Anionic Chain-Growth Polymerization: Mechanism

The anionic polymerization mechanism involves three steps: initiation, propagation, and termination.

In the initiation step, a nucleophilic anion, such as butyllithium, adds to the π bond of a vinylic monomer to generate a carbanion stabilized by the electron‐withdrawing group of the monomer.

In the propagation step, the resulting carbanion acts as a Michael donor. It adds to a second vinylic monomer, a Michael acceptor, to give a dimer that acts as a new carbanion.

The propagation step continues until the reaction is actually terminated by adding a weak acid or an electrophile like carbon dioxide.

In the absence of any terminating agents, the end of each propagating chain will have a stabilized carbanion site, which can continue to polymerize as long as the monomers are available. Therefore, anionic polymers are also called living polymers.