Mechanochemical synthesis of mullite

The specifics of mechanical activation processes in modifying synthetic corundum powder and gels by silicon alkoxide are considered. As Al₂O₃ is milled with a silicon alkoxide additive, Al₂O₃ and SiO₂ reactants are activated with the formation of an intermediate self-organized complex, in which the mechanochemical interaction proceeds accompanied by the formation of mullite.     

Syndiotactic polymerization of methyl methacrylate with Ni(acac)<Subscript>2</Subscript>-methylaluminoxane catalyst

In this study, syndiotactic-rich poly(methyl methacrylate) (PMMA) is obtained by using a soluble nickle acetylacetonate [Ni(acac)₂] and modified methylaluminoxane (MMAO-3A) catalyst system under modest polymerization conditions. The main purpose of this work is concerned with the study of previous conflicting stereospecificity data. Types of MAO, temperature of polymerization, MAO/Ni(acac)₂ (Al/Ni) mole ratio, and various solvents have been investigated in the MMA polymerization. Particularly, high syndiotactic PMMA [(rr) > 91 %] has been obtained when MMAO-3A is used as cocatalyst with Al/Ni ratio of ca. 50 or polymerization temperature ca. 0 °C. As expected, the prepared syndiotacticity-rich PMMA has a higher glass transition temperature (T _g) within 120 ~ 127 °C. The details of the polymerization mechanism, especially in relation to the stereoregularity problems are under investigation.     

The first example of mechanochemical synthesis of organometallic pincer complexes

The synthesis of an organometallic Pd^II pincer complex is realized for the first time via CH bond activation of the bis(thiocarbamate) ligand with PdCl₂(NCPh)₂ under mechanochemical conditions either by grinding of the reactants in a mortar or in a vibration ball mill at gram scale.     

Synthesis and characterization of vinyl silane modified imidazole copolymer as a novel corrosion inhibitor

For a novel corrosion inhibitor for copper, the free radical copolymerization of vinyl imidazole (VI) and vinyl trimethoxy silane (VTS) was carried out in benzene at 68 °C using azobisisobutyronitrile (AIBN) as an initiator. The reactivity ratios of the two monomers were determined from instantaneous composition diagram: r _VI = 3.22 and r _VTS = 0. Thermal stability of poly(VI-co-VTS) depended on VTS mole fraction in the feed rather than the molecular weight of the copolymer, due to the disiloxane bond linkages formed during heating. Received: 21 October 1996/Revised: 9 December 1996/Accepted: 12 December 1996     

Controlled conversion of polyacrylamide into polyacrylic acid

Interaction of nitrous acid with aqua solution of poly(acrylamide) gives poly(acrylic acid) in quantitative conversion yields. The reaction proceeds smoothly, with simultaneous evolution of nitrogen, provided that the temperature below 5 °C. The procedure presented provides a versatile route to prepare acrylic acid-acrylamide copolymers in any desired composition, by adjusting molar ratio of nitrous acid. Received: 29 January 1997/Revised: 24 July 1997/Accepted: 6 August 1997     

Study of energy distribution of active sites in heterogeneous Ziegler-Natta catalysts

New method of investigation of active sites non-uniformity was developed. The method is based on mass-spectrometric study of temperature programmed desorption (TPD) products from the catalyst surface at initial stages of olefin gasphase polymerization. The conditions of polymerization allowed to obtain the short macrochains on catalyst surface. Two well-resolved intense maxima in the temperature ranges of 180 – 210 °C and 280 – 320 °C on the TPD curves from SiO₂/TiCl₄/AlEt₂Cl catalyst surface after polymerization were observed in a result of thermal destruction of active Ti−C bonds and the evolution of hydrocarbon molecules of different length (up to 14 monomer units in chain). One may conclude that in this catalyst there are at least two types of active sites varying in the activation energy of thermal destruction of Ti−C bonds. The distribution of active sites over activation energy of Ti−C bond thermal destruction was calculated. Received: 28 April 1997/Accepted: 12 May 1997     

<Superscript>29</Superscript>Si NMR Chemical Shift Tensor and Electronic Structure of 7-Silanorbornadienes

