The Chemistry of Ionic Chain-Growth Polymerization
Ionic polymerization can be either cationic or anionic. This difference stems from the nature of the carrier ions on the growing polymeric chains. If in the process of growth, the chains carry positive centers, or carbon cations, the mechanism of chain growth is designated as cationic. On the other hand, if the growing chains carry negative ions, or carbanions, then the polymerization is designated as anionic. The two different modes of polymerization can be compared further by examining the two types of initiation. The cationic ones occur by electrophilic attacks of the initiators on the monomers. Conversely, the anionic-ones take place by nucleophilic attacks. The two reactions can be shown as follows:
Generally, the ionic polymerizations depend upon initial formations of positive and negative ions in an organic environment. Propagations in these chain reactions take place through successive additions of monomeric units to the charged or “reactive” terminal groups of the propagating chains [1]. The transition states for the steps of propagation are formed repeatedly in liquid medium systems, containing monomer, initiator, the formed polymer, and frequently a solvent. There are many different types of initiating reactions. These polymerizations, however, never terminate by combination or by disproportionation as they do in free-radical chain-growth polymerizations. Instead, terminations of chain growths are results of unimolecular reactions, or transfers to other molecules, like monomers or solvents, or impurities, like moisture. They can also result from quenching by deliberate additions of reactive terminating species.
