Chain Growth Research Articles

Effects of additives on properties of self-skinning polyurethane foam for automobile steering wheel

Commercialized polyurethane foam products for automobile steering wheels have been committed to the development of products with comprehensive properties such as low density, high production efficiency and environmental protection. Based on this, this article has launched a series of experiments to explore the effects of different types of chain extenders, blowing agents and catalysts on the mechanical properties, cell structure, skin and reactivity of self-skinning polyurethane foams for automobile steering wheels. The results show that the hardness and tensile strength of the foam gradually decrease with the chain growth of small molecular chain extenders (ethylene glycol, 1.3-propanediol, 1.4-butanediol), and the elongation at break increases gradually. The foaming agent formic acid is more conducive than water as it favors the formation of small but compact cells, thus dense skin. The catalytic efficiency of non-reactive catalysts (A1, KC101 and DabcoEG) is higher than that of reactive catalysts (LED-103, ZR-50, DPA). The theoretical research for the development of high-performance and high-efficiency polyurethane foam products for automobile steering wheels.

Supramolecular Polymer Brushes.

Polymer brushes are thin polymer films that consist of densely grafted, chain-end tethered polymers. These thin polymer films can be produced either by anchoring presynthesized chain-end functional polymers to the surface of interest ("grafting to"), or by using appropriately modified surfaces to facilitate growth of polymer chains from the substrate ("grafting from"). The vast majority of polymer brushes that have been prepared and studied so far involved chain-end tethered polymer assemblies that are anchored to the surface via covalent bonds. In contrast, the use of noncovalent interactions to prepare chain-end tethered polymer thin films is much less explored. Anchoring or growing polymer chains using noncovalent interactions results in supramolecular polymer brushes. Supramolecular polymer brushes may possess unique chain dynamics as opposed to their covalently tethered counterparts, which could provide avenues to, for example, renewable or (self-)healable surface coatings. This Perspective article provides an overview of the various approaches that have been used so far to prepare supramolecular polymer brushes. After presenting an overview of the various approaches that have been used to prepare supramolecular brushes via the "grafting to" strategy, examples will be presented of strategies that have been successfully applied to produce supramolecular polymer brushes via "grafting from" methods.

Mass spectrometry insight into the electro-copolymerization mechanism of aniline with o-phenylenediamine

It has been a challenge to accurately characterize the chain growth mechanism during copolymerization. Herein, an electrochemistry-microtube-mass spectrometry (EC-mTube-MS) was developed for on-line monitoring of aniline (ANI) electro-copolymerized with o-phenylenediamine (oPD). Short-lived intermediates and co-oligomers belonging to polyANI-oPD were structural identified with tandem MS method for the first time. The free binding between ANI and oPD monomers determined the presence of more species for a given co-n-mer. Chain growth was found to compete with 2H+, 2e− intramolecular oxidation of co-oligomers, resulting in the units with quinone or phenazine structures. MS signals for polyANI and polyoPD were observed as well, indicating that the homopolymerization occurred simultaneously with the copolymerization, which may affect the formation efficiency even performance of the final copolymer. Investigations on the monomer feed ratio and solution pH provided data support for improving the copolymerization efficiency, and the recommended parameters are cANI: coPD ≥ 10:1 and pH = 7.

Insights into the Fischer–Tropsch mechanism on χ-Fe5C2(510) based on the hydrogen coverage effect

Investigating the mechanism of the Fischer–Tropsch synthesis is beneficial for modifying catalysts, which would result in an improved catalyst performance. However, theoretical calculations have mainly focused on several key elementary reactions and have thus provided fragmented information. Here, we select χ-Fe5C2(510) as representative facet and consider the elementary reactions on as comprehensively as possible via the density-functional theory (DFT)-based kinetic Monte Carlo (kMC) method to generate the fine structure of the Fischer–Tropsch mechanism. The results reveal that the CO activation mechanism is divided into cyclical pathways and termination pathways. The chain growth process and chain dissociation process coexist in the Fischer-Tropsch synthesis while the former follows the CO insertion mechanism, and the latter follows the carbide mechanism. The product methane and ethylene are produced major from the intermediates which are generated in the chain dissociation process rather than the chain grown process as the article reported before. The deep interpretation of the Fischer–Tropsch mechanism from the fine structure could be the guidance to experimentalists in mechanism verification or catalyst design.

