Main Chain Termination Research Articles

Antioxidant activity and mechanism of inhibitory action of gentisic and \u03b1-resorcylic acids

The antioxidant activity of gentisic (GA) and α-resorcylic (α-RA) acids was investigated by considering their molecular structures in various oxidative environments, including DPPH· scavenging assay, stripped olive and soybean oils, and the corresponding oil-in-water emulsions. The mechanism of action in the oils was evaluated in the presence of different concentrations of the antioxidants at 60 °C, using the kinetic parameters the stabilization factor (F), the oxidation rate ratio (ORR), the activity (A), and the average rate of antioxidant consumption (overline{r}_{{{text{AH}}}}). GA was significantly more potent antioxidant than α-RA in all the environments. Although the less polar α-RA showed better activity in the emulsions rather than in the bulk oils, GA with an ortho-hydroxy structure had higher capacity to scavenge DPPH·, and LOO· in the oils and emulsions. The lower performance of α-RA was attributed to its participation in side reactions of chain initiation (AH + LOOH → A· + L· + H2O) and propagation (A· + LH → AH + L·) as competed with the main chain termination reaction (LOO· + AH → LOOH + A·).

Experimental and kinetic modeling investigation on ethylcyclohexane low-temperature oxidation in a jet-stirred reactor

In this work, the oxidation of ethylcyclohexane was studied in a jet-stirred reactor at 780 Torr, 480–780 K and equivalence ratios of 0.5, 1.0 and 2.0. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for the detection of oxidation products, including a series of reactive intermediates such as cycloalkylhydroperoxides, keto-hydroperoxides, alkenal-hydroperoxides and highly oxygenated molecules. Quantum chemistry calculations were performed to obtain ionization energies for the identification of some important intermediates and energy barriers of several important pathways. On the other hand, this work presents the first efforts on developing a low-temperature oxidation model of ethylcyclohexane. The present model can reasonably capture the low-temperature oxidation reactivities and negative temperature coefficient behaviors observed in both present and previous experimental work of ethylcyclohexane oxidation. Modeling analyses were performed to provide insight into the low-temperature oxidation chemistry of ethylcyclohexane. The two-stage O2 addition mechanism is concluded to dominate the chain-branching process in the low-temperature region. The concerted elimination reactions of cycloalkylperoxy radical and “formally direct” chemically activated reactions of cycloalkyl+O2 result in cycloalkenes and HO2 formation at the negative temperature coefficient region and serve as main chain-termination pathways. Compared with smaller cycloalkanes like cyclohexane and methylcyclohexane, the ethyl sidechain structure in ethylcyclohexane reduces the energy barriers of cycloalkylperoxy radical isomerization and facilitates the formation of keto-hydroperoxides which leads to more pronounced low-temperature oxidation reactivity of ethylcyclohexane.

Structural Analysis of Poly(3‐hexylthiophene) Prepared via Direct Heteroarylation Polymerization

This study reports the synthesis and characterization of poly(3‐hexylthiophene) (P3HT) from a direct heteroarylation polymerization of two isomeric monomers, 2‐bromo‐3‐hexylthiophene (monomer 1) and 2‐bromo‐4‐hexylthiophene (monomer 2). Using the Herrmann–Beller catalyst along with P(o‐NMe2Ph)3, the resulting polymers are obtained in excellent yields and exhibit a good number‐average molecular weight (Mn of 33 and 16 kDa, respectively). Detailed 1H NMR analyses reveal less than 1% of homocouplings and no evidence of β‐branching. Dehalogenation is identified as the main chain termination step and preferentially occurs on monomer 2. The melting temperature (237 °C) and hole mobility (up to 0.19 cm2 V−1.s−1) of the nearly defect‐free P3HT obtained from this simple polymerization of monomer 1 are comparable, if not superior, to those obtained with commercially available GRIM and Rieke samples. image

Autoxidation of sulphur dioxide in the presence of alcohols under conditions related to the tropospheric aqueous phase

In this work attempts were made to elucidate the monoterpenic alcohol inhibition of the S(IV) autoxidation by comparing the inhibiting activity of cis -verbenol (C 10 H 15 OH), myrtenol (C 10 H 15 OH) and nopol (C 11 H 17 OH) with that of ethanol (C 2 H 5 OH) and 2-propanol (C 3 H 7 OH). Results of laboratory experiments on the kinetics of S(IV) autoxidation in the presence of these alcohols were interpreted using the equation derived by Alyea and Bäckström (J. Am. Chem. Soc. 51 (1929) 90), brought into relationship with the actual mechanistic knowledge on the reactivity of the inhibitors with respect to sulphate radicals. The rate constants for the reaction: alc+SO 4 · − (+O 2 )→SO 4 2− +H + +HO 2 +ald involving cis -verbenol and myrtenol have been determined as equal to, respectively, 5.4×10 9 and 4.2×10 9 M −1 s −1 . These results were obtained under the assumption that the main chain termination is due to simultaneous scavenging SO 5 · − radical anions by Fe II and SO 4 · − radical anions by an alcohol. In the case of nopol the rough estimate gave for the rate constant of the latter reaction a value of 9.0×10 9 M −1 s −1 burdened with an error caused by the sulphoxy radical induced autoxidation of nopol. The studied airborne compounds of biological origin were shown to be potentially significant modifiers of the acidity formation in clouds and a sink for sulphoxy radicals participating in further transformations of these compounds.

