Reactions of isopropylperoxy radicals with NO and NO2

The rate constants for the reactions have been measured directly by flash photolysis and kinetic spectroscopy. At room temperature, k₃ = (3.4 ± 0.1) × 10⁹ L/mol·s, independent of pressure in the range of 55–400 torr, and k₆ = (2.1 ± 0.2) × 10⁹ L/mol·s.     

Easy Access to Chain‐Length‐Dependent Termination Rate Coefficients Using RAFT Polymerization

Chain‐length‐dependent termination rate coefficients of the bulk free‐radical polymerization of styrene at 80 °C are determined by combining online polymerization rate measurements (DSC) with living RAFT polymerizations. Full k_t versus chain‐length plots were obtained indicating a high k_t value for short chains (2 × 10⁹ L · mol⁻¹ · s⁻¹) and a weak chain‐length dependence between 10 and 100 monomer units, quantified by an exponent of −0.14 in the corresponding power law 〈k_t^i,i〉 = k_t⁰ · P^−b.

Double logarithmic plots of 〈k_t^i,i〉 versus P, evaluated from experimental time‐resolved R_p data according to the procedure described in the text, for different CPDA and AIBN concentrations. The best linear fit for (10 < P < 100) is indicated as full line.     

SP–PLP–EPR Investigations into the Chain‐Length‐Dependent Termination of Methyl Methacrylate Bulk Polymerization

Termination kinetics of methyl methacrylate (MMA) bulk polymerization has been studied via the single pulsed laser polymerization–electron paramagnetic resonance method. MMA‐d₈ has been investigated to enhance the signal‐to‐noise quality of microsecond time‐resolved measurement of radical concentration. Chain‐length‐dependent termination rate coefficients of radicals of identical size, k, are reported for 5–70 °C and up to i = 100. k decreases according to the power‐law expression . At 5 °C, k_t for two MMA radicals of chain‐length unity is k = (5.8 ± 1.3) · 10⁸ L · mol⁻¹ · s⁻¹. The associated activation energy and power‐law exponent are: E_A(k) ≈ 9 ± 2 kJ · mol⁻¹ and α ≈ 0.63 ± 0.15, respectively.

     

Analytical resolution of a parallel second‐order reaction mechanism

The competition among the three different elementary second‐order processes involving the radicals A and B, (1) (2) (3) is frequently established in many combustion and atmospheric chemistry systems. The analytical resolution of the above mechanism for k_AB = 2k_AA and k_AB = 2k_BB, that is, for a cross‐combination ratio ? = k_AB/(k_AAk_BB)^1/2 = 2, is well‐known, but it has been claimed not to exist for ? ≠ 2. In the present paper an analytical resolution of the system (1)–(3), performed under the condition k_AB ≠ 2k_AA and k_AB ≠ 2k_BB, leads to The mathematical procedure leading to this equation and the equation itself are valid independently of the nature of the products of the reactions ( 1 ), ( 2 ) and ( 3 ), that is, recombination or disproportionation products, provided that the reactants and the order of the reactions remain the same. The comparison with numerical integration for exemplary cases is performed. The solutions for the particular cases k_AB ≠ 2k_BB or k_AB ≠ 2k_AA are also presented. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 246–251, 2003     

Beiträge zur Komplexchemie der Phosphine und Phosphinoxide,XXVIII. Übergangsmetall-aminoalkylphosphin-Komplexe Teil 2: Palladium- Und Platinkomplexe

On the Coordination Chemistry of Phosphines and Phosphine Oxides. XXVIII. Transition Metal Aminoalkylphosphine Complexes. Part II: Palladium and Platinum Complexes Aminoalkylphosphines – C₆H₅HP? CH₂ · CH₂? , (C₆H₅)₂P? CH₂ · CH₂ · CH₂? NH₂, (C₆H₅)₂P? CH₂ · CH₂ · CH₂? N?CHC₆H₅ – react with palladium and platinum salts to give coordination compounds of the type MX₂, MX₂()₂, and MX₂()₄ (M = Pd, Pt; X = Cl, BPh₄). The chelating activity of the ligands, structure and properties of the metal complexes are discussed.     

