Anionic polymerization processes variously termed living, controlled, or immortal are used to synthesize polymers having a narrow molecular weight distribution and low polydispersity (1.5). These processes are so named because polymerization generally occurs by addition of monomer units to a constant number of growing polymer chains until all monomer has been consumed; if more monomer is added, polymerization resumes.

Living polymerization is a method by which polymers having a narrow molecular weight distribution may be obtained. Block copolymers may also be synthesized using the method. Block copolymers may display improved mechanical andor chemical properties over corresponding random copolymers. One promising method for free radical polymerization with living characteristics is reversible addition-fragmentation chain transfer (RAFT) polymerization.