In the organic chemistry world, with diversity of chemicals, substances divided into different classes. These substances are polymers. The word "polymers" is Greek and means poly - many, meros – part, that is, organic substances composed of many parts (hundreds, thousands, tens of thousands of parts). Of course, polymers characterized with large and very large molecular mass, that gave them another pervasive name high molecular weight substances. On the pages of the website Entertaining chemistry You can find detailed descriptions about these classes of organic substances: proteins, and polysaccharides, and nucleic acids, and also polyethylene, caoutchouc, plastic etc. The polymers are composed of individual monomers, as already noted, individual component parts. These parts are remnants of molecules of other substances. The number of these residual molecules in the polymer molecule can be huge (from hundreds to several tens of thousands). This chain is the links of the polymer. Degree of polymerization is determined with the number of such links (the parts of the molecules included in the polymer). Many links and a variety of polymers, of course, cannot have the same spatial structure (or the same form of molecules). If, for example, to take one part (or one link for the letter "M", it is possible to distinguish between such forms: linear form ... —̀—̀—̀—̀—̀—̀— ... , in which each link has a connection with 2 adjacent links;
another form – branched, when the molecules of the chain are linked with other molecules (formed triple or more links);
another form - grid, that represents the structure of the polymer in the form of a grid
Among the variety of polymers there are also the so-called asymmetric polymers (CH2=CH—X, where X is any other part of another molecule), which include regular and irregular polymers, which differ with the way of alternating units, for example
All polymers can be divided into artificially got, as synthetic polymers, which are synthesized with directed chemical reactions with use oil and gas and other minerals. Synthetic polymers include, for example, polystyrene and polyethylene. Other natural polymers are got with synthesis occurring in the cells of organisms - plants and animals. Natural polymers are cellulose, starch and etc. Those and other polymers can have an amorphous structure and a crystalline.
A feature of many polymers can be absence of defined melting point, that greatly distinguishes them, for example, from metals. When heated, high molecular weight compounds are softened and easily deformed. This property gave another name for polymers - plastics. Plastic have low density, that is very light, lighter than the lightest metals, but some of them strength like a cast iron (alloy of iron and carbon). The chemical activity of many polymers are resistant to any acid, alkali and other corrosive substances. It to do the plastic excellent structural material.
Many people know, that the plastics - perfect dielectrics, that allows their use in many sectors of the economy.
The process, in that complex molecules of organic substances are arranged in long chains, called polymerization. Process of polymerization is characteristic for alkenes and dienes, in these molecules more than one chemical bond, but not for saturated hydrocarbons (these saturated hydrocarbons have only one chemical bond).
There are 2 mechanisms of polymerization:
- radical polymerization - refers to substances whose molecules are destroyed then heat (decompose into components) free radicals. Free radicals is not molecules, but only part of them, that have very high chemical activity due to the presence free links in it. These radicals attach to molecules of other substances, causing the emergence of new radicals. Thus, is forming large organic molecules and giant radicals. Among such examples may be organic peroxides with form R-O-O-R (where R is various radicals), compounds (R-N=N-R), and oxygen. Reactions of polymerization can be caused with various external influences, such as gamma radiation or ultraviolet ray. The growth of molecules of polymer you can restrict artificialy, if, for example, a radical, which accumulates the molecules and other radicals, accidentally to connect with a radical or molecule, that form a low-activity radical.
- ionic polymerization - another mechanism to form polymers. For the ionic polymerization needs from a large organic molecules (macromolecules) to ged react ions (anions and cations). Depending on the type (cationic or anionic) polymerization reaction takes place at low temperatures with the presence of boron, aluminium chloride and inorganic acid (cationic polymerization). For anionic polymerization requires amides and alkali metals, able to transform individual organic molecules to a chemically active anions (molecules with a free bond), which in consequence are capable to form a large polymer molecules.
Process polymerization of different molecules requires different conditions, for example, styrene is able to polymerize in the open air at normal conditions, other - requires special catalysts, and the third - temperature and high pressure.