Reversible addition-fragmentation synthesis

In 1988 a paper by Zard and coworkers4(, reported that xanlhates were a convenient source of alkyl radicals by reversible addition-fragmentation and used the chemistry for the synthesis of a monoadduct to monomer (a maleimide). Many applications of the chemistry in organic synthesis have now been described in papers and reviews by the Zard group.406 407... 

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

Scheme 3 Synthesis of surface-immobilized diblock copolymer brush (Si/Si02//PS-fc-PDMA) using reverse addition fragmentation transfer polymerization...

And not only for organic synthesis the reversible addition fragmentation to the thiocarbonylthio motif found in xanthates, dithiocarbamates, dithioesters, trithiocarbonates etc., discussed in Scheme 2 for the particular case of xanthates, is now being actively exploited for the synthesis of bloc polymers. For a recent review, see [75] for the original patents on MADIX and RAFT, see [76,77]. The principle of this approach is summarised in Scheme 38 for the synthesis of a diblock polymer 66. The RAFT and MADIX processes, as they are now called, are set to revolutionise the crafting of polymers with well-defined architectures. It is an extremely effective technique that can be applied to essentially all commercial monomers and is tolerant of many functional groups. Scientific papers and patents on the subject now number in the hundreds. 

Figure 13.3 Overview showing the use of bifunctional initiators for block-copolymer synthesis. Two examples using either reversible addition fragmentation chain transfer polymerization or atom transfer radical polymerization combined with eROP are shown.

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

Reversible addition-fragmentation chain transfer (RAFT) polymerization has proven to be a powerful tool for the synthesis of polymers with predetermined molecular weight and low polydispersity [11, 12], In recent years, synthesis of polymers with complex molecular architecture, e.g. block and star copolymers, via the RAFT process have been reported [13],... 

STENZEL-ROSENBAUM M., DAVIS T.P., CHEN V., FANE A.G., Star-polymer synthesis via radical reversible addition-fragmentation chain-transfer polymerization. J. Polym Sci, Part A Polym Chem. (2001), 39 (9), 1353-65. 

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],... 

The development of controlled radical polymerization (CRP) methods,(1,2) including atom transfer radical polymerization (ATRP),(3-6) nitroxide-mediated radical polymerization,(7) and reversible addition fragmentation chain transfer polymerization,(8,9) has led to the synthesis of an unprecedented number of novel, previously inaccessible polymeric materials. Well-defined polymers, i.e., polymers with predetermined molecular weight, narrow molecular weight distribution, and high degree of chain end functionalization, prepared by... 

It is of obvious interest to explore the use of other polymerization techniques that, being more tolerant to the experimental conditions and monomers, can produce amphiphilie azobenzene BCPs with no need for post reactions. Notably, Su et al. have reeently reported the synthesis of such an amphiphilic diblock copolymer with PAA as the hydrophilic block using reversible addition-fragmentation transfer (RAFT) polymerization (structure d in Fig. 6.2) (Su et al., 2007). Using RAFT, they prepared PAA capped with dithiobenzoate and used it as the macro-RAFT transfer agent to polymerize the hydrophobic azobenzene polymer successfully. It ean be expected that more amphiphilic azobenzene BCPs will be synthesized using the eontrolled radical polymerization techniques (ATRP and RAFT) because of their simplicity, versatility, and efficiency. 

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. 

Stenzel, M.H. Davis, T.P. Star polymer synthesis using trithiocarbonate functional P-cyclodextrin cores (reversible addition-fragmentation chain-transfer polymerization). J. Polym. Sci. A 2002,40 (24), 4498-4512. 

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. 

Lowe AB, McCormick CL (2007) Reversible addition-fragmentation chain transfer (RAFT) radical polymerization and the synthesis of water-soluble (co)polymers under homogeneous conditions in organic and aqueous media. Prog Polym Sci 32 283-351... 

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],... 

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