21.12:
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.
21.12:
Anionic Chain-Growth Polymerization: Mechanism
The mechanism for anionic chain-growth polymerization involves initiation, propagation, and termination steps. In the initiation step, a nucleophilic anion, such as butyl lithium, initiates the polymerization process by attacking the π bond of the vinylic monomer. As a result, a carbanion, stabilized by the electron‐withdrawing group, is generated. The resulting carbanion acts as a Michael donor in the propagation step and attacks the second vinylic monomer, which acts as a Michael acceptor. Consequently, a dimer is formed that acts as a new carbanion. The propagation step repeats itself, thereby enabling the polymer chain to extend. The polymer chain continues to grow until a weak acid or an electrophile is added to terminate the polymerization process. Interestingly, in the absence of any terminating agents, the propagating chain’s end acts as a stabilized carbanion site, and the polymerization process continues uniformly as long as the monomers are available. Therefore, anionic polymers are often referred to as living polymers.