Rate coefficient tetrahydrofuran

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

As indicated earlier, the concentration of free anions even in tetrahydrofuran is only 1 % of the total at millimolar concentration so they must be of very high reactivity to influence the propagation rate. The increase in k pp with D at moderately low concentrations of active centres is largely caused by the increased concentration of free anions. Only in dioxane is the observed rate coefficient that of the ion-pair since... 

The formulation of two types of ion-pair is an attractive hypothesis which has been used for other systems [130] to explain differences in reactivity. The polymerization of styrene-type monomers in ether solvents, all of which solvate small cations efficiently, seems to be a particularly favourable case for the formation of thermodynamically distinct species. Situations can be visualized, however, in which two distinct species do not exist but only a more gradual change in properties of the ion-pair occurs as the solvent properties are changed. These possibilities, together with the factors influencing solvent-separated ion-pair formation, are discussed elsewhere [131, 132]. In the present case some of the temperature variation of rate coefficient could be explained in terms of better solvation of the transition state by the more basic ethers, a factor which will increase at lower temperatures [111]. This could produce a decrease in activation energy, particularly at low temperatures. It would, however, be difficult to explain the whole of the fep versus 1/T curve in tetrahydrofuran with its double inflection by this hypothesis and the independent spectroscopic and conductimetric evidence lends confidence to the whole scheme. 

The values both in tetrahydrofuran and tetrahydropyran are of the same order of mj itude as those of polystyrene compounds at 25°C. Only the lithium compound of a-methylstyrene has an appreciably higher than that of styrene. The free anion rate coefficient, 830 1 mole" sec" (extrapolated, 25°C A = 1.5 x 10 , E = 7.2 kcal mole" ) [145] is smaller than for styrene, as are the ion-pair coefficients, but the ratio between the two is roughly the same. A major factor producing low rates appears to be a higher activation energy than is found for styrene polymerization. 

Only one kinetic study exists on initiation of methacrylate polymerization by a sodium compound. The initiator was the disodium oligomer ( tetramer ) of a-methylstyrene and polymerization was investigated at 25°C in toluene in presence of 0.05—0.2 mole fraction of tetrahydrofuran [181]. An internal first order disappearance of monomer was observed, the first order coefficient being directly proportional to active chain and tetrahydrofuran concentrations. The rate coefficients evaluated, e.g. fep = 3.1—13 X 10 1 mole sec at various tetrahydrofuran concentrations, are much lower than those for lithium initiators. They were, however, evaluated using a methyl iodide titration technique to estimate the active chain concentration. In view of the reactivity of tritiated acetic acid with many short chains which are clearly not active in chain propagation, there must be suspicion of similar behaviour with methyl iodide. If this happens, the active chain concentration would be over-estimated and the derived fep value would be too low. Unfortunately no molecular weights of the precipitable polymer were determined, so that it is impossible to check on active chain concentration using this alternative method. 

Here [Pf ] is the concentration of growing centres ending in monomer x and kx y is the absolute rate coefficient of reaction of P with monomer y. Two difficulties arise in anionic polymerization. In hydrocarbon solvents with lithium and sodium based initiators, [Pf ] is not the total concentration of polymer units ending in unit x but, due to self-association phenomena, only that part in an active form. The reactivity ratios determined are, however, unaffected by the association phenomena. As each ratio refers to a common active centre, the effective concentration of active species is reduced equally to both monomers. In polar solvents such as tetrahydrofuran, this difficulty does not arise, but there will be two types of each reactive centre Pf, one an anion and the other an ion-pair. Application of eqn. (22) will give apparent rate coefficients as discussed in Section 4 if total concentrations of Pf are used. Reactivities can change with concentration if defined on this basis. 