The acetyl radicals CH3CO· and CD3CO· studied by flash photolysis and kinetic spectroscopy

Ultraviolet absorption spectra have been characterized for the acetyl-h₃ and acetyl-d₃ radicals, which were generated by the flash photolysis of the corresponding acetones. The spectra are broad and intense, with values of the extinction coefficient at the respective maxima estimated as: ?CH₃CO(215) = (1.0 ± 0.1) × 10⁴ L/mol·cm and ?CD₃CO(207.5) = (1.0 ± 0.05) × 10⁴ L/mol·cm. Rate constants for the reactions of mutual interaction were estimated as: k = 3.5 × 10¹⁰ L/mol·s and k = 3.4 × 10¹⁰ L/mol·s. Rate constants for the reactions of cross interaction were estimated as: k = 8.6 × 10¹⁰ L/mol·s and k = 5.2 × 10¹⁰ L/mol·s. The related values of the cross interaction ratios k/(kk)^1/2 = 2.6 and k/(kk)^1/2 = 1.6 do not differ significantly from the statistical value of 2. The participation of the radical displacement reactions was estimated in terms of the fractions k/k = 0.38 and k/k = 0.47. Corroborative spectra were obtained from the flash photolysis of methyl ethyl ketone and biacetyl, and the relative rates of the competing primary processes were estimated from the relative peak heights of the acetyl and methyl radicals in each system.     

The Radicals and Radical Dianions of Bridged [11]- and [15]Annulenyls as Compared with Those of Benzotropyl and 2,3-Naphthotropyl

The 1,6-methano[11]annulenyl ( 1 ·), 1,6:8, 14-propane-1,3-diylidene[15]annulenyl ( 2 ·), benzotropyl ( 3 ·) and 2,3-naphthotropyl ( 4 ·) radicals have been characterized by their ESR. spectra. The corresponding radical dianions, , , and , have also been studied both by ESR. and ENDOR. spectroscopy. Assignment of the coupling constants a_Hμ to protons in the individual positions μ of these radicals and radical dianions is to a large extent based on investigations of specifically deuteriated derivatives. The radicals 1· , 2· , 3· and 4· exist in temperature-dependent equilibria with ( 1 )₂, ( 2 )₂, ( 3 )₂ and ( 4 )₂, respectively, where ( 1 )₂ to ( 4 )₂ denote mixtures of dimers of 1 · to 4 ·. The dissociation enthalpies, ΔH°, of ( 1 )₂ (102 kJ/mol) and ( 2 )₂ (88 kJ/mol) are considerably smaller than those of ( 3 )₂ and ( 4 )₂ which do not significantly differ from the ΔH° value of bitropyl (139 ± 6 kJ/mol). This finding indicates that the gain in π-electron delocalization energies, Δ(DE)_π, upon dissociation markedly increases on going from bitropyl, ( 3 )₂ and ( 4 )₂ to ( 1 )₂ and ( 2 )₂, and thus points to an additional ‘resonance stabilization’ of 1 · and 2 ·, as compared with 3 · and 4 ·. A more pronounced π-spin localization on the 7-membered ring is observed in 3 ·, 4 ·, and relative to the corresponding species, 1 ·, 2 ·, and . It can be interpreted in terms of simple π-perimeter models without explicitly invoking substantial homoconjugative interactions between the bridged centres in 1 ·, 2 ·, and . However, the shortcomings of these crude models do not allow one to make a clear-cut statement about the contributions of the homotropyl structures to the π-systems of these paramagnetic species. The radical dianions and exhibit considerable hyperfine splittings from one ²³Na or ³⁹K nucleus of the counter-ion, whereas for and such splittings stem from two equivalent alkali metal nuclei. This finding is readily rationalized by different geometries of the bridged annulenyls and their benzo- and naphthotropyl analogues. Hyperfine data are also given for the radical anions of 7 H-benzocycloheptene, ( 3-H )\documentclass{article}\pagestyle{empty}\begin{document}$2^{\ominus \atop \dot{}}$\end{document}, and 6 H-(2,3-naphtho)cycloheptene, ( 4-H )\documentclass{article}\pagestyle{empty}\begin{document}$2^{\ominus \atop \dot{}}$\end{document}, as well as for the radical dianion of 1,6:8,14-bismethano[15]annulenyl, 5 \documentclass{article}\pagestyle{empty}\begin{document}$2^{\ominus \atop \dot{}}$\end{document}.     

