Acyl peroxide initiators, decomposition rates

Initiator decomposition studies of AIBN in supercritical C02 carried out by DeSimone et al. showed that there is kinetic deviation from the traditionally studied solvent systems.16 These studies indicated a measurable decrease in the thermal decomposition of AIBN in supercritical C02 over decomposition rates measured in benzene. Kirkwood correlation plots indicate that the slower rates in supercritical C02 emanate from the overall lower dielectric constant (e) of C02 relative to that ofbenzene. Similar studies have shown an analogous trend in the decomposition kinetics ofperfluoroalkyl acyl peroxides in liquid and supercritical C02.17 Rate decreases of as much as 30% have been seen compared to decomposition measured in 1,1,2-trichlorotrifluoroethane. These studies also served to show that while initiator decomposition is in general slower in supercritical C02, overall initiation is more efficient. Uv-visual studies incorporating radical scavengers concluded that primary geminate radicals formed during thermal decomposition in supercritical C02 are not hindered to the same extent by cage effects as are those in traditional solvents such as benzene. This effect noted in AIBN decomposition in C02 is ascribed to the substantially lower viscosity of supercritical C02 compared to that ofbenzene.18... 

The differences in the rates of decomposition of the various initiators are related to differences in the structures of the initiators and of the radicals produced. The effects of structure on initiator reactivity have been discussed elsewhere [Bamford, 1988 Eastmond, 1976a,b,c Sheppard, 1985, 1988]. For example, k,i is larger for acyl peroxides than for alkyl peroxides since the RCOO- radical is more stable than the RO radical and for R—N=N—R, kd increases in the order R = allyl, benzyl > tertiary > secondary > primary [Koenig, 1973]. 

Most frequently the polymerization process is initiated by free radicals obtained through the decomposition of hydroperoxides, alkyl peroxides, dialkyl peroxides, acyl peroxides, carboxylic ester peracids, salts of (tetraoxo)sulphuric acid, hydrogen peroxide, aliphatic azo compounds and bifunctional azobenzoin initiators. The rate of decomposition of different initiators into free radicals depends on their stmcture and on temperature. A measure of the efficiency of the initiator in the pol5mierization process is the half-decomposition period. 

The effect of activators like FeS04 [11,12] for emulsion polymerization and ferric stearate [13] for bulk polymerization of vinyl monomers in combination with acyl peroxide has been studied. The ferrous ion catalyzed decomposition of BZ2O2 in ethanol has been studied in some detail by Has-egawa and co-workers [14,15]. The cycle, which requires reduction of Fe " by solvent-derived radicals, yields a steady-state concentration of Fe after a few minutes, shown spectroscopically to be proportional to the initial concentration of the ferrous ion [14]. The second-order rate o)nstant for the following reaction was found to be 8.4 L moP sec at 25 C, with an activation energy of 14.2 kcal moP ... 

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