Amide-Functionalized Polyolefins and Facile Post-Transformations

Direct copolymerization of olefins with polar monomers to produce functionalized polyolefins has attracted much attention; however, highly efficient incorporation of amide functions into polyolefins is a long-standing challenge because the amide function is pronounced to retard chain growth. In this contribution, Vince Lactam’s derivatives (VLH, VLMe, and VLBoc) that contain high ring strain and an amide moiety are utilized in ethylene copolymerization mediated by palladium catalysts. Amide-functionalized polyethylenes, poly(E-VLBoc)s, are accessible with key characteristics of high amide incorporations (up to 30.1 mol %), high copolymer molecular weights, and high catalytic activities. The incorporation of the amide comonomer converts crystalline poly(E-VLBoc)s (Tm = 115–125 °C) to noncrystalline and transparent poly(E-VLBoc)s (Tg = 98–196 °C, optical transmittance (T) = 87.6%–90.4%). Both characteristics of cyclic amide functions and high amide incorporations in poly(E-VLBoc) enable facile post-transformations under mild conditions to produce hydrogen bond-containing (-C(O)NH-) poly(E-VLH), difunctionalized (-COOH and -NHR) poly(E-VLNHBoc), and water-soluble ammonium-functionalized poly(E-VLNH3+). The ammonium functionality endows polyolefin with antibacterial properties.

Molecular basis of polyglutamine-modulated ELF3 aggregation in Arabidopsis temperature response.

Some plants, like Arabidopsis thaliana, possess a molecular mechanism that allows them to sense changes in temperature and respond by modifying growth rate, photoperiod length and timing of flowering. Underlying this mechanism is a tripartite protein complex called the Evening Complex (EC) that functions as a DNA transcription repressor targeting growth-related genes. ELF3, a large disordered scaffolding protein, is the primary component of the EC. At low temperature ELF3 sits over DNA targets occluding transcription machinery and halting growth. Rising temperature promote ELF3 condensate formation as it becomes sequestered in large reversible nuclear condensates allowing DNA transcription and plant growth to proceed. A C-terminal prion-like domain (PrD) has been observed to be sufficient for ELF3 aggregation. Within this PrD region lies a poly-glutamine repeat of variable length, the size of which has been found to modulate the thermal responsiveness as measured by hypocotyl (stem) elongation and condensate formation. We use a polymer chain growth approach to build large ensembles and characterize monomeric ELF3-PrD at a range of polyQ lengths and temperatures. We then explore temperature-dependent dynamics of wild-type ELF3-PrD and a mutant, F527A, using chain growth structures as initial conformations for replica exchange (REST2) simulations. We find increased solvent accessibility of expanded polyQ tracts, promotion of temperature-sensitive helices adjacent to polyQ tracts, and exposure of a cluster of aromatic residues at increased temperature, all three of which promote inter-protein interaction. Protein condensate formation is a ubiquitous but poorly understood mechanism implicated in a range of biological functions and malfunction. Our computational approach to understanding the molecular basis for the temperature sensitivity of this system has wider implications on the understanding of phenomena regulating condensate formation more generally.

Advances in the One-Step Approach of Polymeric Materials Using Enzymatic Techniques.