η3‐Benzyl‐Nickel‐Diimine Complex: Synthesis and Catalytic Properties in Ethylene Polymerization

The oxidative addition of benzyl chloride to Ni(cod)₂ in the presence of 1,4‐bis(2,6‐diisopropylphenyl)acenaphthenediimine followed by chloride abstraction affords [(η³‐CH₂C₆H₅)Ni(α‐diimine)][PF₆] (α‐diimine = 1,4‐bis(2,6‐diisopropylphenyl)acenaphthenediimine) in 70% yield. The complex is active in ethylene polymerization in the presence of methylaluminoxane and under mild reaction conditions. The polyethylenes obtained are highly branched, have very low densities, do not show T_m or measurable crystallinity and have molecular weights ranging from 80 × 10³ to 290 × 10³ g · mol⁻¹.

     

Nanoscaled Polyaniline Fibers Prepared by Ferric Chloride as an Oxidant

Summary: Nanoscaled polyaniline (PANI) fibers with 17–30 nm in diameter were successfully prepared by oxidation polymerization using ferric hydrochloride (FeCl₃ · 6H₂O) as an oxidant in the presence of p‐toluenesulfonic acid (p‐TSA), β‐naphthalenesulfonic acid (β‐NSA), and camphorsulfonic acid (CSA) as the dopants. The resulting nanofibers show smaller diameter, higher crystallinity and conductivity (10⁻¹ S · cm⁻¹) compared with the nanofibers oxidized by ammonium persulfate (APS), which may be due to the lower oxidation/reduction potential of FeCl₃.

SEM images of the PANI nanofibers prepared by oxidation polymerization using ferric hydrochloride as an oxidant.     

The addition of methyl radicals to hexafluoroacetone

The reaction of methyl radicals (Me) with hexafluoroacetone (HFA), generated from ditertiary butyl peroxide (dtBP), was studied over the temperature range of 402–433 K and the pressure range of 38–111 torr. The reaction resulted in the following displacement process taking place: where TFA refers to trifluoroacetone. The trifluoromethyl radicals that were generated abstract a hydrogen atom from the peroxide: such that k_6a is given by: where θ = 2.303RT kcal/mol. The interaction of methyl and trifluoromethyl radicals results in the following steps: Product analysis shows that k₁₇/kk = 2.0 ± 0.2 such that k₁₇ = 10^10.4±0.5M^?1 · s^?1. The rate constant k₅ is given by: It is concluded that the preexponential factor for the addition of methyl radicals to ketones is lower than that for the addition of methyl radicals to olefins.     

The reaction of methyl p-nitrophenyl sulfate with hydrogen peroxide anion determination of pKa of hydrogen peroxide in methanol by a kinetic spectrophotometric procedure

A kinetic spectrophotometric investigation of the reaction of the hydrogen peroxide anion with methyl p-nitrophenyl sulfate in methanol solvent resulted in the evaluation of the pK_a of HOOH in methanol at 25°C as 15.8 ± 0.2. Since normal kinetic procedures for the determination of the equilibrium constant K for the process CH₃O^? + H₂O₂ ? CH₃OH + HO were found to be associated with high uncertainty, another procedure was devised to establish the magnitude of K. This method is based on an analysis of the changing slopes of plots of pseudo-first-order rate constants against the total base concentration as the stoichiometric amount of hydrogen peroxide is varied. The method is applicable to any system in which anionic nucleophiles generated in situ compete with solvent anions. Such a corroboration of kinetically determined equilibrium constants is believed essential. The kinetic data allow the specific rate constant k_HOO-for the reaction of methyl p-nitrophenyl sulfate with hydrogen peroxide anions to be evaluated and yield the rate constant ratio k/k = 8.8 ± 2.2. This confirms the existence of an α effect at saturated carbon in this system.     

