What is polymerization in organic chemistry
Polymerization is a process of obtaining from onetype of initial monomers of a large molecule. 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 polymerization is in chemistry, we note that this reaction occurs in the presence of a catalyst (a 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 complexchemical process, the molecular weight of the resulting polymer may have a different index. In high-molecular compounds there are "terminal groups", which differ from repeating fragments. Such groups are contained in the structure of the polymer in a small amount, so their nature is not taken into account when analyzing the chemical and physical properties of the polymer substance obtained.
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, resulting in the formation of carbocation, which has a longer chain.
Multiple repetition of the process takes place, resulting in the formation of a carbocation with a high molecular weight.
The probability of a cationic center breaking is high, which can be provoked by a detachment from the proton molecule.
Let us consider in more detail the question of what ispolymerization 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-donor 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?class of unsaturated hydrocarbons? If concentrated sulfuric acid is chosen as the catalyst, a mixture of two dimers is observed after the polymerization. After two molecules of isobutylene are joined together, 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.
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 withacrylonitrile over a double bond, resulting in the formation of 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 take place atpresence 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 isseveral structural units of ethylene. It is this structure that characterizes the valuable properties of this high-molecular compound, which 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 byfree radical mechanism. All polymers obtained in the chemical industry find their application in various branches of engineering, pharmaceuticals, and medicine. The most common polymers obtained from unsaturated hydrocarbons of the ethylene series are polyethylene itself, as well as its homologue-propylene.