What is Anionic Polymerization? An In-Depth Analysis

What is Anionic Polymerization?

What is Anionic Polymerization?

Anionic polymerization is a form of addition polymerization or chain-growth polymerization that includes the polymerization of monomers initiated with anions of vinyl monomers with strong electromagnetic groups. This polymerization process is mainly utilized to produce thermoplastic styrene elastomers, synthetic polydiene rubbers, and solution styrene-butadiene rubbers (SBR).

It all begins with a nucleophilic attack on a monomer, making a carbanion. Primarily, all vinyl monomers with sturdy electronegative replace (one or more) hydrogen atoms on the parent chain of a hydrocarbon (also called substituents) in the presence of these anions.

Some electron-replacing substituents equilibrate the negative charge using charge delocalization, making the anionic polymerization stable and smoother – -C6H5, -CH=CH2, and CN are good examples for the same.

That’s why aldehydes, epoxides, dienes, styrenes, acrylates, and methacrylates easily go through anionic polymerization reactions.

The initiators are either strong anions or electron transfer agents. The shifting of an electron from an initiator to a vinyl monomer triggers the shaping of an anion radical, better known as carbonian.

Typical Initiators – 

Typical initiators are alkali metals like lithium and sodium. Other notable initiators are amides, amines, hydroxides, phosphines, covalent, alkoxides, and organometallic compounds such as alkyl lithium compounds and Grignard reagents.

The initiation is started by adding either neutral (B:) or negative (B:-) nucleophile to the monomer. Below is an example of the initiation of styrene with potassium amide:

KNH2 ⇔ K+ + NH2- NH2- + M → NH2M- NH2Mn- + M → NH2Mn+1- NH2Mn- + NH3 → NH2MnH + NH2-

Intersting Read – What is Condensation Polymerization? | The Definitive Guide

Important Consequences –

An anionic polymerization under Inert solvents and pure reactors don’t go through termination reactions. Thus, the chains will remain active for good unless there is an external termination or chain transfer. That has two consequences, and they are a follows:

  1. The average molecular weight can be calculated by analyzing the amount of initiator and consumed monomer because polymerization is highly dependent on the ratio of moles of monomer and initiators. The equation – MWn = MW0 [M0] / [I]. MW0 is for the molecular weight of the repeat unit, and [M0] and [I] are for the initial molar concentrations of both monomers and initiators.
  2. As all the chains are initiated roughly simultaneously, polymer synthesis can be done in a more sophisticated manner. In point of fact, it is the sole process that leads to a well-described and nearly mono-disperse molecular weight distribution and structural integrity.

The type of reaction has many manifestations, but with vinyl monomers, it is called living polymerization.

What is Living Anionic Polymerization?

Living anionic polymerization is a living polymerization mechanism entailing an anionic propagating species. Szwarc and co-workers first exhibited it in 1956. The initial work was completely dependent on the polymerization of styrene and dienes. One of the standout features of living polymerization is that it doesn’t involve any termination step.

The following criteria in living polymerization concluded 4 things:

  1. Polymerization until the monomer is completely consumed and will further monomer is added.
  2. The mono-disperse molecular weight distribution of molecular weight
  3. The constant number of propagating species.
  4. Chain end functionalization can be carried out finitely.

The Outcome of Living Polymerization – 

The two main outcomes are Block copolymers and End group termination/functionalization.

Block Copolymer: 

Block polymer synthesis is one of the most crucial applications of living polymerization, offering an incredible level of control over its structure. The nucleophilicity of the consequential carbanion will determine the order of monomer addition; as the monomer forms, the less nucleophilic propagating species may show the addition of the more nucleophilic monomer becoming a chain.

End Group Termination/Functionalization:

As mentioned earlier, one of the more amazing features of living anionic living polymerization is the absence of an organic termination step. Without impurities, the carbanion will remain active, anticipating the addition of a new monomer. However, unintentional termination may occur by quenching impurities such as trace amounts of oxygen, carbon dioxide, or water.

A substitute approach towards operating end-groups is to start polymerization with a functional anionic initiator.

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 Interesting History of Anionic Polymerization – 

The first anionic polymerization was proposed in 1936 by Karl Ziegler. It utilized styrene and butadiene by consecutive addition of monomer to an alkyl lithium initiator occurred without termination. Roughly 20 years later, living polymerization got widespread demonstration by Szwarc and coworkers. Szwarc explained that electron transfer occurred from radical anion sodium naphthalene to styrene in of the significant events in polymer science.

The outcome was forming an organosodium species, which rapidly added styrene to form a “two–ended living polymer.” A crucial part of the work was aprotic solvent tetrahydrofuran-associated kinetics and the thermodynamics of the process in great detail.

He also explored and explained the structure-property relationship of several ion pairs and radical ions involved. The work proved to be the base of polymer synthesis with more control over molecular weight, weight distribution, and compositional integrity.

FAQs – 

1. Which catalyst is used in anionic polymerization?

Ans. For styrene polymerization, sodium naphthalene processing by electron transfer. It ensured a defined reaction and created a form of naphthalene radical anion to the monomer.

2. What are the two types of polymerization?

Ans. Generally, there are two types of polymerization reactions – addition polymerization and condensation polymerization. In addition to polymerization, the polymers are formed with monomer with no by-products; on the other hand, in condemnation copolymerization, the polymers are formed due to the condemnation of more than one different monomer resulting in the creation of small molecules. such as HCl, water, ammonia, etc

3. Why is anionic polymerization called living polymerization?

Ans. Anionic polymerization involves living anionic addition polymerization of vinyl monomers and living anionic ring-opening polymerization of cyclic monomers. And since the propagating chains maintain their reactivity and are still “living,” the resulting polymer and the polymerizing system is called “living polymer.” and “living polymerization,” respectively.

4. Which monomers will undergo anionic polymerization?

Ans. The monomers suitable for anionic polymerization include vinyl monomers with electron-withdrawing groups. Prime examples are methyl methacrylate, acrylonitrile, and hydrocarbon monomers like styrene and 1,3-butadiene.

5. Why does styrene undergoes anionic polymerization easily?

Ans. It occurs with monomers having electrons withdrawing groups such as nitrile, phenyl, carboxyl, and vinyl. These polymerizations are started by nucleophilic addition and through the formation of carbanion.

Suggested Read – 

Final Thoughts – 

That was all my insights on anionic polymerization and all related attributes. The anionic polymerization technique created havoc in the polymer science field when it was first introduced. It changed the way people started looking at polymers and the area of polymer science itself.

A hearty thanks for reading, Kindly share your reviews in the comment box.

Have an incredible day.

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