Polymerization Site Research Articles

Insight into HTPB pyrolysis mechanism under high-temperature: A reactive molecular dynamics study

Hydroxyl-terminated polybutadiene (HTPB) is widely utilized in solid and hybrid rocket propellants due to its mechanical properties and combustion performance. Insight into its pyrolysis process is key to enhancing combustion efficiency in rocket engines. This study employs ReaxFF molecular dynamics (MD) simulations to explore the pyrolysis mechanism of HTPB under extreme conditions, with temperatures ranging from 1000 K to 2000 K. The results identify the primary degradation products and elucidate their formation mechanisms. The simulation reveals that C-C bond cleavage at polymerization sites is the initial step, followed by the formation of linear oligomers and butadiene. Subsequent reactions, including hydrogenation and dehydrogenation, lead to the generation of smaller molecular species. The kinetic analysis confirms that HTPB pyrolysis follows first-order reaction kinetics, with an activation energy of 8.12 kcal/mol. The findings are compared with existing experimental data, highlighting the influence of thermal environments on pyrolysis mechanisms and product distributions.

Influence of enforced carbonation on alkali-silica reaction: Performance and multi-scale mechanisms

Alkali-silica reaction (ASR) has emerged as one of the major concrete deterioration issues. Despite the extensive effort investments in ASR mitigation, the interaction between ASR and carbonation remains unclear. To investigate the role and mechanisms of early-age enforced carbonation in ASR, enforced carbonation conditions at 50 °C and 95 % RH with different CO2 concentrations up to 20 % were employed at two stages. The ASR mitigation efficacy was evaluated by monitoring the volume expansion and cracking behavior of mortars containing reactive aggregates. A comprehensive understanding was obtained by analyzing the evolutions of ASR gel's composition, molecular structure, and hygroscopicity. The results indicate that ASR can be effectively suppressed under carbonation with a 97.9 % decrease in ASR-induced volume expansion and fully suppressed cracking when the mortars were cured at 20 % CO2 for 30 days. The decreases in the Q3 polymerization sites and [K + Na]/Si ratios of the ASR products, as well as the 30.3 % reduction in water uptake capacity of ASR gels under carbonation, uncovered the underlying mechanisms for the mitigated ASR at multiple length scales.

Inducing and Switching the Handedness of Polyacetylenes with Topologically Chiral [2]Catenane Pendants

AbstractTo explore the chirality induction and switching of topological chirality, poly[2]catenanes composed of helical poly(phenylacetylenes) (PPAs) main chain and topologically chiral [2]catenane pendants are described for the first time. These poly[2]catenanes with optically active [2]catenanes on side chains were synthesized by polymerization of enantiomerically pure topologically chiral [2]catenanes with ethynyl polymerization site and/or point chiral moiety. The chirality information of [2]catenane pendants was successfully transferred to the main chain of polyene backbones, leading to preferred‐handed helical conformations, while the introduction of point chiral units has negligible effect on the overall helices. More interestingly, attributed to unique dynamic feature of the [2]catenane pendants, these polymers revealed dynamic response behaviors to solvents, temperature, and sodium ions, resulting in the fully reversible switching on/off of the chirality induction. This work provides not only new design strategy for novel chiroptical switches with topologically chiral molecules but also novel platforms for the development of smart chiral materials.

Inducing and Switching the Handedness of Polyacetylenes with Topologically Chiral [2]Catenane Pendants.

To explore the chirality induction and switching of topological chirality, poly[2]catenanes composed of helical poly(phenylacetylenes) (PPAs) main chain and topologically chiral [2]catenane pendants are described for the first time. These poly[2]catenanes with optically active [2]catenanes on side chains were synthesized by polymerization of enantiomerically pure topologically chiral [2]catenanes with ethynyl polymerization site and/or point chiral moiety. The chirality information of [2]catenane pendants was successfully transferred to the main chain of polyene backbones, leading to preferred-handed helical conformations, while the introduction of point chiral units has negligible effect on the overall helices. More interestingly, attributed to unique dynamic feature of the [2]catenane pendants, these polymers revealed dynamic response behaviors to solvents, temperature, and sodium ions, resulting in the fully reversible switching on/off of the chirality induction. This work provides not only new design strategy for novel chiroptical switches with topologically chiral molecules but also novel platforms for the development of smart chiral materials.