The formulation in which biochemical enzymes are administered in polymer science plays a key role in retaining their catalytic activity. The one-step synthesis of polymers with highly sequence-controlled enzymes is a strategy employed to provide enzymes with higher catalytic activity and thermostability in material sustainability. Enzyme-catalyzed chain growth polymerization reactions using activated monomers, protein-polymer complexation techniques, covalent and non-covalent interaction, and electrostatic interactions can provide means to develop formulations that maintain the stability of the enzyme during complex material processes. Multifarious applications of catalytic enzymes are usually attributed to their efficiency, pH, and temperature, thus, progressing with a critical structure-controlled synthesis of polymer materials. Due to the obvious economics of manufacturing and environmental sustainability, the green synthesis of enzyme-catalyzed materials has attracted significant interest. Several enzymes from microorganisms and plants via enzyme-mediated material synthesis have provided a viable alternative for the appropriate synthesis of polymers, effectively utilizing the one-step approach. This review analyzes more and deeper strategies and material technologies widely used in multi-enzyme cascade platforms for engineering polymer materials, as well as their potential industrial applications, to provide an update on current trends and gaps in the one-step synthesis of materials using catalytic enzymes.

Ring-Opening Polymerization of rac-Lactide Catalyzed by Octahedral Nickel Carboxylate Complexes

To date, nickel(II) complexes have not been practically investigated as catalysts in ring-opening polymerization (ROP) of lactide to produce biodegradable poly(lactic acid), which is in demand in biomedicine and industry. In this study, carboxylate complexes of nickel(II) containing various N-donor ligands with different nuclearity, metal core rigidity and nature of carboxylate ligands were synthesized and studied by infrared spectroscopy, X-ray diffraction, elemental and thermogravimetric analyses. The obtained complexes were examined in the ROP of the rac-lactide in bulk and in toluene solution with and without the addition of a benzyl alcohol initiator. In the series of complexes studied, the complex [Ni(DBED)2(O2CC(CH3)3)2]·(CH3)3CCO2H (DBED is N,N′-dibenzylethylenediamine) was a syndioselective catalyst and showed the highest catalytic ability in the polymerization without the addition of benzyl alcohol. For this complex, according to 1D DOSY 1H NMR spectroscopy and mass spectrometry with electrospray ionization, polymerization is initiated by a free secondary amine, DBED, leaving the metal’s inner coordination sphere. Based on the experimental data obtained, a comprehensive density functional theory (DFT) study of the ROP pathways including the initiation and first chain growth cycle steps with a detailed description of the intermediates and evaluation of the energy barriers of the steps was carried out. It was shown that one of the key roles in the reaction process is played by carboxylate ligands, which act as proton carriers from the initiator molecule and have a significant influence on the reactivity of the catalytic metal complexes.

In vitro fermentation assay on the bifidogenic effect of steviol glycosides of Stevia rebaudiana plant for the development of dietetic novel products

The relationship between excessive sugar consumption and many diseases such as dental caries, obesity, diabetes and coronary heart has been increasing in recent years. In this study, utilization of natural sugar replacer steviol glycosides and bifidogenic effect by Bifidobacterium animalis subsp. lactis was assayed in vitro model system. The basal medium (non-carbohydrate containing MRS, Man, Rogosa and Sharpe Agar) were supplemented with 0.025% and 1% stevia, 0.025% stevia + 1% inulin, %1 stevia + 1% inulin. The medium which contained no carbohydrate was designated as negative control, whereas the medium containing 1% glucose or inulin were evaluated as positive and evaluated on the 0, 12, 24, 36 and 48 h of fermentation. Steviol glycosides in both system significantly stimulated the growth of Bifidobacterium animalis subsp. lactis to varying degrees with highest prebiotic activity score, short chain fatty acid production and growth parameters as much as glucose and prebiotic inulin. The viability of the probiotic bacteria was determined within the bio-therapeutic level with potential prebiotic effects depending on the probiotic bacterial strain growing and the type of carbohydrate source utilized. In the study, stevia at lower concentration showed a higher growth rate of with inulin. In conclusion, stevia can be used as functional ingredients for the modulation of the gut microbiota and design of synbiotic systems as a prebiotic substrate and sugar substitute.

