Polymerization of acrylic acid initiated by a hexavalent chromium-organic sulfur compounds reducing agent system

The kinetics of polymerization of acrylic acid initiated by Cr⁶⁺-thiourea, Cr⁶⁺-thioacetamide, Cr⁶⁺-2-aminoethane thiol, Cr⁶⁺-cysteine, and Cr⁶⁺-thioglycollic acid have been studied at 30, 35, and 40°C in nitrogen. The rates of polymerization were measured. Chromic acid alone did not initiate the polymerization under deaerated and un-deaerated conditions. On the basis of the experimental observation of the dependence of the rate of polymerization R_p on various variables, a suitable kinetic scheme is proposed.     

Effects of ligands on the “living” radical polymerization initiated by the aged Cr2+ plus benzoyl peroxide system

The “living” radical polymerization of methyl methacrylate with the aged Cr²⁺ plus benzoyl peroxide (BPO) system in the presence of various amines as ligand has been studied in N,N′-dimethylformamide. Aliphatic amines such as ethylenediamine diminished the rate of polymerization, while dipyridyl (dipy) and o-phenanthroline (phen) accelerated the polymerization rate as follows: phen > dipy > pyridine ? none. Specifically, the rate of polymerization in the presence of phen had a maximum value at [phen]/[Cr²⁺] = 0.5. The retardation of polymerization by aliphatic amines was explained by the interaction of BPO with free and coordinated amines. The latter result may support the mechanism involving the complexed radical proposed for the living radical polymerization with the aged Cr²⁺ + BPO system. In the presence of phen the effects of aging period and aging temperature as well as polymerization temperature were studied and the nature of complexed radicals is discussed.     

Effect of amines on vinyl polymerization initiated by chromous acetate–alkyl halide systems

The radical polymerization of vinyl monomers initiated by Cr²⁺–RX in the presence of various amines was studied in DMF at 30°C. Polyamines able to form the chelate complex with Cr²⁺ accelerated the rate of polymerization of styrene in the following order: ethanolamine > triethylenetetramine > diethylenetriamine > ethylenediamine. However, aliphatic monoamine, hexamethylenediamine, and aromatic diamine did not have any effect on the polymerization. These results suggest that the effect of multidentate ligands may be associated with chelating effects which affect the electron transfer ability of the metal complex. An apparent activation energy of 8.2 kcal/mole for the polymerization of styrene was obtained in the presence of ethanolamine. With the Cr²⁺–CHCl₃ system, on addition of ethanolamine, the polymerization of methyl methacrylate was accelerated, and acrylonitrile and vinyl chloride, could be polymerized.     

Kinetics and Mechanism of Cr(VI) Reduction in a Water Cathode Induced by Atmospheric Pressure DC Discharge in Air

The process of reduction of Cr⁶⁺ ions (solution of potassium dichromate, K₂Cr₂O₇) in a water cathode was studied during a DC discharge in air. The concentration range of Cr⁶⁺ was (5.7–19) ×10^?5 mol/l and discharge current range was 20–80 mA. Cr⁶⁺ ions were shown to be reversibly reduced under a discharge action. The equilibrium degree of reduction increased with increasing initial concentration of the solution at fixed discharge current. At fixed initial concentration the reduction degree increased with increasing discharge current. The reduction degrees so obtained were 0.34–0.84. A kinetic scheme of the processes taking place in a solution was proposed. The calculated data obtained as a result of application of this scheme described well the experimental results on Cr⁶⁺ kinetics. The main processes of Cr⁶⁺ reduction and Cr³⁺ oxidation were revealed. HO ₂ ^· radicals and hydrogen peroxide were shown to be responsible for Cr⁶⁺ reduction whereas ^·OH radicals and O₂ molecules provide the reverse process of Cr³⁺ oxidation to Cr⁶⁺. The mechanism of action of phenol additives improving the process efficiency is discussed. The efficiency of phenol action as a radical scavenger was shown to be determined with its mass-transfer to the reaction area rather than chemical reaction rate.     

Effects of acids on vinyl polymerization initiated by chromous acetate-alkyl halide

Effects of various acids, i.e., acetic, chloroacetic, hydrochloric, and sulfuric acids on the polymerization of styrene initiated by Cr²⁺–CHCl₃ in DMF was studied. These acids reduced the rates of polymerization by Cr²⁺–CHCl₃ initiator systems. This phenomenon could be explained by a decrease of the reducing power of Cr²⁺ for alkyl halide. On the other hand, chloroacetic acids could initiate the polymerization of styrene without CHCl₃ because these acids have active chlorine in the molecule for Cr²⁺. It was found that the polymer obtained by this initiator system had an endgroup containing chromium carboxylate, therefore this polymer was green in color and insoluble in benzene.     