Elucidating the role of magnesium in alkali-silica reaction: Performance and mechanisms

Alkali-silica reaction (ASR) is a major concrete deterioration causing volume expansion, cracking, and hence degradations in the strength and permeability of concrete structures. Despite a variety of ASR mitigation approaches having been investigated, the successful use of chemical admixture is limited to lithium salts, which have negative impacts on cement hydration and shrinkage behavior of concrete. To explore alternative substances for effective and economic ASR mitigation, the role of magnesium nitrate in mortars containing reactive aggregate is investigated at varying Mg/alkali ratios from 0 to 3.0 in terms of volume expansion, cracking behavior, silica dissolution, and modifications of ASR products. The results indicate that, at a Mg/alkali ratio of 0.74, the expansion and surface cracking density of the ASR-impacted mortar were decreased by 21.1 % and 39.1 %, respectively, which is superior to the efficacy of lithium nitrate. The 51.3 % less silica dissolution from aggregate, suppressed ASR gel formation, fewer Q3 polymerization sites in the silicate chains, less packed structure, and reduced alkali/Si ratio of the reaction products elucidate the underlying mechanisms across multiple length scales.

Piezoresistive pressure sensor based on conductive MWCNTs@PANI fiber network for motion sensing and human–machine interface

AbstractWearable smart sensors have broad prospects in medical health and physical exercise. In this study, the core–shell conductive fiber membrane was prepared by adding multiwalled carbon nanotubes during polymerization of aniline on spun fibers. In comparison to the conventionally utilized polymerized aniline, the integration of carbon nanotube particulates enhances the conductivity and surface area, provides more aniline polymerization sites, and simultaneously increases the stability of the composite film. The pressure sensor has a sensitivity of 61.12, 40.32, and 23.17 kPa−1 in the range of 0–7.1 kPa, 7.1–11.1 kPa, and 11.1–19.54 kPa. In addition, it has a fast response time (loading time of 120 ms and unloading time of only 22 ms) and exhibits excellent cyclic stability through numerous loading–unloading cycles. These properties enable the sensor to be applied to real‐time monitoring of human sports health, such as knee and elbow bending, mouse clicking, beaker holding, and so on. This research method provides a new strategy and technology for conducting polymers as materials for flexible electronic equipment.

Electropolymerization of Donor-Acceptor Conjugated Polymer for Efficient Dual-Ion Storage.

Rationally designed organic redox-active materials have attracted numerous interests due to their excellent electrochemical performance and reasonable sustainability. However, they often suffer from poor cycling stability, intrinsic low operating potential, and poor rate performance. Herein, a novel Donor-Acceptor (D-A) bipolar polymer with n-type pyrene-4,5,9,10-tetraone unit storing Li cations and p-type carbazole unit which attracts anions and provides polymerization sites is employed as a cathode for lithium-ion batteries through in situ electropolymerization. The multiple redox reactions and boosted kinetics by the D-A structure lead to excellent electrochemical performance of a high discharge capacity of 202mA h g-1 at 200mA g-1, impressive working potential (2.87 and 4.15 V), an outstanding rate capability of 119mA h g-1 at 10A g-1 and a noteworthy energy density up to 554Whkg-1. This strategy has significant implications for the molecule design of bipolar organic cathode for high cycling stability and high energy density.

Solvent-free conversion of CO2 in carbonates through a sustainable macroporous catalyst

The novelty of this work consists of synthesizing and exploiting a heterogeneous catalyst containing ammonium chloride as part of the polymeric sponge sites for CO2 capture. To this aim, the polymerization of 2-acryloyl(oxyethyl)trimethylammonium chloride was performed in cryo-condition, in the presence of a crosslinking agent, obtaining a lightweight macroporous freestanding material. Its efficiency in converting aromatic and aliphatic epoxides to the corresponding carbonates was successfully proved by using proton Nuclear Magnetic Resonance (1H NMR). Remarkably, the conversion of styrene oxide (SO) to styrene carbonate (SC) reached a yield of 99 % after 24 h of reaction. The calculated yield versus the aliphatic cyclohexene oxide is 71 %. Similar results were obtained by substituting the resin counter anion with Br−, although the conversion kinetic was slower than the chloride. It is worth noticing that reactions took place in the mixture without adding the tetrabutylammonium bromide (TBAB), typically used as a co-catalyst to convert epoxides into carbonates. The recyclability of the as-prepared catalyst was evaluated for four reaction cycles, evidencing stable properties without significant depletion of CO2 capture efficiency. Most importantly, the post-cleaning of the catalytic sponge is not required to be reused. Finally, the green chemistry metrics applied to the process demonstrated that our approach significantly mitigates risks and reduces environmental impact, thus elevating the overall cleanliness of our proof of concept.

