7 junio, 2024

Monomers: what they are, characteristics, structure, types, examples

What are monomers?

The monomers They are small or simple molecules that constitute the basic or essential structural unit of larger or more complex molecules called polymers. Monomer is a word of Greek origin meaning bunone and Merepart.

As one monomer bonds with another, a dimer is formed. When this in turn unites with another monomer, it forms a trimer, and so on, until it forms short chains called oligomers, or longer chains, which are called polymers.

Monomers join or polymerize by forming chemical bonds by sharing pairs of electrons; that is, they are joined by covalent bonds.

This union of monomers is known as polymerization. Monomers of the same or different types can be joined, and the number of covalent bonds that they can establish with another molecule will determine the structure of the polymer they form (linear or inclined chains or three-dimensional structures).

There is a wide variety of monomers, among which are those of natural origin. These belong to and design the organic molecules called biomolecules, present in the structure of living beings.

For example, the amino acids that make up proteins; the monosaccharide units of carbohydrates and the mononucleotides that make up nucleic acids. There are also synthetic monomers, which make it possible to produce an innumerable variety of inert polymeric products, such as paints or plastics.

Characteristics of the monomers

Monomers are joined by covalent bonds

The atoms that participate in the formation of a monomer are held together by strong and stable bonds such as the covalent bond. Likewise, monomers polymerize or join with other monomeric molecules through these bonds, giving the polymers strength and stability.

These covalent bonds between monomers can be formed by chemical reactions that will depend on the atoms that make up the monomer, the presence of double bonds, and other characteristics of the monomer structure.

The polymerization process can occur by one of the following three reactions: condensation, addition, or free radicals. Each of them carries its own mechanisms and mode of growth.

Functionality of the monomers and structure of the polymer

A monomer can bind with at least two other monomer molecules. This property or characteristic is what is known as functionality of the monomers, and it is what allows them to be the structural units of macromolecules.

The monomers can be bifunctional or polyfunctional, depending on the active or reactive sites of the monomer; that is, of the atoms of the molecule that can participate in the formation of covalent bonds with the atoms of other molecules or monomers.

This characteristic is also important, since it is closely linked to the structure of the polymers that they form, as detailed below.

Bifunctionality: linear polymer

Monomers are bifunctional when they have only two binding sites with other monomers; that is, the monomer can only form two covalent bonds with other monomers and forms only linear polymers.

Among the linear polymers, ethylene glycol and amino acids can be mentioned as examples.

Polyfunctional monomers – three-dimensional polymers

There are monomers that can be joined with more than two monomers and constitute the structural units with the highest functionality.

They are called polyfunctional and are those that produce branched, network or three-dimensional polymeric macromolecules; like polyethylene, for example.

Skeleton or central structure

With a double bond between carbon and carbon

There are monomers that present in their structure a central skeleton made up of at least two carbon atoms joined by a double bond (C=C).

In turn, this central chain or structure has laterally attached atoms that can change to form a different monomer (R2C=CR2).

If any of the R chains are modified or replaced, a different monomer is obtained. Also, when these new monomers come together they will form a different polymer.

As examples of this group of monomers, propylene (H2C=CH3H), tetrafluoroethylene (F2C=CF2) and vinyl chloride (H2C=CClH) may be mentioned.

Two functional groups in the structure

Although there are monomers that have only one functional group, there is a wide group of monomers that have two functional groups in their structure.

Amino acids are a good example of this. They have an amino functional group (-NH2) and a carboxylic acid functional group (-COOH) attached to a central carbon atom.

This characteristic of being a difunctional monomer also gives it the ability to form long polymer chains, such as the presence of double bonds.

Functional groups

In general, the properties that polymers present are given by the atoms that form the side chains of the monomers. These chains make up the functional groups of organic compounds.

There are families of organic compounds whose characteristics are given by the functional groups or side chains. As an example is the carboxylic acid functional group R–COOH, the amino group R–NH2, the alcohol R–OH, among many others that participate in polymerization reactions.

