Determination Of Absolute Rate Constants Research Articles

Kinetic study of the reaction CH(X 2Π)+H2⇄CH2(X 3B1)+H in the temperature range 372 to 675 K

The reaction CH(X 2Π)+H2⇄CH2(X 3B1)+H was studied over the temperature range 372 to 675 K. Laser-induced fluorescence detection of CH radicals allowed direct determination of absolute rate constants for the forward reaction k1. The data are well fit by the Arrhenius expression k1=(2.38±0.31)×10−10 exp[−(1760±60)/T] cm3/s over the temperature range studied. A transition state theory (RRKM) calculation yields the following form for k1, k1=(5.5±1.5)×10−16 T1.79±0.04 exp[−(840±30)/T] cm3/s for the temperature range 300 to 2500 K, thus allowing cautious extrapolation to combustion temperatures. The rate constants for the reverse reaction are evaluated by utilization of the equilibrium and the forward rate constants. The data for k−1 are well fit by k−1=(4.7±0.6) ×10−10 exp[−(370±60)/T] cm3/s over the temperature range 372 to 675 K. A transition state theory calculation yields the following form for k−1, k−1=(6.4±1.1) ×10−15 T1.54±0.02 exp[(430±20)/T] cm3/s for the temperature range 300 to 2500 K. The heat of formation of CH2 at 0 K was determined to be ΔHfo {CH2}=92.6±0.5 kcal/mol, assuming that the CH2+H radical recombination reaction has no activation energy.

Determination of absolute rate constants for free radical polymerization of ethyl α‐fluoroacrylate and characterization of the polymer

AbstractThe absolute rate constants for propagation (kp) and for termination (kt) of ethyl α‐fluoroacrylate (EFA) were determined by means of the rotating sector method; kp = 1120 and kt = 4.8 × 108 L/mol.s at 30°C. The monomer reactivity ratios for the copolymerizations with various monomers were obtained. By combining the kp values for EFA from the present study and those for common monomers with the monomer reactivity ratios, the absolute values of the rate constants for cross‐propagations were also evaluated. Reactivities of EFA and poly(EFA) radical, being compared with those of methyl acrylate and its polymer radical, were found to be little affected by the α‐fluoro substitution. Poly(EFA) prepared with the radical initiator was characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Although the glass transition temperature obtained by DSC for poly(EFA) resembled that of poly(ethyl α‐chloroacrylate), its TGA thermogram showed fast chain de polymerization to EFA that was distinct from complicated degradation of poly(ethyl α‐chloroacrylate).

Determination of Absolute Rate Constants for Radical Polymerization and Copolymerization of Ethyl α-Chloroacrylate: Effects of Substituents on Reaction Rates of Monomer and Polymer Radical

Abstract The propagation and termination rate constants (kp and kt) for the radical polymerization of ethyl a-chloroacrylate (ECA) were determined by the rotating sector method kp = 1660 and kt = 3.33 × 108 L/mol•s at 30° C. The absolute rate constants for cross-propagations in copolymerization were evaluated from the kp determined for ECA or those for common monomers and the monomer reactivity ratios. The reactivities of ECA and poly-(ECA) radicals estimated as the rate constants of cross-propagations were accounted for by using equations relating these rate constants to the polar and resonance effects of the substituents. ECA was highly reactive toward various polymer radicals as expected from the resonance effects of the carbethoxy and chloro substituents. The poly(ECA) radical was found to be more reactive than common polymer radicals. The reactivity of a polymer radical in cross-propagation seemed to increase with increasing electron-accepting power by facilitating electron transfer from a monomer required for the new C-C bond formation.

