What is polymerization in organic chemistry?

Polymerization is the process of obtaining a large molecule from one kind of initial monomers. The resulting polymer is a macromolecule that consists of a sufficient number of repeating units.

The main types of polymerization

Answering the question about what a polymerization is in chemistry, we note that this reaction occurs in the presence of a catalyst (process accelerator).

Homopolymer - a high-molecular compound, which is obtained during the combination of the same starting monomers.

Heteropolymer is a reaction product of different starting materials.

Structure of polymer

Given that polymerization is a complex chemical process, the molecular weight of the resulting polymer can have a different index. In high-molecular compounds there are "end groups" that differ from repeating fragments. Such groups are contained in the structure of the polymer in a small amount, so their character is not taken into account when analyzing the chemical and physical properties of the polymer substance obtained.

Cationic Polymerization

What is cationic polymerization? This process under the influence of Lewis acids (a mixture of aluminum chloride, boron fluoride, acids) by a mechanism that is analogous to the process of electrophilic addition with a multiple (double) bond.

At the first stage, the proton is attached to the alkene, resulting in the formation of carbocation.

Further, due to the electronic connection, another alkene molecule enters the interaction, eventually forming a carbocation with a longer chain.

Multiple repetition of the process takes place, as a result of which a carbocation is formed, which has a high molecular weight.

There is a high probability of breaking the cation center, which can be provoked by separation from the proton molecule.

Examples

Let us consider in detail the question of what is polymerization in chemistry. Examples of such a process are given on the basis of polymerization of isobutylene. The process proceeds through the cationic mechanism, since there are alkyl electron-donating groups.

The formation of the chain of polyisobutylene occurs in such a way that the most stable carbocation forms at its end. As a result, there is an orderly joining of "head to tail".

During the chemical interaction, practically without the presence of water, formation of a polymer having a long chain is observed.

What is the polymerization of this representative of the class of unsaturated hydrocarbons? If concentrated sulfuric acid is chosen as the catalyst , a mixture of two dimers is observed after the polymerization. After the two molecules of isobutylene are connected, the chain breaks off. In this case, the intermediate complex (carbocation) gives its proton to the water before it enters into interaction with another molecule of unsaturated alkene.

Anionic polymerization

What is anionic polymerization? This mechanism assumes the polymerization of the alkene at one of the ends of a multiple bond. Polymerization is possible when an alkene molecule is attacked by certain nucleophilic reagents.

For example, the amide anion interacts with acrylonitrate in a double bond, resulting in a carbanion in which a negative charge is localized between the cyanide and the carbon atom. If the complex turns out to be stable, its addition to the next alkene molecule is observed.

The termination of a growing chain is possible in a reaction leading to the elimination of a negative charge at the end of the carbon chain.

Free radical polymerization

The process of ethylene polymerization can proceed in the presence of peroxides (at elevated pressure and high temperature). The breakdown of the chain is facilitated by the process of recombination or disproportionation of radicals.

In the hydrocarbon chain of polyethylene there are several structural units of ethylene. It is this structure that characterizes the valuable properties of this high-molecular compound, contributes to its wide application in the production of a packaging film on an industrial scale. In addition, polyethylene is needed to create products by molding and casting.

In the case of a free radical type of polymerization in a nonsymmetric alkene, the growing end will be represented as a stable radical, and the process is represented by the "head to the tail" type.

Isobutylene and propylene do not polymerize on the free-radical mechanism. All polymers obtained in the chemical industry find their application in various branches of machine building, pharmaceuticals, and medicine. The most common polymers obtained from unsaturated hydrocarbons of the ethylene series are polyethylene itself, as well as its homologue-propylene.