Union of monomers of the same or different type

Union of the same monomers

Monomers can form different classes of polymers. Monomers of the same type or of the same type can be joined and so-called homopolymers can be generated.

As an example we can mention styrene, the monomer that forms polystyrene. Starch and cellulose are also examples of homopolymers made up of long branched chains of the glucose monomer.

Union of different monomers

The union of different monomers forms copolymers. The units are repeated in a different number, order or sequence throughout the structure of the polymeric chains (ABBBAABAA-…).

As an example of copolymers we can mention nylon, a polymer formed by repeating units of two different monomers. These are the dicarboxylic acid and a diamine molecule, which are joined via condensation in equimolar (equal) proportions.

Different monomers can also be joined in unequal proportions, as in the case of the formation of a specialized polyethylene whose basic structure is the 1-octene monomer plus the ethylene monomer.

Types of monomers

There are many characteristics that allow establishing various types of monomers, among which are their origin, functionality, structure, the type of polymer they form, how they polymerize, and their covalent bonds.

natural monomers

There are monomers of natural origin such as isoprene, which is obtained from the sap or latex of plants, and which is also the monomeric structure of natural rubber.
Some amino acids produced by insects form fibroin, or silk protein. Likewise, there are amino acids that form the keratin polymer, which is the wool protein produced by animals such as sheep.
Among the natural monomers are also the basic structural units of biomolecules. The glucose monosaccharide, for example, joins with other glucose molecules to form different types of carbohydrates such as starch, glycogen, cellulose, among others.
Amino acids, on the other hand, can form a wide range of polymers known as proteins. This is because there are twenty types of amino acids, which can be linked in any arbitrary order; and therefore, they end up forming one or another protein with its own structural characteristics.
Mononucleotides, which make up the macromolecules called DNA and RNA nucleic acids respectively, are also very important monomers within this category.

synthetic monomers

Among the artificial or synthetic monomers (which are numerous), we can mention some with which different varieties of plastics are made, such as vinyl chloride, which forms polyvinyl chloride or PVC; and ethylene gas (H2C=CH2), and its polymer polyethylene. It is well known that a wide variety of containers, bottles, household objects, toys, construction materials, among others, can be built with these materials.
Tetrafluoroethylene monomer (F2C=CF2) is forming the polymer called and known commercially as Teflon.
The toluene-derived caprolactam molecule is essential for the synthesis of nylon, among many others.
There are several groups of acrylic monomers that are classified based on composition and function. Among these are acrylamide and methacrylamide, acrylate, fluorine acrylics, among others.

Apolar and Polar Monomers

This classification is made according to the difference in electronegativity of the atoms that form the monomer. When there is a marked difference, polar monomers are formed; for example, polar amino acids such as threonine and asparagine.

When the electronegativity difference is zero, the monomers are nonpolar. There are non-polar amino acids such as tryptophan, alanine, valine, among others; and also nonpolar monomers such as vinyl acetate.

Cyclic or linear monomers

According to the shape or organization of the atoms within the structure of the monomers, these can be classified as cyclic monomers, such as proline or ethylene oxide; and linear or aliphatic, such as the amino acid valine or ethylene glycol, among many others.

Examples of monomers

In addition to those already mentioned, there are the following additional examples of monomers:

Formaldehyde
furfural
Cardanol
Galactose
styrene
polyvinyl alcohol
isoprene
fatty acids
epoxies
And although they were not mentioned, there are monomers whose structures are not carbonated, but sulfurized, phosphorous, or have silicon atoms.

References

Carey F. (2006). Organic Chemistry. (6th ed.). Mexico: Mc Graw Hill.
The Editors of Encyclopedia Britannica (2015). Monomer: Chemical Compound. Taken from: britannica.com
Mathews, Holde, and Ahern (2002). Biochemistry (3rd ed.). Madrid: PEARSON
Polymers and Monomers. Retrieved from: materialsworldmodules.org
Wikipedia (2018). Monomer. Taken from: en.wikipedia.org

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