Degenerative chain transferring

These methods are based on the idea of establishing equilibrium between the active and dormant species in solution phase. In particular, the methods include three major techniques called stable free-radical polymerization (SFRP), atom transfer radical polymerization (ATRP), and the degenerative chain transfer technique (DCTT) [17]. Although such syntheses pose significant technical problems, these difficulties have all been successively overcome in the last few years. Nevertheless, the procedure of preparation of the resulting copolymers remains somewhat complicated. 

Coelho and co-workers have recently reported another use of water in living polymerisation in the synthesis of a block copolymer [poly(vinyl chloride)-b-poly(/z-butyl acrylate)-b-poly(vinyl chloride)].3 The new material was synthesised by single electron transfer/degenerative chain transfer-mediated living radical polymerisation (SET-DTLRP) in two steps. 

Fig. 2. The mechanisms of (1) stable free radical polymerizations, (2) reversible redox polymerizations (i.e., ATRP), and (3) degenerative chain transfer...

Degenerative chain transfer with alkyl iodides [50,51] or dithioesters (RAFT or MADIX) [52-55] (3 in Fig. 2)... 

Percec et al. [233-237] recently reported the synthesis of a/o-diiodo poly(vinyl chloride) (PVC) by combination of single electron transfer (SET) and degenerative chain transfer (ITP) (Scheme 42). 

There are four principal mechanisms that have been put forward to achieve living free-radical polymerization (1) Polymerization with reversible termination by coupling, the best example in this class being the alkoxyamine-initiated or nitroxide-mediated polymerization, as first described by Solomon et al. (1985) (2) polymerization with reversible termination by hgand transfer to a metal complex (usually abbreviated as ATRP),(Wang and Matyjaszewski, 1995) (3) polymerization with reversible chain transfer (also termed degenerative chain transfer)-, and (4) reversible addition/ffagmentation chain transfer (RAFT). 

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.)...

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

The activation of P-I occurs not only by the catalytic process (Scheme 2) but also by degenerative chain transfer (Scheme lb). However, for example, the system with PE-I (80 mM) and BPO (20 mM) but without Gel4 (entry 11 iodide-mediated polymerization) only gave a PDI as large as 1.55 for 4 h at 80 °C, while that with Gel4 (5 mM) (entry 1) achieved a fairly small PDI of 1.17 (with other conditions set the same). This means that the catalytic activation plays a main role in the Gel4 system, with a small contribution of degenerative chain transfer. 

As mentioned, in the presence of Gel4 (deactivator XA), PSt-I can be activated by degenerative chain transfer (Scheme lb rate constant fcex) and the catalytic process with Gel (activator A ) (Scheme 2 rate constant k. ). Thus, the pseudo-first-order activation rate constant kact is given by... 

After the induction period, the polymerization takes place and is dominated by the degenerative chain transfer mechanism. A typical evolution of the molecular weight and polydispersity index with conversion is given in Figure 4 in the case of RITP of... 

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