Main features of the polymerization process
Mechanism of forming polymers
The polymerization process is a reaction in which a polymer molecule is formed from a low-molecular substance without the discharge of byproducts.
In the polymerization process, the links of a polymer connect, and by its composition the polymer itself does not differ in any way from the original monomers. The elemental composition of the polymer and monomer are absolutely identical.
Polymers, in their turn, can be divided into 2 large groups depending on the number of monomers taking part in polymerization:
- homopolymers (only one type of monomer takes place in formation);
- copolymers (several monomers take part in their formation simultaneously).
An interesting feature of polymers is that unlike various low-molecular substances, polymers do not have a precise melting point, although at high temperatures they gradually soften.
In melting, a polymer passes through 3 stages of different states:
- highly elastic
Polymers can be divided into 3 main groups by electrical conductivity: dielectrics, semiconductors and conductors. The conductivity of dielectrics increases as a polymer is heated to melting point. The heat shrinkage of polymers is around 15 times greater than for any metal. Click here to find out more interesting facts about polymers.
Mechanism of forming polymers
Polymerization usually takes place according to a chain radical mechanism. This mechanism is used in obtaining polymers for industrial purposes. The process consists of several stages. It is easiest to examine the whole mechanism based on the example of the polymerization of ethylene.
Initiation. In this situation radicals strive to attach some substances to themselves, as they have free electrons.
Growth of the chain. Initially the process starts with the monomer attaching a radical particle to itself. As a result, a new radical particle forms, then it once more bonds with the monomer. These transformations may take place many times. At this moment the active center moves to other links in the chain, and there is also a formation of several active centers at once.
Break in the chain. This may take place at any moment. This phenomenon is explained by the fact that radicals interact with one another at this time. At this moment the destruction of active centers takes place.
If the active centers are free radicals, then this polymerization is called radical. If they are located in ions or polarized molecules, then this will be ionic polymerization. As a result, a macromolecule of the substance forms. Its main distinguishing feature is that it consists of the most varied molecules of substances, and not of numerous identical small molecules. This phenomenon is called polydispersity of a polymer. This feature is displayed by all polymers, regardless of the way they are formed.
Main properties of polymers
In examining this topic, it is also important to pay attention to the main functions of polymers, which must be taken into account when working with these substances:
- polymers are usually either in a solid or liquid state. Gaseous states are not characteristic for them;
- polymers are a very durable material. Many organic polymers display durability even in such mediums as alkalis and concentrated acids;
- unlike metals, they are far less subject to corrosion. They display more prominent and lengthy durability to corrosion, both in chemical form and in electric form;
- the greater the molecular mass of a polymer, the worse it dissolves. Large molecules practically do not dissolve in solvents, not even in solvents of an organic nature. Polymers with a spatial structure are barely subject to the impact of organic solvents;
- compared with materials made of iron, polymers have greater plasticity, but they are less strong;
- polymers have the greatest heat capacity and lowest heat conductivity compared with other materials used in industry;
- from lengthy stress, the durability of polymers gradually deteriorates;
- usually the processing of all polymers takes place in the period when they are in a viscous flowing state;
- when polymers are used over a long time, certain chemical and physical changes takes place in them, as a result of which polymers age, i.e. they undergo a partial gradual loss of their main properties. Provoking factors capable of accelerating aging are light, and also frequent change of temperatures;
- to slow down the aging of polymers, stabilizers are usually added to them. This makes the material more durable to the impact of the environment;
- before starting manufacture of polymers, they are tested for various kinds of durability. Only after this can proper manufacture begin.
Here is how to make a polymer using resorcinol and formaldehyde:
In conclusion, polymers are one of the most important materials used in everyday life and industry. Rubber is also a polymer which is used in the most varied spheres of activity. The material is popular because unlike metals, polymers are more durable to the impact of the environment. They are therefore a more universal material for outdoor work, and also for application in places with aggressive weather conditions. The properties of polymers also make it possible to use them as insulation material in places with high voltage.