Free radical initiators decomposition rates

Predicting Rates of Decomposition of Free-Radical Initiators... 

A number of studies of simple reactions using DSC have given values for rate constants and activation energy in good agreement with those determined by methods utilising chemical analysis. An example of this is the work of Barrett18) on the thermal decomposition of free radical initiators. 

Whether initiated by radiation or by the thermal decomposition of free radical initiators, and whether in the bulk or in the imbibed state, the mechanisms of free radical polymerizations of monomers in wood should be essentially similar. As with any free radical polymerization, three basic steps must be involved initiation, propagation, and termination also, chain transfer reactions may occur, depending on the monomer, additives, and on the mode of initiation (Chapiro, 1962 Siau et al, 1965a see also Appendix A, Chapter 1). In such cases, the rate of polymerization should depend on the square root of the concentration of initiating radicals, which, in turn, should depend on the dose or on the concentration of free radical initiator ... 

Any one of these expressions gives the rate of initiation Rj for the particular catalytic system employed. We shall focus attention on the homolytic decomposition of a single initiator as the mode of initiation throughout most of this chapter, since this reaction typifies the most widely used free-radical initiators. Appropriate expressions for initiation which follows Eq. (6.6) are readily derived. 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

Predictive equations for the rates of decomposition of four families of free radical initiators are established in this research. The four initiator families, each treated separately, are irons-symmetric bisalkyl diazenes (reaction 1), trans-phenyl, alkyl diazenes (reaction 2), tert-butyl peresters (reaction 3) and hydrocarbons (reaction 4). The probable rate determining steps of these reactions are given below. For the decomposition of peresters, R is chosen so that the concerted mechanism of decomposition operates for all the members of the family (see below)... 

Equation 6 would hold for a family of free radical initiators of similiar structure (for example, the frarw-symmetric bisalkyl diazenes) reacting at the same rate (at a half-life of one hour, for example) at different temperatures T. Slope M would measure the sensitivity for that particular family of reactants to changes in the pi-delocalization energies of the radicals being formed (transition state effect) at the particular constant rate of decomposition. Slope N would measure the sensitivity of that family to changes in the steric environment around the central carbon atom (reactant state effect) at the same constant rate of decomposition. 

The most important results of the linear free energy equations in this study (Table IV) are the applications to which they can be used. For a new free radical initiator, belonging to any of the four radical forming reactions of this study, equation 6 should be useful to predict the rate of decomposition with reasonable accuracy. All that is needed is an HMD calculation to obtain the pi-delocalization energy for the radical formed in the reaction (R ) and an estimate of the steric A values for groups bonded to the central carbon of R. ... 

Steps (29) and (30) are those involved also in the decomposition of N205. Initial rates of decomposition are apparently higher than at later stages in the reaction because, initially, the free-radical reaction is not inhibited by the fast step (29). When sufficient nitric oxide is present, either initially added or formed by N02 decomposition, the free-radical reaction path is suppressed. Ashmore et al.212 213 found indeed that the value of the second-order rate coefficient of decomposition kd, depends on the [N0]/[N02] ratio in agreement with the relation... 

Azobisnitriles are efficient sources of free radicals for vinyl polymerizations and chain reactions, e.g., chlorinations. See also Initiators (Free-Radical) Initiators (Anionic) and Initiators (Cationic). These compounds decompose in a variety of solvenls al nearly first-order rates to give free radicals with no evidence of induced chain decomposition, They can be used in bulk, solution, and suspension polymerizations and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

Dixon, K.W. (1999) Decomposition rates of organic free radical initiators. In J. Brandrup, E.H. Immergut and E.A. Grulke (Eds) Polymer Handbook (4th edn). Wiley, New York, p. 1-76. 

Several years later, DeSimone and co-workers (Guan et al., 1993) examined the decomposition of the free-radical initiator 2,2 azobis(isobutyro-nitrile) (AIBN, Scheme 4.6) in sc C02 as a function of pressure. The rate constant was found to increase with increasing pressure (reaching a maximum at approximately 250 bar). At higher pressures, the rate constant... 

