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-CH2Ph and S=C(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 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.