Literature data are summarized for the chain‐length‐dependence of the termination rate coefficient in dilute solution free‐radical polymerizations. In essence such experiments have yielded two parameter values: the rate coefficient for termination between monomeric free radicals, k, and a power‐law exponent e quantifying how kt values decrease with increasing chain length. All indications are that the value e ≈ 0.16 in good solvent is accurate, however the values of k which have been deduced are considerably lower than well‐established values for small molecule radicals. This seeming impasse is resolved by putting forward a ‘composite’ model of termination: it is proposed that the value e ≈ 0.16 holds only for long chains, with e being higher for small chains – the value 0.5 is used in this paper, although it is not held to dogmatically. It is then investigated whether this model is consistent with experimental data. This is a non‐trivial task, because although the experiments themselves and the ways in which they are analyzed are elegant and not too complicated, the underlying theory is sophisticated, as is outlined. Simulations of steady‐state polymerization experiments are first of all carried out, and it is shown that the composite model of termination both recovers the e values which have been found and beautifully explains why these experiments considerably underestimate the true value of k. Simulations of pulsed‐laser polymerizations find the same, although not quite so strikingly. It is therefore concluded that our new termination model, which retains the virtue of simplicity and in which all parameter values are physically reasonable, is consistent with experimental data. Taking a wider view, it seems likely that the situation of the exponent e varying with chain length will not just be the case in dilute solution, but will be the norm for all conditions, which would give our model and our work a general relevance.Normalized chain length distributions from PLP simulations.imageNormalized chain length distributions from PLP simulations.

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The asymmetric epoxidation of indene using aqueous NaOCl, catalyzed by Jacobsen's chiral manganese salen complex, provides indene oxide in 90% yield and 85-88% enantioselectivity. The axial ligand, 4-(3-phenylpropyl)pyridine N-oxide (P(3)NO), increases the rate of epoxidation without affecting enantioselectivity and also stabilizes the catalyst. These two effects afford a reduction in catalyst loading to <1%. The turnover-limiting step in the catalytic cycle has been determined to be the oxidation of the manganese catalyst, based on reaction orders of 0 in indene and 1 in catalyst and also based on the dependence of the rate on the hypochlorite concentration. In the presence of the ligand P(3)NO, this rate-limiting oxidation occurs in the organic phase with HOCl as oxidant, as shown by the dependence of the rate on the NaOH concentration. P(3)NO assists the transport of HOCl to the organic layer as demonstrated by titration studies and by measuring the rates of oxidation of a redox indicator, diphenylbenzidine. On the other hand, stirring speed studies indicate that, in the absence of the ligand, oxidation occurs at the interface. Thus, the axial ligand plays at least two roles in the epoxidation of indene: it stabilizes the catalyst, presumably by ligation, and it increases the epoxidation reaction rate by drawing the active oxidant, HOCl, into the organic layer.

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The flash photolysis-resonance fluorescence technique was employed to determine the temperature dependencies of the rate constants for the reaction of 0(3P) and OH radicals with HCI. These reactions were studied under pseudo-first-order conditions and in the absence of interfering secondary reactions. The Arrhenius expression for each bimolecular rate constant is given as follows in units of cm3 molecule"1 s'1: kx = (3.3 ± 0.3) X 10"12 exp[-(937 ± 78) cal mo1"7RT] 250-402 K, OH + HCI -* H20 + Cl (1) and k2 = (5.2 ± 1.0) X 10"11 exp[-(7510 ± 750) cal mol_1/i?T] 350-454 K, 0(3P) + HCI --OH + Cl (2). The stratospheric implications of this new rate data are discussed.