Different actual systems generate the small parameter e in different ways. For example, in homogeneous chain reactions, the small parameter is a ratio of rate coefficients. It arises because the reactions in which free radicals, which are unstable and thus are short-lived, participate are much faster than the other reactions. [Pg.97]

As noted above, the duration of the retarding action of an inhibitor is directly proportional to the / value. In systems with a cyclic chain termination mechanism, the / coefficient depends on the ratio of the rate constants for two reactions, in which the inhibitor is regenerated and irreversibly consumed. In the oxidation of alcohols, aminyl radicals are consumed irreversibly via the reaction with nitroxyl radical formation (see earlier) and via the following reaction [11] [Pg.565]

One of the few systematic investigations of this matter is a study of the electron-transfer reactions between various acceptors and excited pyrene molecules [13,d], in a series of straight-chain hydrocarbons from n-heptane (C7H10) to squalane (C30H62) the diffusion coefficients of the reactants were measured as well as the rate constants in each solvent. The rate constants deviated markedly from Equation (2.3), based on viscosity, by a factor of up to 10, but much less from Equation (2.1) based on diffusion coefficients. With nitrobenzene as acceptor the ratio of the observed values of k to the Smoluchowski value 4 r was constant within a few per cent. [Pg.73]

The simple copolymer model is a first-order Markov chain in which the probability of reaction of a given monomer and a macroradical depends only on the terminal unit in the radical. This involves consideration of four propagation rate constants in binary copolymerizations, Eqs. (7-2)-(7-4). The mechanism can be extended by including a penultimate unit effect in the macroradical. This involves eight rate constants. A third-order case includes antepenultimate units and 16 rate coefficients. A true test of this model is not provided by fitting experimental and predicted copolymer compositions, since a match must be obtained sooner or later if the number of data points is not saturated by the adjustable reactivity ratios. [Pg.272]

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