In a more recent study Sigwalt et al. [41] investigated the use of carbazyl sodium to initiate PS polymerization in tetrahydrofuran (THF). This initiator produced only one living end per chain. Excellent agreement with the results obtained on amphianionic polymer was obtained. At levels of living ends below 10 molel the rate coefficient found for ion pairs was 3 x 10" 1 mole sec and for free ions 4 1 mole sec . The dissociation constant (determined kinetically) was 6.4 x 10 mole 1 . ... 

Rate coefficients of reaction (24), fep, and (66), kg, in tetrahydrofuran at 25°C (The figure in the lactam anion indicates the number of ring atoms.)... 

The addition of sodium to aromatic hydrocarbons produces only the corresponding radical anion [121]. Alkali metals, however, dissolve in aliphatic ethers, e.g. tetrahydrofuran, dioxan, producing the characteristic blue colour [122], The solutions are diamagnetic because of the formation of higher ion pairs. Flash photolysis of solutions of sodium in ethers forms the ion pair consisting of the solvated electron and a sodium cation [123]. Three transients are formed in the flash photolysis of sodium pyrenide in tetrahydrofuran [124]. These have been identified as the solvated electron, es"oiv. the ion pair, esoiv> Na, and the sodium atom, Na°. Rate coefficients of reactions of es ,iv with various compounds relative to the rate of reaction with NjO have been determined recently by the 7-radiolysis of 2-methyl-tetrahydrofuran [125]. 

Rate coefficients for reaction of OH with tetrahydrofuran have been reported near 300 K by Winer et al. (1977), Ravishankara and Davis (1978), Wallington et al. (1988e), and over a range of temperatures by Moriarty et al. (2003), see table III-E-6 and figure III-E-3 for a summary. Values near 300 K lie in the range (15 - 19) x 10... 

Figure lll-E-3. Arrhenius plot of the rate coefficient for the reaction of OH with tetrahydrofuran. 

Table lll-E-7. Rate coefficients (k, cm molecule s ) for reaction of NO3 with tetrahydrofuran cyclo-C4lisO)... 

An isocratic HPLC method for screening plasma samples for sixteen different non-steroidal anti-inflammatory drugs (including etodolac) has been developed [29]. The extraction efficiency from plasma was 98%. Plasma samples (100-500 pL) were spiked with internal standard (benzoyl-4-phenyl)-2-butyric acid and 1 M HC1 and were extracted with diethyl ether. The organic phase was separated, evaporated, the dry residue reconstituted in mobile phase (acetonitrile-0.3% acetic acid-tetrahydrofuran, in a 36 63.1 0,9 v/v ratio), and injected on a reverse-phase ODS 300 x 3.9 mm i.d. column heated to 40°C. A flow rate of 1 mL/min was used, and UV detection at 254 nm was used for quantitation. The retention time of etodolac was 30.0 minutes. The assay was found to be linear over the range of 0.2 to 100 pg/mL, with a limit of detection of 0.1 pg/mL. The coefficients of variation for precision and reproducibility were 2.9% and 6.0%, respectively. Less than 1% variability for intra-day, and less than 5% for inter-day, in retention times was obtained. The effect of various factors, such as, different organic solvents for extraction, pH of mobile phase, proportion of acetonitrile and THF in mobile phase, column temperature, and different detection wavelengths on the extraction and separation of analytes was studied. 

The solvent effects on rates shown by these two reactions were determined employing the solvents chloroform, dichloromethane, acetonitrile, ethyl acetate, benzene, tetrahydrofuran and dioxane. Solvents which react with TCNE, such as nitromethane, dimethylformamide and protic solvents, as well as cyclohexane, carbon tetrachloride and tetrachloroethylene, in which the reactants have very low solubility, were deliberately excluded from the study. The observed solvent effects were virtually identical for both Diels-Alder and [2 + 2] cycloaddition processes. Statistical correlations of rate data using a multiparameter equation with dependencies based on acceptor properties, polarizability and inherent polarity of the solvents gave nearly identical coefficients through the regression analyses for each term for both reactions, and excellent linear fits to the rate data. 

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