Degenerative transfer

The cleavage of C-I bond can be achieved from various methods [373-375]. However, according to well chosen monomers, two main ways have been developed in order to control telomerisation from alkyl iodides iodine transfer polymerisation (ITP) and degenerative transfer. ITP can be easily applied to fluorinated monomers whereas degenerative transfer concerns the controlled polymerisation of methyl methacrylate, butyl acrylate [376] or styrene [377] and will not be discussed in this chapter. 

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,... 

The controlled free-radical miniemulsion polymerization of styrene was performed by Lansalot et al. and Butte et al. in aqueous dispersions using a degenerative transfer process with iodine exchange [91, 92]. An efficiency of 100% was reached. It has also been demonstrated that the synthesis of block copolymers consisting of polystyrene and poly(butyl acrylate) can be easily performed [93]. This allows the synthesis of well-defined polymers with predictable molar mass, narrow molar mass distribution, and complex architecture. 

If an overall conclusion could be made, it might be considered that the counterradicals vary considerably (Scheme 3). They can either be stable (e.g., nitroxyls, arylazooxyls), semi persistent (e.g., from thiourams) and also metallic (e.g., acetoacetato metals). In addition, if these radicals either terminate or transfer, non-living (or inactive) species will be produced. But, in order to preserve the living character, the radicals must propagate and in specific cases (e.g., iodine transfer polymerization or degenerative transfer) active species will be obtained. The more that one of these latter steps is favored, the more living is the tendency of the radical polymerization, with a very high kinetic control of this reaction. 

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. 

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,... 

The mechanism of Co(acac)2-mediated polymerization of Vac is still an open question. On the basis of an early work by Wayland and coworkers on the controlled radical polymerization of acrylates by complexes of cobalt and porphyrins, Debuigne and coworkers proposed a mechanism based on the reversible addition of the growing radicals P to the cobalt complex, [Co(II)], and the establishment of an equilibrium between dormant species and the free radicals (equation 8). Maria and coworkers, however, proposed that the polymerization mechanism depends on the coordination number of cobalt . Whenever the dormant species contains a six-coordinated Co in the presence of strongly binding electron donors, such as pyridine, the association process shown in equation 8 would be effective. In contrast, a degenerative transfer mechanism would be favored in case of five-coordinated Co complexes (equation 9). 

This technique for controlling radical polymerizations is based on one of the oldest technique, that of chain transfer, and has often been used in telomeriza-tion [83]. Similar to the concept of degenerative transfer with alkyl iodides [50, 51, 84], reversible addition fragmentation chain transfer with dithio esters (RAFT) [52-55, 85] is successful because the rate constant of chain transfer is faster than the rate constant of propagation. Analogous to both nitroxide-medi-... 

RAFT has also been used to prepare copolymers. The copolymerization of MMA with nBA in the presence of cumyl dithiobenzoate as the transfer agent resulted in a polymer with a gradient of composition along the backbone, well-defined molecular weights, and low polydispersities [53]. Several copolymers were made by degenerative transfer with alkyl iodides [133]. 

First block copolymers by degenerative transfer technique were prepared by using alkyl iodides/AIBN in polymerization of BA and St. For example, at 70 °C pSt with Mn=2500 and Mw/Mn=1.45 was extended to pSt-b-pBA with Mn=8630 and Mw/Mn=2.20. Although polydispersities were relatively high, a clean shift of the entire MW distribution was observed. In a similar way pBA with Mn=2820 and Mw/Mn=1.60 was extended to pBA-b-pSt with Mn=6290 and Mw/Mn=1.75. Conversion of the first block was above 95% and the second 80% in both cases [51]. 

Block copolymers prepared using the degenerative transfer with alkyl iodides and RAFT systems include methacrylate, acrylate, and styrene combinations as... 

Like RAFT, ITP is a degenerative transfer polymerization using alkyl halides [10,11]. ITP was developed in the late 1970s by Tatemoto et al. [226-229]. In ITP, a transfer agent RI reacts with a propagating radical to form the dormant polymer chain P -1. The new radical R can then reinitiate the polymerization. In ITP, the concentration of the polymer chains is indeed equal to the sum of the concentrations of the transfer agent and of the initiator consumed. The newly formed polymer chain P- can then propagate or react with the dormant polymer chain P -1 or R -1 [230]. The mechanism of ITP with alkyl iodide is shown in Scheme 41. 

More recently, several investigations have shown that ITP can produce telechelic oligomers. The degenerative transfer process then requires the use of diiodide compounds instead of the iodide compounds usually employed in ITP. Noteworthy, the Dupont [231] and Ausimont [232] companies were first attracted by this concept (using IC4F8l as the transfer agent) in the CRP of... 

To get the diiodo telechelic structure, the authors used iodoform and methylene iodide as degenerative CTAs. SET involves the production of radical ions generating free radicals and anions or cations. Iodoform and methylene iodide can be activated both by degenerative transfer mediated by the growing PVC radicals and by SET. The reaction may be catalyzed by sodium dithionite (Na2S204). These catalyzed reactions allow the suppression of side reactions but also the scavenging of oxygen. The LRP of vinyl chloride resulted in diiodo PVC with Mn of 6000- 10 000 g mol 1 and with PDI of about 1.6. 

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.)...

Living polymerization is another rapidly developing area where the degenerative transfer of xanthates and related derivatives (e.g. dithioesters, dithiocarbamates, trithiocarbonates) is having a significant impact [38] (see also Volume 1, Chapter... 

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