New ways to polysilanes

New cyclosilanes with substituents, which are functional groups, open the possibility of forming new polycyclic silanes in an aimed synthesis. This is possible on five- and six-membered rings. Also, mixed systems with different ring sizes were prepared. The UV spectra are discussed. An annelid dicycle Si₁₀Me₁₈ was also prepared. With this compound a radical anion can be formed. New aspects of the electrochemical synthesis of polysilanes are presented and a chlorine stable anode is proposed for the first time. The polymerization reaction of hydrogen-containing methyldisilanes is discussed. This reaction yields polysilanes with a general composition (SiMeH) _n and a ceramic yield over 88%. The reaction is very different from the known reaction of monosilanes. Disilanes react faster and to high polymers. Two reaction mechanisms seem to be working simultaneously, i.e., a mechanism as postulated in monosilanes and a disproportionation reaction. These mechanisms are discussed.     

Conductivities of polysilanes

Measurements of the electrical conductivities of three types of polysilanes were carried out over a range of dc fields and temperatures. These polymers are varied in substituents and structures with formulas of [C₆H₅SiCH₃]_n, [(C₆H₅SiCH₃)₇₀(CH₃SiH)₃₀]_n, and [(C₆H₅SiCH₃)₅₀(C₆H₅SiH)₃₀(CH₃Si)₂₀]_n. Undoped polysilanes behaved as insulators since their conductivities were observed in the range of 10⁻¹⁰ to 10⁻¹³ S cm⁻¹, while SbF₅-doped polysilanes of all kinds behaved as semiconductors with conductivities in the range of 10⁻² to 10⁻⁴ S cm⁻¹. No significant difference in conductivities was observed among three SbF₅-doped polysilanes although these polymers are very different in chemical properties. These experimental results suggest that electrical conductivities of polysilanes are associated with the Si Si main-chain backbone rather than with the side groups. It is evident that the dopant is able to diffuse throughout the bulk of the polymer and the conductivity of the doped polymer is a function of the dopant concentration from the result of in situ monitoring of the resistance of the silane homopolymer during SbF₅ doping at room temperature. The conductivities of polysilanes appeared to be temperature-dependent. The activation energy for the conduction of SbF₅-doped silane copolymer was found to change at its glass transition temperature. © 1996 John Wiley & Sons, Inc.     

Polysilanes Bearing a Phenoxypropyl Side Chain

A series of polysilanes bearing a 3‐phenoxypropyl side chain was prepared applying the Wurtz coupling reaction of dichlorosilanes. All prepared polymers ([R‐Si(CH₂)₃OPh]_n, R = Me, Pent, Hex, Hept, (CH₂)₃OPh, Ph) were investigated by GPC, UV, fluorescence, and ¹H, ¹³C, ²⁹Si NMR spectroscopy. Among these polysilanes only [HeptSi(CH₂)₃OPh]_n ( 3d ) exhibits a thermochromic behaviour showing at least two transitions.     

用作陶瓷先驱体的可溶性聚硅烷的合成与表征

以甲基氢二氯硅烷、二甲基二氯硅烷为原料，利用均聚和共聚的方法，合成了几种不同组成的可溶性聚硅烷，研究了反应条件对可溶性聚硅烷产率的影响，并用IR、XRD对产物进行了表征。结果表明：甲基氢二氯硅烷在反应体系中的比例越高，可溶聚硅烷的产率越高，增加反应时间和降低反应温度可提高可溶聚硅烷的产率。     

Cooking schedule of alkyd resin preparation. I. Effect of cooking schedule on viscosity-molecular weight relationship

The two constants in the equation log η = A + C′ M?_n^1/2 (η is the viscosity of molten alkyd and M?_n the number average molecular weight) were determined at 110°C for two kinds of alkyd resin prepared with the same formulation but with different cooking schedules. It was found that the magnitude of the slope C′ was larger for the alkyd which was prepared by raising the reaction temperature directly up to 230°C, in comparison with that of the alkyd which was prepared by maintaining the temperature at 170°C for an hour and then raising it up to 230°C. Measurements of η and M?_n were carried out until the gelation occurred. Both upward and downward breaks were observed in log η vs. M?_n^1/2 plot. Based on these viscometric data, the gel point mechanism was discussed. Disagreements between the molecular weight observed and that calculated from the Flory's theoretical equation became more remarkable as the esterification proceeded. This suggested that a large extent of intraesterification reaction is taking place in alkyd synthesis.     