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity,Induction Period and Polymer Properties

The Cr/SiO₂ Phillips catalyst has taken a central role in ethylene polymerization ever since its discovery in 1953. This catalyst is unique compared to other ethylene polymerization catalysts, since it is active without the addition of a metal-alkyl co-catalyst. However, metal-alkyls can be added for scavenging poisons, enhancing the catalyst activity, reducing the induction period and altering polymer characteristics. Despite extensive research into the working state of the catalyst, still no consensus has been reached. Here, we show that by varying the type of metal-alkyl co-catalyst and its amount, the Cr redox chemistry can be tailored, resulting in distinct catalyst activities, induction periods, and polymer characteristics. We have used in-situ UV-Vis-NIR diffuse reflectance spectroscopy (DRS) for studying the Cr oxidation state during the reduction by tri-ethyl borane (TEB) or tri-ethyl aluminum (TEAl) and during subsequent ethylene polymerization. The results show that TEB primarily acts as a reductant and reduces Cr⁶⁺ with subsequent ethylene polymerization resulting in rapid polyethylene formation. TEAl generated two types of Cr²⁺ sites, inaccessible Cr³⁺ sites and active Cr⁴⁺ sites. Subsequent addition of ethylene also revealed an increased reducibility of residual Cr⁶⁺ sites and resulted in rapid polyethylene formation. Our results demonstrate the possibility of controlling the reduction chemistry by adding the proper amount and type of metal-alkyl for obtaining desired catalyst activities and tailored polyethylene characteristics.     

Studies on the hydrolytic polymerization of chromium (III) ion: IX. Revision of graphical method for the reaction in the presence of maleic acid by computer

The hydrolytic polymerization of Cr²⁺ in the presence of maleic acid was studied by equilibrium pH method at 60°C and constant ionic strength. Both maleic acid and Cr³⁺ are of three different concentrations: 0.006, 0.008, 0.01 mol·L^?1. The state of Cr³⁺ in aqueous solution were determined by graphical method and pqr analysis. The following species were found (CrA)^+, Cr-(OH)A and Cr₂(OH)₂A₂. The results by graphical method were revised by computer calculation with data of about fifty experimental points. Hydrolysis constants of all species were obtained with good fitting. It is obvious that the results obtained by program LEMIT are more accurate than those by graphical method. Mechanism of Cr³⁺ polymerization in the presence of maleic acid is also discussed.     

时间分辨红外光谱研究Cr价态在La,Cr共掺杂SrTiO3光催化剂中的作用

使用时间分辨红外光谱研究了经H₂和O₂处理的La和Cr共掺杂SrTiO₃的光生电子动力学. X射线光电子能谱和Raman光谱结果表明, H₂处理后样品中的Cr均是+3价, 而O₂处理后Cr为+3和+6价. 使用355和532 nmm激光激发样品所得时间分辨红外光谱表明, 相比在Cr⁶⁺存在时, 光生电子衰减速率在Cr³⁺存在的情况下要慢, 这说明Cr³⁺更有利于抑制光生电子空穴的复合, 从而增加光催化产氢的活性.     

Grafting of Acrylamide On 1, 1, 2, 2-Tetra-Chloroethane Preswelled Polyethylene Terephthalate) Films Using Azobisisobutyronitrile Initiator

Abstract

Graft polymerization of acrylamide (AAm) on 1, 1, 2, 2 tetrachloro-ethane (TCE) preswelled poly(ethylene terephthalate) (PET) films were performed with chemical initiation method using asobisiso-butyronitrile (AIBN) initiator. Temperature was found to have a greater effect on the swelling then the swelling time. Variation of the graft yield with polymerization temperature, time, AIBN concentration, AAm concentration, AIBN and AAm inclusion times were investigated. The optimum temperature for grafting was found to be 70°CC. The graft yield was observed to increase with polymerization time, AAm concentration, initiator and monomer diffusion time up to a saturation graft yield and then leveled off. An increase in AIBN concentration first enhanced the percent grafting then showed a decrease. The addition of some salts (Ni²⁺, Cr³⁺, Co²⁺, Cu²⁺) on the rate of grafting was also investigated. From the temperature dependence of the initial rate of grafting, the overall activation energy was found to be 4. 1 kcal/mol and relevant rate equation have been derived. The effect of grafting on film propities, such as water absorption capacity, intrinsic viscosity were determined. Grafted films were characterized by FTIR spectros-copy and scanning electron microscopy (SEM).     

