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This type of polymerization occurs via carbanion intermediates. A monomer in this process must give a stabilized carbanion when it reacts with the anionic end of the growing chain. Monomers selected for this type of polymerization should have at least one electron withdrawing groups like carbonyl group, a cyano group or a nitro group.

This type of polymerization is initiated by bases. Catalysts normally employed for this process includes alkali metals, alkali metal alkyls, alkali metal amides and Grignard reagents. The growth of the chain is started by the conjugate addition of the initiator to the monomer molecule.

A single structural formula sometimes cannot explain all the properties of a compound that is given. In such cases, the compound may be represented by two or more structural formulae which differ from each other only in the arrangement of electrons. None of this structural formula alone can explain all the observed properties of the compound. The compound is then said to show resonance. The various structures are known as resonating structures.

The initiator (butyllithium) adds to the monomer to form an anion (carbanion) in the initiation step.

In the propagation step, the carbanion formed in the initiation step reacts with another monomer molecule and this step is repeated until a polymeric molecule is formed.

Polymerization goes so quickly and easily because the anion that is formed after deprotonation is stabilized by resonance.