Carbon dioxide: a renewable feedstock for the synthesis of fine and bulk chemicals

The syntheses of carbon dioxide (CO₂) based industrially important chemicals have gained considerable interest in view of the sustainable chemistry and “green chemistry” concepts. In this review, recent developments in the chemical fixation of CO₂ to valuable chemicals are discussed. The synthesis of five-member cyclic carbonates via, cycloaddition of CO₂ to epoxides is one of the promising reactions replacing the existing poisonous phosgene-based synthetic route. This review focuses on the synthesis of cyclic carbonates, vinyl carbamates, and quinazoline-2,4(1H,3H)-diones via reaction of CO₂ and epoxide, amines/phenyl acetylene, 2-aminobenzinitrile and other chemicals. Direct synthesis of dimethyl carbonate, 1,3-disubstituted urea and 2-oxazolidinones/2-imidazolidinones have limitations at present because of the reaction equilibrium and chemical inertness of CO₂. The preferred alternatives for their synthesis like transesterification of ethylene carbonate with methanol, transamination of ethylene carbonate with primary amine and transamination reaction of ethylene carbonate with diamines/β-aminoalcohols are discussed. These methodologies offer marked improvements for greener chemical fixation of CO₂ in to industrially important chemicals.     

Effect of initial compositions on boron carbide synthesis and corresponding growth mechanism

ABSTRACT

The yield of boron carbide (B₄C) synthesised by carbothermal reduction (CTR) is low (40–50%) owing to the inappropriate initial composition. In this paper, thermodynamic equilibrium simulations for effects of initial compositions on B₄C synthesis were performed, while initial compositions on the quality and yield of B₄C synthesised by CTR from B₂O₃-C mixture at atmospheric pressure were systemically investigated. The reaction mechanisms for B₄C synthesis process were thermodynamically analysed. Moreover, growth mechanisms for B₄C particles were speculated based on the thermodynamic and experimental results. The results show that the simulations agree well with experimental results, and optimal initial composition is 1.906 (B₂O₃/C weight ratio). Under optimal condition, B₄C powder prepared at 1973?K is high-quality with well crystallinity, purity of 93.58% and yield of 90.05%. Additionally, the prepared B₄C particles show two types of morphologies, aggregate of small equiaxed particles and needle-like particle, which are formed through different reaction mechanisms, respectively.     

Synthesis of poly[methyl(phenyl)silanediyl] containing 1,3,4-oxadiazole electron-transporting moieties

Summary The new oxadiazole derivative containing amino group, 2-(4-aminophenyl)-5-(biphenyl-4-y1)- 1,3,4-oxadiazole (Oxad-NH₂), was synthesized and attached to formylated poly[methyl(phenyl)silanediyl] (PMPSi) by acid-catalyzed condensation of the aldehyde with the amino group yielding a Schiff base. The aldehyde groups were incorporated into the parent PMPSi by the reaction with dichloromethyl methyl ether in the presence of Lewis acid (SnCl₄). GPC analyses before and after modification reaction revealed that the attachment of Oxad-NH₂ proceeded without degradation of the silicon backbone. We reached 75 % conversion of aldehyde groups to oxadiazole Schiff base. The condensation reaction was followed and the prepared compounds and polymers were characterized by ¹H NMR, UV-vis, FT IR, GPC and elemental analyses. Fluorescence behavior of oxadiazole-modified PMPSi was studied in tetrahydrofuran. Broad emission band with maximum at 440 nm was observed. Fluorescence quantum yield of modified polysilanes increases with the increasing content of the attached oxadiazole moiety. Received: 10 February 2003/Revised version: 8 April 2003/Accepted: 8 April 2003 Correspondence to Drahomír Vyprachticky     

On the formation of permethylcyclopolysilanes in the würtz-type reaction of Me2SiCl2: The influence of solvent and complexing agent

Permethylcyclopolysilanes are most often prepared by Würtz-type coupling of Me₂SiCl₂ with Na K alloy in THI: When nonpolar solvents like benzene were used, only linear polysilanes (Me₂Si)_n were produced, but the addition of a catalytic amount of crown ether 18-crown-6 was found to increase significantly the amount of cyclic oligomers. The ratio of (Me₂Si)₆ to (Me₂Si)₅ decreased from 18:1 to 6:1 in the presence of crown ether in a typical synthesis in THF. An explanation based on solvated electron theory has been proposed.     

