Cationic and anionic polymerization

Polymerization refers to the process of joining these monomers to form massive macromolecules of various sizes and forms. In thermodynamic terms, polymerization is usually very advantageous, mostly on energetic grounds, because arranging molecules into linked chains is a process that reduces entropy.

The nature of the catalyst used and the way the chains expand to generate the final product are two ways to look at polymerization. Polymerizations can be carried out in a gaseous, liquid, or solid state to synthesize highly directed macromolecules.

Cationic polymerization 

The Cationic polymerization begins when a cation transfers its ion to a monomer, resulting in the formation of a more reactive species. The monomer utilized here will become exceedingly reactive, reacting with other monomers similar, and therefore all monomers will become reactive.

A fixed monomer can only aid this cationic polymerization chemical reaction. Only alkenes with an electron-donating substituent and the heterocyclic molecule can be employed with the monomers used in the cationic polymerization method.

Monomer unit

Cationic polymerization can only use two types of monomers: alkene and heterocyclic monomers. Cationic polymerization of both types of monomers occurs only if the whole reaction is thermally favorable.

This is due to the isomerization of the monomer double bond in alkenes, the release of monomer ring strain, and, in certain situations, the isomerization of repeating units in heterocycles. Cationic polymerization monomers are nucleophilic and polymerize to generate a stable cation.

Initiators

Initiators used in cationic polymerization reactions include electrophilic substances such as halohydric acids. Lewis acids and other chemicals that can generate carbonium ions can also start polymerization.

Mechanism of polymerization

Initiation:  The initial stage in the cationic polymerization process is initiation, which produces a carbenium ion from a polymer chain. There are many different initiators available for this procedure, and some of them require a co-initiator to generate a certain cation.

Propagation: Because of the stability of the carbocation and steric concerns, most propagation is done through the head to tail addition.

Termination: The propagating chains have the same charge; termination by two propagating chains interacting is not conceivable, unlike in free radical polymerization. Instead, ion-pair rearrangement results in the release of H+ and a terminal C=C bond formation. Even though the terminated chain still retains a double bond that can theoretically continue to polymerize, sterics will limit the reactivity due to the substitution of that terminal carbon.

Anionic polymerization 

When the intermediate anionic species is substantially stable, anionic polymerization is more efficient than cationic polymerization. Nucleophilic binding to the monomer’s double bond initiates these polymerizations. An ion such as hydroxide, alkoxide, cyanide, or carbanion can be used as a nucleophile.

 An organometallic species, such as alkyl lithium or Grignard reagent, can produce the carbanion. Electron transfer, which occurs when alkali metals or related species are the initiators, is another method of initiation. Simple alkenes are not polymerized efficiently by the anionic technique because alkyl groups are donor groups.

Monomer unit

Monomers that can produce stable carbanionic species in polymerization conditions are vulnerable to anionic polymerization. The most researched monomers are styrenic, dienic, and cyclic styrenic. The double bond should have substituents that can stabilize the negative charge by removing a charge from the anions, making them more resistant to nucleophilic assault from other species.

As a result, strong electrophilic groups or proton donating relatively acidic groups, such as amino-, carboxyl-, hydroxyl-, or acetylene functional groups, will interfere with anionic polymerization and must be removed from the monomer unit or protected by an appropriate derivative.

Initiators

In anionic polymerization, nucleophiles; alkoxide, hydroxide ,cyanide can be used as initiators. Also,alkyl lithium or Grignard reagent can produce carbanion. 

Mechanism of polymerization

Anionic polymerization is a chain-growth polymerization in which a charge is transferred from an anionic initiator to a reactive vinyl monomer. To build a linear polymer, each reactive monomer reacts with other monomers. Unless contaminations such as water, alcohol, or carbon dioxide are present or purposely supplied to end the reactions, there is no termination phase in these reactions.

Furthermore, suppose chain transfer is not present. In that case, anionic techniques produce an unusually narrow molecular weight distribution because the rate of polymerization is nearly the same for all active chains, and chain growth of all growing chains begins at roughly the same time when the catalyst is added to the monomer.

When all monomers have been utilized, the polymerization process ends. As a result, this polymerization is frequently referred to as anionic “alive” polymerization. Because each anion commences polymerization, the catalyst’s efficiency is usually quite high, and the active carbanions will not react because of strong ionic repulsion.

Conclusion

Anionic polymerization and cationic polymerization are the different types of chain-growth polymerization reactions that are used to make different kinds of polymers. The reaction mechanism is the same in both of these reactions, but the reaction initiator is different. The solvent employed in both of these polymerization events is important.