Computer-aided ligand screening to develop polymer-free catalyst in Cr-catalyzed ethylene oligomerization

Preventing the formation of undesirable polymer in ethylene oligomerization is a major challenge since the presence of these insoluble materials can cause severe reactor fouling. Based on previous experimental data, the concomitant formation of these solid compounds can be divided in two types. Detailed discussion in combination with mathematical analysis revealed that the low-molecular weight waxes (low-Mw waxes) are most likely the tail products of the Schulz-Flory distribution while high-molecular weight polyethylene (high-Mw PE) may be catalyzed by multiple active sites independent from the oligomerization cycle. Controlling the Schulz-Flory coefficient (K value) to remain in a lower range is crucial in achieving plentiful C4–C8 components and inhibiting polymer formation at the same time. Attempting to predict the K value, the energetic disparities of key transition states which control 1-butene selectivity have been extracted as a chemical descriptor to establish a general model. Promising ligands derived from computer-aided screening are expected to avoid the inherent waxes formation. Plausible strategies for the elimination of external polymerization sites are also suggested. The combination of them may provide a chance to develop more polymer-free catalysts to meet the requirements of industrial manufacturing.

The influence of water on size of the monodisperse polystyrene microspheres prepared by dispersion polymerization

AbstractPolystyrene microspheres with uniform sizes have a wide range of applications in biomedical engineering. However, detailed and systematic investigations on the influence of water content on alcohol/water systems are relatively scarce. In this study, the impact of trace water content on microsphere size was comprehensively examined, and a systematic exploration of the varying effects of different hydration levels on particle nucleation and growth mechanisms was conducted. When the water content increased from 0.1% to 0.4%, the microsphere diameter rapidly decreases from 3.6 to 2.71 μm. Correspondingly, the molecular weight increased from 29,797 to 69,186. However, within the water content range of 0.5%–14.5%, alterations in water content induced only slight variations in the microsphere diameter. The microsphere size, molecular weight, conversion rate, and reaction rate were compared in two stages. It was observed that the diminishing influence of water on the system was due to the changes in the main polymerization sites. Subsequently, the addition of water content up to 33.5% revealed an exponential decrease in the microsphere size with increasing water content in ethanol. This pattern was also observed in methanol and isopropanol, demonstrating its universality and predictability, making it applicable for precise prediction of microsphere size in different solvents.

Efficacy of functionalized sodium-montmorillonite in mitigating alkali-silica reaction

Alkali-silica reaction (ASR) is a fatal deterioration that can cause volume expansion, cracking, and premature failure of concrete. In this study, the efficacy of sodium montmorillonite (NaMt) organically functionalized with two non-ionic surfactants (ONaMts) in mitigating ASR is investigated by determining the expansion and cracking behavior of mortars containing reactive aggregates. The underlying mitigation mechanisms were analyzed through the quantification of reaction products and in-situ characterizations of ASR gels. The results revealed that, compared with raw NaMt, ASR-induced expansion and cracking can be more substantially mitigated in the presence of ONaMts, which is supported by the improved consumption of portlandite and reduced formations of both crystalline and amorphous ASR gels. The functionalized ONaMts appeared to further suppress the formation of Q3 polymerization sites, decrease the [K + Na]/Si atomic ratio and increase the Al/Ca in ASR gels.