Materials availability and supply chain considerations for vanadium in grid-scale redox flow batteries

Redox flow batteries (RFBs) are a promising electrochemical storage solution for power sector decarbonization, particularly emerging long-duration needs. While the battery architecture can host many different redox chemistries, the vanadium RFB (VRFB) represents the current state-of-the-art due to its favorable combination of performance and longevity. However, the relatively high and volatile price of vanadium has hindered VRFB financing and deployment opportunities. Here we evaluate the vanadium supply chain to understand how it enables or constrains VRFB advancement and assess opportunities for accelerated growth. We find that – while vanadium may not be scarce – its abundance is confounded by highly concentrated production coupled with the disperse nature of sources suitable for potential supply increase. These factors challenge rapid growth, limiting deployment rate and magnitude. We estimate gigawatt-hour deployment scales are feasible over the next decade, which would represent marked expansion of the RFB industry and drive down system costs substantially, though this would require growth rates to vanadium production above historical averages. Accordingly, we review opportunities to accelerate supply chain growth and economic strategies to stabilize the market. Finally, we posit terawatt-hour deployment scales will be challenged by vanadium market conditions and resource availability, motivating the continued efforts developing next-generation RFB chemistries.

Protecting-Group-Free Iterative Divergent/Convergent Method for Preparing Sequence-Defined Polymers

A new iterative growth method is reported to prepare monodisperse sequence-defined polymers by combining the chain growth mechanisms of the traditional iterative sequential growth (ISG) and iterative exponential growth (IEG) strategies. Using three orthogonal click reactions to couple three groups of monomers, each containing two orthogonal clickable groups. This method grows monodisperse sequence-defined polymers in a protecting-group-free divergent/convergent way. Each iterative cycle of this method involves two divergent sequential growth steps and one convergent exponential growth step, which endows the new method with the chain growth advantages of both traditional ISG and IEG. The two divergent sequential growth reactions introduce new monomer structures into main chains and provide the resultant sequence-defined polymers with varied monomer sequences, while the convergent exponential growth reaction guarantees the method a fast chain growth speed. The repetition of the abovementioned iterative cycle allows this method to grow monodisperse sequence-defined polymers with a fast chain growth manner of 3 × 2n–2.

Efficient Synthesis and PISA Behavior of Molecular Bottlebrush Block Copolymers via a Grafting-From Strategy through RAFT Dispersion Polymerization

Precise synthesis of molecular bottlebrushes (MBBs) with high yield via the grafting-from strategy using a reversible deactivation radical polymerization (RDRP) technique has remained elusive. Herein, the molecular bottlebrush block copolymers (MBB BCPs) with high yield and varying chemical compositions were synthesized via a polymerization-induced self-assembly (PISA)-assisted grafting-from strategy by reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization. First, the poly(norbornene-g-poly(ethylene oxide))-b-poly(norbornene-g-chain transfer agent) [P(NB-g-PEG45)-b-P(NB-CTA)] BCPs with varied chemical compositions were synthesized by sequential ring-opening metathesis polymerization (ROMP) of norbornene PEG macromonomers (NB-PEG45) and norbornene chain transfer agent (NB-CTA) monomers. Subsequently, these P(NB-g-PEG45)m-b-P(NB-CTA)n BCPs were used as multifunctional macro-CTAs for the synthesis of MBB BCPs with a high yield (up to 80% styrene monomer conversion) and a low dispersity (Đ < 1.15) via a grafting-from strategy through RAFT dispersion polymerization. The PISA behaviors of these MBB BCPs were also investigated, and the results indicated that the MBB BCPs assemblies were transformed from spherical micelles into two-dimensional disk-like micelles because the rigidity of the PS MBB segment increased with the growth of PS side chains.

Divergent chain growth of poly(dithiocarbonate)s through arylmethyl triflate-mediated ring-opening polymerization of cyclic dithiocarbonate

Divergent chain growth of poly(dithiocarbonate)s through arylmethyl triflate-mediated ring-opening polymerization of cyclic dithiocarbonate

Abatement of Aromatic Contaminants from Wastewater by a Heat/Persulfate Process Based on a Polymerization Mechanism.

