Polymerization is the process of obtaining a large molecule from one type of starting monomer. The resulting polymer is a macromolecule, which consists of a sufficient number of repeating units.
The main types of polymerization
Answering the question of what polymerization is in chemistry, we note that this reaction proceeds in the presence of a catalyst (process accelerator).
A homopolymer is a high molecular weight compound that is obtained by combining the same starting monomers.
A heteropolymer is a reaction product of the interaction of various starting materials.
Polymer structure
Given that polymerization is a complex chemical process, the molecular weight of the resulting polymer may have a different indicator. In high molecular weight compounds, there are “end groups” that differ from repeating fragments. Such groups are contained in the polymer structure in an insignificant amount; therefore, their nature is not taken into account when analyzing the chemical and physical properties of the obtained polymer substance.
Cationic polymerization
What is the polymerization of a cationic species? This process proceeds under the influence of Lewis acids (a mixture of aluminum chloride, boron fluoride, acids) by a mechanism that is similar to the process of electrophilic addition via multiple (double) bonds.
At the first stage, the proton joins the alkene, as a result of which a carbocation is formed.
Further, due to electronic communication, another alkene molecule enters into the interaction, as a result, a carbocation with a longer chain is formed.
A multiple repetition of the process occurs, resulting in the formation of a carbocation having a high molecular weight.
There is a high probability of termination of the cationic center, which can be triggered by separation from the proton molecule.
Examples
Let us consider in more detail the question of what polymerization is in chemistry. Examples of such a process are based on the polymerization of isobutylene. The process proceeds according to the cationic mechanism, since alkyl electrodonor groups are present.
The polyisobutylene chain is formed so that a maximally stable carbocation is formed at its end. As a result, there is an ordered attachment “head to tail”.
During the chemical interaction, practically without the presence of water, the formation of a polymer having a long chain is observed.
What is the polymerization of a given member of the unsaturated hydrocarbon class ? If concentrated sulfuric acid is selected as the catalyst, the formation of a mixture of two dimers is observed after polymerization. After two isobutylene molecules are interconnected, the chain breaks. In this case, an intermediate complex (carbocation) gives its proton to water before it interacts with another unsaturated alkene molecule.
Anionic Polymerization
What is anionic type polymerization? This mechanism involves the polymerization of alkene at one end of a multiple bond. Polymerization is possible by attacking an alkene molecule with certain nucleophilic reagents.
For example, the amide anion interacts with acrylonitrate in a double bond, resulting in the formation of a carbanion in which a negative charge is localized between the cyanide and the carbon atom. If the complex is stable, its attachment to the next alkene molecule is observed.
An open chain termination is possible in a reaction leading to the elimination of a negative charge at the end of the carbon chain.
Free radical polymerization
The ethylene polymerization process can occur in the presence of peroxides (at elevated pressure and high temperature). The chain termination 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 macromolecular compound, contributes to its widespread use in the manufacture of industrial packaging film. In addition, polyethylene is necessary to create products by molding and molding.
In the case of a free-radical type of polymerization in an asymmetric alkene, the growing end will be represented as a stable radical, and the process will be presented in a head-to-tail fashion.
Isobutylene and propylene do not have polymerization by the free radical mechanism. All polymers obtained in the chemical industry find their application in various branches of mechanical engineering, pharmaceuticals, and medicine. The most common polymers derived from unsaturated hydrocarbons of a number of ethylene are polyethylene itself, as well as its homologue, propylene.