Synthesis and reactivity of metal-containing monomers 49. Synthesis and structure of low-valence transition metal acrylates and their polymers

The methods for synthesis of Cu¹⁺, Fe²⁺, Cr²⁺, and V³⁺ acrylates were elaborated. The compounds obtained were characterized by elemental analyses, data of IR, X-ray photoelectron, and Mössbauer spectroscopy, magnetochemistry, and mass spectrometry. Polymeric products were obtained by liquid-phase radical polymerization of metal acrylates. Changes in the electronic state of the ions and their nearest ligand environment during polymerization were observed.     

Intrinsic Characteristics of Cr6+-Resistant Bacteria Isolated from an Electroplating Industry Polluted Soils for Plant Growth-Promoting Activities

The Cr⁶⁺-resistant plant growth-promoting bacteria was isolated from soil samples that were collected from an electroplating industry at Coimbatore, India, that had tolerated chromium concentrations up to 500?mg Cr⁶⁺/L in Luria-Bertani medium. Based on morphology, physiology, and biochemical characteristics, the strain was identified as Bacillus sp. following the Bergey's manual of determinative bacteriology. Evaluation of plant growth-promoting parameters has revealed the intrinsic ability of the strain for the production of indole-3-acetic acid (IAA), siderophore, and solubilization of insoluble phosphate. Bacillus sp. have utilized tryptophan as a precursor for their growth and produced IAA (122???g/mL). Bacillus sp. also exhibited the production of siderophore that was tested qualitatively using Chrome Azurol S (CAS) assay solution and utilized the insoluble tricalcium phosphate as the sole source of phosphate exhibiting higher rate of phosphate solubilization after 72?h of incubation (1.45???g/mL). Extent of Cr⁶⁺ uptake and accumulation of Cr⁶⁺ in the cell wall of Bacillus sp. was investigated using atomic absorption spectrophotometer and scanning electron microscope-energy dispersive spectroscopy, respectively. The congenital capability of this Cr⁶⁺-resistant plant growth-promoting Bacillus sp. could be employed as bacterial inoculum for the improvement of phytoremediation in heavy metal contaminated soils.     

Reduction–Oxidation of Chromium Ions in Aqueous Solution by Treatment with Atmospheric-Pressure Direct-Current Discharge in Argon

Kinetic features of the oxidation–reduction processes of chromium ions in a potassium dichromate aqueous solution subjected to treatment in argon atmospheric-pressure dc discharge have been investigated in the discharge current range of 20–80 mA and the dichromate concentration range of 0.1–0.5 mmol/L. The solution served as the discharge cathode. It has been revealed that the discharge treatment stimulates the reduction reactions of Cr⁶⁺ to Cr³⁺ ions and the reverse reactions of oxidation of Cr³⁺ to Cr⁶⁺ ions; i.e., the oxidation–reduction reactions are reversible. The limiting degree of reduction of Cr⁶⁺ ions has been found to depend on the discharge current and the initial concentration of the solution. The apparent rate constants of oxidation and reduction have been determined, and the energy efficiency of the process has been evaluated. The data are compared with the results obtained earlier for a discharge in air.     

The dependence of the kinetics of some simple outer-sphere electrode reactions on the nature of the electrode material

The kinetics of a number of simple inorganic electrode reactions that are known or expected to follow outer-sphere pathways have been examined at mercury, silver, platinum, and gold-aqueous interfaces in order to explore the effects of varying the electrode material on outer-sphere reactivity. The electroreduction kinetics of Co(III) ammine complexes exhibited only mild dependences on the nature of the electrode material which were compatible with the expected variations in double-layer effects. However, the electrooxidation of Cr²⁺ proceeded at strikingly higher overpotentials on the solid surfaces compared with mercury electrodes. Similar effects were also seen for the electrooxidation of V²⁺, Eu²⁺ and Ru²⁺ in the presence of Cr²⁺. Much larger rate constants were observed for these aquo reactions at solid surfaces in the absence of Cr²⁺, although Cr²⁺ had no influence on Co(NH₃)₆³⁺ electroreduction, or any reaction at mercury electrodes. It is speculated that the very large substrate effects upon the electrode kinetics of aquo couples arise from the influence of the inner-layer water structure on the reactant-solvent interactions experienced by these “structure-making” reactants at their plane of closest approach. The inhibiting influence of Cr²⁺ may be due to its ability to efficiently remove adsorbed catalytic contaminants by incorporation into a substitutionally inert Cr(III) electrooxidation product by means of a ligand-bridge mechanism.     