Gas phase reaction of heptafluoroisopropyl radicals with carbon tetrachloride

The reaction was investigated in the gas phase over the range 80–225°C using the photolysis of heptafluoroisopropyl iodide as the source of radicals. The rate constant, based on the value of 10^13.36 cm³ mol^?1 s^?1 for the recombination of i-C₃F₇ radicals, is given by where θ = 2.303 RT/cal mol^?1. Arrhenius parameters for chlorine abstraction from CCl₄ by CF₃, C₂F₅, n-C₃F₇, and some hydrogenated radicals are compared.     

Kinetic Simulations of Reversible Chain Transfer Catalyzed Polymerization (RTCP): Guidelines to Optimum Molecular Weight Control

Kinetic simulations of reversible chain transfer catalyzed polymerization (RTCP) were performed using the program package Predici. Mimicking the RTCP of styrene in bulk at 80 °C, the full molecular weight distributions, the polydispersities of resulting polymer and the time evolutions of monomer conversion and participating species were simulated. The influence of the kinetic coefficients governing the RTCP equilibrium – specifically, the rate coefficients of activation, k_a, and deactivation, k_da – on the controlled polymerization behavior was probed in detail by varying their respective simulation input values over five orders of magnitude. It was found that optimum results for molecular weight control are obtained for K = k_a/k_da in the range 1 to 10 and with k_a and k_da being of the order of 10⁶ L · mol⁻¹ · s⁻¹ or above. The influence of degenerative chain transfer on the process was found to be significant only in poorly controlled systems, but is small in well‐controlled RTCP. Based on the finding that the catalyst is depleting during the polymerization due to cross‐termination, guidelines for obtaining high molecular weight material via repeated addition of catalyst were developed.

     

Determination of Propagation Rate Coefficient of Acrylates by Pulsed‐Laser Polymerization in the Presence of Intramolecular Chain Transfer to Polymer

Unusual difficulties are faced in the determination of propagation rate coefficients (k_p) of alkyl acrylates by pulsed‐laser polymerization (PLP). When the backbiting is the predominant chain transfer event, the apparent k_p of acrylates determined in PLP experiments for different frequencies should range between k_p (propagation rate coefficient of the secondary radicals) at high frequency and k at low frequency. The k value could be expressed from kinetic parameters: , where k_fp is the backbiting rate coefficient, k_p2 is the propagation rate coefficient of mid‐chain radicals, and [M] is the monomer concentration.

Apparent propagation rate coefficients determined for different frequencies by simulating the PLP of n‐butyl acrylate at 20 °C. Horizontal full lines show the values of k_p and k.     

Metal Salt‐Induced Ferroelectric Crystalline Phase in Poly(vinylidene fluoride) Films

In this Communication, the effect of varying mass fractions (0–20 wt.‐%) of calcium chloride (CaCl₂) salt on the α‐ and β‐phase content of poly(vinylidene fluoride) (PVDF) as‐cast films were investigated. Spectral and X‐ray studies revealed the maximum ferroelectric β‐phase for the addition of 15 wt.‐% of CaCl₂ in PVDF compared to neat PVDF samples. The dense β‐phase dominant PVDF–CaCl₂ (15 wt.‐%) thick film used as a ferroelectric insulator in one‐capacitor (1C) type random access memory device exhibited a remnant polarization of 3.1 µC · cm², and is a good indication that the unoriented PVDF–CaCl₂ films can be used in electronic applications without further stretching process.

