Chain propagation energetics

The results of the calculation could not distinguish hydroxyl or propyl radicals as the desorbed species, since they produced very similar product distributions. This is because they were both involved in the chain propagation reactions. Since the desorption of a hydroxyl radical required cleavage of metal-oxygen bonds in the oxide lattice, it would be energetically less favorable. Thus it was concluded that propyl radicals were the most likely desorbed species. 

Compare the relative energetic feasibilities of these chain-propagation steps with those of other possible mechanisms. 

Write a possible mechanism for the reaction, showing the propagation steps with (CH3)3CO as the chain-propagating radical. Use the bond-dissociation energies of Table 4-6 to determine whether your mechanism is energetically and kinetically feasible. Assume the O-CI bond-dissociation energy of fert-butyl hypochlorite is 61 kcal mole-1. 

Both propagation steps for the addition of HBr are exothermic, so propagation is exothermic (energetically favorable) overall. For the addition of HCl or HI, however, one of the chain-propagating steps is quite endothermic, and thus too difficult to be part of a repeating chain mechanism. Thus, HBr adds to alkenes under radical conditions, but HCl and HI do not. 

The essence of the energetic studies on TS and 4-BCMU is contained in Fig. 9. In TS formation of the chain initiating species -- a dimer — requires an energy of 1.0 eV. It can be supplied thermally or optically via monomer excitation. In the former case it is this chain initiation reaction that controls the thermal reactivity and its temperature-dependence. Chain initiation can also be produced optically at a yield of order 10 per absorbed UV-quantum. In this case it is chain propagation that determines the temperature dependence of the polymerization yield. However, the activation energy E" need not be and in general is not identical with the energy... 

In the chain propagation reaction a 71-bond is changed into a 0-bond by addition of an adjacent monomer molecule to the intermediates. In this way in every reaction step about 0.9 eV are released. The resulting energy level scheme of the polymerization reaction is shown in Fig. 26. It represents the energetic positions of the resulting DR or AC intermediates characterized by the general notation M and the transient pair states M M and The addition reaction steps may be induced optically... 

DFT calculations combined with molecular mechanics methods have been used to study the first (R = Me) and the second (R = propyl) insertion of the ethylene monomer into the Ti-R bond of (CpSiMe2NBut)(R)Ti(/t-Me)B(C6F5)3. The influence of the counterion and the solvent effects on the energetic profile of the polymerization have been evaluated. Theoretical investigations have also been directed at mechanistic aspects of olefin polymerizations catalyzed by mono-Cp titanium complexes. The chain propagation mechanism, the chain termination and... 

If the blradical dimer Is the intermediate in both the ther mal polymerization and photopolymerization, then the energy released on addition of a monomer unit to a growing chain should be the same for both processes. Unfortunately, both 4BCMU and ETCD are thermally inactive and a direct comparison is not possible. Just the opposite is true for TS. The high thermal reactivity of TS allows easy measurement of the thermal heat of polymerization, but the low photosensitivity, compared to 4BCMU, prohibits any quantitative determination of the enthalpy of the photoreaction. Therefore a direct proof that the energetics of chain propagation is independent of the mode of initiation is not possible with these materials. 

Some idea of the rather rudimentary state of our concepts at the time is shown by the fact that at the Boston ACS meeting in September, 1939 I gave the first explanation, based on the energetics of chain propagation steps, why radical chain reactions had been observed with HBr, but not with other halogen acids. 

Under oxidation conditions, this isomerization is a stage of intramolecular chain propagation. Competition between reactions (8) and (9) depends, naturally, on the structure of hydrocarbon and its concentration Vg/V9 = 8[RH]/ifc9. The ratio k%lk = 0.024 1/mol for 2,4-dimethylpentane and 0.71 1/mol for n-pentane (373 K), i.e., in hydrocarbons with two (or more) tertiary C—bonds in the P- and y-positions the intramolecular attack of RO-2 is more energetic. 

Chain transfer by the HS CHa CF CI CI Cl would be energetically unfavorable as it would involve Cle transfer from the initiator to the propagating cation, i.e., the creation of a relatively less stable initial benzyl carbocation by sacrificing a more stable propagating tertiary benzyl cation58. ... 

In total, many possibilities of propagation result for the ions formed by the attack on the C = 0 double bond. According to the calculations, 4 of the structures which can be formed theoretically by interaction of an acrolein chain end with an acrolein monomer possess energetic preference. Two of them are the structures c and d. These results agree with the experimental cationic polymerizability ofacroleine(R = —CHO), as well as with the fact that in the cationically polymerized polyacroleine the following structures alternate with each other88) ... 

Isomerization polymerizations are polyaddition reactions where the propagating species rearranges to energetically preferred structures prior to subsequent chain growth. 

The peroxides and peracids formed in autocatalytic systems are highly energetic molecules. We now see that the Co/Mn/Br catalyst serves to rapidly relax this energy in increasingly lower steps winding up with a highly selective bromide(O) radical (probably as a complex with the metal). The bromide(O) transient species quickly reacts with methylaromatic compounds to form PhCHj radicals and hence continues to propagate the chain sequence. 

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