Activation energy, free radical initiator decomposition

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. 

Table 20-1. Half-Lives and Activation Energies of Decomposition of Some Free Radical Initiators AI BN, Azobisisobutyronitrile BPO, Dibenzoyl Peroxide MEKP, Methyl Ethyl Ketone Peroxide IPP, Diisopropyl Peroxide Dicarbonate Dicup, Dicumyl Peroxide CuHP, Cumyl Hydroperoxide...

When the catalyst system consists of vanadium oxychloride, triisobutylaluminum, and tetrahydrofuran, the activation energy of the process is found to be 16.1 kcal/mole, similar to that found for other Ziegler-Natta systems. The PVC prepared by this system has a decomposition temperature of 275°-335°C, compared to 250 -295°C for PVC prepared by free-radical initiation [202,203]. 

In later work, Shantarovicli and Pavlov122 found that CIK decomposition at 850-1000°C. is first order and proceeds by a free-radical chain mechanism with an effective activation energy of 85 kcal. The initiating step was stated to be reaction (9), with an activation energy of 92-94 kcal. 

Organic peroxides, which readily decompose into free radicals under the effect of thermal energy, are used under high pressures as initiators for radical polymerizations. The measurement of the influence of pressure on the rate of decomposition gives rise to the determination of the activation volume, which, in turn, allows conclusions to be drawn on the decomposition mechanism and the transition state. 

Free radical scavenging, peroxide decomposition, and regeneration of inhibitors interfere with propagation reactions by (1) stopping active free radicals, and by (2) decomposing unstable peroxides into nonactive species. In the fourth mode of action, ultraviolet absorbers preferentially absorb incident light energy before it can initiate further chain reactions and produce active radicals. 

An example of initiation can be thermal decomposition of the above mentioned di-azo reagents represented by AIBN. Heat supplies the energy needed for splitting the molecule into two isobutyronitrile radicals (plus a molecule of nitrogen). Each free radical has an unpaired electron further acting as an active site for propagation. 

Many polymerizations are carried out at temperatures between 0 and 100°C. Initiation at the required rates under these conditions is confined to compounds with activation energies for thermal homolysis in the range 1(X)-165 kJ/mol. If the decomposition process is endothermic, the activation energy can be considered to be approximately equal to the dissociation energy of the bond which is being split. It can be expected, then, that useful initiators will contain a relatively weak bond. (The normal C—C sigma bond dissociation energy is of the order of 350 kJ/mol, and alkanes must be heated to 3(X)-500°C to yield radicals at the rates required in free-radical polymerizations.)... 

For a given tube radius there exists a particular wall temperature that gives maximum conversions in free-radical polymerizations. This can be seen qualitatively from the following considerations. If the tube wall is loo cool, the initiator will be slowly decomposed and some of it will leave the reactor unconsumed. However, the activation energy for initiator decomposition exceeds that for consumption of monomer (Section 6.16.1), and the initiator can be entirely decomposed at low monomer conversions if the wall temperature is too high for the particular reaction system [2]. 

Free radical polymerization offers a convenient approach toward the design and synthesis of special polymers for almost every area. In a free radical addition polymerization, the growing chain end bears an unpaired electron. A free radical is usually formed by the decomposition of a relatively unstable material called initiator. The free radical is capable of reacting to open the double bond of a vinyl monomer and add to it, with an electron remaining unpaired. The energy of activation for the propagation is 2-5 kcal/mol that indicates an extremely fast reaction (for condensation reaction this is 30 to 60 kcal/mol). Thus, in a very short time (usually a few seconds or less) many more monomers add successively... 

As described previously, thermooxidative degradation of polyolefins proceeds by a typical free-radical chain mechanism in which hydroperoxides are key intermediates because of their thermally-induced hemolytic decomposition to free radicals, which in turn initiate new oxidation chains. However, since the monomolecular hemolytic decomposition of hydroperoxides into free radicals require relatively high activation energies, this process becomes effective only at temperatures in the range of 120°C and higher. 

Redox initiators produce polymerization-inducing free radicals by reaction of a reducing agent with an oxidizing agent. The required thermal activation energy is quite low, so that polymerizations can be induced at much lower temperatures than is the case for purely thermal decomposition of peroxides or peresters. Five kinds of redox systems can be distinguished ... 

The rate of initiator decomposition in the absaioe of any other process vMch consumes free radicals can be followed by hydrogen ion evolved. By measviring the choige in rate of decomposition with tenperature. the activation energy for free radiced. generation was determined as 10.900 caly nole. 

In an attempt to get a more basic understanding of the initiator system, we undertook a study of the kinetics of de -composition of aqueous potassium persulfate.In O.lN potassium hydroxide buffer, the apparent activation energy for decomposition was found to decrease from 33.5 kcal/mole to 19.9 kcal/mole upon the addition of 18-crown-6. A more detailed investigation has now shown this to be a radical chain decomposition in which crown is being oxidized. The accelerated decomposition seems to be due to the reaction of the crown cation radical and persulfate dianion (Scheme 2). Addition of a free radical trap, in this case methacrylonitrile, suppressed the rate of persulfate disappearance to that normally observed in the absence of crown. 

The front velocity is a function of the initial temperature and the AT of the reaction, where AT is determine by the DH X Mo/Cp. The value of AT is also affected by the presence of any inert material. Goldfeder et al. derived an expression for the front velocity in terms of the parameters for a free-radical polymerization. The velodty is a function of k, the thermal diffusivity (0.0014 cm s" ), Tb, fed = the preexponential factor for the initiator decomposition (4xl0 s" ), i = d = the energy activation for the dissociation constant for the initiator = 27 kcal mol" g is the ideal gas constant. 

Because of the high value of the energy of activation of the dissociation step (about 130kJ-mor ), this increase in temperature favors an even faster decomposition of the initiator, adding more free radicals into the medium. 

Initiators with high activation energies show large increases in decomposition rate for a small temperature rise, and hence the half-life is shorter. In terms of reaction kinetics tv is a first order reaction. As will becmne rqrparent, free radical formation does not follow the ideal for first order kinetics, due to the influence of factors other than temperature on the decomposition rate. In practice, the rate of decomposition has been found to lie between that predicted for first order and second order reactions. 

Persulfate initiators generate free radicals upon the thermally induced scission of O—O bonds, thus resembling the organic peroxides dealt with above. For potassium persulfate, the decomposition rate constant is 9.6 X 10 sec at 80°C and the activation energy amounts to 140 kJ mol" (in 0.1 mol L NaOH) [90]. 

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