Determination of absolute rate constants for radical polymerization and copolymerization of ethyl α‐cyanoacrylate in the presence of effective inhibitors against anionic polymerization

AbstractThe absolute rate constants of propagation kp and of termination kt of ethyl α‐cyanoacrylate (ECNA) were determined in bulk at 30°C by means of the rotating sector method under conditions to suppress anionic polymerization; kp = 1 622 1 · mol−1 · s−1 and kt = 4,11 · 108 1 · mol−1 · s−1 for the polymerization in the presence of acetic acid, and kp = 1610 1 · mol−1 · s−1 and kt = 4,04 · 108 l · mol−1 · s−1 for the polymerization in the presence of 1,3‐propanesultone. The magnitude of k/kt determined was 6,39 · 10−3 l · mol−1 · s−1. The absolute rate constants for cross‐propagation in ECNA copolymerizations were also evaluated. Quantitative comparison of the rate constants with those of common monomers and polymer radicals shows that the strong electron‐withdrawing power of the ethoxycarbonyl and cyano groups enable the poly(ECNA) radical to add to monomers as fast as the other polymer radicals. The relatively high reactivity of ECNA, regardless of the type of attacking polymer radical, is interpreted by a transition state greatly stabilized by both the ethoxycarbonyl and the cyano groups.

EPR determination of absolute rate constants for the reactions of h and oh radicals with hydrogen bromide

The absolute rate constants for the reactions of H atoms and OH radicals with HBr were measured by the discharge flow technique with EPR detection. The rate constants at room temperature are k 1 = (6.3 ± 0.5) × 10 −12 cm 3 molecule −1 s −1 for H + HBr → H 2 + Br and K 2 = (9.2 ± 0.7) × 10 −12 cm 3 molecule −1 s −1 for OH + HBr → H 2O + Br. There is a discrepancy between measurements carried out by the EPR-discharge flow and flash photolysis-resonance fluorescence techniques.

Effect of solvent on radical polymerization of vinyl benzoate

AbstractAbsolute rate constants of the vinyl benzoate polymerization have been measured by use of the intermittent illumination method in various aromatic solvents and ethyl acetate at 30°C. The determination of absolute rate constants showed that effects of solvent on the polymerization rate of vinyl benzoate were mainly ascribed to the variation of kp values with solvents rather than that of kt values. The kp values for solvents used increased in the order: benzonitrile < ethyl benzoate < anisole < chlorobenzene < benzene < fluorobenzene < ethyl acetate. There was an eightfold difference between the largest and the smallest values The large variation among kp values was explained neither by the copolymerization through solvents nor the chain transfer to solvents, but by a reversible complex formation between the propagating radical and aromatic solvents. This explanation was supported by a correlation between kp values and calculated delocalization stabilizations for the complexes.

Determination of absolute rate constants for an excited triplet state

Absolute values for the rate constants for intersystem crossing to each of the triplet sublevels have been determined. This has been achieved by monitoring both the phosphorescence and the fluorescence in an optically detected magnetic resonance experiment. Using the above absolute values togethr with the measured value for the phosphorescence quantum yield allows us to determine the absolute values for the radiative and nonradiative rate constants for decay from the individual sublevels of the triplet state.(AIP)

Absolute Rate Constants in Free-Radical Polymerization. III. Determination of Propagation and Termination Rate Constants for Styrene and Methyl Methacrylate

Abstract A spatially intermittent polymerization (SIP) reactor has been used for determination of absolute rate constants in photo-initiated, free-radical polymerization of styrene (STY) and methyl methacrylate (MMA). Experimental data are reported in the temperature range 15-30°C and in the high molecular weight region for MMA and STY. Additional experimental data are reported at 30° C and various lower molecular weights for STY which indicate that the propagation rate constant K is independent of polymer molecular weight, and K is dependent on molecular weight, especially at low molecular weight, approaching an approximately constant value at high molecular weight.