A novel approach to the abatement of pollutants consisting of their conversion to separable solid polymers is explored by a heat/persulfate (PDS) process for the treatment of high-temperature wastewaters. During this process, a simultaneous decontamination and carbon recovery can be achieved with minimal use of PDS, which is significantly different from conventional degradation processes. The feasibility of this process is demonstrated by eight kinds of typical organic pollutants and by a real coking wastewater. For the treatment of the selected pollutants, 30.2-91.9% DOC abatement was achieved with 24.8-91.2% carbon recovery; meanwhile, only 5.2-47.0% of PDS was consumed compared to a conventional degradation process. For the treatment of a real coking wastewater, 71.0% DOC abatement was achieved with 66.0% carbon recovery. With phenol as a representative compound, our polymerization-based heat/PDS process is applicable in a wide pH range (3.5-9.0) with a carbon recovery of >87%. Both SO4•- and HO• can be initiators for polymerization, with different contribution ratios under various conditions. Phenol monomers are semioxidized to form phenolic radicals, which are polymerized via chain transfer or chain growth processes to form separable solid phenol polymers, benzenediol polymers, and cross-linked polymers.

Polypeptide Chain Growth Mechanisms and Secondary Structure Formation in Glycine Gas-Phase Deposition on Silica Surfaces.

Peptide formation by amino acids condensation represents a crucial reaction in the quest of the origins of life as well as in synthetic chemistry. However, it is still poorly understood in terms of efficiency and reaction mechanism. In the present work, peptide formation has been investigated through thermal condensation of gas-phase glycine in fluctuating silica environments as a model of prebiotic environments. In-situ IR spectroscopy measurements under a controlled atmosphere reveal that a humidity fluctuating system subjected to both temperature and water activity variations results in the formation of more abundant peptides compared to a dehydrated system subjected only to temperature fluctuations cycles. A model is proposed in which hydration steps result in the hydrolysis and redistribution of the oligomers formed during previous deposition in dry conditions. This results in the formation of self-assembled aggregates with well-defined secondary structures (especially β-sheets). Upon further monomers feeding, structural elements are conserved in newly growing chains, with indications of templated polymerization. The structural dynamics of peptides were also evaluated. Rigid self-assembled structures with a high resistance to further wetting/drying cycles and inaccessibility to isotopic exchange were present in the humidity fluctuating system compared to more flexible structures in the dehydrated system. The resistance and growth of self-assembled structures were also investigated for an extended duration of Gly deposition using isotope labeling.

Glycine Peptide Chain Formation in the Gas Phase via Unimolecular Reactions

Peptide chain formation from amino acids such as glycine is a key step in the emergence of life. Unlike their synthesis by living systems, how peptide chains grow under abiotic conditions is an open question given the variety of organic compounds discovered in various astrophysical environments, comets and meteorites. We propose a new abiotic route in the presence of protonated molecular dimers of glycine in a cold gaseous atmosphere without further need for a solid catalytic substrate. The results provide evidence for the preferential formation of mixed protonated dimers of glycine consisting of a dipeptide and a glycine molecule instead of pure protonated glycine dimers. Additional measurements mimicking a cosmic-ray impact in terms of internal excitation show that a single gas-phase collision induces polymerization via dehydration in both the mixed and pure dimer ions. Peptide chain growth is thus demonstrated to occur via a unimolecular gas-phase reaction in an excited cluster ion.

Living Cationic Polymerization of ε-Caprolactone Catalyzed by a Metal-free Lewis Acid of Trityl Tetrafluoroborate

Research on metal-free catalytic synthesis of polycaprolactone (PCL) is fueled by the many practical needs for biodegradable and biocompatible materials. In this regard, the pursuit of effective and economical catalysts as well as precise control over the polymerization has become the main challenge. Herein, we report a living cationic ring-opening polymerization of ε-caprolactone catalyzed by a safe, inexpensive, and metal-free Lewis acid of trityl tetrafluoroborate. The polymerization is initiated by alcohol or water, which is corroborated by the presence of initiator segments at the chain ends of PCL polymers via 1H NMR and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Kinetic experiments show that polymerization is a first-order reaction to monomers. This result, together with the predictable molar mass, narrow molar-mass dispersity, and controllable chain growth and extension, confirms the synthesis to be a living polymerization. This work expands the catalytic toolbox of PCL synthesis and represents a rare example of cationic living polymerization of ε-caprolactone catalyzed by Lewis acids.

