Anionic Chain-Growth Polymerization: Mechanism

In the initiation step, a nucleophilic anion such as butyllithium attacks the π bond of a vinylic monomer. This attack generates a carbanion stabilized by the electron-withdrawing group of the monomer. The stabilized carbanion then serves as the reactive species for the propagation step.

During propagation, the carbanion acts as a Michael donor and attacks a second vinylic monomer, which functions as a Michael acceptor. This produces a dimer that becomes a new carbanion. The propagation step repeats continuously, extending the polymer chain with each monomer addition until termination occurs.

Anionic polymerization terminates when a weak acid or an electrophile like carbon dioxide is added to the reaction. These terminating agents react with the stabilized carbanion at the chain end, stopping further monomer addition and stabilizing the final polymer product.

Anionic polymers are called living polymers because in the absence of terminating agents, the stabilized carbanion site at the chain end remains active. The polymerization continues uniformly as long as monomers are available, allowing controlled chain growth and potential for further reactions.

The electron-withdrawing group on the vinylic monomer stabilizes the carbanion intermediate formed during initiation and propagation. This stabilization is crucial for maintaining the reactive carbanion species throughout the polymerization process, enabling efficient chain growth and monomer incorporation into the growing polymer chain.

Anionic polymerization uses nucleophilic anions to initiate the reaction and generate carbanion intermediates, whereas cationic polymerization uses electrophilic cations to generate carbocation intermediates. The two mechanisms operate through opposite charge polarities and require different monomer types, initiators, and terminating agents for successful polymer synthesis.

Vinylic monomers with electron-withdrawing groups are suitable for anionic polymerization because these groups stabilize the carbanion intermediate formed during initiation and propagation. The electron-withdrawing effect facilitates carbanion formation and stabilization, making these monomers highly reactive toward nucleophilic attack and enabling efficient polymerization.