MONOMERS: CHARACTERISTICS, TYPES AND EXAMPLES - CHEMISTRY - 2025

The monomers are small or simple molecules which constitute the basic or essential structural unit larger or complex molecules called polymers. Monomer is a word of Greek origin that means mono, one and mere, part.

As one monomer joins with another, a dimer forms. By joining this in turn with another monomer, it forms a trimer, and so on until it forms short chains called oligomers, or longer chains that are called polymers.

Source: Ardonik via Flickr

Monomers bond or polymerize by forming chemical bonds by sharing pairs of electrons; that is to say, they are united by bonds of covalent type.

In the image above, the cubes represent the monomers, which are linked by two faces (two bonds) to give rise to a leaning tower.

This union of monomers is known as polymerization. Monomers of the same or different type 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, inclined chains or three-dimensional structures).

Polystyrene molecule. Monomer example (red rectangle)

There is a great 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 polymer products, such as paints and plastics.

Two of the thousands of examples that can be given can be mentioned, such as tetrafluoroethylene, which forms the polymer known as Teflon, or the monomers phenol and formaldehyde, which form the polymer called Bakelite.

Monomer Characteristics

Monomers are linked 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 bind 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 that have the structure of the monomer.

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

Monomers functionality and Polymer structure

A monomer can bind with at least two other monomer molecules. This property or characteristic is what is known as the functionality of monomers, and 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 make up, 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.

Examples of linear polymers include ethylene glycol and amino acids.

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 double bond between carbon and carbon

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

In turn, this chain or central structure has laterally bonded atoms that can change to form a different monomer. (R ₂ C = CR ₂).

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

Examples of this group of monomers include propylene (H ₂ C = CH ₃ H), tetrafluoroethylene (F ₂ C = CF ₂) and vinyl chloride (H ₂ C = CClH).

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 (-NH ₂) and the 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. An example is the carboxylic acid functional group R – COOH, the amino group R – NH ₂, the alcohol R – OH, among many others that participate in polymerization reactions.

Union of the same or different monomers

Union of equal monomers

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

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

Union of different monomers

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

As an example of copolymers, nylon, a polymer formed by repeating units of two different monomers, can be mentioned. 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 formation of a specialized polyethylene having the 1-octene monomer plus the ethylene monomer as its basic structure.

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. Also, there are amino acids that form the polymer keratin, which is the protein in wool produced by animals such as sheep.

-Among the natural monomers are also the basic structural units of biomolecules. The monosaccharide glucose, for example, binds 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 form 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; like vinyl chloride, which forms polyvinyl chloride or PVC; and ethylene gas (H ₂ C = CH ₂), and its polyethylene polymer.

It is well known that with these materials a wide variety of containers, bottles, household objects, toys, construction materials, among others, can be built.

-The tetrafluoroethylene monomer (F ₂ C = CF ₂) is found forming the polymer known commercially as Teflon.

-The caprolactam molecule derived from toluene is essential for the synthesis of nylon, among many others.

-There are several groups of acrylic monomers that are classified according to composition and function. Among these are acrylamide and methacrylamide, acrylate, acrylics with fluorine, among others.

Nonpolar and polar monomers

This classification is carried out according to the electronegativity difference of the atoms that make up the monomer. When there is a noticeable difference, polar monomers are formed; for example, polar amino acids such as threonine and asparagine.

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

Cyclic or linear monomers

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

Examples

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

-Formaldehyde

-Furfural

-Cardanol

-Galactose

-Styrene

-Polyvinyl alcohol

-Isoprene

-Fatty acids

-Epoxides

-And although they were not mentioned, there are monomers whose structures are not carbonated, but sulfurized, phosphorous, or have silicon atoms.

References

1. Carey F. (2006). Organic Chemistry. (6th ed.). Mexico: Mc Graw Hill.
2. The Editors of Encyclopedia Britannica. (2015, April 29). Monomer: Chemical Compound. Taken from: britannica.com
3. Mathews, Holde and Ahern. (2002). Biochemistry (3rd ed.). Madrid: PEARSON
4. Polymers and Monomers. Recovered from: materialsworldmodules.org
5. Wikipedia. (2018). Monomer. Taken from: en.wikipedia.org