Effect of the Chiral Center Position of an Optically Active Terminal Group on the Induction of Optical Activity in Polysilanes

Summary Polysilanes with an optically active alkoxy group, i.e., (S)-(+)-2-butoxy, (R)-(-)-2-butoxy, (S)-(-)-2-methyl-1-butoxy, and (S)-(+)-3,7-dimethyl-1-octoxy, at the terminal positions, the chiral carbon centers of which were located at the α, β, and γ positions relative to the oxygen, respectively, were prepared, and the effect of the position of chiral center of the terminal optically active group on the induction of optical activity in polysilanes was investigated. The circular dichroism (CD) spectra of these polymers showed positive Cotton signals around 340 nm at temperatures below -20 °C, but the intensities were small, indicating that the optically active groups at the terminal positions have some ability, albeit small, to induce optical activity to the polysilanes. Further, the optically active (S)-(+)-2-butoxy and (R)-(-)-2-butoxy groups did not control the helical sense direction of the polymers, despite the different chiral stimuli from the 2-butoxy groups introduced to the terminal positions. To control the helical structure of polysilanes by the use of optically active terminal groups, appropriate optically active groups are required.     

Preparation of polycarbosilanes and polysilanes from (trimethylsilylmethyl)chlorosilanes

The one-step conversion of (Me₃SiCH₂)SiCl₃ to polycarbosilanes by reaction with three molar equivalents of the alkali metals lithium, sodium, or sodium potassium alloy is reported. The expected network polysilanes were not isolated. Under similar conditions (Me₃SiCH₂)SiHCl₂ gives polysilanes. The low molecular weight polymers have been characterized by elemental analyses, gpc, and FT-IR, NMR, UV-vis, and mass spectroscopies.Presented in part at the XXVIth Silicon Symposium, Indiana University-Purdue University at Indianapolis, March 26–27, 1993.     

Characterisation of BaZr0.9Y0.1O3‐δ Prepared by Three Different Synthesis Methods: Study of the Sinterability and the Conductivity

BaZr_0.9Y_0.1O_3‐δ has been synthesised by three different methods: the solid‐state reaction, the spray pyrolysis and the spray drying. Significantly different apparent lattice parameters (between 0.4192 nm for the sample prepared by the solid‐state reaction method and sintered at 1,500 °C and 0.4206 nm for the sample prepared by the solid‐state reaction method and sintered at 1,720 °C) are observed after calcination and sintering, depending on the synthesis method and the sintering temperature. The bulk conductivity values also vary over several orders of magnitude (–7.2< log σ_b <–3.6 at 300 °C) depending on the synthesis method and the sintering temperature. These variations of the bulk conductivity and also the activation energy are correlated with variations of the apparent lattice parameter. The influence of the preparation method on the electrical properties is discussed. The grain boundaries are more resistive than the bulk. The variation of the grain boundary conductivity could be correlated to the microstructure in terms of the grain size.     

Electrodeposition of Ni-transition alloys for the oxygen evolution reaction

The oxygen evolution reaction (OER) is the anodic reaction in several industrial electrolytic processes. The objective of this work was to develop a new electrocatalytic material for long-lasting and economical high performance electrodes. New electrodes were prepared by electrodeposition of nickel, nickel-ruthenium and nickel-iridium alloys. They were then activated by anodic polarization at 100 mA cm^–2 to form an oxide layer. The electrocatalytic activity was characterized for the OER in 5M KOH solution. The results show that nickel-iridium alloys provide greater electrocatalytic activity for the OER and better corrosion resistance than nickel-ruthenium in alkaline solution. The effects of transition elements on improving the performance of the nickel electrode are then discussed.Notation b Tafel slope - i _ex exchange current density - i _dp electrodeposition current density - i _OER oxygen evolution reaction current density - C _dl double-layer capacity - o₂ oxygen overpotential     

New Efficient Dielectric and Antistatic Materials Based on Oligoaminophenols

New polyconjugated oligomers—oligoaminophenols—have been prepared by the oxidative polycondensation reaction of aminophenols (APs) in the presence of oxidants (H₂O₂, NaOCl, KOH plus O₂). It has been established that the reactivity of AP increases with the series of m- < o- < p-isomers in this reaction. The oligoaminophenols synthesized show a high reactivity in electron-exchange processes and in reactions with the oxirane ring of epichlorohydrin and epoxy resins. They are highly efficient antioxidants and antistatic agents. © 1997 SCI     

