Search results for "CONST"

1990

The effect of lithium tert-butoxide (t-BuOLi) on rate constants and equilibrium constants of reactions involved in the initial stage of the anionic polymerization of methyl methacrylate (MMA) in tetrahydrofuran, initiated by methyl α-lithioisobutyrate (P) was investigated at 23 ± 3°C. Addition of t-BuOLi decreases the rate constants of initiation and propagation by one order of magnitude; the rate constants of termination by cyclization decrease by two orders of magnitude. This leads to an overall tenfold higher preference of propagation with respect to termination and explains the favourable effect of alkoxide in the polymerization reported earlier. Within experimental error, the equilibri…

ISomerization of xylene: MINDO/3 study of the cyclization of benzenium cation into bicyclo[3.1.0]hexenyl cation

1989

Abstract A MINDO/3 study of thermal electrocyclic closing of the benzenium cation to form a bicyclo[3.1.0]hexenyl cation is presented. The calculated mechanism discusses the possibility of cyclization of xylenium cations on a silica catalyst or in superacid media. The structure and energy of the transition state, the enthalpies of isomerization and barriers of activation were calculated for the following processes: isomerization of 2,6-dimethylbenzenium cation into 2,6-dimethylbicyclo[3.1.0]hexenyl cation, isomerization of 2,4-dimethylbenzenium cation into 2,4-dimethylbicyclo[3.1.0]-hexenyl cation, and isomerization of benzenium cation into bicyclo[3.1.0]-hexenyl cation. Equilibrium and kin…

Energetics and Dynamics of Naphthalene Polyaminic Derivatives. Influence of Structural Design in the Balance Static vs Dynamic Excimer Formation

2003

Two new fluorescent macrocyclic structures bearing two naphthalene (Np) units at both ends of a cyclic polyaminic chain were investigated with potentiometric, fluorescence (steady-state and time-resolved) and laser flash photolysis techniques. The fluorescence emission studies show the presence of an excimer species whose formation depends on the protonation state of the polyamine chains implying the existence of a bending movement (occurring in both the ground and in the first singlet excited state), which allows the two naphthalene units to approach and interact. For comparison purposes, one bis-chromophoric compound containing a rigid chain (piperazine unit) was also investigated. Its em…

Anionische Polymerisation in Schwach Solvatisierenden Lösungsmitteln

2007

The anionic polymerization of styrene initiated by cumylsodium and carried out in cumyl methyl ether has been examined with use of conversion kinetics and molecular weight distribution. The solvent has a weaker solvation power than THF and THP, which have been studied with the same experimental technique. The polymerization rate is first order relative to monomer and the rate constant has a value of ca. 1 mole−1 1. sec.−1 at − 20.5°C. The constant is only slightly dependent on the initiator concentration, but the molecular weight distribution is broader than a Poisson function. Added electrolyte has little effect on the propagation constant but it greatly alters the molecular weight distrib…

Untersuchungen zum oxidativen abbau nativer cellulose, 4. Stöchiometrische verhältnisse zwischen der anzahl der entstehenden spaltstellen, dem sauers…

1978

An investigation of the kinetics of cellulose degradation confirms the scheme proposed by G. V. Schulz and Mertes with the following modifications: the fast pre-stage of the degradation is missing, because there are no faster cleaving bonds in the native cellulose; the first step of the degradation (pre-oxidation) in agreement with the above mentioned authors consists in the uptake of an oxygen atom: in the second step, the scisson proper, the scheme of Staudinger and Roos is replaced by the newer and better founded scheme of Haskins and Hogsed, assuming an alkoxy-elimination; the third step (post-oxidation) finally consists in a limited number of substeps leading to a splitting off of abou…

Rate constants for ion-pair dissociation and association in the anionic two-state polymerization of styrene in THF

1965

ChemInform Abstract: Ternary Complexes of Cimetidine and Phenobarbital with Cu(II) in Methanolic Solution.

1986

The formation constants of the binary complexes Cu(CM)2+ and Cu(CM) 2 2+ as well as those of the ternary complexes Cu(CM)L + and Cu(CM)2 L + (CM=Cimetidine=N-Cyano-N′-methyl-N″[(5-methyl-1H-imidazol-4-yl)methyltioethyl]-guanidine; HL=Phenobarbital=5-ethyl-5-phenyl-barbituric acid) have been determined in 0.1 and 1.0 mol dm−3 NaClO4 methanol solutions at 25±0.2°C. The values of logX, log βstat.., and Δ logK confirm the stability of the ternary complexes.

Electrochemical determination of the stability of complexes formed by proton-ionizable ligands of 3,5-disubstituted 1H-pyrazole with phenethylamine

1999

The application of two extreme models for diffusion in two-component systems to electrochemically determine equilibrium constants is discussed. The application of cyclic voltammetric, diferential pulse and rotating-disc electrode voltammetric data to elucidate the stoichiometry and formation constant of complex species by applying a generalization of the molar-ratio method is described. Molar-ratio experiments permit the distinction between the limiting diffusive regimes. The values of the equilibrium stability constants for complexation of phenethylamine and phenethylammonium ions by a 26-membered dioxotetraester crown of 3,5-disubstituted 1H-pyrazole as free ligand 1[L] and as dipyrazolat…

Ternary complexes of cimetidine and phenobarbital with Cu(II) in methanolic solution

1986

The formation constants of the binary complexes Cu(CM)2+ and Cu(CM) 2 2+ as well as those of the ternary complexes Cu(CM)L + and Cu(CM)2 L + (CM=Cimetidine=N-Cyano-N′-methyl-N″[(5-methyl-1H-imidazol-4-yl)methyltioethyl]-guanidine; HL=Phenobarbital=5-ethyl-5-phenyl-barbituric acid) have been determined in 0.1 and 1.0 mol dm−3 NaClO4 methanol solutions at 25±0.2°C. The values of logX, log βstat.., and Δ logK confirm the stability of the ternary complexes.

1965

Bei der Polymerisation von Trioxan bei 70°C in der Schmelze mit HClO4 · H2O als Initiator nimmt die Bruttopolymerisationsgeschwindigkeit exponentiell mit steigender Wasserkonzentration ab; gleichzeitig nimmt die Lange der Induktionszeit entsprechend zu. Die Retardierung durch Wasser wird auf eine Hydratation aktiver Zentren zuruckgefuhrt. Wasser ist ein wirkungsvoller Polymerisationsubertrager; seine relative Ubertragungskonstante betragt bei 25°C in Methylenchlorid ca. 2,6. Jedoch werden die Verhaltnisse durch die Bildung von Methylenglykol aus Wasser und Formaldehyd kompliziert. In polymerization of trioxane in the melt at 70°C. with HClO4 · H2O as initiator the rate of polymerization dec…