Thiocarbonylthio Compounds (SC(Z)S−R) in Free Radical Polymerization with Reversible Addition-Fragmentation Chain Transfer (RAFT Polymerization). Effect of the Activating Group Z
2003-03-15T00:00:00Z (GMT) by
Free-radical polymerization in the presence of suitable addition−fragmentation chain transfer agents [SC(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 [SC(Z)S−CH2Ph and SC(Z)S−C(Me)2CN]. The chain transfer coefficients decrease in the series where Z is Ph > SCH2Ph ∼ SMe ∼ Me ∼ N-pyrrolo ≫ OC6F5 > N-lactam > OC6H5 > 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 CS 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 CS 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.