Multifunctional transfer agents

The most efficient transfer agents are the mercaptans. Practially every —SH group is used for transfer, and does not retard polymerization [4]. Molecules with several mercaptan groups are copolymerizing cross-linking agents of a well-defined reaction mode [25] 

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Multifunctional transfer agents are of no practical importance. They can, however, contribute significantly to our knowledge of transfer to polymer. 

Each polymer is really a potential multifunctional transfer agent. It depends only on the reactivity of the growing radicals or ions under the given conditions as to how often it will be attacked. Transfer to polymer has many variants. In principle, the reaction is always of the type ... 

Cross-linking can occur in free-radical polymerizations because of chain transfer to polymer (Section 6.8.4) or monomer (Section 6.8.2), as well as the presence of multifunctional monomers. When it is desireable to retard or suppress crosslinking, chain transfer agents are added to the polymerization mixture. Ek]uation (7-67) shows why this is helpful. 

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],... 

In this case, the chains are grown away from the core, and attached when undergoing transfer reactions hence, stars only contain branches in a dormant form. One advantage of this technique is that complications such as star-star couplings, as encountered in the core-first star synthesis, can be avoided. A potential problem, however, is the reduced accessibility to the RAFT moieties of the core by polymeric arms (shielding effect). Some typical multifunctional RAFT agents used in the Z-approach are shown in Figure 27.4. 

It may be noted that many of the ingredients are multifunctional. For example the soap serves to emulsify the monomers, solubilize the monomers in the micelles and to stabilize the latex particles formed. The mercaptan is not only a chain transfer agent but also a promoter of polymerization in the case of the hot rubber. 

RAFT polymerization is unique among the most prominent RDRP techniques because the inherently bifunctional TTC group can be employed in RAFT-type chain transfer agents. As a result of this bifunctional nature, the TTC moiety can be easily incorporated into the backbone of linear multifunctional RAFT agents [100, 101] or cyclic RAFT agents [102], both of which permit the formation of multiblock polymers with narrowly distributed block lengths. In such systems, the RAFT mechanism causes a continuous redistribution of all blocks and RAFT... 

Jesberger, M., Earner, L. Stenzel, M.H. Malmstrom, E., and Davis, T.P., Bamer-Kowollik, C. 2003. Hyperbranched polymers as scaffolds for multifunctional reversible addition fiagmentation chain-transfer agents A route to polystyrene-core-polyesters and polystyrene-block-poly(butyl acrylate)-core-polyesters. J. Polym. Sci. Part A Polym. Chem. 41 3847-61. 

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