Autoacceleration reactions

S02 is a very stable oxide and its thermal decomposition is only measurable at the very high temperatures attained in a shock tube. A study357 of the time-dependence of light emission from shock-heated S02/Ar mixtures in the region of 3000 °K has shown that S02 is removed in accordance with a sigmoid-shaped concentration-time curve typical of a chain or autoaccelerated reaction. The induction period observed357 prior to the onset of detectable decomposition corresponded closely with the time for the formation of a fixed concentration of O (or SO) calculated from the rate expression (Table 24) for the unimolecular decomposition... 

PhotoDSC has also been used to study the kinetics of acrylates with functionality of 1 to 6 [52], The author suggested an autocatalytic or autoaccelerated reaction. 

It would appear that the calcination of ti kinetics and the statistics of reactions in atactic chains could be simidified if some of the ten constants were equivalent or if there were some other restrictions. Rsmen calculated the kinetics a polymer-analogous reaction in atactic chains for the particular case kj) = ko = kg=k, kj =ki = ki,k i =k = k2, kj = 2 k2-ko,k5 = kx +k2 —ko,l4 = 2ki — k and k2 > ki > ko (autoaccelerating reaction). 

It should be pt ted out in conclusion that intramolecular ctoss-linldng is an autoaccelerated reaction, and that the initial rate and the degree of autoacceleration increase with an increase in chain Imgth. Another inqiortant condusion is the existence of a uniform relationsliip between the kinetics of tire reaction and the equQibrium j perties of partially cross-linked drains. 

This accelerating behavior appears similar to the autoaccderation phenonomen observed for the 8 2 addition due to the initiation reaction (Rxn. 36), yet there are substantial and fundamental differences between the two phenomena. First, in the autoaccelerating reaction the increasing active catalyst concentration is directly proportional to the extent of epoxy reaction. This is not the case for the epoxy-phenol reaction. The active complex formation reaction, Rxn. 36, is independent of the propagation reaction, Rxn. 37. thus, any observed proportionali is simply coincidental. Second, the rate of production of active catalytic complex will decrease as the tertiaiy amine or phosphine is consumed in Rxn. 36. The maximum concentration of active catalytic complex is limited by the initial charge of catalyst. These considerations are not accounted for in the autoacceleration model. 

The character of the kinetic curves cannot be explained from the mechanism presented by the reactions (Equation 3.72-74). According to these reactions, based on the direct interaction of hydroperoxides with NO, the rate of decomposition is maximal at the beginning of the process and decreases with a decrease in the concentration of ROOH. The shape of the kinetic curves is characteristic of autoaccelerated reactions with an induction period. 

Since, according to the above, at e a < 1 the reaction is relatively slow and at sa > 1 it is a fast autoaccelerated reaction ending in explosion, the condition for the transition from the stationary to the non-stationary state, i.e. the condition for the ignition limit (limits) can be formulated as follows... 

Semenov [427] was the first to find that an autoaccelerated reaction resulting in explosion at low p and T can develop at a constant temperature. This is referred to as chain self-ignition or chain (isothermal) explosion. At high p and T the rate of heat removal may be insufficient the temperature and the reaction heat will increase in progression and the reaction would end in explosion. Such an explosion is referred to as thermal. 

The explanation for autoacceleration is as follows. As polymerisation proceeds there is an increase in the viscosity of the reaction mixture which reduces the mobility of the reacting species. Growing polymer molecules are more affected by this than either the molecules of monomer or the fragments arising from decomposition of the initiator. Hence termination reactions slow down and eventually stop, while initiation and propagation reactions still continue. Such a decrease in the rate of the termination steps thus leads to the observed increase in the overall rate of polymerisation. 

Antioxidants shift the autoaccelerating increase of chemiluminescence intensity to higher time. This is due to reactions 12 and 13 of the Bolland-Gee mechanism (Section 1, Scheme 2) in which peroxyl radicals and hydroperoxides are scavenged until antioxidants InFl and D are consumed. A typical example of such a behavior occurs for samples of PP containing 0.1 %wt. of Irganox 1010 (a sterically hindered phenol) (Figure 18 and Table 4). The presence of antioxidants usually reduces the maximum chemiluminescence intensity [61,62]. This may be explained by the quenching effect of the antioxidant on excited carbonyls, but it may be also related to the mechanism of oxidation of stabilized PP. Stabilizers in... 