The structure and bonding of 7-silanorbornadienes was investigated using X-ray Diffraction (XRD), solid-state NMR spectroscopy and density functional calculations. The solid state structures of four benzo-7-silanorbornadienes (4a, c, d, e) and of one dibenzo-7-silabenzonorbornadiene (5a) are reported and compared with the results of previous structural investigations. The most prominent features of the molecular structures of all 7-silanorbornadienes are very long Si-C(bridgehead) bonds (d(SiC) = 190.6–196.8 pm) and very acute CSiC bond angles α (α(CSiC) = 78.7–83.9°). All newly investigated 7-silanorborndienes show for tetracoordinated silicon nuclei extremely deshielded ²⁹Si NMR resonances (δ²⁹Si = 65.6–31.6). Solid State NMR investigations for 7-silanorbornadienes anti-4a, b reveal highly anisotropic chemical shift tensors of axial or nearly axial symmetry (4a: δ₁₁ = 161, δ₂₂ = δ₃₃ = −11; 4b: δ₁₁ = 113, δ₂₂ = 14, δ₃₃ = −15). The dominating, strongly deshielding δ₁₁ component is oriented almost perpendicular to the plane spanned by the two bridgehead carbon atoms and the bridging silicon atom. The DFT calculations suggest that the origin of the strong deshielding is a small energy difference between the frontier orbitals, which are strongly localized at the silicon atom. In addition the computations reveal that both the long SiC bonds and the strongly deshielded ²⁹Si NMR chemical shift are direct consequences of the bonding situation in 7-silanorbornadienes which are characterized by through space interaction of the C=C double bonds and by hyperconjugation between the SiC σ-bonds and the unoccupied orbitals of the C=C double bonds.     

Influence of cocatalyst on the stereoselectivity and productivity of styrene polymerization reactions

Influence of different cocatalysts on the polymerization reaction of styrene using heterogeneous nanoparticle NA-MgO and NA-TiO2 (anatase) supported bis (cyclopentadienyl) zirconium dichloride catalysts is studied. Methyaluminoxane, trityl tetrakis(pentafluorophenyl)borate(1), dimethylanilinium tetrakis (pentafluoro-phenyl)borate (2) and tris(pentafluorophenyl)borane (3) are used as cocatalysts for this study. The productivity and stereoselectivity of the catalysts systems are found to be highest with MAO and lowest with the borane 3 (MAO > 1> 2 > 3). Catalysts derived from the borane 3 yield amorphous atactic polystyrenes but those from cocatalysts MAO, 1, or 2 yield crystalline, syndiotactic polystyrenes under the same reaction conditions. Effects of addition of various scavengers and solvents with different polarities on styrene polymerizations are also reported here. Characterization of the obtained polymers is done by Gel Permeation Chromatography, ¹³C-NMR spectroscopy and Differential Scanning Calorimetry.     

Dispersion polymerization of acrylamide with 2-acrylamide-2-methyl-1-propane sulfonate in aqueous solution of sodium sulfate

The copolymer of acrylamide (AM) and 2-acrylamide-2-methyl-propane sulfonate (AMPS) was synthesized through free radical dispersion polymerization in an aqueous solution of ammonium sulfate and poly(2-acrylamide-2-methyl-1-propane sulfonate) as a steric stabilizer. The average particle size of the copolymer ranged from 1.0 μm to 4.0 μm, and the molecular weight was 2.0 × 10⁶ ~ 7.0 × 10⁶ g mol⁻¹. The swelling property of the dispersion copolymer was studied by characterizing the apparent viscosity and particle size distribution. When the dispersion was diluted with salt aqueous solution in which the ammonium sulfate concentration kept equal with that of the original dispersion, particle size and particle size distribution of the diluted dispersion changed little, compared with that of the original dispersion. While diluted with deionized water, particle size and particle size distribution could expand several times. The effects of the AMPS/AM molar ratios, the molecular weight of stabilizer, and the initiation temperature were investigated. It was found that with the increase of the AMPS/AM ratios, the molecular weight of copolymers increased, and then decreased. The particle size and conversion of monomers increased. The stability of the copolymer dispersion increased with increasing the molecular weight of stabilizer for a fixed stabilizer concentration. With the increase of the initiation temperature, the molecular weight of copolymer increased at first and then decreased gradually, but the particle size and conversion increased. The optimum conditions for the stable AMPS/AM dispersion were as follows: the AMPS/AM molecular ratio was 15/85, the molecular weight of the PAMPS stabilizer 3.0 × 10⁵ ~ 4.0 × 10⁵, and the initiation temperature 50 °С, respectively.     

