RAFT polymerization transfer coefficients

N-dialkyl dithiocarbamates, electron-withdrawing substituents on Z can enhance the activity of RAFT agents so that they are more effective in polymerization of styrenic and (meth) acrylic monomers. Thus transfer coefficients of the 0-aryl xanthates are higher than those of simple 0-alkyl xanthates. Those of fluorinated xanthates (158 and 159) are higher than those of nonfluorinated analogs (160 and 166, respectively). 

The presence of withdrawing groups on Z, which lead to higher transfer coefficients, increases the likelihood of side reactions such as hydrolysis or aminolysis and participation in cycloaddition reactions such as the hetero-Diels Alder reaction with diene monomers and 1,3-dipolar cyclo-addition. This is an important consideration in some RAFT agent syntheses, can be critical to the choice of RAFT agent for specific polymerization conditions e.g., in aqueous media or in emulsion polymerization), and determines the ease of end group transformation processes that may be required post-RAFT polymerization. 

Temperatures reported for RAFT polymerization range from ambient to 180 There is evidenee with that retardation, when observed, is less at higher temperatures. There is also some data that show lower dispersities can be achieved at higher temperatures. This is eonsistent with the rate coefficients for fragmentation of the RAFT intermediates and transfer eonstants of RAFT agents all inereasing with inereasing reaetion temperature. 

RAFT agent. The growing radicals produced from the fragmentation of the RAFT agent exit the particles and reenter into the continuous phase to form new particles before the precipitation of the existing particles, thus increased the exit rate coefficient with RAFT concentration. This induces the retardation of the polymerization due to the transfer of the RAFT agent to the particles and so the particle size decreases with the RAFT concentration. In the emulsion polymerization of styrene, the particle diameter decreased and the size distribution became narrower with the RAFT concentration. However, a partial... 

Benzyl thionobenzoate (58) is believed to be ineffective as a transfer agent in MMA polymerization because of an unfavorable partition coefficient. Poly(methyl methacrylate) radical (PMMA ) is a much better radical leaving group than benzyl radical. Analogous benzyl thiocarbonylthio compounds (e.g., benzyl dithiobenzoate or dibenzyl trithiocarbonate) are also ineffective as RAFT agents in MMA polymerization. 

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