An optimal Fe-C coordination ensemble for hydrocarbon chain growth: a full Fischer-Tropsch synthesis mechanism from machine learning.

Fischer-Tropsch synthesis (FTS, CO + H2 → long-chain hydrocarbons) because of its great significance in industry has attracted huge attention since its discovery. For Fe-based catalysts, after decades of efforts, even the product distribution remains poorly understood due to the lack of information on the active site and the chain growth mechanism. Herein powered by a newly developed machine-learning-based transition state (ML-TS) exploration method to treat properly reaction-induced surface reconstruction, we are able to resolve where and how long-chain hydrocarbons grow on complex in situ-formed Fe-carbide (FeCx) surfaces from thousands of pathway candidates. Microkinetics simulations based on first-principles kinetics data further determine the rate-determining and the selectivity-controlling steps, and reveal the fine details of the product distribution in obeying and deviating from the Anderson-Schulz-Flory law. By showing that all FeCx phases can grow coherently upon each other, we demonstrate that the FTS active site, namely the A-P5 site present on reconstructed Fe3C(031), Fe5C2(510), Fe5C2(021), and Fe7C3(071) terrace surfaces, is not necessarily connected to any particular FeCx phase, rationalizing long-standing structure-activity puzzles. The optimal Fe-C coordination ensemble of the A-P5 site exhibits both Fe-carbide (Fe4C square) and metal Fe (Fe3 trimer) features.

Controlled reductive C-C coupling of isocyanides promoted by an aluminyl anion.

We report the reaction of the potassium aluminyl, K[Al(NON)] ([NON]2- = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) with a series of isocyanide substrates (R-NC). In the case of tBu-NC, degradation of the isocyanide was observed generating an isomeric mixture of the corresponding aluminium cyanido-κC and -κN compounds, K[Al(NON)(H)(CN)]/K[Al(NON)(H)(NC)]. The reaction with 2,6-dimethylphenyl isocyanide (Dmp-NC), gave a C3-homologation product, which in addition to C-C bond formation showed dearomatisation of one of the aromatic substituents. In contrast, using adamantyl isocyanide Ad-NC allowed both the C2- and C3-homologation products to be isolated, allowing a degree of control to be exercised over the chain growth process. These data also show that the reaction proceeds through a stepwise addition, supported in this study by the synthesis of the mixed [(Ad-NC)2(Dmp-NC)]2- product. Computational analysis of the bonding within the homologised products confirm a high degree of multiple bond character in the exocyclic ketenimine units of the C2- and C3-products. In addition, the mechanism of chain growth was investigated, identifying different possible pathways leading to the observed products, and highlighting the importance of the potassium cation in formation of the initial C2-chain.

Growth of 1D ClAlPc molecular chains mediated by graphene moiré patterns.

The on-surface formation of iso-oriented 1D molecular architectures, with high structural perfection, on 2D materials has been a long-sought objective. However, such realization has been troublesome and limited, and it still remains an experimental challenge. Here, the quasi-1D stripe-like moiré pattern, arising at the interface of graphene grown on Rh(110), has been used to guide the formation of 1D molecular wires of π-conjugated, non-planar, chloro-aluminum phthalocyanine (ClAlPc) molecules, brought together by van der Waals interactions. Using scanning tunnelling microscopy (STM) under ultra-high vacuum (UHV) at 40 K, the preferential adsorption orientations of the molecules at low coverages have been investigated. The results shed light on the potential signature of graphene lattice symmetry breaking, induced by the incommensurate quasi-1D moiré pattern of Gr/Rh(110), as the subtle mechanism behind this templated growth of 1D molecular structures. For coverages close to 1 ML, the molecule-molecule interactions favor a closely packed square lattice arrangement. The present work provides new insights to tailor 1D molecular structures on graphene grown on a non-hexagonal metal substrate.