Stimuli-responsive rotaxane-branched dendronized polymers with tunable thermal and rheological properties

Aiming at the creation of polymers with attractive dynamic properties, herein, rotaxane-branched dendronized polymers (DPs) with rotaxane-branched dendrons attached onto the polymer chains are proposed. Starting from macromonomers with both rotaxane-branched dendrons and polymerization site, targeted rotaxane-branched DPs are successfully synthesized through ring-opening metathesis polymerization (ROMP). Interestingly, due to the existence of multiple switchable [2]rotaxane branches within the attached dendrons, anion-induced reversible thickness modulation of the resultant rotaxane-branched DPs is achieved, which further lead to tunable thermal and rheological properties, making them attractive platform for the construction of smart polymeric materials.

Revisiting dispersion and reactivity of active sites via a restricted random distribution model over supported vanadia catalysts for NO reduction

The structure and dispersion of active sites substantially impact the performance of supported metal oxide catalysts, as exemplified by the V2O5/TiO2 catalysts used for NH3-SCR reaction. Due to the complexity of the active site dispersion on the catalyst surface, the intrinsic law of this structure–activity relationship remains controversial. Herein, we proposed a restricted random distribution model to describe the stochastic anchoring behavior of active sites on the TiO2 surface during the loading process of vanadia. The structures of monomeric, dimeric, and polymeric VOx sites were determined by DFT calculations and verified with spectroscopy and adsorption information experimentally. The calculated reaction pathways and energy profiles for these structures indicated that the dual sites configuration and terminal V(=O)2 bond are responsible for the high reactivity of aggregated VOx sites. Combining the reactivity of these various sites and the random distribution method, we built a general profile of the correlation between the V2O5/TiO2 catalyst surface and its reaction performance. The reactivity results obtained from experimental in situ DRIFTS and kinetic tests validate our theoretical model. The random distribution method links DFT calculations and experiments, coupling the structure and dispersion of the VOx active sites to the catalyst reactivity.

(Invited) Hard Polymers: Irreversible Synthetic Routes to Ultra-Strong Two-Dimensional Polymers in Bulk

Polymers that extend covalently in two dimensions have attracted recent attention as a means of combining the mechanical strength and in-plane energy conduction of conventional two-dimensional (2D) materials with the low densities, synthetic processability, and organic composition of their one-dimensional counterparts. Efforts to date have proven successful in forms that do not allow full realization of these properties, such as polymerization at flat interfaces or fixation of monomers in immobilized lattices. A frequently employed synthetic approach is to introduce microscopic reversibility, at the cost of bond stability, to achieve 2D crystals after extensive error correction. Herein we demonstrate a synthetic route to 2D irreversible polycondensation directly in the solution phase, resulting in a covalently bonded 2D polymer that is chemically stable and highly processable. Further fabrication offers highly oriented, free-standing films which exhibit exceptional 2D elastic modulus and yield strength at 12.7 ± 3.8 GPa and 488 ± 57 MPa, respectively. The highly planar asymmetry of 2D molecules is evidenced by chemical atomic force microscopy, and molecular alignment in nanofilms is supported by a polarized photoluminescence centered at 580 and 680 nm from different dipole transitions. We have also made substantial progresses in the theory of 2D chain growth. We performed a chemical kinetic simulation study to understand 2D polymerization in homogeneous solution with irreversible chemical steps. We show that reaction sites for polymerization in 2D always scale unfavourably compared to 3D, growing as molecular weight to the 1/2 power versus 2/3 power for 3D. However, certain mechanisms can effectively suppress out-of-plane defect formation and subsequent 3D growth. We consider two such mechanisms, which we call bond-planarity and templated autocatalysis. In the first, although single bonds can easily rotate out-of-plane to render polymerization in 3D, some double-bond linkages prefer a planar configuration. In the second mechanism, stacked 2D plates may act as van der Waals templates for each other to enhance growth, which leads to an autocatalysis. When linkage reactions possess a 1000:1 selectivity (γ) for staying in plane versus rotating, solution-synthesized 2D polymers can have comparable size and yield with those synthesized from confined polymerization on a surface. Autocatalysis could achieve similar effects when self-templating accelerates 2D growth by a factor β of 106. A combined strategy relaxes the requirement of both mechanisms by over one order of magnitude. We map the dependence of molecular weight and yield for 2D polymer on the reaction parameters, allowing experimental results to be used to estimate β and γ. Our calculations show for the first time from theory, the feasibility of producing two-dimensional polymers from irreversible polymerization in solution. These new synthetic routes, now experimentally validated, provides exciting, new opportunities for 2D polymers in applications ranging from composite structures to barrier coating materials.

