Reversible addition-fragmentation chain transfer technique

RAFT Reversible Addition-Fragmentation Chain Transfer Technique... 

Baum M, Brittain WJ Synthesb of polymer brushes on silicate substrates via reversible addition fragmentation chain transfer technique. Macromolecules 35 610—615, 2002. 

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. 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

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-... 

The first step for the core-first stars is the synthesis of multifunctional initiators. Since it is difficult to prepare initiators that tolerate the conditions of ionic polymerization, mostly the initiators are designed for controlled radical polymerization. Calixarenes [39, 58-61], sugars (glucose, saccharose, or cyclodextrins) [62-68], and silsesquioxane NPs [28, 69] have been employed as cores for various star polymers. For the growth of the arms, mostly controlled radical polymerizations were used. There are only very rare cases of stars made from nitroxide-mediated radical polymerization (NMRP) [70] or reversible addition-fragmentation chain transfer (RAFT) techniques [71,72], In the RAFT technique one has to differentiate between approaches where the chain transfer agent is attached by its R- or Z-function. ATRP is the most frequently used technique to build various star polymers [27, 28],... 

Besides the ATRP method, other controlled radical polymerization techniques such as reversible addition/fragmentation chain transfer polymerization (RAFT) (Zhang et al., 2007) and nitroxide-mediated polymerization (NMP) (Yoshida and Ohta, 2005), have also been explored to synthesize azo BCs. 

CRP provides a versatile route for the preparation of (co) polymers with controlled molecular weight, narrow molecular weight distribution (i.e., Mw/Mn, or PDI < 1.5), designed architectures, and useful end-functionalities. Various methods for CRP have been developed however, the most successful techniques include ATRP, stable free radical polymerization, " and reversible addition fragmentation chain transfer (RAFT) polymerization. " " CRP techniques have been explored for the synthesis of gels " " and cross-linked nanoparticles of well-controlled polymers in the presence of cross-linkers. 

Taking into account all of the above mentioned applications, the synthesis of magnetic latex will be discussed in two parts first, the preparation of iron oxide nanoparticles and, second, the preparation of magnetic latex. Depending on the aim of researchers, many polymerization techniques are applied such as suspension, dispersion, emulsion, microemulsion and miniemulsion polymerization in combination with controlled radical polymerization techniques like atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) and nitroxide-mediated radical polymerization (NMP). The preparation of hybrid magnetic latex by emulsion polymerization will be the focus of this review. 

Anionic and later cationic pol3Tnerization gave most of examples of living pol3rmerization systems until recently, when more sophisticated methods of manipulation with free-radical polymerization processes become available. These methods are based on the use of the compounds which reversibly react with propagating radical and convert it to the so-called dormant species . When the equilibrium between the active and dormant species is regulated by special catalysts based on a transition metal, this process is called atom transfer radical polymerization (ATRP). If this equilibrium is provided by stable radicals such as nitroxides, the process is called stable free-radical polymerization (SFRP). In the case when dormant species are formed via a chain transfer rather than reversible termination reactions, this process is referred to as reversible addition fragmentation chain transfer (RAFT) polymerization. All these techniques allow to produce macromolecules of desired architecture and molecular masses. 

With the development of controlled radical polymerization techniques like nitroxide-mediated radical polymerization (NMRP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization (see Section 3.2), the field of linear glycopolymers has significantly flourished, especially as control of molar mass and monomer sequence has become available, even for functionalized monomers. This enables incorporation of new and more complex glycomonomers as well as allows controlled dispersity, end group functionality, and monomer sequences in block, star-shaped, and graft copolymers, and eventually... 

In this review, the term macromer is used to describe oligomer or polymer precursors that undergo reversible association to form supramolecular polymers or networks. Macromer synthesis, although a crucial aspect of supramolecular science, is also out of the scope of this review. Several comprehensive reviews of the synthesis of H-bonding polymers are available [10, 11,42] and primarily describe the application of controlled radical polymerization techniques, including atom-transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP). For synthesis of telechelic polymers, avoiding monofunctional impurities that can act as chain stoppers is crucially important [43],... 

It is probably interesting to note that tmtil now in this chapter, no reference was made to a specific type of controlled/LRP. What would be the difference among the three prevailing techniques, that is, NMP, ATRP, and reversible addition-fragmentation chain-transfer (RAFT)-mediated polymerization Especially for the case of ATRP, there have been reports on reactivity ratios deviating from values for... 

In recent years, focus has moved to the use of living radical polymerization techniques such as atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) for the production of well-defined polymer coatings. Both the grafting from and grafting to approaches have been explored for the attachment of MPC-based polymers onto a variety of substrates. Grafting from requires the incorporation of initiator sites onto the biomaterial surface prior to the polymerization reaction. This usually involves a series of... 

---