A novel tridentate nitrogen donor as ligand in copper catalyzed ATRP of methyl methacrylate

A tridentate ligand, BPIEP: 2,6‐bis[1‐(2,6‐diisopropyl phenylimino) ethyl] pyridine, having central pyridine unit and two peripheral imine coordination sites was effectively employed in controlled/“living” radical polymerization of MMA at 90°C in toluene as solvent, Cu^IBr as catalyst, and ethyl‐2‐bromoisobutyrate (EBiB) as initiator resulting in well‐defined polymers with polydispersities M_w/M_n ≤ 1.23. The rate of polymerization follows first‐order kinetics, k_app = 3.4 × 10^?5 s^?1, indicating the presence of low radical concentration ([P*] ≤ 10^?8) throughout the reaction. The polymerization rate attains a maximum at a ligand‐to‐metal ratio of 2:1 in toluene at 90°C. The solvent concentration (v/v, with respect to monomer) has a significant effect on the polymerization kinetics. The polymerization is faster in polar solvents like, diphenylether, and anisole, as compared to toluene. Increasing the monomer concentration in toluene resulted in a better control of polymerization. The molecular weights (M_n,SEC) increased linearly with conversion and were found to be higher than predicted molecular (M_n,Cal). However, the polydispersity remained narrow, i.e., ≤1.23. The initiator efficiency at lower monomer concentration approaches a value of 0.7 in 110 min as compared to 0.5 in 330 min at higher monomer concentration. The aging of the copper salt complexed with BPIEP had a beneficial effect and resulted in polymers with narrow polydispersitities and higher conversion. PMMA obtained at room temperature in toluene (33%, v/v) gave PDI of 1.22 (M_n = 8500) in 48 h whereas, at 50°C the PDI is 1.18 (M_n = 10,300), which is achieved in 23 h. The plot of lnk_app versus 1/T gave an apparent activation energy of polymerization as (ΔE^≠_app) 58.29 KJ/mol and enthalpy of equilibrium (ΔH⁰_eq) to 28.8 KJ/mol. Reverse ATRP of MMA was successfully performed using AIBN in bulk as well as solution. The controlled nature of the polymerization reaction was established through kinetic studies and chain extension experiments. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4996–5008, 2005     

Microwave promoted methylation of plant polysaccharides

O-Methylation is of outstanding importance in structural polysaccharide chemistry. A novel method for the methylation of polysaccharides using microwave (MW) irradiation is described. Seed gum from Cyamopsis tetragonolobus (Guar) was fully methylated with dimethyl sulphate and sodium hydroxide using 100% microwave power for 4 min in 68% yield. The completely methylated seed gum thus obtained was hydrolyzed by 70% formic acid followed by 0.5N H₂SO₄ under full microwave power for 1.16 and 1.66 min, respectively. The partially methylated monosaccharides were separated and identified.     

Stereospecific adduct ion formation in the chemical ionization mass spectra of E- and Z-1,2,3-triaryl-2-propen-1-ones

Stereospecific adduct ion formation has been observed in the chemical ionization mass spectra (positive and negative) of certain E- and Z-1,2,3-triaryl-2-propen-1-ones. The Z isomers are found to give higher relative abundances of adduct ions than the E isomers. This has been interpreted in terms of the differences in the proton affinities of the isomers originating from their different degrees of enone resonance. Halide ion (CI^? and Br^?) attachment spectra of these compounds also show stereochemical differences in the relative abundances of [M]^?˙ and [M+halide]^? ions, though the effect is not as pronounced as in the case of the positive ion spectra.     

Pervaporation from a dense membrane: roles of permeant-membrane interactions, Kelvin effect, and membrane swelling

Dense polymeric membranes with extremely small pores in the form of free volume are used widely in the pervaporative separation of liquid mixtures. The membrane permeation of a component followed by its vaporization on the opposite face is governed by the solubility and downstream pressure. We measured the evaporative flux of pure methanol and 2-propanol using dense membranes with different free volumes and different affinities (wettabilities and solubilities) for the permeant. Interestingly, the evaporative flux for different membranes vanished substantially (10-75%) below the equilibrium vapor pressure in the bulk. The discrepancy was larger for a smaller pore size and for more wettable membranes (higher positive spreading coefficients). This observation, which cannot be explained by the existing (mostly solution-diffusion type) models ofpervaporation, suggests an important role for the membrane-permeant interactions in nanopores that can lower the equilibrium vapor pressure. The pore sizes, as estimated from the positron annihilation, ranged from 0.2 to 0.6 nm for the dry membranes. Solubilities of methanol in different composite membranes were estimated from the Flory-Huggins theory. The interaction parameter was obtained from the surface properties measured by the contact angle goniometry in conjunction with the acid-base theory of polar surface interactions. For the membranes examined, the increase in the "wet" pore volume due to membrane swelling correlates almost linearly with the solubility of methanol in these membranes. Indeed, the observations are found to be consistent with the lowering of the equilibrium vapor pressure on the basis of the Kelvin equation. Thus, a higher solubility or selectivity of a membrane also implies stronger permeant-membrane interactions and a greater retention of the permeant by the membrane, thus decreasing its evaporative flux. This observation has important implications for the interpretation of existing experiments and in the separation of liquid mixtures by pervaporation.     

