjacent events with spatio-temporal proximity, or a broadcasting group. Weibo data serves as a third-party dataset to verify the above anomaly clusters. Results show that the framework is effective to detect outlier clusters, with recall value being 0.86. Findings may assist public safety departments in taking pro-active actions to interrupt chaos from broadcasting.

Mechanism and kinetics study of the chemically initiated oxidative polymerization of hexafluoropropylene

AbstractChemically initiated oxidative polymerization stands out as the most suitable method for the large‐scale and controllable synthesis of perfluoropolyether (PFPE). However, the mechanism and related reaction kinetics of this synthesis reaction remain elusive. In this study, PFPE was synthesized through the copolymerization of hexafluoropropylene and oxygen, initiated by fluorine. Subsequently, the synthesis mechanism of this chemically initiated oxidative polymerization was first explored using density functional theory. Simulation results yielded a comprehensive reaction network of the synthesis process, including chain initiation, propagation, decomposition, transfer, and termination. Meanwhile, a detailed kinetic model was constructed based on theoretical reaction rates of relevant elementary reactions. The effects of reaction operating conditions on the molecular weight of PFPE were experimentally investigated, with results in good agreement with the kinetic model. This work stablishes a solid foundation for optimizing and controlling the PFPE synthesis process.

Solvent Engineering of Thermo-Responsive Hydrogels Facilitates Strong and Large Contractile Actuations.

Thermo-responsive hydrogels can generate the actuation force through volumetric transitions in response to temperature changes. However, their weak mechanical properties and fragile actuation performance limit robust applications. Existing approaches to enhance these properties have typically depended on additional components, leading to an unavoidable interference to the actuation performance. In this work, robust thermo-responsive hydrogels are fabricated through solvent engineering. A particular solvent, N-methylformamide, interacts affinitively with the carbonyl group of N-isopropylacrylamide monomer, solubilizes the monomer with extremely high concentration, stabilizes chain propagation during polymerization, and greatly increases chain lengths and entanglements of the resulting polymer. The synthesized hydrogels are highly elastic, strong, and tough, displaying remarkable thermo-responsive contractile actuation. The simple synthetic process can broaden its applicability in designing robust functional hydrogel applications.

Enhancing the Kinetics of Vapor-based Polymerization by Pulsed Filament Approach.

Initiated chemical vapor deposition is a versatile technique for synthesizing conformal polymer films on both planar and porous surfaces. It can retain functional groups and avoid undesired cross-linking. However, there is still room for enhancing its performance without altering the feed parameters. Here, we investigate a pulsed iCVD approach to improve the deposition process, achieved by switching on and off the resistively heated filament periodically. By strategically switching off the filament, a shortage of thermally activated primary radicals was created, which allowed uninterrupted chain propagation with fewer termination reactions and potentially increased monomer conversion rates. This has caused significantly faster deposition kinetics with a higher molecular weight and longer chain length for poly(glycidyl methacrylate) compared to continuous deposition. Spectra analyses confirmed that the functionality and stoichiometry ratios remained intact throughout the pulsed deposition process. The pulsed iCVD method is therefore a competitive and sustainable tool, demonstrating fast deposition kinetics and a well-preserved functionality.

Photoinduced Intermolecular Radical Hydroalkylation of Olefins via Ligated Boryl Radicals-Mediated Halogen Atom Transfer.

Light-mediated Halogen-Atom Transfer (XAT) has become a significant methodology in contemporary synthesis. Unlike α-aminoalkyl and silyl radicals, ligated boryl radicals (LBRs) have not been extensively explored as halogen atom abstractors. In this study, we introduce NHC-ligated boranes as optimal radical chain carriers for the intermolecular reductive radical hydroalkylation and hydroarylation of electron-deficient olefins by using direct UV-A light irradiation. DFT analysis allowed us to rationalize the critical role of the NHC ligand in facilitating efficient chain propagation.

BCN‐Supported CoFe Alloy Catalysts for Enhanced C─C Coupling in Photothermocatalytic CO Hydrogenation

AbstractThe high selectivity of C─C coupling reactions in Fischer–Tropsch synthesis (FTS) is often limited due to the difficulty in the regulation of transition metals acting as active sites to balance between C─C chain propagation and over hydrogenation. Herein, BCN‐supported CoFe alloy catalyst has been successfully constructed for promoting C─C chain propagation. When exposed to light irradiation, the CoFe‐BCN catalyst exhibits a higher CO conversion of 18.4% with the enhanced selectivity toward multi‐carbon (C2+) hydrocarbons that increases from 22.4% to 64.1%, and the reduced over hydrogenation to CH4 that decreases from 74.8% to 25.4% in contrast to Co‐BCN catalyst. Structural characterizations indicate that introducing Fe to create CoFe alloy can decrease the d‐band center of Co, which significantly promotes C─C coupling reactions but weakens hydrogenation in FTS process. The findings underscore the potential of modifying catalysts with metal atoms to optimize their electronic structure to regulate reaction pathways in CO hydrogenation for high‐value products formation.

