Degenerative chain transfer polymerization

Another method of control is based on degenerative chain transfer polymerization (DT), the term originally introduced a few years earlier. It is discussed in detail by Moad et... 

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. 

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

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.)...

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

VAc has been successfully polymerized via controlled/ living radical polymerization techniques including nitroxide-mediated polymerization, organometallic-mediated polymerization, iodine-degenerative transfer polymerization, reversible radical addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization. These methods can be used to prepare well-defined various polymer architectures based on PVAc and poly(vinyl alcohol). The copper halide/t is an active ATRP catalyst for VAc, providing a facile synthesis of PVAc and its block copolymers. Further developments of this catalyst will be the improvements of catalytic efficiency and polymerization control. 

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. 

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... 

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]. 

SCFIEME 8.7 Mechanism of isotactic/atactic stereoblock propylene polymerization with 9, through degenerative chain transfer. (Reprinted with permission from Harney, M. B. Zhang, Y. Sita, L. R. Angew. Chem. Int. Ed. 2006, 45, 6140-6144. Copyright 2006 John WUey Sons, Inc.)... 

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... 

Iodide-mediated polymerization is a simple and robust LRP that can be performed in experimental conditions dose to those of conventional RP. However, it has, at this moment, limited applicability, since it does not give polymers with a very low polydispersity. This is due to generally small values of feact achievable by tbis polymerization. Mechanistically, it includes a DT process. In what follows, we will describe the kinetic features of iodide-mediated polymerization of styrene in some detail for its importance as a model degenerative chain transfer-mediated polymerization (DTMP). 

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