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 non-polar 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 equilibriums, however, are mobile in character [193]. The driving force in the propagation reaction is similar to that in the initiation. In non-polar 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. In homogeneous anionic polymerizations of simple vinyl monomers steric placement is also temperature dependent, just as it is in cationic polymerizations. Syndiotactic placement is favored in polar solvents at low temperature. In non-polar solvents, however, isotactic placement predominates at the same temperatures. Here too, this results mainly from the degree of association with the counterion . Much of our current knowledge of the propagation reaction is based on studies carried out in highly solvating ether solvents. Less information is available about homogeneous reactions in non-polar medium. Generally, though, the rate of propagation increases with solvent polarity and with the degree of ion pair dissociation. Organolithium compounds undergo the greatest degree of solvation when changed from hydrocarbons to polar ether solvents. Cesium compounds, on the other hand, are least affected by changes in solvent polarity. In addition, NMR studies of polystyryl carbanion structures associated with lithium, potassium, and cesium counter cations were studied in different solvents and at different temperatures . The results show an interaction between the larger radius cations and the phenyl rings of the ultimate monomer units in the chains. The structures with potassium and cesium counterions, judging from model compounds, were found to be planar with sp2-hybridized a-carbon. It suggests that in the presence of the larger counterions, rotation of the terminal phenyl ring in styrene polymerization is strongly hindered. Propagation rates also depend upon the structures of the monomers. For polymerizations initiated by alkali amides the following order of reactivity was observed :
Also, methyl substitution on the a-carbon tends to decrease the reaction rate due to the electron releasing effect of the alkyl group. This tends to destabilize the carbanion and also to cause steric interference with solvation of the chain end and with the addition of the monomer .