Photo-enhancement of Cr(VI) reduction by fungal biomass of Neurospora crassa

Various organisms such as fungus are capable of reducing Cr(VI) to less toxic Cr(III). However, light-induced Cr(VI) reduction by fungus is less reported and needs to be explored since anthropogenic or natural activities may bring these two reactants into a sunlit environment. In this study, the interactions and reaction mechanisms of Cr(VI) on a model fungus, Neurospora crassa, were evaluated in the presence or absence of light. The influence of ferric ion, a widely distributed metal, on Cr(VI) reduction by the fungus was also investigated under illumination. The results show that 20–54% of added Cr(VI) (96.2 μM) was removed by 1 g of dead fungal biomass (i.e., 1–2.7 mg Cr(VI) reduction by 1 g biomass) at pH 1–3, after 6 h reaction in the dark. However, 96.2 μM Cr(VI) disappeared completely (i.e., 5 mg Cr(VI) reduction by 1 g biomass) under the same reaction time and experimental conditions when light was present. The rapid disappearance of Cr(VI) in solution was due to the reduction of Cr(VI) by the excited biomass upon light absorption, and the rates of redox reactions increased with a decrease at pH. Cr(VI) reduction could be further increased with the addition of 89.5 μM Fe(III) because the formation of Fe(II) from the photolysis of Fe–organic complexes enhanced Cr(VI) reduction. Spectroscopic studies indicated that the amide, NH, and carboxyl groups of N. c.-biomass may be responsible for initiating Cr(VI) reduction; comparatively, the cyclo-carbons of chitin, glucan, and their derivatives were more persistent to the oxidation by Cr(VI). Accordingly, fungi containing high amount of carboxyl, amide, and NH groups may be preferable as efficient reductants for scavenging Cr(VI) from environment. Upon the absorption of a renewable light source, Cr(VI) could be converted rapidly by the biomaterials to the less toxic Cr(III).     

Synthesis of sterically hindered ethyl vinyl ethers containing electron acceptors and their polymerization behaviors

3,5-Dimethoxy-4-(2′-vinyloxyethoxy)benzylidenemalononitrile (2) and methyl 3,5-dimethoxy-4-(2′-vinyloxyethoxy)benzylidenecyanoacetate (3) were prepared and polymerized by radical and cationic initiators. Bifunctional monomers 2 and 3 did not polymerize by radical initiators, but copolymerized readily with ethyl vinyl ether in γ-butyrolactone solution at 65 °C. The trisubstituted terminal double bond participated in the vinyl polymerization and radical polymerization of 2 and 3 with ethyl vinyl ether led to swelling polymers 8 and 9 that were not soluble in common solvents due to cross-linking. Under the same polymerization conditions ethyl vinyl ether polymerized well with model compounds of 3,4,5-trimethoxybenzylidenemalononitrile (10) and methyl 3,4,5-trimethoxybenzylidenecyanoacetate (11), respectively, to give 1 : 1 alternating copolymers 12 and 13 in high yields. Polymers 8 and 9 showed a thermal stability up to 300 °C without any characteristic T_g peaks in DSC thermograms. Alternating copolymers 12 and 13 were soluble in common solvents such as acetone and DMSO, and the inherent viscosities of the polymers were in the range of 0.33 – 0.55 dL/g. Films of polymers 12 and 13 cast from acetone solution were cloudy and tough and T_g values obtained from DSC thermograms were in the range of 163 – 168 °C. Received: 2 August 1996/Revised version: 17 September 1996/Accepted: 20 September 1996     

Effect of an Iron Catalyst and Process Parameters on Si-Based Ceramic Materials Synthesized From Rice Husks