To overcome the unconventional polymerization behavior of single-site symmetrical metallocene catalyst in the presence of borate

The β-H transfer of a propagating chain shows an unconventional polymerization behavior with single-site catalysts. In this work, we seek to address these challenges in the presence of different alkylaluminum cocatalysts. Adjusting the amount and type of cocatalyst in the polymerization process can affect molecular weight distribution (MWD) and chain ends formed via β-H transfer. The cocatalyst's elevated triethylelaluminum (TEA) by more than 50 % resulted in a drop in isoprene (IP) concentration, Mw, and activity, while pure TEA exhibited a smaller [C*]/[Zr] % than both pure Triisobutylaluminum (TIBA) and TIBA/TEA mixture. These results indicate that the type of alkylaluminum and its varying ratio affect the activation of metallocene catalysts. Secondly, this study also includes chain termination and transfer reactions and how comonomers participate in overcoming these issues. For this complicated process, we additionally performed the quench-labeling metal-polymer bonds technique (quantitative experiments (catalyst's active site)) leading to building a kinetic model that plays a significant role in mechanistic studies. The quantitative of active sites results indicate that the lower propagation rate constant (kp) was increased by nearly 20 % with TEA (25 %), and continuously increased with the TEA amount, meaning that TEA is a noticeable essential component in the cocatalyst for higher kp. The higher kpE and kpIP with TIBA might be associated with faster propagation and lower chain transfer reactions to Al−iBu and active centers and faster re-initiation of Al−iBu. The active center and 3,4 connections of IP are increased with increased TEA, meaning that the active site based on Al−Et bonds is facilitating more coordinated-to-IP through 3,4 connections. Finally, to clarify the impact of cocatalysts on MWD, a detailed theoretical splitting of the corresponding GPC was conducted. However, compared to PE MWD, TEA decreases the MWD of the polymers and becomes narrow. It is evident that introducing IP as a comonomer and TEA as a combined cocatalyst significantly reduces the chain transfer reaction that occurs during PE.

Real-time monitoring of vegetable oils photo-oxidation kinetics using differential photocalorimetry

This study introduced differential photocalorimetry (DPC) as a method for real-time monitoring of the photo-oxidation kinetics of vegetable oils. DPC measures the heat flow generated during the oxidation of oils upon light exposure. Experiments conducted with stripped linseed oil (SLSO), an oil depleted from its natural antioxidants, showed no induction time (τ). Conversely, spiking SLSO with increasing concentrations of trans-ferulic acid resulted in an induction time (τ) proportional to the antioxidant concentration (R2 = 0.99). A comparative study among different vegetable oils revealed that rice bran oil exhibited the highest resistant to photo-oxidation, followed by corn, soybean, and sunflower oils. The results are discussed in terms of sample oxidizability and antioxidant efficiency (A.E.), and validated through high-performance liquid chromatography with diode array detection (HPLC-DAD). Furthermore, the measured heat flow enabled the determination of the rates of inhibited (Rinh) and uninhibited (Runi) periods, as well as the rate constant of propagation (kp) and inhibition (kinh) reactions.

Primary Radical Effectiveness: Do the Different Chemical Reactivities of Hydroxyl and Chlorine Radicals Matter for Tropospheric Oxidation?

The atmospheric oxidation of organics occurs primarily via reaction cycles involving gas phase radical species, catalysed by nitric oxide (NO), which result in the production of secondary pollutants such as ozone. For these oxidation cycles to occur, they must be initialized by a primary radical, i.e., a radical formed from non-radical precursors. Once formed, these primary radicals can result in the oxidation of organic compounds to produce peroxy radicals that, providing sufficient NO is present, can re-generate "secondary" radicals which can go on to oxidize further organics. Thus, one primary radical can result in the catalytic oxidation of multiple organics. Although the photolysis of ozone in the presence of water vapor to form two hydroxyl (OH) radicals is accepted as the dominant tropospheric primary radical source, multiple other primary radical sources exist and can dominate in certain environments. The chemical reactivity of different radicals to organic and inorganic compounds can be very different, however, and how these differences in radical chemistry impact atmospheric organic oxidation under different atmospheric conditions has not been previously demonstrated. In this work, we use a series of model simulations to investigate the impact of the chemical reactivity of the primary radical on the effectiveness in initializing organic oxidation and thus the production of the secondary pollutant ozone. We compare the chemistries of the OH and atomic chlorine (Cl) radicals and their effectiveness at initializing organic oxidation under different nitrogen oxide and organic concentrations. The OH radical is the dominant tropospheric radical, with both primary and secondary sources. In contrast, Cl has primary sources that show significant spatial heterogeneity throughout the troposphere but is not typically regenerated in catalytic cycles. Both primary OH and Cl can initiate organic oxidation, but this work shows that the relative effectiveness with which they oxidize organics and produce ozone depends on their balance of propagation vs termination reactions which is in turn determined by the chemical environment in which they are produced. In particular, our work shows that in high NOx radical-limited environments, like those found in many urban areas, Cl will be more efficient at oxidizing organics than OH.

Organo-Photocatalytic Anti-Markovnikov Hydroamidation of Alkenes with Sulfonyl Azides: A Combined Experimental and Computational Study.

The construction of C(sp3)-N bonds via direct N-centered radical addition with olefins under benign conditions is a desirable but challenging strategy. Herein, we describe an organo-photocatalytic approach to achieve anti-Markovnikov alkene hydroamidation with sulfonyl azides in a highly efficient manner under transition-metal-free and mild conditions. A broad range of substrates, including both activated and unactivated alkenes, are suitable for this protocol, providing a convenient and practical method to construct sulfonylamide derivatives. A synergistic experimental and computational mechanistic study suggests that the additive, Hantzsch ester (HE), might undergo a triplet-triplet energy transfer manner to achieve photosensitization by the organo-photocatalyst under visible light irradiation. Next, the resulted triplet excited state 3HE* could lead to a homolytic cleavage of C4-H bond, which triggers a straightforward H-atom transfer (HAT) style in converting sulfonyl azide to the corresponding key amidyl radical. Subsequently, the addition of the amidyl radical to alkene followed by HAT from p-toluenethiol could proceed to afford the desired anti-Markovnikov hydroamidation product. It is worth noting that mechanistic pathway bifurcation could be possible for this reaction. A feasible radical chain propagation mechanistic pathway is also proposed to rationalize the high efficiency of this reaction.