Activation energy, free radical initiator

What is the usual value for the energy of activation of free radical initiation ... 

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. 

Oxidation of cyclohexanol with a mixture of 02 and 03 at 80—100° C proceeds by a chain mechanism [70], The rate of free radical formation is 1000 times lower than that of ozone consumption and the activation energy for chain initiation by ozone is 11 kcal mole-1. The cyclohexanone formed is oxidized by ozone without formation of free radicals. 

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...

In the presence of UV radiation polymer molecules become excited, i.e. acquire excess energy which causes breakdown of the weakest bonds in the polymer chain and the resulting active free radical initiates the degradation process. Additives are used to prevent, or minimize, polymer... 

The essential criterion for FP is that the system must have an extremely low rate of reaction at the initial temperature but a high rate at the front temperature such that the rate of heat production exceeds the rate of heat loss. In other words, the system must react slowly or not at all at room temperature, have a large heat release, and have a high energy of activation. For free-radical polymerization, the peroxide or nitrile initiator provides the large activation energy. It is not possible to create a system that has a long pot life at room temperature and a rapid reaction at any arbitrary temperature if the system follows Arrhenius kinetics. 

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]. 

Termination occurs when the active sites of two growing chains meet, as sho vn in Fig. 2.3 d). The unpaired electrons form a bond that couples the ends of the chains. Alternatively, disproportionation may occur. This happens when one chain transfers a hydrogen atom to the other and the electrons on both species rearrange themselves to satisfy the octet rule. There are many varieties of free radical initiators. Chemical initiators decompose to create radicals examples include organic peroxides, azo compounds, or even oxygen. More rarely we initiate polymerization via a physical condition, such as heat or high energy radiation, to create free radicals directly from the monomers. 

In radiation curing processes, addition reactions are activated by various forms of radiation infrared, microwave, radio frequency, gamma rays, ultraviolet (UV), and electron beam [2,3]. Basically, the energies of the first three are such that they simply thermally activate the system, that is, heat it, and are used in conjunction with the usual free-radical initiators. 

The occurrence of the three possible structures depends upon the activation energies for the different reactions. Using free-radical initiators it is generally found that about 20 percent of the polymer has the 1,2 structure, whereas most of the polymer has the trans-l,4 structure. If the reaction temperature is increased the amount of the 1,2 structure remains almost constant whereas the proportion of ds-1,4 increases. 

Mn(C 511702)2 as free radical initiator temperature range 295-313 K, value estimated from overall activation energy with Ei = 58.6 kJ/mol ... 

In the case of free radical polymerization with chemical initiation, tenperature exerts an influence at all stages of the process, with the activation energy associated with initiator deconposition generally greater than that of either the propagation or termination steps. In the case of photopolymerization, the initiator dissociation rate is expected to depend primarily on UV light intensity rather than temperature. This is evident in the data for the plateau value of G shown in Figure 2, where an increase with temperature is observed primarily at the lowest UV intensities. 

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

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