Degenerative Transfer DT

As mentioned above, in both NMP and ATRP the exchange between the active and the dormant state is based on a reversible (although different) termination mechanism. Therefore, the exchange directly affects the radical concentration. In LRP by DT, instead, this exchange is carried out by direct transfer of the (o-end group between an active and a dormant chain. When an iodine atom is used as end group, the reaction can be summarized by Eq. (c), where R I indicates the generic dormant species with iodine. 

Only a few papers have appeared in the literature dealing with LRP by DT [64— 

and the applications are almost completely limited to the homopolymerization of styrene. In this case, it was possible to obtain good control of the final CLD, with polydispersity values as low as 1.3-1.4. Better performances are difficult to obtain with styrene, mainly because of the limited transfer activity of the iodine atoms. 

This is the main reason for the very poor results obtained when applying this process to the polymerization of acrylates (for example, n-butyl acrylate), and for the complete lack of control reported for other monomers [64—66]. 

4 Reversible Addition-Fragmentation Transfer (RAFT) Polymerization 

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In fact, the RAFT process resembles the degenerative transfer (DT) process [274]. In a polymerization in which an alkyl iodide is used as the degenerative transfer agent, the iodine atom is exchanged between a polymeric radical and a dormant chain, similar to the dithiocarbonate exchange in RAFT. However, in the case of degenerative transfer there is a direct equilibrium between the dormant and growing chains, without formation of an intermediate radical. 

Control by degenerative transfer (DT) involves perhaps the smallest change from a eonventional free radical process of all the controlled/living polymerization proeesses developed to date. A recent review of various methods of telomer synthesis [180] diseusses the different types of transfer agents and monomers and the contribution of the teehniques of telomerization to CRP (includes discussion of iodine transfer polymerization, RAFT, and macromolecular design through interchange of xanthates (MADIX)) [181,182]. 

The /fd. /fex. and kp are the rate constant tor reversible termination (RT) (Scheme 7(a)), degenerative transfer (DT) (Scheme 7(b)), and propagation, respectively. Cex is the degenerative chain transfer constant (=kjkp). 

Reversible termination (RT) and degenerative transfer (DT) mechanisms for LRP are clearly distinguished by the source and concentration of radicals in solution as well as through the origin of the living character manifested by the process. The dormant complex (X-P) is the exclusive source of radicals (P ) for all RT processes which encompass the subcategories of homolytic X-P dissociation (5.1) and atom transfer (5.2). The ideal radicals X and Y depicted in eqn (1) and (2) are unable either to dimerize or initiate polymerization. 

Organo-cobalt Porphyrins in LRP of Methyl Acrylate by a Degenerative Transfer (DT) Mechanism... 

First order rate plots for the polymerization of methyl acrylate initiated by V-70 and AIBN at 333 K in the presence of (TMP)Co and (TMP)Co-CH(C02CH3)CH3 are illustrated in Figure 5.7. AIBN has a half life of about 18 hours at 333 K in benzene and the much slower entry of radicals into solution compared to V-70 (ti/2(333K) = 11 minutes) permits attaining a near constant steady state radical concentration which simplifies the kinetic analysis. The rate of MA polymerization after the induction period follows first order rate behavior where the slope of ln([M]o/[M]/) versus time is proportional to the square root of the initiator concentration ([AIBN] ). This demonstrates that the rate of MA polymerization is controlled by the AIBN concentration and not by the organo-Co(TMP) mediator complex which is a signature criterion for a degenerative transfer (DT) process. 

Six different series of cobalt(ii) complexes that have been evaluated as LRP mediators are shown in Scheme 5.4. Several of the important features for a specific OMRP system include electronic effects that influence M-C and M-H bond energetics, steric effects on access to the metal center for H-abstraction from radicals and M-C bonding, and the meehanistic capability to have reversible termination (RT) and degenerative transfer (DT) operate in mediating an LRP process. 

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