Understanding anionic polymerization initiators

Anionic addition polymerization is a type of chain-growth polymerization in which vinyl monomers with substantial electronegative groups are polymerized. Synthetic polydiene rubbers, thermoplastic styrenic elastomers, and solution styrene-butadiene rubbers are all made using this method of polymerization.

Chain-growth procedures wherein the active center to which consecutive monomers are introduced is a negative ion connected with a positive counterion.

Anionic addition polymerization initiators

An initiator starts the anionic addition  polymerization process. An anion, an ion with a negative electrical charge, initiates the anionic polymerization process. In anionic vinyl polymerization, a variety of initiators are utilized, but the most common is butyl lithium, a seemingly innocuous chemical.

Types of anionic addition polymerization initiators

Nucleophilic initiators

To initiate anionic addition polymerization, many nucleophilic essential initiators are being used. Alkoxides, cyanides, amines, phosphines, and organometallic compounds like n-C4H9Li and fMgBr are examples of covalent or ionic metal amides. 

The introduction of a neutral or negative nucleophile base to a monomer is the first step in the process. The most effective of such initiators are alkyl lithium compounds, which are used in the polymerizations commercially. Adding the alkyl to the monomer initiates the reaction.

Alkyl derivatives

Anionic addition polymerization has also been initiated using alkyl derivatives of alkaline-earth metals. Organomagnesium compounds have a lower activity than organolithium compounds due to a less polarized metal-carbon link. They can only polymerize more active monomers than styrene and 1,3-dienes, as 2- and 4-vinyl pyridines and acrylic and methacrylic esters. 

Organostrontium and organobarium compounds may polymerize styrene and 1,3-dienes along with the more active monomers because they have more polar metal-carbon linkages.

Neutral nucleophiles

The hypothesized propagation component in the rare anionic polymerizations triggered by neutral nucleophiles such as tertiary amines or phosphines is a zwitterion. Primary and secondary amines are ineffective in comparison to tertiary amines. The zwitterion transmitting species has the obvious flaw of growing electrostatic repulsion as transmission progresses.

 The positive terminal of one zwitterion propagation chain may operate as the counterion of the carbanion end of some other zwitterion propagating chain to stabilize zwitterion species. If initiation is postulated as occurring via the hydroxide ion generated by the amine interaction with adventitious water, the use of a zwitterion spreading species is eliminated.

Nucleophilic attack

The amount of initiator needed to polymerize monomers is determined by its response to nucleophilic attack. The capability to hold the carbanion charge enhances the monomer response. To polymerize monomers with significantly weaker electron-withdrawing substituents, powerful nucleophiles are required.

 Relatively weak nucleophiles, such as hydroxide ions, can polymerize monomers with highly electron-withdrawing substituents like acrylonitrile, and methyl vinyl ketone, however, at a reduced efficiency than stronger nucleophiles. Very weak nucleophiles can polymerize a monomer with two electron-withdrawing substituents, such as methyl-a-cyanoacrylate.

Electron transfer

Aromatic radical anions such as sodium naphthalene generate fascinating and beneficial polymerizations. The effective initiator, the naphthalene radical–anion, is formed prior to the start of the reaction. An electron is shifted from the alkali metal to naphthalene in this reaction. The carbanion origin of the anion–radical has been shown by their reactivity with carbon dioxide to generate the carboxylic acid derivative. 

The radical character of the anion–radical has been established by electron spin resonance spectroscopy. The equilibrium is determined by the hydrocarbon’s binding energy and the solvent’s donor characteristics. Biphenyl is less beneficial than naphthalene because it has a lower radical equilibrium than naphthalene. Even though it generates the anion–radical quickly, anthracene is less helpful.

Alkaline earth metals

A more polar solvent than THF, such as hexamethylphosphoramide, is required for the less electropositive alkaline-earth metals. The styryl radical–anion is formed when the naphthalene anion–radical, which is tinted greenish-blue, transmits an electron to a monomer such as a styrene.

 The anion and radical centers are alternating on the alpha and beta carbon atoms in the styryl radical–anion, which is represented as a resonance hybrid of the forms. The dicarbanion is formed when the styryl radical–anion dimerizes. Electron spin resonance studies, which demonstrate the full elimination of radicals in the solution followed by monomer addition, demonstrate that this reaction occurs.

Radical anions

It is often used to initiate electron transfer from radical anions generated by the reaction of sodium with non enolizable ketones, azomethine, nitriles, azo, and azoxy chemicals. When such alkali metals are used in liquid ammonia, initiation through electron transfer has been demonstrated in addition to radical–anions.

 Alkali metal-induced polymerizations in liquid ammonia follow two distinct pathways. The amide ion generated in some processes, such as the polymerization of styrene and methacrylonitrile by potassium, initiates the polymerization. Methacrylonitrile polymerization is similar to that of Grignard reagents.

These polymerizations are similar to alkali amide polymerizations. The amide ion cannot cause polymerization in plenty of other systems. Thus, lithium initiates the polymerization of methacrylonitrile in liquid ammonia at a significantly quicker pace than lithium amide initiates the polymerization of methacrylonitrile in liquid ammonia.

Ionizing radiations

Ionizing radiation causes electron transfer, which leads to initiation. The solvent or the monomer in the reaction system initiates radiolysis to produce a cation–radical and solvated electron. If an electron-withdrawing compound is included in the monomer, polymerization happens via adding the electron to the monomer, dimerization to the dicarbanion, and transmission.

Conclusion

Although initiation in specific systems may entail creating organic anions by electrolytic reaction of some constituent of the reaction system, efficient electron addition to the monomer produces the radical monomer.