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Free-radical polymerization in the presence of suitable addition−fragmentation chain transfer agents [S&dbd;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&dbd;C(Z)S−CH2Ph and S&dbd;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&dbd;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&dbd;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.

The synthesis of multiarm star polymers by radical polymerization with reversible addition−fragmentation chain transfer (RAFT) is described. When the precursor RAFT agents to star polymers are trithiocarbonate derived, fragmentation of the radical intermediate can lead to different products depending on the leaving abilities of the two groups attached to sulfur. To demonstrate this, two types of RAFT agents, ones that allow growth of arms away from the core and the other attached to the core during propagation, were designed, and an example of each was synthesized. While both star RAFT agents gave excellent molecular weight control and low polydispersities (typically <1.2), the one that grows its arms away from the core offers polymers free from star−star coupled products. Thus, the direction of fragmentation should be considered when designing efficient star RAFT agents. The living nature of the arms of these polymers was demonstrated by extending the arms with a different monomer to afford a star block copolymer. The RAFT agents described are easily synthesized from commercially available reagents by a simple experimental procedure.

Living radical polymerization with reversible addition−fragmentation chain transfer (RAFT polymerization) can be achieved with the use of dithiocarbamate derivatives that have the nonbonded electron pair of the nitrogen included as part of an aromatic system. These compounds have been shown to be highly effective in RAFT polymerization of styrene and (meth)acrylate esters to produce polymers of predetermined molecular weight and narrow polydispersity (usually <1.2). By contrast, simple N,N-dialkyl dithiocarbamates (those compounds previously described as “photoiniferters”) are ineffective as RAFT agents. The reasons for the different behavior are discussed.