The effects of PVP surfactant in the direct and indirect hydrothermal synthesis processes of ceria nanostructures

CeO₂ nanostructures with completely different morphologies were successfully prepared using the same cerium source and mineralizer through the direct and indirect hydrothermal methods with different introducing strategies of PVP surfactant. The CeO₂ nanostructures tend to form the morphologies of nano-flowers and nano-cubes through the indirect and direct hydrothermal methods, respectively. X-ray diffraction (XRD) analyses revealed that both as-prepared nanostructures are composed of CeO₂ with a standard fluorite structure. The different synthesis mechanisms and corresponding chemical evolutions of the as-prepared CeO₂ nanostructures are discussed based on the different introducing strategies of PVP surfactant in the direct and indirect hydrothermal processes. Investigation of the UV-shielding ability of both CeO₂ nanostructures suggested that the UV absorbance of the nano-flowers is much higher than that of the nano-cubes.     

Characterization and catalytic properties of mesoporous CuO/SBA-16 prepared by different impregnation methods

CuO/SBA-16 catalysts were prepared by two different routes - the conventional impregnation method and the modified impregnation method with pH adjustment. These catalysts were characterized by X-ray diffraction (XRD), atomic absorption spectrometry (AAS), N₂ physisorption and hydrogen temperature programmed reduction (H₂-TPR) measurements which reveal that the cubic cagelike (Im3m) pore structure of the parent SBA-16 molecule sieves was well maintained throughout the synthesis. After introduction of Cu, a different CuO dispersion exists on these catalysts. The CuO/SBA-16 prepared by modified impregnation method has a single highly dispersed CuO which is considered as a highly efficient species for hydroxylation of phenol with H₂O₂. CuO/SBA-16 prepared by the conventional impregnation method shows the presence of bulk CuO species which is undesirable for this reaction.     

Formation of active phases in MoVTeNb oxide catalysts for ammoxidation of propane

The method of drying, heat evaporation or spray drying, of the aqueous suspension of starting chemicals has a pronounced effect on the phase composition of the final MoVTeNb catalyst, which ultimately influences the catalytic properties in propane ammoxidation reaction. The sample synthesized by spray drying is active and selective; it contains two main crystalline phases, orthorhombic M1 and hexagonal M2. The activity of the sample prepared by heat evaporation is low. This sample does not contain the active M1 phase and consists of hexagonal M2, TeMo₅O₁₆, and Mo_5−x(V/Nb)_xO₁₄ phases. The different mechanisms of phase composition formation in the samples synthesized by heat evaporation or spray drying arise from the different chemical nature of corresponding solid precursors.     

Solvothermal One-Step Synthesis and Effect of Carbon on Properties of Ruthenium Sulfide Catalysts

Abstract  

Solvothermal one-step synthesis method was used for the preparation of ruthenium sulfide catalysts supported on the carbon black by the reaction of Ru₃(CO)₁₂ and sulfur powder with the carbon black in i-propanol. The carbon black acted as not only a supporting substrate but also a dispersing agent for ruthenium sulfide nanoparticles in the solvothermal one-step synthesis. The oxygen reduction properties of the products prepared by the solvothermal one-step synthesis were improved in comparison with those of the catalysts prepared by physically mixing the carbon black with the solvothermally prepared ruthenium sulfide powder.     

Preparation and degradation of polysilylenes

Mechanistic aspects of preparation and degradation of polysilylenes (polysilanes) are discussed. Reductive coupling of disubstituted dichlorosilanes at ambient temperatures in the presence of ultrasound leads to monomodal polymers with relatively narrow molecular weight distributions (M _w/M _n from 1.2 to 1.5) and relatively high molecular weights (M _n from 50,000 to 100,000). Ring-opening polymerization of 1,2,3,4,-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane initiated with carbanions and silyl anions provides polymers with molecular weights from 10,000 to 100,000 and gives potential possibility of the microstructure control. The dearylation of phenyl containing polysilylenes with triflic acid provides polymers with strong electrophilic silyl triflate moieties. They can react with any nucleophiles such as alcohols, amines, carbanions, organometallics, etc., and produce various functional polysilylenes. Synthesis and solid-state transitions in random copolysilylenes are discussed. Thermal, mechanical, and chemical degradation of polysilylenes is described.This review was presented at the Second International Topical Workshop, Advances in Silicon-Based Science.