Initiation of polymerization with substituted ethanes—12. Free radical polymerization of methyl methacrylate and styrene initiated with 3-methoxycarbonyl-3-methyl-2,2,5,5-tetraphenylhexanedinitrile

The title compound, the first member of a homologous series of oligomers formed by free radical polymerization of methyl methacrylate initiated by tetraphenylbutanedinitrile, was synthesized and its structure was determined by X-ray diffraction. The longest CC bond of this molecule measures 1.628 Å and can be thermally split to give radicals. The activation energy of this dissociation was found to be 150 kJ/mol. The kinetics of the free radical polymerization of methyl methacrylate and styrene initiated with the title compound shows remarkable deviations from the ideal polymerization kinetics, indicating that primary radical termination might be the main chain termination reaction.

The addition of trimethylgermanium radicals to fluoroethylenes

The light induced addition of trimethylgermanium hydride to fluoroethylenes has been studied. Reactant concentration, light intensity and temperature have been varied. The results are interpreted in terms of a chain mechanism in which reaction (2) is reversible. The main chain termination ( CH 3) 3 Ge+E⇄( CH 3) 3 GeE· ( CH 3) 3 GeE·+( CH 3) 3 GeH→( CH 3) 3 GeE H+( CH 3) 3 Ge· process is the combination of two trimethylgermanium radicals, but evidence is present to show that reaction (4) is also reversible under the experimental conditions. The reversibility of this reaction 2( CH 3) 3 Ge· ⇄ h ( CH 3) 3 GeGe( CH 3) 3 makes it impossible to determine the rate constants k 2, k −2 and k 3. The orientation of addition to unsymmetric olefins thus depends on concentration, temperature and light intensity and although data for one set of conditions are reported their interpretation is only possible in general terms.

Radiation-sensitized Thermal Cracking of n-Butane. I. Radical Reactions

The radiolysis of n-butane was investigated at temperatures ranging from 17 to 548 °C in both static and flow systems.It was concluded that in the radiation-sensitized thermal cracking region the main products, methane, ethane, ethylene, and propylene, were formed by a radical chain mechanism. The conclusion was reached from comparison with the thermal cracking products, the effect of ammonia addition, the dose rate dependence, and in particular the correlation between the temperature change of the type of the main chain-termination reaction and that of the activation energy of propylene formation. The value of the activation energy for propylene formation showed that the main chain-termination reaction at temperatures between 410 and 520 °C was a combination reaction of ethyl radicals. The major part of 1-butene, trans-2-butene, and cis-2-butene, formed in the chain region, was shown to result from the thermal decomposition of the chain carrying butyl radicals.Rate parameters for some of the reactions involved were calculated.

Free radical addition to olefins. Part 6.—Addition of sulphur chloride pentafluoride to fluoro-olefins

The light-induced addition of SF5Cl to fluoro-olefins (E) has been studied in a large series of experiments in which light intensity, temperature and relative concentrations have been varied. The reaction mechanism is the same as that established for ethylene with the following principal chain carrying steps : [graphic ommitted] and the combination of two SF5· radicals (k6) the main chain termination step. The addition to tetrafluoro-, trifluoro- and 1,1-difluoro-ethylene has been investigated and from studies on the latter two compounds the following rate expressions were obtained. [graphic ommitted] *R the gas constant taken to be 1.9872 cal deg.–1 mol–1 and subsequent energies expressed in cal mol–1(1 cal = 4.1855 J).

The pyrolysis of propylene

Detailed analyses of the reaction products of the pyrolysis carried out in the temperature range 555 to 640°C, at initial pressures between 7 and 300 mmHg, and measurements of overall pressure change have shown that the overall pyrolysis may be described by the expression -d[C3H6]/dt= 1014.06[C3H6]1.4exp (-58600/RT) mole ml.-1s-1. Twenty three primary and three secondary products of the pyrolysis at 600°C and an initial pressure of 103 mmHg have been determined at six extents of reaction up to 12%. On the basis of these measurements a long chain free radical mechanism is proposed in which reactions of the 1-methyl-4-pentenyl radical are of prime importance. The main chain termination reaction is found to be combination of methyl and allyl radicals. It is concluded that radical combination reactions involving allyl are considerably slower than those involving alkyls. Steady-state treatment of the data is precluded by their complexity. Speculative routes to the formation of the many higher products are suggested.