Most polymerizations proceed with autoacceleration as the increasing viscosity of the reaction system results in an increase in the kp/kj.1/2 ratio. Both kp and kt decrease with increasing viscosity but k(- decreases more than does kp since termination involves reaction between two large-sized species while propagation involves reaction between one large and one small species. 

The autoxidation of ethers occurs with self-acceleration as autoxidation of hydrocarbons. The kinetics of such reactions was discussed earlier (see Chapter 2). The autoacceleration of ether oxidation occurs by the initiating activity of the formed hydroperoxide. The rate constants of initiation formed by hydroperoxides were estimated from the parabolic kinetic... 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1], The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16], Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction [6], However, the addition of an aqueous solution of Na2C03 or NaOH in combination with vigorous agitation of this system accelerates the oxidation process [1-17]. The addition of an aqueous phase accelerates the oxidation and withdrawal of water retards it [6]. The addition of surfactants such as salts of fatty acids accelerates the oxidation of cumene in emulsion [3], The higher the surfactant concentration the faster the cumene autoxidation in emulsion [17]. The rates of cumene emulsion oxidation after an induction period are given below (T = 353 K, [RH] [H20] = 2 3 (v/v), p02 = 98 kPa [17]). 

During conventional polymerizations of both HEMA and DEGDMA, complications resulting from diffusion limitations to termination and propagation are observed. Features such as autoacceleration, autodeceleration and incomplete conversion of double bonds characterize the rate behavior of these polymerizations. As TED is added to the reacting system, the carbon-DTC radical termination reaction is introduced. Diffusion limitations to carbon-DTC radical combination are lower than those to carbon-carbon radical termination as the DTC radical is smaller and much more mobile than a typical polymeric carbon radical. As a result, the cross-... 

Similar trends are observed in the case of DEGDMA (Figure 2) however, the autoacceleration peak begins very early in the reaction (less than 2% conversion) and to a greater extent than HEMA. Also, the autodeceleration takes place at lower... 

It can be observed that the initial rate of polymerization decreases and the autoacceleration peak is suppressed as the TED concentration is increased. The TED molecules generate dithiocarbamyl (DTC) radicals upon initiation. As a result, termination may occur by carbon-carbon combination which leads to a dead polymer and by carbon-DTC radical reaction which produces a reinitiatable ( living ) polymer. The cross-termination of carbon-DTC radicals occurs early in the reaction (with the carbon-carbon radical termination), and this feature is observed by the suppression of the initial rate of polymerization. As the conversion increases, the viscosity of the system poses mass transfer limitations to the bimolecular termination of carbon radicals. As has been observed in Figure 3, this effect results in a decrease in the ktCC. However, as the DTC radicals are small and mobile, the crosstermination does not become diffusion limited, i.e., the kinetic constant for termination of carbon-DTC radicals, ktCS, does not decrease. Therefore, the crosstermination becomes the dominant reaction pathway. This leads to a suppression of the autoacceleration peak as the carbon-DTC radical termination limits the carbon radical concentration to a low value, thus limiting the rate of polymerization. This observation is in accordance with results of previous studies (10) with XDT and TED, where it was found that when there was an excess of DTC radicals, the carbon radical concentration was lower and the cross-termination reaction was the dominant termination pathway. 

From these experimental and modeling studies, the mechanism of the living free radical polymerizations initiated by a combination of TED and DMPA have been elucidated. The TED produces DTC radicals that preferentially cross-terminate with the propagating carbon radicals. By this cross-termination reaction, the carbon radical concentration is kept low (as was shown in figure 6) and the rate of polymerization is decreased, as is the autoacceleration effect. This suppression of the autoacceleration peak in HEM A polymerizations and, interestingly, in DEGDMA polymerization has been observed to increase as the TED concentrations are increased. This behavior has been predicted successfully by the model as well. 

The bulk polymerization of acrylonitrile in this range of temperatures exhibits kinetic features very similar to those observed with acrylic acid (cf. Table I). The very low over-all activation energies (11.3 and 12.5 Kj.mole-l) found in both systems suggest a high temperature coefficient for the termination step such as would be expected for a diffusion controlled bimolecular reaction involving two polymeric radicals. It follows that for these systems, in which radicals disappear rapidly and where the post-polymerization is strongly reduced, the concepts of nonsteady-state and of occluded polymer chains can hardly explain the observed auto-acceleration. Hence the auto-acceleration of acrylonitrile which persists above 60°C and exhibits the same "autoacceleration index" as at lower temperatures has to be accounted for by another cause. 

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