Thiocarbonylthio Compounds (SC(Z)S−R) in Free Radical Polymerization with Reversible Addition-Fragmentation Chain Transfer (RAFT Polymerization). Effect of the Activating Group Z

Free-radical polymerization in the presence of suitable addition−fragmentation chain transfer agents [S C(Z)S−R] (RAFT agents) possess the characteristics of a living polymerization (i.e., polymer products can be reactivated for chain extension and/or block synthesis, molecular weights are predetermined by RAFT agent concentration and conversion, narrow polydispersities are possible). Styrene polymerizations (110 °C, thermal initiation) were performed for two series of RAFT agents [S C(Z)S−CH 2 Ph and S C(Z)S−C(Me) 2 CN]. The chain transfer coefficients decrease in the series where Z is Ph > SCH 2 Ph ∼ SMe ∼ Me ∼ N -pyrrolo ≫ OC 6 F 5 > N -lactam > OC 6 H 5 > O(alkyl) ≫ N(alkyl) 2 (only the first five in this series provide narrow polydispersity polystyrene (< 1.2) in batch polymerization). More generally, chain transfer coefficients decrease in the series dithiobenzoates > trithiocarbonates ∼ dithioalkanoates > dithiocarbonates (xanthates) > dithiocarbamates. However, electron-withdrawing substituents on Z can enhance the activity of RAFT agents to modify the above order. Thus, substituents that render the oxygen or nitrogen lone pair less available for delocalization with the C S can substantially enhance the effectiveness of xanthates or dithiocarbamates, respectively. The trend in relative effectiveness of the RAFT agents is rationalized in terms of interaction of Z with the C S double bond to activate or deactivate that group toward free radical addition. Molecular orbital calculations and the estimated LUMO energies of the RAFT agents can be used in a qualitative manner to predict the effect of the Z substituent on the activity of RAFT agents.