Autoinitiation reaction

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

The interval 3-lO s < t < 510 s corresponds to the non-autoinitiated reaction. Within this interval the main step of free radical generation is presented by reaction (1). linhibition of ethylbenzene oxidation is basically conditioned by step (15), the disproportionation of phenoxyl radicals. 

Intensification of the autoinitiation reaction with increase in the initial concentration of TH interacting with lipohydroperoxide... 

The kinetic analysis proves that formation of very active radical from intermediate product can increase the reaction rate not more than twice. However, the formation of inactive radical can principally stop the chain reaction [77], Besides the rate, the change of composition of chain propagating radicals can influence the rate of formation and decay of intermediates in the oxidized hydrocarbon. In its turn, the concentrations of intermediates (alcohols, ketones, aldehydes, etc.) influence autoinitiation and the rate of autoxidation of the hydrocarbon (see Chapter 4). 

Depending on the oxidation conditions and its reactivity, the inhibitor InH and the formed radical In can participate in various reactions determining particular mechanisms of inhibited oxidation. Of the various mechanisms, one can distinguish 13 basic mechanisms, each of which is characterized by a minimal set of elementary steps and kinetic parameters [38,43 15], These mechanisms are described for the case of initiated chain oxidation when the initiation rate v = const, autoinitiation rate fc3[ROOH] -C vy and the concentration of dissolved dioxygen is sufficiently high for the efficient conversion of alkyl radicals into peroxyl radicals. The initiated oxidation of organic compounds includes the following steps (see Chapter 2). 

This problem was first approached in the work of Denisov [59] dealing with the autoxidation of hydrocarbon in the presence of an inhibitor, which was able to break chains in reactions with peroxyl radicals, while the radicals produced failed to contribute to chain propagation (see Chapter 5). The kinetics of inhibitor consumption and hydroperoxide accumulation were elucidated by a computer-aided numerical solution of a set of differential equations. In full agreement with the experiment, the induction period increased with the efficiency of the inhibitor characterized by the ratio of rate constants [59], An initiated inhibited reaction (vi = vi0 = const.) transforms into the autoinitiated chain reaction (vi = vio + k3[ROOH] > vi0) if the following condition is satisfied. 

The mechanism of inhibitory action of aryl phosphites seems to be relatively complex. Phosphites reduce hydroperoxide and thus decrease chain autoinitiation. The formed peroxyl and alkoxyl radicals react with phosphites to form aroxyl radicals. The latter terminates the chains by reaction with peroxyl radicals. On the other hand, phosphites are hydrolyzed with... 

The aforesaid suggests that a flow graph inherently enables the systematization of intuitive, qualitative and incompletely quantitative knowledge of a researcher and switch over the equation language. Let us illustrate this by the example of liquid-phase oxidation of benzaldehyde by dioxygen, which will be considered in more detail in Chapter 6 [29,56]. Let us suppose that the researcher possesses the following preliminary information about this reaction (i) the peroxyacid is an intermediate product that results in benzoic acid directly or by interaction with the initial aldehyde (ii) the oxidation is autoinitiated by the peroxyacid. 

In order to avoid overtiring the reader we will skip the kinetic equations that are derived from the detailed reaction mechanism of the liquid-phase oxidation of ethylbenzene. This is if, for example, referring to [2,57] one can make certain of the equivalence between the equations derived by the empirical and non-empirical ways. Similar equations might be obtained if one supposed from the outset that the quadratic termination of the chain carriers is peculiar to the reaction considered. Consequently, this would result in a square root dependence for the accmnulation rates of reaction species on the concentration of hydroperoxide, which is responsible for the autoinitiation of reactions. 

The interval 510 s < t < 510 s corresponds to the autoinitiation mode within the induction period of ethylbenzene oxidation. Generation of free radicals takes place by steps (6) and (13) with the participation of hydroperoxide formed during the interaction. Within this time interval the eontribution of step (14), the reaction between phenoxyl and peroxyl radicals, in the inhibition of the oxidation process becomes considerable. 

The experiment performed at 120 °C (see Figure 7.5) also points to the existence of such an equilibrium. The reaction comes out from the induction period at the expense of the hydroperoxide accumulation, responsible for the autoinitiation of the oxidation at an incomplete conversion of the inhibitor. This is evidence of the inhibitor regeneration by reaction (17). 

Changes in the molecular structure of the inhibitor leading to the decrease in the dissociation energy of die 0-H bond, consequently an increase in the rate constant and a decrease in the degree of reversibility for the reaction of peroxyl radicals with the inhibitor, apparently will not give the desired result. The matter is that in this case the role of the autoinitiation step (20) involving the inhibitor s molecule is considerably enhanced at concentrations 10 h-10 M that are found in practical use. 

From data presented in Table 7.6 it is easy to note that increasing the initial concentration of BHT causes some extending in the appropriate base mechanism. So, with increasing the BHT concentration the steps (6),(18),(25),(26) with the participation of the hydroperoxyl radical, as well as the steps (33) and (34) - the reactions of the hydrogen peroxide with BHT and the phenoxyl radical, and the autoinitiation step (20) with the participation of BHT molecule are involved in the base mechanism. 

Value contributions for harmonic oscillations corresponding to the above-mentioned sequences A, B and C, as well as at a stationary reaetion mode are illustrated in Figure 8.6. As during the reaction (1) presented in Table 8.1, the eonsumption of Br yields the aetivator X (HBr02), where its value eontribution is positive. In sequence A, as expected, the most important role is played by reaction (1) and (2), leading to the consumption of the inhibitor, the Br ion, while the role of the autoinitiation reaetion (3) is not important. 

Now let us specify the role of individual steps in sequence B. In the autoinitiation stage the role of reaction (1) and (2) responsible for the inhibitor, Br , consumption decreases signifieantly, and the opposite is true for the autoinitiation sequence (3), ensuring the positive feedbaek. 

If oxidation is carried out without an initiator, that is, in the autoxidation regime, it occurs with self-acceleration due to the initiation rate increasing during the reaction. It is very important that the temp of acceleration depends on the rate of chain oxidation, i.e., there is a positive feedback between the processes of autoinitiation and chain oxidation of RH. This relationship is also manifested in the inhibition oxidation of organic compounds. 

In reality, NMP is not as simple as portrayed in Scheme 4.5 (reactions 1-7). Side reactions of the nitroxide or alkoxyamine (reactions 8-10) can occur, and there may be additional sources of alkyl radicals, e.g., from deliberately added conventional radical initiator (in order to speed up the polymerization) or from autoinitiation of the monomer. Furthermore, free nitroxide may be present or added at the start of the polymerization. The effects of these side reactions are discussed in Section 3.5. 

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