A polymer is a large molecule that appears as a long chain to which different branches can be linked. The structure is made up of many basic units such as in the macro world could be a pearl necklace. The basic units are single molecules, called monomers, and they can be aggregated in groups of two, three, four, or more (and then they are called dimers, trimers, tetramers) or hundreds (high polymers). Some organic molecules are very large in size; however, their structure can be described relatively easily, as they derive from the bond of many simple units. These units can all be the same or different from each other but of the same chemical nature, are called monomers.
The monomers, to form the polymer, are linked together by means of one or more covalent bonds. The reaction leading to the formation of polymers is called polymerization. To understand in detail what polymers are, just think of cellulose, which presents itself as one of the best-known polymers in nature.
Polymers are generally linear, fibers, or branched. The possible presence of double bonds, unsaturation in the polymer chain allows the connection with other monomers and the formation of three-dimensional structures, crosslinking.
The molecule obtained by binding a lot of monomers is called a polymer. Some very common polymers are, for example, starch and cellulose. Both of these molecules are formed as a result of the union of many glucose molecules; they are, therefore, polymers of glucose (the monomer). Other biological polymers are proteins, formed from amino acids, and nucleic acids, formed by nucleotides. Starch and cellulose belong to the category of carbohydrates, in particular, polysaccharides.
Many artificial polymers also exist and are important, such as plastics (polyethylene, polyvinyl chloride, etc.).
There are several methods of classification of polymers commonly used in the chemical industrial sector. If we stop to analyze their origin, the polymers can be divided into:
- Natural polymers
- Artificial polymers
- Synthetic polymers
A further classification criterion takes into account the type of primary structure with which these chains of monomeric units occur, making the necessary distinctions in:
- Linear polymers
- Branched polymers
An example of a branched polymer is starch, while linear polymers include wool, cotton, polyethylene, vinyl polymers, and polyamides.
The following table summarizes the main biological polymers (biopolymers).
MONOMERS → POLYMERS EXAMPLES
Monosaccharides (e.g. glucose) → Polysaccharides Starch, cellulose, glycogen
Amino acids → Protein Enzymes, hemoglobin, antibodies.
Nucleotides → Nucleic acids DNA, RNA
Let us now see how monomers bind in the formation of a polymer. The bonds are of different types depending on the chemical nature of the monomers. However, it is possible to describe all these bonds with a general scheme:
R- OH + H -R ' → R-R' + H 2 O
The letter "R" indicates a residue (or radical); it represents the part of the molecule on which the highlighted groups are linked. In other words, the symbol R is used when it is not necessary to specify the complete formula of the molecule. In the reaction, two molecules bonded by eliminating a group –OH from one molecule and hydrogen –H from the other, which are found in the products in the form of water. This is the pattern by which all biopolymers are formed, and the reaction is called CONDENSATION.
Condensation involves the functional groups of organic molecules; some bonds are typical and take specific names:
ALCOHOLIC GROUP + CARBOXYLIC GROUP → FOREIGN R- O H + HO CO-R ' → R- O -CO-R' + H2O
AMINE GROUP + CARBOXYLIC GROUP → AMIDE R- N H H + HO CO-R ' → R- N H-CO-R' + H2O
ALCOHOLIC GROUP + ALCOHOLIC GROUP → ETHER R- O H + HO -R ' → R- O -R' + H2O
CARBOXYLIC GROUP + CARBOXYLIC GROUP → ANHYDRIDE R-CO O H + HO CO-R ' → R-CO O -CO-R' + H2O
The monomers bind, forming long chains, sometimes branched. This implies that each monomer, excluding those that form the ends of the chain, is condensed with two or three other monomers nearby. The polymer can be assimilated to a chain formed by many rings, represented by the monomers. In many cases, the number of monomers is determined (and determinant) very precisely (e.g., proteins, nucleic acids), in other cases, not (e.g., polysaccharides).
Here is an important example. Amino acids are molecules that have both the carboxylic group and the amino group. They can bind in hundreds by forming long chains by joining the carboxylic group of one amino acid with the amino group of the next one. This particular amide bond is called a peptide bond; the chain is called a polypeptide. The proteins are complete and functional polypeptide, manufactured by living cells. Many proteins are made up of multiple polypeptides.
The inverse reaction of condensation is called HYDROLYSIS:
R-R ' + H 2 O → R- OH + H -R'
A typical high natural polymer is cellulose, made up of many sugar units. A cotton fiber (almost pure cellulose), for example, is made up of 3,500 sugar monomers, while polyvinyl chloride has a chain of 25,000 monomers. Other polymers are rubbers and plastics, wool, starch.
Today it is possible to establish a priori the order of monomers in the chain and modify it as desired to obtain polymers with different characteristics. How do the monomers stick together to form the complex structure of the desired polymer? Each "pearl of the necklace" is joined to the others with a chemical bond, characteristic of each polymer. Once the solvent suitable for dissolving it is found, the chain depolymerizes, that is, it breaks and frees the units. Vice versa, in polymerization, the structure is recreated, this is what happens. For example, when repairing the tire of a bicycle and, with special mastic, vulcanize the rubber of the inner tube and the patch.