Determination of absolute rate constants of addition of ethyl radicals to olefins

Determination of absolute rate constants of addition of ethyl radicals to olefins

Spatially intermittent polymerization

AbstractA novel reactor has been designed which permits the precise determination of absolute rate constants in photoinitiated free‐radical vinyl polymerization. A solution of monomer and initiator flows through a dark tubular reactor past regularly spaced slots through which light shines. The alternating dark and light regions produce spatially intermittent polymerization (SIP) and make the system analogous to the well‐known rotating‐sector technique. However, the SIP reactor has the advantage of producing large volumes of reaction product, at low conversion, suitable for analysis of both conversion and molecular weight. This supplies the necessary data, from a single set of experiments, for the simultaneous determination of the rate constants for propagation and termination. Experimental data are reported at 25°C for methyl methacrylate which indicate that kp = 315 I./mole‐sec, independent of polymer molecular weight, and kt is dependent on molecular weight especially at low molecular weight, approaching a lower value of kt = 30 × 106 I./mole‐sec at a molecular weight of 106. For styrene, measurements being made only at high molecular weight, kp = 74 ± 5 and kt = 37 ± 0.3 × 106 l./mole‐sec at 25°C.

Determination of absolute rate constants for the various steps of a chain reaction by means of flash photoinitiation

Determination of absolute rate constants for the various steps of a chain reaction by means of flash photoinitiation

Determination of absolute rate constants of chain propagation and termination in the polymerization of N-vinylpyrrolidone

Determination of absolute rate constants of chain propagation and termination in the polymerization of N-vinylpyrrolidone

Novel Chemiluminescence Methods for Determination of Absolute Rate Constants for Elementary Radical Reactions in the Liquid Phase

THE recombination of peroxide radicals is usually accompanied by luminescence (see, for example, refs. 1 and 2). In the presence of certain luminescent substances (activators) that do not affect the reaction rate the luminescence will increase due to energy transfer3. The luminescence intensity is a function of the concentration of peroxide radicals:

Reactions of the hydroxyl radical. Part 2.—Determination of absolute rate constants

The first page of this article is displayed as the abstract.

Determination of absolute rate constants of free radical reactions: Addition of trichloro-bromo-methaneto cyclohexane and to heptene-1

Determination of absolute rate constants of free radical reactions: Addition of trichloro-bromo-methaneto cyclohexane and to heptene-1

Pulse Radiolysis: Fast Reaction Studies in Radiation Chemistry

Developments in the determination of absolute rate constants, made possible by the availability of electron accelerators that can deliver a highly intense electron pulse of microsecond duration, are reviewed. Characteristics of the electron pulse and the physical arrangement for irradiation of samples and detection of transient species at Argonne National Laboratory are indicated. Important observations arising from the Argonne technique are discussed. Among these are the primary species in irradiated water, a variety of aromatic and aliphatic free radicals in both water and organic liquids, and one of the primary species formed in the radiolysis of polar organic liquids. Implications of the pulse radiolysis method for future research are presented. (H.M.G.)

Kinetics of anionic copolymerization. Determination of absolute rate constants

AbstractLack of termination in the “Living” polymer systems enables one to determine directly the absolute rate constant of propagation and of copolymerization. Anionic polymerizations in tetrahydrofuran are usually very fast, and a flow technique was developed to determine the rate of these reactions. The method is described and its use illustrated by studies on the following systems: styrene–p‐methylstyrene, styrene–p‐methoxystyrene, and styrene‐2‐vinylpyridine. The results indicate that the polarity of the reacting monomer is the dominant factor in determining the value of the copolymerization rate constant as long as the polarities of the monomers involved are similar. However, the results obtained on the system styrene–2‐vinylpyridine clearly demonstrate that the terminal carbanion plays a major role when the polarities of the two monomers are very different. The technique can also be used to study the effect of the penultimate unit of a chain on the rate constant of propagation or copolymerization. Moreover, the chain length of the “living” polymer can be caried and its effect of the rate constant determined.

Determination of absolute rate constants of copolymerization in anionic polymerization

Determination of absolute rate constants of copolymerization in anionic polymerization

On the photodegradation of methyl methacrylate and determination of absolute rate constants

On the photodegradation of methyl methacrylate and determination of absolute rate constants

Determination of Absolute Rate Constants for Olefinic Oxidations by Measurement of Photochemical Pre- and After-Effects. I. At “High” Oxygen Pressures

Determination of Absolute Rate Constants for Olefinic Oxidations by Measurement of Photochemical Pre- and After-Effects. I. At “High” Oxygen Pressures