     

An evaluation of the kinetic parameters for the reaction of trifluoromethyl radicals with CH3Cl in the gas phase in the temperature range from 416 to 636 K

The hydrogen and chlorine atom abstraction reactions from CH₃Cl by CF₃ radicals produced by the photolysis of hexafluoroacetone (HFA) and CF₃I were studied relative to the recombination of CF3 radicals (I) The Arrhenius parameters obtained in the temperature range 416 to 578 K are: where Θ = 2,303.RT cal mol^?1 and k₂ is the recombination rate constant for the CF₃ radicals. The factors that influence the transfer processes of chlorine and hydrogen are analyzed in a series of reactions of halomethanes with CF₃ and CH₃ radicals. © 1993 John Wiley & Sons, Inc.     

Reaction of pentafluoroethyl radicals with hydrogen cyanide

The reaction of C₂F₅ radicals with HCN has been studied over the range of 533–673 K using the pyrolysis of pentafluoroethyl iodide as the free-radical source. Arrhenius parameters for the reaction relative to C₄F₁₀ recombination are given by where θ = 2.303RT kJ/mol and k_H/k is in cm^3/2/mol^1/2·s^1/2.     

Formation of crystalline polyelectrolytic complexes on matrix polymerization

Zwitterionic matrix stepwise polymerization of 4-vinylpyridine (VP) using isotactic poly(acrylic acid) (i-PAA) as a template results in formation of crystalline polycomplexes consisting of i-PAA and of ionene (I): Depending on the concentration of VP and i-PAA, the two types of crystalline polycomplexes can be prepared. The complex containing long ionene sequences with n ? 2 is formed when [VP]₀ = [i-PAA]₀ ≥ 0.05M. At [VP]₀ = [i-PAA]₀ ? 0.01M, the complex formed evidently contains the ionene dimer (n = 2). The x-ray structures of the crystalline complexes are identical with those obtained by mixing of i-PAA and the corresponding dimer and polymer presynthesized in the absence of the template. The matrix polymerization of VP in the presence of syndiotactic and atactic PAA at the same conditions results in formation of amorphous polycomplexes.     

Ambipolar Field‐Effect Transistor (FET) and Redox Characteristics of a π‐Conjugated Thiophene/1,3,4‐Thiadiazole CT‐Type Copolymer

Summary: A π‐conjugated charge transfer‐type copolymer consisting of an electron‐donating thiophene and an electron‐accepting 1,3,4‐thiadiazole, P(ThdzTh), underwent facile electrochemical p‐ and n‐doping, as revealed by cyclic voltammetry. The copolymer gave a new ambipolar field‐effect transistor (FET), which showed typical I_DS (source–drain current)–V_DS (source–drain voltage) curves in both a p‐type working mode and an n‐type working mode. In the n‐type working mode, the polymer showed a carrier mobility of about 5 × 10⁻³ cm² · V⁻¹ · s⁻¹ and an on/off ratio of about 3 × 10⁴.

n‐Channel field‐effect transistor characteristics of P(ThdzTh).     

Assessing the RAFT Equilibrium Constant via Model Systems: An EPR Study

Reversible addition‐fragmentation chain transfer (RAFT) equilibrium constants, K_eq, for the model system cyano‐iso‐propyl dithiobenzoate (CPDB) – cyano‐iso‐propyl radical (CIP) have been deduced via electron paramagnetic reso1494nce (EPR) spectroscopy. The CIP species is produced by thermal decomposition of azobis‐iso‐butyronitrile (AIBN). In solution of toluene at 70 °C, K_eq has been determined to be (9 ± 1) L · mol⁻¹. Measurement of K_eq = k_ad/k_β between 60 and 100 °C yields ΔE_a = (–28 ± 4) kJ · mol⁻¹ as the difference in the activation energies of k_ad and k_β. The data measured on the model system are indicative of fast fragmentation of the intermediate radical produced by addition of CIP to CPDB.