All About Anionic Polymerization

Free radical or ionic polymerization can be used for the carbon-carbon double bond polymerization reaction. The concept differs, whereas a vinyl monomer’s p-bond can react to the activator species in two ways: homolytic or heterolytic bond breakdown. 

Whereas cationic, radical, and anionic initiators are all utilized in chain polymerizations, none of them operate for all monomers. Monomers have varied preference levels when it comes to the point of the reactive center, which will trigger their polymerization. 

Monomers have a strong affinity for ionic initiators. Some monomers will not polymerize when used with cationic initiators, while others will not polymerize when used with anionic initiators. To produce polymers, coordination polymerization necessitates the use of a coordination catalyst.

Characteristics of ionic chain polymerization

•         Ionic polymerizations have a high degree of selectivity.
•         In ionic polymerization, the active core of a propagating chain is accompanied by a counter ion.
•         Ionic polymerizations are very susceptible to the presence of small amounts of contaminants and other ionic reaction products because they occur at such a fast rate.
•         The reaction elements are adequately characterized, and easier to purify anionic systems that seem more repeatable.
•         Because of the stringent conditions for the stability of anionic and cationic propagating molecules, ionic polymerization is selective.
•         In ionic polymerization, the counter ion can impact stereochemistry and polymerization rate.
•         Because the active centers have the same charge, they cannot be terminated by the reaction.

Anionic polymerization

Anionic polymerization is the type of addition polymerization that involves the polymerization of monomers with strong electromagnetic groups triggered by anions of vinyl monomers. Anionic polymerization generates a carbanion when an initiator undergoes nucleophilic additions to a monomer to generate a propagating chain.

The thermoplastic synthetic polydiene rubbers, styrene elastomers, and solution styrene-butadiene rubbers are all made using this polymerization technique. It all starts with a nucleophilic attack on a monomer which results in the formation of a carbanion. In the context of these anions, all vinyl monomers with a vital electronegative substitute hydrogen atoms on the parent chain of a hydrocarbon.

Anionic polymerization mechanism involves when a charge is transferred from an anionic initiator to a vinyl monomer, which becomes reactive. To build a linear polymer, each reactive monomer reacts with other monomers

History of anionic polymerization

Karl Ziegler proposed the first anionic polymerization in 1936. It used styrene and butadiene as monomers sequentially added to an alkyl lithium initiator without being terminated. Living polymerization was widely demonstrated by Szwarc and collaborators roughly 20 years later. One of the critical events in polymer science, according to Szwarc, was electron transfer from the radical anion sodium naphthalene to styrene.

Ion association in anionic polymerization

FUOSS-WINSTEIN ION PAIRS

The extent and kind of active ion pairs generated while polymerization, which are dependent on the characteristics of the solvent and ions, determine the pace of anionic polymerization of vinyl monomers. Anions can associate in one of two ways; claims Fuoss et al.: they can be strongly connected with cations or loosely coupled.

Contact and solvent ion pairs are the two types of ion pairs named by Winstein et al. Marcus proposed a third type of solvated ion pair, known as solvent bridged ion pairs, in which ions share solvent molecules. The three kinds of pairs of ions are in a stable equilibrium with one another and with ions.

The polymerization kinetics will be affected by the various states of carbanions aggregation in polar and nonpolar solvents. The polymerization is often dominated by one of the 3 pairs. The type of carbanion utilized in polymerization and the reaction media used will determine which ion pairs are most prevalent.

Kinetics of anionic polymerization

Non polar monomer in non polar solvent

Because of the numerous distinct forms of ion pairs and aggregate that might form upon initiation and act as centers, the general process of anionic polymerization is challenging. When styrene is polymerized in a nonpolar solvent, the involvement of one type of ion-pair in the rate of polymerization exceeds all others.

Worsfold et.al. were the first to investigate the kinetics of this form of polymerization in 1960. The order of reaction concerning the propagating anion is affected by the initiator or solvent, according to them and others. In hydrocarbon solvents, polystyrene lithium anions are accepted as accurate as nonreactive dimers in equilibrium with energetic contact ion pairs.

Non polar monomer in polar solvent

The process of anionic propagation is influenced by the type of the solvent and monomer as well as the polymerization parameters. Free solvated carbanions, solvent-separated ions pairs, and Contact ion pairs reside in a dynamic balance in a polar organic solvent.Contact ion pairs seem to be the most numerous ion pairs in the polymerization of styrene in THF and they are in balance with free ions. Even though the latter has a low concentration, their proportion of the total rate of propagation can be substantial since unbound carbanions have much higher rate constants than ion pairs.

Polar monomer in polar solvent

Because of their superior solubility, polar monomers like (meth)acrylates are frequently polymerized in polar solvents. The carbanions are accumulated to variable levels in these solvents. Metal ester enolates generate dimeric or tetrameric self-aggregates when methacrylate molecules interact with ion pairs of organometallic initiators. The non-aggregated ion pairs unimers are in a dynamic balance with these aggregates.

Conclusion

When it was initially introduced, the anionic polymerization process caused mayhem in the polymer science sector. It altered people’s perceptions about polymers and the field of polymer research as a whole.