Chain reactions degenerative

Terminally brominated PE as PE macroinitiator can be produced by other methods. It has been reported that vinyl terminated PE produced by a bis(phenoxy-imine)metal complex and MAO catalyst system (Mn = 1800, Mw/Mn = 1.70) was converted to terminally 2-bromoisobutyrate PE through the addition reaction of 2-bromoisobutyric acid to the vinyl chain end. Polyethylene-Wodc-poly( -bulyl acrylate) (PE-fo-PnBA) from terminally brominated PE by ATRP procedure has also been produced [68]. It was reported that degenerative transfer coordination polymerization with an iron complex can be used to prepare terminally brominated PE as a macroinitiator [69]. A Zn-terminated PE prepared using an iron complex and diethylzinc,... 

As mentioned above, in both NMP and ATRP the exchange between the active and the dormant states is based on a reversible (although different) termination mechanism. Therefore, the exchange directly affects the radical concentration. In LRP by degenerative transfer, instead, this exchange is carried out by direct transfer of the w-end group between an active and a dormant chain. When an iodine atom is used as end group, the reaction can be expressed as follows ... 

The RAFT process can be regarded as a special case of degenerative transfer. As shown in Scheme 6.4, the reaction proceeds through the direct interaction of an active and a dormant chain with the formation of a reaction intermediate involving both chains [9a,b]. At this stage, the reaction can either go back,... 

Figure 6.26 Reaction scheme of controlled free-radical polymerization, based on degenerative chain transfer, of butyl acrylate (R = C4H9COO-) in the presence of secondary alkyl iodide (R = CH3CH(Ph)-, X = I) as the degenerative transfer agent. The latter alone does not initiate polymerization. (After Matyjaszewski et al., 1995.)...

The polymer-nitroxyl adduct P-X reversibly dissociates thermally, in process 1 into the polymer radical P and the nitroxyl radical X. The rate constants of dissociation and combination are and kc, respectively. The, so-called, degenerative transfer takes place in process 11. The second-order rate constant for active species in either direction is k y. Here all the rate constants are assumed to be independent of chain length. Since the frequency of cleavage of the P-X bond is proportional to [P-X] in process 1 and to [P l [P -X]] in process 11, the overall frequency,/ per unit time and per unit volume, of the bond-cleaving or activation reactions, may be expressed by [277] ... 

What is meant by degenerative chain transferring Illustrate back-biting. The telomerization reaction. 

There are three general classifications of living radical polymerization based on differences in the reversible activation reaction step described in the previous section. These three mechanisms are termed dissociation-combination, atom transfer and degenerative chain transfer, respectively [17, 18]. 

Scheme 13.6 Reversible activation step for degenerative chain transfer reactions.

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