Silica and carbon are naturally occurring in rice husks (RH) and these were used for the production of ceramic materials made of Si, C, N and O as the main constituents. The various “silicon-based” ceramics were produced from the thermal decomposition of rice husks and posterior heat treatment at temperatures varying from 1200–1450 °C under a pure nitrogen atmosphere. FeSO₄ in various concentrations was introduced to the decomposed rice husk prior to the heat treatment. The formation of various Si/C/N/O ceramics in general and silicon nitride in particular, were studied with respect to the concentration of FeSO₄ (4–10%) as well as temperature (1200–1450 °C). The formation of different phases were confirmed by XRD and FT-IR analysis. Morphology and surface properties of the products have been studied using SEM. The maximization of whiskers of the ceramic products was also studied through microstructural analysis. Elemental analysis of the whiskers product was analyzed through EDX. The chemical analysis of raw rice husk was also carried out.     

Formation of highly porous materials in the combustion wave of systems based on 5-aminotetrazole with potassium perchlorate

The formation of highly porous materials in the combustion wave of mixtures of 5-aminotetrazole with potassium perchlorate is studied. Under normal conditions with an oxidizer deficiency, the combustion of such mixtures proceeds in an unsteady oscillatory regime with formation of condensed products of incomplete oxidation of 5-aminotetrazole. It is shown that in the presence of metallic iron or its oxide, the completeness of interaction of the mixture components increases sharply and combustion becomes steady-state. In the presence of silicon dioxide, a highly porous solid residue forms which retains the shape of the initial block. At a certain ratio of iron and silicon dioxide in the initial composition, the solid combustion products have the form of interwoven thin fibers about 1 µm in diameter and 10–30 µm long.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 2, pp. 75–80, March–April, 2005.     

Acid‐Free Nickel Catalyst for Stereo‐ and Regioselective Hydrophosphorylation of Alkynes: Synthetic Procedure and Combined Experimental and Theoretical Mechanistic Study

The nickel catalyst prepared in situ from nickel bis(acetylacetonate) [Ni(acac)₂] precursor and bis(diphenylphosphino)ethane (DPPE) ligand has shown excellent performance in the hydrophosphorylation of alkynes. Markovnikov‐type regioselective addition to terminal alkynes and stereoselective addition to internal alkynes were carried out with high selectivity without an acidic co‐catalyst (in contrast to the palladium/acid catalytic system). Various H‐phosphonates and alkynes reacted smoothly in the developed catalytic system with up to 99% yield. The mechanisms of catalyst activation and C P bond formation were revealed by experimental (NMR, ESI‐MS, X‐ray) and theoretical (density functional calculations) studies. Two different pathways of the alkyne insertion in the coordination sphere of the metal are reported for the first time.     

Mechanochemical Nucleation and Stability of Newly-Formed Phases

Nucleation of new phases originated by mechanochemical activation of the reactants is considered. The decrease in nucleation energy of high-melting powders modified with silicon alkoxide is achieved via formation of carbon clathrates in SiO₂ and self-organized high-activity ensemble with a specified proportion of cybotactic groups. The stability of newly-formed -SiC and mullite synthesized by a mechanochemical method is discussed.     

Macroporous polymeric hydrogels formed from acrylate modified polyvinyl alcohol macromers

Macroporous polymeric hydrogels for the last several years have found broad application in areas connected with medicine, especially in such new disciplines as cell and tissue engineering. In present work a novel combine approach is proposed for preparation of polyvinyl alcohol macroporous hydrogels by cross-linking of polyvinyl alcohol acrylic derivatives in the presence of heterophase of frozen aqueous media. Hydrogels prepared using this method does not need additional structure fixing and are characterized by high thermal stability in swollen state sustaining even heating to more than 100 °С. The influence of different factors and reaction conditions on the cross-linked hydrogel formation process was studied. The high yield of products (80 ÷ 95%) was observed when reaction was conducted at temperature range −12 ÷ −18 °С, concentration of macromer 4–12 weight %, and amount of initiator 0.8 ÷ 1.6 mg/ml. Moreover, the equilibrium swelling of synthesized macroporous hydrogels was investigated and it was shown that synthesized cross-linked hydrogels are characterized by high water absorption which is weakly depended on solution pH and ionic force values.     

