Molecule assisted homolysis

The two most widely accepted mechanisms for the spontaneous generation of radicals from S are the biradical mechanism (top half of Scheme 3.61) first proposed by Flory314 and the Mayo315 or MAH (molecule assisted homolysis) mechanism (lower pari of Scheme 3.61). 

The Mayo mechanism involves a thermal Diels-AIder reaction between two molecules of S to generate the adduct 95 which donates a hydrogen atom to another molecule of S to give the initiating radicals 96 and 97. The driving force for the molecule assisted homolysis is provided by formation of an aromatic ring. The Diels-AIder intermediate 95 has never been isolated. However, related compounds have been synthesized and shown to initiate S polymerization."110... 

Involvement in molecule assisted homolysis where, for example, decomposition of hydroperoxides is accelerated by presence of organic xenobiotic. 

Part 7. Molecule-Assisted Homolysis, One-Electron Transfer, and Non-Concerted Cycloaddition Reactions... 

In the textbook Free Radicals", ref. 1, I used the expression "molecule-induced homolysis" and the abbreviation MIH, a phraseology retained by Harmony (9). More recently, I have used "molecule-assisted homolysis" to avoid confusion of MAH processes with the induced decomposition of initiators caused by radicals, a propagation step, rather than an initiation process ... 

A general review is presented of the mechanisms for reactions in which radicals are formed at accelerated rates by the interaction of closed-shell molecules. We have grouped these mechanisms into three classes molecule-assisted homolysis, electron transfer, and non-concerted cycloaddition reactions. 

The spontaneous thermal polymerization of styrene is postulated to involve an initiation step in which the reactive Diels-Alder dimer of styrene (AH in Figure lA) undergoes a molecule-assisted homolysis reaction with another styrene molecule. The evidence for this process is reviewed, and it is concluded that it is not at all conclusive. Furthermore, methylenecyclohexadiene... 

We have studied the kinetics of this reaction in CFCI3 at -4 C and have obtained the kinetic isotope effect and an approximate activation energy. Using these data, both an electron-transfer reaction and a concerted 1,3-dipolar insertion reaction can be eliminated as mechanistic possibilities for the peroxide-ozone reaction. We conclude the mechanism involves a molecule-assisted homolysis ... 

The second mechanism, proposed by Mayo (116), involves the Diels-Alder reaction of two styrene molecules to form a reactive dimer (DH) followed by a molecular assisted homolysis between DH and another styrene molecule. 

Mayo [16] proposed an alternative mechanism that is currently widely supported. Figure 7.7 shows a schematic of the Mayo mechanism. A Diels-Alder reaction between two styrene molecules produces an intermediate dimer (DH), also referred to as Mayo dimer . DH is highly reactive and has never been isolated. To complete the auto-initiation, DH reacts with a third styrene molecule via molecular assisted homolysis [17] to form a phenyltetraline radical (D ) and a phenethyl radical (SH ). A second reaction involving DH is to undergo chain transfer with a growing radical chain to produce a dead polymer chain (PS-H) and a new growing radical. The chain transfer constant (A ct) of DH has been estimated at 10, which is the highest Kcl ever reported for a molecule that contains no heteroatoms [18,19]. 

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