The polymerization of vinyl monomers is an exothermic reaction and a considerable amount of heat is released, about 18 kCal per mole. In both the catalyst-heat and gamma radiation processes the heat released during polymerization is the same for a given amount of monomer. The rate at which the heat is released is controlled by the rate at which the free radical initiating species is supplied and the rate at which the chains are growing. As pointed out above, the Vazo and peroxides are temperature dependent and the rate of decomposition, and thus the supply of free radicals, increases rapidly with an increase in temperature. Since wood is an insulator due to its cellular structure, heat flow into and out of the wood-monomer-polymer material is restricted. In the case of the catalyst-heat process heat must be introduced into the wood-monomer to start the polymerization, but once the exothermic reaction begins the heat flow is reversed. 

Other reactions may be taken into consideration, with an effect on polymer structure, namely the formation of short- and long-chain branches. A complete list of reactions in S-PVC polymerization may be found in Kiparissides et al. [5]. On the above basis kinetic equations may be written. To keep it simple the chain transfer, back-biting and inhibition reactions are disregarded, while termination is considered to occur only by disproportionation. The elementary reaction rates for initiator decomposition and free radicals generation are as follows ... 

Table 5-8. Solvent influence on rates of monomolecular decomposition of various free-radical initiators [164],...

In general, initiation should be as fast as is practical to produce as much polymer as possible per unit of reaction time. The reaction cannot be allowed to proeeed more quickly than the rate at which the exothermic heat of polymerization can be removed from the system, however. The decomposition rates of free-radical initiators are very temperature sensitive (the fi/2 of benzoyl peroxide drops from 13 hat 70°C to 0.4 h at 100°C), and a runaway reaction can result from overheating if the rate of initiation is not limited appropriately. 

As would be predicted from Equation 1, the rate of dissociation of free radical initiators is decreased by the application of pressure. Thus azobisisobuty-ronitrile dissociates with a rate constant equal to 4.47 X 1(H sec." at 1500 atm. but at 1 atm. the dissociation rate constant is 5.5 X 10 sec. (8). Studies concerning the effect of pressure on the decomposition of benzoyl peroxide reveal that the rate of this reaction also decreases with increasing pressure (II, 18). The extent to which the radical-induced decomposition of this peroxide at high pressures affects the rate is not clear, but it appears that some complications arise from this cause. 

The hydroxyl radical ( OH) generation from decomposition of ozone in alkaline pH mainly depends on the hydroperoxide free-radical initiating step (rate-determining step) in the chain reaction and the regeneration of the superoxide radical ion Oi ... 

On the first part of this research, Advanced Chemical Oxidation, a quantitative estimation of direct ozonation and indirect free radical oxidation of dyes with assorted chromophores was studied through the examination of reaction kinetics in the ozonation process. The reaction kinetics of dye ozonation under different conditions was determined by adjusting the ozone doses, dye concentration, and reaction pH. The ozonation of dyes was found dominant by pseudo first-order reaction, and the rate constants decreased as the dye/ozone ratio increased. For all selected azo dyes, the dye decay rates increased as the initial pH of the solution increased, yet the decay rates of anthraquinone dyes would decrease in the same situation because of their insensible structure for ozone oxidation, formation of leuco-form, and higher solubility at a lower pH. The ozonation of dyes at a high pH contributed by hydroxyl free radicals was qualitatively verified by the use of a free radical scavenger. A proposed model, in another way, quantitatively determines the fraction of contribution for dye decomposition between free radical oxidation and direct ozonation. 

CsHjoOj Combustible liquid. Forms explosive mixture with air [explosion limits in air (vol %) 1.6 to uel unknown flash point 149°F/65°C Fire Rating 1]. Unless inhibited (200 ppm hydroquinone recommended), polymerization may occur avoid exposure to high temperatures, ultraviolet light, free-radical initiators. Reacts with water with release of heat may not be violent if not contained. Strong oxidizers may cause fne and explosions. Reacts violently with sodium peroxide, uranium fluoride. Incompatible with strong acids, nitrates. Incompatible with sulfuric acid, nitric acid, caustics, aliphatic amines, isocyanates, boranes. Thermal decomposition releases toxic acrid fumes of acrolein and acrylic acid. On small fires, use dry chemical powder (such as Purple-K-Powder), water spray, alcohol-resistant foam, or CO2 extinguishers. 

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