Fibre-optic fluorosensor for sulphur dioxide

A fibre-optic sensor for continuous measurement of sulphur dioxide is described. It is based on the dynamic quenching of the fluorescence of a polynuclear aromatic hydrocarbon [benzo(b)fluoranthene] which is immobilized in silicone polymer. Sulphur dioxide is shown to be an efficient quencher; Stern-Volmer graphs are given which describe the relation between SO₂ concentration and relative fluorescence. Detection limits are about 0.01% (v/v) SO₂ in air; the useful range is from 0.01–6% (v/v). Other gases likely to occur in air were found to be inert, except for oxygen which also acts as a dynamic quencher. Its interference is negligible for SO₂ levels below 6% in air at constant oxygen pressure, because the quenching efficiency of SO₂ is about 26 times higher than that of oxygen. For varying oxygen levels, a two-sensor technique is suggested.     

Titrations with hypovanadous salts

Summary In the present investigations, the standardization of vanadium(II) solution against ferric and cupric salts has been described and a few visual indicators have been shown to be applicable in the above titrations. When iron(III) solution is added to vanadium(II) solution neutral red marks the end point for V^II to V^III change. This change is also noted when vanadium(II) solution is added to ferric solution in the presence of excess of concentrated hydrochloric acid at an elevated temperature and neutral red and phenosafranine as visual indicators. Under the same conditions, the end point with eupric solution also corresponds to this change.When vanadium(II) solution is added to ferric solution at an elevated temperature in the presence of methylene blue or gallocyanine as visual indicators, the end point marks V^II to V^IV change. A mixture of iron and copper has also been titrated successfully with the help of visual indicators.

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Zusammenfassung Zur Einstellung von Vanadium(II)-lösungen mit Eisen(III)- oder Kupfer(II)-salzen werden als visuelle Indicatoren Neutralrot, Phenosafranin, Methylenblau oder Gallocyanin benutzt. Bei Zugabe von Eisen(III)-lösung zur Vanadium(II)-lösung zeigt Neutralrot den Übergang V^II V^III an. Dieser Übergang wird ebenfalls von Neutralrot und Phenosafranin angezeigt, wenn Vanadium(II)-lösung in Gegenwart von überschüssiger konz. Salzsäure bei erhöhter Temperatur zu Eisen(III)- oder Kupfer(II)-lösung gegeben wird. Der Übergang V^II V^IV wird von Methylenblau oder Gallocyanin angezeigt, wenn Eisen(III)-lösung bei erhöhter Temperatur mit Vanadium(II)-lösung titriert wird. Die Titration eines Gemisches von Eisen(III) und Kupfer(II) wird ebenfalls ausgeführt.

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Thanks of the authors are due to the C.S.I.R. authorities for providing a Junior Research Fellowship to one of them (R.K.M.).     

Mechanism of iridium(III) catalysis in the oxidation of diols by N-bromoacetamide

Summary The kinetics of iridium(III)-catalysed oxidation of 1,2-ethanediol and 1,4-butanediol by N-bromoacetamide (NBA) in HClO₄ in the presence of [Hg(OAc)₂] as a scavenger for Br^– have been investigated. The reactions are zero-order with respect to both diols, and first-order in NBA at low NBA concentrations, tending to zero order at high concentrations. The order in Ir^III decreases from unity to zero at high iridium(III) concentrations. A positive effect on the oxidation rate is observed for [H⁺] and [Hg^II] whereas a negative effect is observed for acetamide and [Cl^–]. Ionic strength does not influence the oxidation rate. (H₂OBr)⁺ is postulated as the oxidizing species. A mechanism consistent with the observed kinetic data is proposed.     

Reaction of sulphate radical anion (SO<Superscript>.</Superscript><Subscript>4</Subscript><Superscript>-</Superscript>) with hydroxy-and methyl-substituted pyrimidines: a pulse radiolysis study

Reactions of sulphate radical anion (SO·₄ ^-) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 6-methyl uracil (MU) and 5,6-dimethyl uracil (DMU) have been studied by pulse radiolysis at pH 3 and at pH 10. The transient intermediate spectra were compared with those from the reaction of hydroxyl radical (·OH). It is proposed that SO·₄ ^- produces radical cations of these pyrimidines in the initial stage. These radical cations are short-lived except in the case of DMHP where a relatively longer lived radical cation is proposed to be formed. When there is a hydrogen atom attached to the N(1) or N(3) position, a deprotonation from these sites is highly favored. When there is no hydrogen attached to these sites, deprotonation from a substituted methyl group is favored. At acidic pH, deprotonation from nitrogen is observed for DHMP, MU and DMU. At basic pH, the radical cation reacts with OH^- leading to the formation of OH adducts.