Structural basis of peptidoglycan synthesis by E. coli RodA-PBP2 complex

Peptidoglycan (PG) is an essential structural component of the bacterial cell wall that is synthetized during cell division and elongation. PG forms an extracellular polymer crucial for cellular viability, the synthesis of which is the target of many antibiotics. PG assembly requires a glycosyltransferase (GT) to generate a glycan polymer using a Lipid II substrate, which is then crosslinked to the existing PG via a transpeptidase (TP) reaction. A Shape, Elongation, Division and Sporulation (SEDS) GT enzyme and a Class B Penicillin Binding Protein (PBP) form the core of the multi-protein complex required for PG assembly. Here we used single particle cryo-electron microscopy to determine the structure of a cell elongation-specific E. coli RodA-PBP2 complex. We combine this information with biochemical, genetic, spectroscopic, and computational analyses to identify the Lipid II binding sites and propose a mechanism for Lipid II polymerization. Our data suggest a hypothesis for the movement of the glycan strand from the Lipid II polymerization site of RodA towards the TP site of PBP2, functionally linking these two central enzymatic activities required for cell wall peptidoglycan biosynthesis.

Synergistic effect of amine and borane in immobilized metallocene catalysts on ethylene polymerization

The effect of incorporating amines and borane into the supported metallocene catalyst systems was investigated for the polymerization of ethylene and copolymerization of ethylene-1-hexene. Two methods, the conventional method and the one-pot method, were employed to prepare immobilized metallocene catalysts. The synthesized catalysts underwent characterization using scanning electron microscopy (SEM), elemental analysis (EA), inductively coupled plasma (ICP) analysis, and X-ray photoelectron spectroscopy (XPS). The results revealed that the inclusion of polyethyleneimine (PEI) increased the Zr content in the catalyst and yielded polyethylene with higher molecular weight. PEI also exhibited the ability to enhance the comonomer response of supported metallocene catalysts. Additionally, the immobilization of both PEI and borane demonstrated a significant improvement in polymerization activity. XPS analysis demonstrated the electronic effects of amines and borane on the catalyst, indicating enhanced anionic properties of methylaluminoxane and the formation of cationic zirconocenium species. These findings suggest that the presence of PEI and borane promotes the formation of more active metallocene sites for ethylene polymerization.

Protein Modelling and Molecular Docking Analysis of Fasciola hepatica β-Tubulin's Interaction Sites, with Triclabendazole, Triclabendazole Sulphoxide and Triclabendazole Sulphone.

Fasciola hepatica is a globally distributed trematode that causes significant economic losses. Triclabendazole is the primary pharmacological treatment for this parasite. However, the increasing resistance to triclabendazole limits its efficacy. Previous pharmacodynamics studies suggested that triclabendazole acts by interacting mainly with the β monomer of tubulin. We used a high-quality method to model the six isotypes of F. hepatica β-tubulin in the absence of three-dimensional structures. Molecular dockings were conducted to evaluate the destabilization regions in the molecule against the ligands triclabendazole, triclabendazole sulphoxide and triclabendazole sulphone. The nucleotide binding site demonstrates higher affinity than the binding sites of colchicine, albendazole, the T7 loop and pβVII (p < 0.05). We suggest that the binding of the ligands to the polymerization site of β-tubulin can lead a microtubule disruption. Furthermore, we found that triclabendazole sulphone exhibited significantly higher binding affinity than other ligands (p < 0.05) across all isotypes of β-tubulin. Our investigation has yielded new insight on the mechanism of action of triclabendazole and its sulphometabolites on F. hepatica β-tubulin through computational tools. These findings have significant implications for ongoing scientific research ongoing towards the discovery of novel therapeutics to treat F. hepatica infections.

RNA Polymerase Ribozyme That Recognizes the Template-Primer Complex through Tertiary Interactions.

