Chain transfer homolytic substitution

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

General aspects of chain Iransfer have been reviewed by Chiefari and Rizzardo, Barson/ Farina, Bastmond and Palit et a . The use of chain transfer in producing lelechelic and other functional polynners has been reviewed by Boutevin, Heitz, Corner and Starks and is discussed in Section 7.5.2. I here are two main mechanisms which should be considered in any discussion of chain transfer (a) atom or group transfer by homolytic substitution (Section 6.2.2) and... 

In this ca.se, the chain transfer step involves formation of an intermediate adduct. Other examples thought to involve a transfer by homolytic substitution are iodine transfer polymerization (Section 9.5.4) and TERP (telluride-mediated polymerization, Section 9.5.5). 

There are at least two basic mechanisms for chain transfer (1) atom or group transfer and (2) addition-fragmentation (169). The two mechanisms differ mainly in the nature of the intermediate that is formed by the two participating molecules. The atom transfer mechanism—the most common mechanism—proceeds via an S—2 mechanism (substitution, homolytic, bimolecular) as depicted in equation 26. 

Radical polymerizations that involve a reversible chain transfer step for chain equilibration and that displayed the characteristics of living polymerizations were first reported in 1995. ° The mechanism of the reversible chain transfer step may involve homolytic substitution (Scheme 73) or addition-fragmentation (RAFT) (Scheme 74). An essential feature is that the product of chain transfer is also a chain transfer agent with similar activity to the precursor transfer agent. The process has also been termed degenerate or degenerative chain transfer... 

Scheme 6 Reversible homolytic substitution chain transfer. Reproduced from Moad, G. Rizzardo, E. Thang, S. H., Radical addition-fragmentation chemistry in polymer synthesis. Polymerl888, 49,1079-1131. ...

Early on, it was demonstrated that aromatic nitro compounds may form radical anions in alkaline solutions [4], with the possibilities of photochemical reactions [5]. There followed the development of the radical chain mechanism [6]. An interesting early danonstration of reaction by this mechanism was in the reaction of ort/io-halogenoanisoles with potassium amide in liquid ammonia [7]. Reaction by the benzyne mechanism gives predominantly the uiera-substituted product due to the electronic influence of the methoxy group. Howeva, with an access of potassium metal, which promotes electron transfer, the pathway predominates yielding ort/io-anisidine, as shown in Scheme 6.4. The mechanism now forms an important synthetic pathway, and this and other homolytic processes are covered in Chapters 9 and 10. 

---