The polymerization mechanism of lactide initiated with zinc (II) acetylacetonate monohydrate

The paper presents the mechanism of lactide polymerization initiated with zinc (II) acetylacetonate monohydrate. However, the actual initiator of this reaction is the complex containing a metal-oxygen bond, formed by the exchange reaction of acetylacetonate ligand with deprotonated lactide derivative. The described reaction results in the release of free acetylacetonate and formation of transitional zinc complex with metal-oxygen bond connecting the zinc atom with derivative of lactide, incorporated - as a new, active in polymerization ligand. Polylactide chain propagation process, which constitutes the following stage of the reaction, is caused by a typical, well known, coordination-insertion ring opening polymerization. The proceeding polymerization maximum yield at the applied conditions does not exceed about 70% in benzene solution and 90% at bulk.     

Synthesis and characterization of potassium humate–acrylic acid–acrylamide hydrogel

A novel potassium humate–acrylic acid–acrylamide (KHA–AA–AM) superabsorbent polymer was prepared from the reaction among leonardite potassium humate, acrylic acid and acrylamide by free radical initiating process using ammonium persulfate as the initiator and N, N′-methylene bisacrylamide as the crosslinker. Various effects of synthesis conditions on superabsorbent polymer were studied and the optimal reaction condition was obtained with crosslinker concentration 0.44–0.74 wt%, initiator concentration 1.12–2.22 wt%, n(KOH)/n(AA) 0.51–0.70, monomer concentration 10.95–12.59 wt%, graft reaction temperature 83 ± 1°C, monomer mole ratio of acrylic acid to acrylamide 1.42–2.30, and potassium humate content 17.54 wt%. Under the optimal conditions, the solution absorbency of KHA–AA–AM superabsorbent polymer to deionized water, tap water, 0.5% carbamide solution and 0.9% NaCl solution were 733–756, 161–284, 786–825, and 76–83 g/g, respectively.     

Separation of block copolymers from parent homopolymers by means of liquid chromatography at the critical adsorption point

The characterization of a block copolymer consisting of blocks of tetrahydrofuran, THF, and methyl methacrylate, MMA, was attempted with the aid of liquid chromatography, LC, at the critical adsorption point, CAP. The conditions applying to the CAP at 30 °C for the homopolymers are: PTHF: eluent composition: THF : acetonitrile = 50 : 50, w/w; column: Nucleosil C₁₈. PMMA: eluent composition: THF : n-hexane = 81.8 : 18.2, w/w; column: bare silica gel. The presence of homo PTHF and of homo PMMA in the crude block copolymer was evidenced. Moreover, chromatography at the CAP for PMMA showed that the block copolymer contains multiblock (triblock) fractions. The formation of triblock copolymer is expected when the polymerization of MMA is terminated (partially) by combination. In conclusion, LC at the CAP is a powerful tool to characterize block copolymers regarding the presence of homopolymer and multiblock (triblock) fractions. Received: 13 February 1997/Revised: 18 March 1997/Accepted: 24 March 1997     

Preparation of Ultrafine Fe–Pt Alloy and Au Nanoparticle Colloids by KrF Excimer Laser Solution Photolysis

We prepared ultrafine Fe–Pt alloy nanoparticle colloids by UV laser solution photolysis (KrF excimer laser of 248 nm wavelength) using precursors of methanol solutions into which iron and platinum complexes were dissolved together with PVP dispersant to prevent aggregations. From TEM observations, the Fe–Pt nanoparticles were found to be composed of disordered FCC A1 phase with average diameters of 0.5–3 nm regardless of the preparation conditions. Higher iron compositions of nanoparticles require irradiations of higher laser pulse energies typically more than 350 mJ, which is considered to be due to the difficulty in dissociation of Fe(III) acetylacetonate compared with Pt(II) acetylacetonate. Au colloid preparation by the same method was also attempted, resulting in Au nanoparticle colloids with over 10 times larger diameters than the Fe–Pt nanoparticles and UV–visible absorption peaks around 530 nm that originate from the surface plasmon resonance. Differences between the Fe–Pt and Au nanoparticles prepared by the KrF excimer laser solution photolysis are also discussed.