In Vitro Selection of Chromium‐Dependent DNAzymes for Sensing Chromium(III) and Chromium(VI)

Chromium is a very important analyte for environmental monitoring, and developing biosensors for chromium is a long‐standing analytical challenge. In this work, in vitro selection of RNA‐cleaving DNAzymes was carried out in the presence of Cr³⁺. The most active DNAzyme turned out to be the previously reported lanthanide‐dependent Ce13d DNAzyme. Although the Ce13d activity was about 150‐fold lower with Cr³⁺ than that with lanthanides, the activity of lanthanides and other competing metals was masked by using a phosphate buffer; this left Cr³⁺ as the only metal that could activate Ce13d. With 100 μm Cr³⁺, the cleavage rate is 1.6 h^?1 at pH 6. By using a molecular beacon design, Cr³⁺ was measured with a detection limit of 70 nm , which was significantly lower than the United States Environmental Protection Agency (EPA) limit (11 μm ). Cr⁴⁺ was measured after reduction by NaBH₄ to Cr³⁺, and it could be sensed with a similar detection limit of 140 nm Cr⁴⁺; this value was lower than the EPA limit of 300 nm . This sensor was tested for chromium speciation analysis in a real sample, and the results supported its application for environmental monitoring. At the same time, it has enhanced our understanding of the interactions between chromium and DNA.     

Heavy metal-induced inhibition of Aspergillus niger nitrate reductase: applications for rapid contaminant detection in aqueous samples

Enzyme inhibition assays have the potential to rapidly screen and identify heavy metals in environmental samples. Inhibition of nitrate reductase (NR) was examined as a method for detecting toxic metals. The activity of NR (EC 1.6.6.2) from Aspergillus niger was assayed as a function of metal concentration in the presence of Cd²⁺, Cr³⁺, Cr⁶⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺. NR exhibited sensitivity to these metals at concentrations below 10 μM. Various buffers were screened for their ability to protect NR activity from metal inhibition, and 3-(N-morpholino) propanesulfonic acid (MOPS) was selected as the buffering system for the NR assays as it exhibited the least interference with metal inhibition, thus providing increased assay sensitivity. The hypothesis that chelating agents could prevent the inhibition of NR activity by metal ions was also tested. Results indicated that 10 mM ethylenediaminetetraacetic acid (EDTA) could protect NR activity from inhibition by Cr³⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Zn²⁺ at concentrations below 100 μM, but that the EDTA had no effect on NR inhibition by Cr⁶⁺. An amount of 10 mM nitrilotriacetic acid (NTA) prevented NR inhibition by Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ at metal concentrations below 100 μM. However, 10 mM NTA was unable to protect the enzyme from inhibition by either Cr³⁺ or Cr⁶⁺. These results indicated that through specific metal chelation, a NR-based method for individually quantifying Cr³⁺ and Cr⁶⁺ species in aqueous solutions could be developed. The ability to restore activity to NR which been previously inhibited by exposure to 100 μM Pb²⁺, Cd²⁺, Zn²⁺, Cu²⁺, and Cr³⁺ was explored to determine whether NR activity could be recovered by EDTA additions for use in consecutive metal inhibition assays. The results showed NR activity could not be regained after exposure to Cr³⁺ or Cu²⁺, but did partially recover activity after Cd²⁺, Pb²⁺, and Zn²⁺ exposure.     

The reaction of chromate with diphenylcarbazide. II

Job's method was employed for ascertaining the detailed mechanism of the chromato-carbazide reaction. Three different systems (1) chromatc-carbaxidc, (2) chromate-carbastone and (3) chromous-earbazone, leading to the formation of the same coloured complex were investigated. From studies in cquimolecular solutions, the maximum composition values for the reactions were found to be 2:3, 1:3 and 1:1, respectively. Proceeding from the assumption that the reaction (3) is a case of simple displacement, the stoichiometry of all the three systems has been formulated. The role of chromate in the chromatc-carbay.ide reaction, has been found to be primarily that of an oxidising agent followed by complex formation. The tentative formulation for the three systems is given below. (1) 2Cr⁺⁶ + 3 carbazide ? 2 [Cr⁺² carbazone^-2 complex ] + carbacliazone + 4 H⁺ (2) Cr⁺⁶ + 3 carbazone ? [Cr⁺² carbazone^-2 complex] + 2 carbadiazone + 2 H⁺ (3) Cr⁺² + carbazone ? [Cr⁺² carbazone^-2 complex] + 2 H⁺ The proposed mechanism seems to explain the relevant facts observed in the course of investigation of the three Systems, particularly the rapid discharge of the colour produced when excess of chromium was added and the greater sensitivity of carbaxide as compared to carbaxonc were accounted for, The determination of K values from studies in non-equimolccular solutions of the three reactions also corroborates the aforesaid mechanism. The apparent dissociation constant of the Cr⁺²-carbazonc complex was found to be 2.7.10^-6 and the equilibrium constants of (1) and (2) were calculated as 5.7.10^-12 and 2.3.10^-7, respectively. The existence of bivalent Cr⁺² ion in the complex molecule has also been independently confirmed from magnetochemical measurements. The molar susceptibility value of 4.6 Bohr magnetons calculated on the basis of 1:1 chromous-carbastonc ratio, indicates ionic bonding in the complex.     

