Autoinitiation

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. 

As the alkene monomers can absorb oxygen from the air, forming peroxides (c/. p. 329) whose ready decomposition can effect autoinitiation of polymerisation, it is usual to add a small quantity of inhibitor, e.g. quinone, to stabilise the monomer during storage. When subsequent polymerisation is carried out, sufficient radical initiator must therefore be added to saturate the inhibitor before any polymerisation can be initiated an induction period is thus often observed. 

KINETICS OF AUTOINITIATED HYDROCARBON OXIDATION 4.7.1 Initial Stage of Autoxidation... 

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). 

Therefore, aldehyde autoxidation produces an efficient autoinitiator. 

If hydroperoxide is the main autoinitiator, oxidation can be retarded by the introduction of chemical compounds capable to decompose hydroperoxide without the formation of free radicals. 

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... 

Acidic products (S02, H2S04, RS02H, RS03H) break hydroperoxides into molecular products, thereby inhibiting autoinitiation. At the same time, they act as initiators by breaking hydroperoxide with the formation of free radicals. 

One inhibitor breaks chains, and another diminishes the rate of autoinitiation by breaking down hydroperoxide into molecular products or deactivating a catalyst that breaks hydroperoxide into radicals. 

Thus, the introduction of S into RH oxidized in the presence of InH leads to the following events. The concentration of hydroperoxide and the rate of autoinitiation decrease, whereas the duration of the InH-induced inhibitory period increases. When added at a sufficiently high concentration, S leads to a quasistationary regime of oxidation. If an inhibitory mechanism implies the occurrence of critical phenomena, the addition of S decreases the critical concentration [InH]cr (see Chapter 14). For mechanism III,... 

To understand this effect requires a discussion of the mechanism of autoinitiation. Of the mechanisms proposed, two receive most of the discussion in the literature. A mechanism proposed by Flory [15] involves a 1,4-cyclobutane diradical intermediate and has been supported experimentally by polymerization in the presence of a free radical scavenger. Several of the dimers found in thermally initiated polystyrene are also attributed to a Flory initiation mechanism. 

Fukuda et al.36b-48-49 covered a situation that is more often encountered in practical polymerizations than the previous scenario. There, transient radicals are generated not only by the dissociation of R —Y but also with an additional rate r. These additional radicals are provided by a deliberately added conventional initiator, by impurity derived radical sources such as peroxides, or by the autoinitiation of the monomer. Now, the kinetic equations for the radical concentrations read... 

Figure 6 shows calculated polymerization rates ln-([M]o/[M]) for various ratios r/Ad[I]o. Even for very small ratios of rand Ad[I]o, rather large rate enhancements are obtained. Moreover, the time dependence of ln([M]0/[M]) becomes linear. Since some additional initiation by impurities or autoinitiation may always occur, the nonlinear behavior for the ideal case of Figure 3 may be difficult to observe in actual polymerizations, unless kd[I]0 is sufficiently large. 

With certain Lewis acids of higher acid strength such as AJCI3 and TiCl4 autoinitiation or self-ionization may occur. In such cases the initiator and coinitiator are the same, and the initiation is usually represented by... 

The rates of initiation depend on the type of activation ch.osen. In photochemical initiation, Vs = 2 0/, where I is the absorbed light intensity and 0 = the efficiency coefficient. With an average intensity, rates of initiation of approximately 10"7 mole l 1 s 1 are attained. In thermal activation, autoinitiation by interaction between oxygen and aldehyde gives low values of approximately 10 9 mole l-1 s"1 under standard laboratory conditions. When azonitrile is used, since the thermal decomposition rate of this product is approximately first order [62], Vi is given by... 

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