RNA enzymes (ribozymes) often rely on specific base-pairing interactions to engage RNA substrates, which limits the substrate sequence generality of these enzymes. An RNA polymerase ribozyme that was previously optimized by directed evolution to operate in a more efficient and sequence-general manner can now recognize the RNA template, RNA primer, and incoming nucleoside 5'-triphosphate (NTP) entirely through tertiary interactions. As with proteinaceous polymerases, these tertiary interactions are largely agnostic to the sequence of the template, which is an essential property for the unconstrained transmission of genetic information. The polymerase ribozyme exhibits Michaelis-Menten saturation kinetics, with a catalytic rate of 0.1-1 min-1 and a Km of 0.1-1 μM. Earlier forms of the polymerase did not exhibit a saturable substrate binding site, but this property emerged over the course of directed evolution as the ribozyme underwent a structural rearrangement of its catalytic center. The optimized polymerase makes tertiary contacts with both the template and primer, including a critical interaction at the C2' position of the template nucleotide that opposes the 3'-terminal nucleotide of the primer. UV cross-linking studies paint a picture of how several portions of the ribozyme, including regions that were remodeled by directed evolution, come together to position the template, primer, and NTP within the active site for RNA polymerization.

Construction of a Reciprocal-Supporting Phenol-amine@CuNW Network for Antisedimentation Conductive Ink.

Homogeneously dispersed copper nanowire (CuNW) materials are the basis for practical applications in many types of electronic devices. At present, the dispersion of CuNWs in water is achieved through polymeric spatial site resistance effects primarily and the electrostatic dispersion mechanism in a few. However, the electrical conductivity of CuNWs could be weakened by the excessive addition of polymers; therefore, it is difficult to maintain a stable dispersion enduringly for surface charge modifiers. Based on the coagulation mechanism of colloids, a novel antisedimentation mechanism is refined by this work. Directed by this mechanism, a stable reciprocal-supporting antisedimentation conductive CuNW ink was achieved enduringly and a uniform conductive coating (1.81-5.65 Ω·sq-1) was successfully manufactured. The tannic acid-polyethylene imine (TA-PEI) could support copper nanowires to maintain a stable height of 61.4% after 15 days best, while CuNWs in other systems would settle completely in one day. Meanwhile, the TA-PEI composite cluster antisedimentation network not only provided massive spatial potential resistance for CuNWs but also modified the surface charge of CuNWs. CuNWs were dispersed stably in this phenol-amine@CuNW network. Furthermore, the CuNWs were crosslinked more tightly with each other relying on the vigorous adhesive properties of TA-PEI. With this antisedimentation mechanism and simple treatment process, CuNW ink will be utilized in more applications.

Identification of Polymerase and Proofreading Exonuclease Domains in the DNA Polymerases IA, IB and Nuclear-Encoded RNA Polymerase of the Plant Chloroplasts

Chloroplast plays a crucial role in all photosynthetic plants and converts the light energy to chemical energy. It is a semi-autonomous organelle and is mostly controlled by its own genome and partly by the nuclear imports. To replicate its own genome, it uses two DNA polymerases, viz. polymerases IA and IB. DNA polymerase IA showed 72.45% identity to polymerase IB, but only 35.35% identity to the E. coli DNA polymerase I. Multiple sequence alignment (MSA) analysis have shown that the DNA polymerases IA and IB and the E. coli DNA polymerase I possess almost identical active sites for polymerization and proofreading (PR) functions, suggesting their possible common evolutionary origin. The nuclear-encoded RNA polymerase (NEP) is imported from the nucleus and involves in the transcription of all the four subunits of the chloroplast RNA polymerase. The polymerase catalytic core of the DNA polymerases IA, IB and the NEP are remarkably conserved and is in close agreement with other DNA/RNA polymerases reported already, and possess a typical template-binding pair (-YG-), a basic catalytic amino acid (K) to initiate catalysis and a basic nucleotide selection amino acid R at -4 from K. The DNA polymerases IA and IB are very similar to prokaryotic DNA polymerases, except in possessing a zinc-binding motif (ZBM) in them, like the eukaryotic replicases. Interestingly, the PR exonucleases of all three polymerases belong to the DEDD-superfamily of exonucleases. The DNA polymerases IA and IB belong to the DEDD(Y)-subfamily, whereas the NEP belongs to the DEDD(H)-subfamily.