Adsorption applications of synthetically prepared PANI-CuO based nanocomposite material

In this work, we have used a polyaniline and cupric oxide-based nanocomposite material (PANI-CuO) for the adsorption of Pb²⁺, Cd²⁺, and Cr⁶⁺ ions from contaminated water. FTIR, FESEM, EDX and XRD methods were used in the characterisation of PANI-CuO. The maximum percentage removal of Pb²⁺, Cd²⁺, and Cr⁶⁺ ions was found to be 79.9%, 78.9% and 82.1% at 1 g of PANI-CuO for 100 mL of contaminated water. The suitability order of the isotherm model was observed as Langmuir > Freundlich > Elovich, whereas the pseudo first order model was best fitted as compared to the pseudo second order model. The rate constants were evaluated as 1.441, 1.801 and 1.661 g mg^?1 min ^?1, respectively. The values of ΔG⁰ were calculated to be ?287.4, ?849.6, and ?728.9 kJ mol^?1 for Pb²⁺, Cd²⁺ and Cr⁶⁺ ions, respectively.     

Surface phenomena in conducting materials based on lanthanum chromite

The composition of the surface of a solid solution based on lanthanum chromite, La_0.975Ca_0.025·Cr_0.9Al_0.1O₃, and prepared by solid-phase synthesis followed by vacuum sintering and annealing in air was studied by X-ray photoelectron spectroscopy. The spectral line of chromium differs from the lanthanum line in the X-ray photoelectron spectrum in that the left wing of the chromium line is more gently sloping. The gentle slope of the line wing indicates that the given atom occurs in different charge states. For chromium, it appears possible to resolve the Cr³⁺ and Cr⁶⁺ bands in the X-ray photoelectron spectra, which allows estimation of the Cr³⁺ and Cr⁶⁺ concentrations.     

Photocatalytic activity and physicochemical characteristics of modified potassium polytitanates in the reaction of decomposition of aqueous-alcoholic solutions

Study of the photocatalytic activity of nanomaterials based on potassium polytitanate modified with transition metal ions (Ni²⁺, Cr³⁺, Cu²⁺) demonstrated that the cation composition, morphology of photocatalyst particles, and their size strongly affect the photocatalytic activity. The rate of the photoinduced hydrogen evolution from the aqueous-alcoholic solution grows in the series of potassium polytitanates modified with Cu²⁺, Ni²⁺, Cr³⁺ ions, and that from pure water, for potassium polytitanates modified with Ni²⁺,Cr³⁺, Cu²⁺ ions. The capacity for intercalation of water into the interlayer space does not strongly affect the rate of the photoinduced hydrogen evolution. It was shown that potassium polytitanate modified with Cr³⁺ ions has the maximum quantum efficiency (30%).     

Mechanism of Polymerization of α-Olefins on Oxide Catalysts

The polymerization of ethylene on a chromic oxide catalyst with and without a solvent has been studied. It was found that the active catalyst surface is formed exclusively as a result of its interaction with ethylene. This interaction is accompanied by the formation of products which poison the surface of the catalyst when they are sorbed on it in the absence of a solvent. A catalyst which contains no Cr⁺⁶ atoms as a result of reduction by alcohol is inactive. On the other hand, a catalyst which contains only Cr⁺⁶ atoms becomes active only after it has been partially reduced. The most probable product of this reduction is trivalent chromium atoms. The results obtained have given grounds for the assumption that the active complex contains Cr⁺⁶ and Cr⁺³ atoms. A possible mechanism of the reaction is discussed. Owing to the oxidative action of CrO₃ on the ethylene molecules, part of the Cr⁺⁶ is reduced to Cr⁺³, and the trivalent chromium becomes alkylated. The monomer molecule is added at the Cr⁺³—C bond thus formed. A strong Lewis acid, CrO₃, lowers the electron density on the Cr⁺³ atom. This increases the strength of the Cr⁺³—C bond and the ability of the Cr⁺³ atom to coordinate with the monomer molecule. The monomer molecule enters the chain at the moment when the strength of the Cr^?3—C bond is weakened due to coordination of this molecule with the Cr⁺³ atom.