RAFT polymerization agent synthesis

RAFT polymerization of two anionic acrylamido monomers sodium 2-acrylamido-2-methylpropane-sulfonate, AMPS, and sodium 3-acrylamido-3-methyl-butanoate, AMBA, (Scheme 29) was conducted in water at 70 °C using 4,4/-azobis(4-cyanopentanoic acid) as the initiator and 4-cyanopentanoic acid dithiobenzoate as the RAFT chain transfer agent [80]. The synthesis was initiated either from one monomer or the other leading to narrow molecular weight distributions in both cases (Mw/Mn < 1.2). 

CRP is a powerful tool for the synthesis of both polymers with narrow molecular weight distribution and of block copolymers. In aqueous systems, besides ATRP, the RAFT method in particular has been used successfully. A mrmber of uncharged, anionic, cationic, and zwitterionic monomers could be polymerized and several amphiphilic block copolymers were prepared from these monomers [150,153]. The success of a RAFT polymerization depends mainly on the chain transfer agent (CTA) involved. A key question is the hydrolytic stability of the terminal thiocarbonyl functionaHty of the growing polymers. Here, remarkable progress could be achieved by the synthesis of several new dithiobenzoates [150-152]. 

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. 

Semsarilar, M.L. Vincent-Perrier, S. Synthesis of a cellulose supported chain transfer agent and its application to RAFT polymerization. J. Polym. Sci. A 2010,48 (19), 4361-4365. 

The first example of nanogel synthesis by direct RAFT polymerization under precipitation/dis-persion polymerization condition was reported by An et al. in 2007 (Figure 54.27). Two types of poly(A,A -dimethylacrylamide)s (PDMAs) bearing a trithiocarbonate group were first synthesized by RAFT solution polymerization and were subsequently used as both stabilizers and RAFT agents for nanogel synthesis by RAFT precipitation/dispersion polymerization. These two types of... 

Houillot, L. Bui, C. Save, M. Charleux, B. Farcet, C. Moire, C. Raust, J.A. Rodriguez, I. Synthesis of well-defined polyacrylate particle dispersions in organic medium using simultaneous RAFT polymerization and self-assembly of block copolymers. A strong influence of the selected thiocarbonylthio chain transfer agent. Macromolecules 2007, 40 (18), 6500-6509. 

Finally, the synthesis of a BAB triblock copolymer, where A is a PDMA block and B is a PNIPAM block, has been realized via RAFT polymerization using a symmetrical bistrithiocarbonate as the bifimctional chain transfer agent for the polymerization of the middle block [40]. The polydispersity indices for the synthesized triblocks, determined by SEC, were in the range 1.19-1.31. The high Mw/M values could be also correlated with the possible interaction of the copolymers with the column material, as well as to the association of block copolymer chains in the carrier solvent. A series of copolymers with an identical middle block and outer blocks with different molecular weight have been synthesized. However, the presence of diblocks or unreacted homopolymers in the final product could not be excluded. 

Finally, the synthesis of multiblock DHBCs has been also reported. The desired multiblocks have been obtained via RAFT polymerization, using polytrithiocarbonate as the chain transfer agent [50]. The synthesis of two mulhblocks with different molecular characteristics was achieved. Both block copolymers were consisted of PDMA and PNIPAM sequences. The molecular characteristics of the synthesized macromolecules were studied... 

While not related exclusively to block copolymer synthesis, the formation of many of the more complex architectures available through RAFT polymerization - including those based on a single monomer - shares the characteristics and caveats of linear block copolymer formation. One technique to obtain such structures (aldn to the triblock synthesis mentioned above) is the use of higher-level, multifunctional RAFT agents. A synthetic approach with a multifunctional core or a RAFT agent-functionalized polymer backbone allows... 

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