Living free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization): Approaches to star polymers

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.