Addition-fragmentation chain transfer

Watanabe et al,25-5 52s applied AMS dimer (116) as a radical trap to examine the reactions of oxygen-centered radicals (e.g. r-butoxy, cumyloxy, benzoyloxy). AMS dimer (116) is an addition fragmentation chain transfer agent (see 6.2.3.4) and reacts as shown in Scheme 3,96. The reaction products are macromonomers and may potentially react further. The reactivity of oxygen centered radicals towards 116 appears to be similar to that of S.2 1 Cumyl radicals are formed as a byproduct of trapping and are said to decay mainly by combination and disproportionation. 

This equation can be solved numerically to give values of Clr and Ctr.404 For reversible addition-fragmentation chain transfer (RAFT) (Scheme 6.5), the rate constant for the reverse reaction is defined as shown in eq. 22 ... 

Penultimate unit effects are also important in both substitution40"41 and in addition-fragmentation chain transfer.42"44 Some examples are provided in Sections 6.2, 6.2.2.4, 6.2.3.4 and 9.5. 

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

Although the term RAFT (an acronym for Reversible Addition-Fragmentation chain Transfer)38" is sometimes used in a more general sense, it was coined to describe, and is most closely associated with, the reaction when it involves thiocarbonylthio compounds. RAFT polymerization, involving the use of xanthates, is also sometimes called MADIX (Macromolccular Design by Interchange of Xambate) 96 The process has been reviewed by Rizzardo et [Pg.502]

For addition-fragmentation chain transfer, the rate constants for the forward and reverse reaclions are defined as shown in eqs. 21 and 22 respectively. 

RAFT reversible addition-fragmentation chain transfer... 

Synthesis of Block Copolymers by Reversible Addition-Fragmentation Chain Transfer Radical Polymerization, RAFT... 

Reverse transcriptase, 21 281 Reverse water-gas shift reactions, 5 14-15 Reversible addition-fragmentation chain transfer (RAFT), 7 621, 623 Reversible addition-fragmentation chain transfer (RAFT) polymerization,... 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

Fijten MWM, Meier MAR, Hoogenboom R, Schubert US (2004) Automated parallel inves-tigations/optimizations of the reversible addition-fragmentation chain transfer polymerization of methyl methacrylate. J Polym Sci Part A Polym Chem 42 5775-5783... 

Paulus RM, Fijten MWM, de la Mar MJ, Hoogenboom R, Schubert US (2005) Reversible addition-fragmentation chain transfer polymerization on different synthesizer platforms. QSAR Comb Sci 24 863-867... 

Chiefari J, Chong YK, Ercole F (1998) Living free radical polymerization by reversible addition-fragmentation chain transfer -the RAFT process. Macromolecules 31 5559-5562... 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

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