Polymerization and polycondensation reactions
Main characteristics and stages
Polymerization and polycondensation reactions are widely used in organic chemistry for the synthesis of new substances. Polymers obtained in the polymerization reaction have enormous significance for the industrial sphere, so the topics of polymerization and polycondensation are especially important.
Polymerization is the process of forming a high-molecular compound (polymer). In the course of the process, low-molecular compounds gradually attach to each other. Initially they start to attach to the active center, which is located at the very start of the growing chain.
Polymerization is often classified according to the following characteristics:
by the number of monomers participating in the process:
- homopolymerization (only 1 monomer participates);
- copolymerization (several monomers participate);
by the nature of the active center:
- radical (active center formed by a free radical);
- ionic (active centers formed by ions).
The polymerization process also has the following main characteristics:
- usually only substances with multiple bonds enter into polymerization;
- the process often takes place with the release of heat in the course of reactions;
- polymerization can take place with substances which may open cyclic groups;
- the polymerization process may be carried out by various methods depending on the state of the substance (in a solution, casting polymerization, gaseous or solid polymer, in a liquid medium).
Usually polymerization takes place in several successive stages:
- Initiation. At this stage, certain parts of the molecule (monomers) turn into active centers. This is possible under the impact of specially introduced substances (catalysts). At this moment, we may observe the release of light, heat, electric current and energy in the process of polymerization.
- Growth of the chain. The gradual successive attachment of monomers to the active center of the chain takes place.
- Break of the chain. Takes place from the destruction of the active center of the macromolecule. This is possible because of the collision of two active centers.
It is also possible for the active center to shift to another particle of the molecule (monomer or solvent). In this case, the growth of a new chain begins (splitting).
If the first two stages are always present in the course of any polymerization process, a break in the chain or the shift of its active center may not even take place at all in some cases.
Interesting demonstration of polymerization:
Polycondensation is the process of forming polymers by the combination of different monomers. The process is frequently accompanied by the release of various subsidiary low-molecular products (water, alcohol, salt).
For polycondensation, the following monomers are characteristic: compounds with molecules of at least 2 functional groups. They are usually divided for convenience into three groups:
- identical functional groups which do not react among each other (diamines)
- different functional groups which may react among each other and thus form polymers (amino acids);
- identical functional groups which may react among each other, forming simple polyethers;
In this process, reactions of functional groups of monomers are sometimes possible not only with other groups, but among each other. This explains why so many polymers can be formed.
Polycondensation is a process of several successive stages. Monomers are used up relatively quickly (at the early stage of reaction). After this, a high-molecular polymer is formed from other oligomers which were previously formed by functional groups. In this process, different exchange reactions are possible. In the polycondensation process, numerous polymers are formed which take part in metabolism and various biological processes in the human body.
It may seem that polymerization and polycondensation are very similar reactions. In fact, their only similarity lies in the synthesis of polymers. In polycondensation, besides the main groups (polymers), secondary substances are also formed – low-molecular compounds (water, alcohol, ammonia).
If in the polymerization process, the composition of the monomer and polymer is identical, in the polycondensation process the composition of the final macromolecule may significantly differ from the original composition of the monomer.
Importance of indicators
Polymerization and polycondensation have a direct influence on the quality of the materials that are manufactured in these processes.
The durability of the material depends directly on the average degree of polymerization. The higher the degree of polymerization, the more viscous, durable and long-lasting the material can be obtained. The flexibility of the raw material is reduced. But if truly dense polymers are required, the temperature of the process must be increased, in order to evaporate the solvent (during polymerization in a solution) and raise the concentration of monomers. This indicator is decisive in choosing raw materials for the manufacture of final products. For example, if the polymerization degree of cellulose is less than 100, this material is not considered suitable for further use.
Here you’ll find more articles about polymers.
Polycondensation also has great importance for the synthesis of polymers. Besides the synthesis of the polymers themselves, subsidiary products may also be obtained, or in other words, monomers may be purified of unnecessary additives to obtain a more concentrated polymer.
Polymerization in industry is used much more frequently. This is the method used to obtain 75% of synthetic polymers manufactured worldwide (polystyrene, polyvinyl chloride).