Propagation in anionic chain-growth polymerization

The propagation reaction consists of successive additions of monomer molecules to the active centers of the growing chains  

No matter what the mechanism of initiation is, the propagation reaction takes place strictly between the monomer and the growing polymeric chain with or without a counterion. 

When the reaction occurs in nonpolar solvents, the propagation step is not hampered as much by a tendency of ion pairs to cluster into aggregates, as is encountered in initiation. For instance, in butyllithium-initiated polymerizations of styrene in benzene, the propagation step is much faster than the initiation. This is probably due to an absence of aggregates. Some association between the growing polymeric chains, however, does occur. It may be shown as follows  

These association equilibria, however, are mobile in character. The driving force in the propagation reaction is similar to that in the initiation. In nonpolar solvents the reaction with the incoming monomers are similar to those in the initiation step. The monomers coordinate with the cations at the end of the chains first. This is followed by intramolecular rearrangements that lead to regenerations of new metal carbon linkages  

In polar solvents, on the other hand, these reactions can go to the other extreme. The propagation can simply consist of successive additions of the monomers to the growing anions. 

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