POLYMERS: HISTORY, POLYMERIZATION, TYPES, PROPERTIES - CHEMISTRY - 2025

The polymers are molecular compounds characterized by having a high molar mass (ranging from thousands to millions) and consist of a large number of units, called monomers, which are repeated.

Because they have the characteristic of being large molecules, these species are called macromolecules, which gives them unique qualities that are very different from those observed in smaller ones, only attributable to this type of substances, such as the propensity they have for shaping glass structures.

In the same way, as they belong to a very large group of molecules, the need arose to grant them a classification, for which they are divided into two types: polymers of natural origin, such as proteins and nucleic acids; and those of synthetic manufacture, such as nylon or lucite (better known as Plexiglas).

Scholars began their investigations of the science behind polymers in the 1920s, when they observed with curiosity and bewilderment how substances such as wood or rubber behave. So scientists of the time began to analyze these compounds so present in everyday life.

By reaching a certain level of understanding about the nature of these species, it was possible to understand their structure and advance in the creation of macromolecules that could facilitate the development and improvement of existing materials, as well as the production of new materials.

Likewise, it is known that numerous significant polymers contain nitrogen or oxygen atoms in their structure, linked to carbon atoms, forming part of the main chain of the molecule.

Depending on the main functional groups that are part of the monomers, they will be given their names; for example, if the monomer is formed by an ester, a polyester is created.

History of polymers

The history of polymers must be approached starting with references to the first known polymers.

In this way, certain materials of natural origin that have been widely used since ancient times (such as cellulose or leather) are mainly made up of polymers.

XIX century

Contrary to what one might think, the composition of polymers was an unknown until a couple of centuries ago, when it began to determine how these substances were formed, and they even sought to establish a method to achieve artificial manufacture.

The first time the term "polymers" was used was in 1833, thanks to the Swedish chemist Jöns Jacob Berzelius, who used it to refer to substances of an organic nature that have the same empirical formula but have different molar masses.

This scientist was also in charge of coining other terms, such as "isomer" or "catalysis"; although it should be noted that at that time the concept of these expressions was completely different from what they mean today.

After some experiments to obtain synthetic polymers from the transformation of natural polymeric species, the study of these compounds was acquiring greater relevance.

The purpose of these investigations was to achieve the optimization of the already known properties of these polymers and the obtaining of new substances that could fulfill specific purposes in different fields of science.

Twentieth century

Observing that the rubber was soluble in a solvent of organic nature and then the resulting solution exhibited some unusual characteristics, the scientists were concerned and did not know how to explain them.

Through these observations, they inferred that substances like this exhibit a very different behavior from smaller molecules, as they were able to observe while studying rubber and its properties.

They noted that the studied solution had high viscosity, a significant decrease in the freezing point and a small osmotic pressure; From this it could be deduced that there were several solutes of very high molar mass, but scholars refused to believe in this possibility.

These phenomena, which were also manifested in some substances such as gelatin or cotton, made scientists of the time think that these types of substances were made up of aggregates of small molecular units, such as C ₅ H ₈ or C ₁₀ H ₁₆, bound by intermolecular forces.

Although this wrong thinking persisted for some years, the definition that persists to this day was the one given to it by the German chemist and Nobel Prize winner in Chemistry, Hermann Staudinger.

XXI century

The current definition of these structures as macromolecular substances linked by covalent bonds was coined in 1920 by Staudinger, who insisted on devising and conducting experiments until he found evidence for this theory over the next ten years.

The development of the so-called “polymer chemistry” began and since then it has only been capturing the interest of researchers around the world, counting among the pages of its history very important scientists, including Giulio Natta, Karl Ziegler, Charles Goodyear, among others, in addition to those previously named.

At present, polymeric macromolecules are studied in different scientific areas, such as polymer science or biophysics, where substances resulting from linking monomers through covalent bonds with different methods and purposes are investigated.

Certainly, from natural polymers such as polyisoprene to those of synthetic origin such as polystyrene, they are used very frequently, without diminishing the importance of other species such as silicones, made up of monomers based on silicon.

Also, a large part of these compounds of natural and synthetic origin are made up of two or more different classes of monomers, these polymeric species have been given the name of copolymers.

Polymerization

In order to delve into the subject of polymers, we must begin by talking about the origin of the word polymer, which comes from the Greek terms polys, which means "a lot"; and mere, which refers to the "parts" of something.

This term is used to designate molecular compounds that have a structure made up of many repeating units, this causes the property of a high relative molecular mass and other intrinsic characteristics of these.

Thus, the units that make up polymers are based on molecular species that have a relatively small relative molecular mass.

In this vein, the term polymerization only applies to synthetic polymers, more specifically to the processes used to obtain this type of macromolecules.

Therefore, polymerization can be defined as the chemical reaction that is used in the combination of monomers (one at a time) to, from these, produce the corresponding polymers.

Thus, the synthesis of polymers is carried out through two main types of reactions: addition reactions and condensation reactions, which will be described in detail below.

Polymerization by addition reactions

This type of polymerization has the participation of unsaturated molecules that have double or triple bonds in their structure, especially those of carbon-carbon.

In these reactions, the monomers undergo combinations with each other without the elimination of any of their atoms, where the polymeric species synthesized by breaking or opening the ring can be obtained without generating the elimination of small molecules.

From a kinetic point of view, this polymerization can be seen as a three-step reaction: initiation, propagation and termination.

First, the initiation of the reaction occurs, in which heating is applied to a molecule considered as an initiator (denoted as R ₂) to generate two radical species as follows:

R ₂ → 2R ∙

If the production of polyethylene is used as an example, then the next step is propagation, where the reactive radical formed tackles an ethylene molecule and a new radical species is formed as follows:

R ∙ + CH ₂ = CH ₂ → R – CH ₂ –CH ₂ ∙

This new radical is subsequently combined with another ethylene molecule, and this process continues successively until the combination of two long-chain radicals to finally give rise to polyethylene, in the reaction known as termination.

Polymerization by condensation reactions

In the case of polymerization through condensation reactions, the combination of two different monomers generally occurs, in addition to the consequent elimination of a small molecule, which is generally water.

Similarly, polymers produced by these reactions often have heteroatoms, such as oxygen or nitrogen, as part of their backbone. It also happens that the repeating unit that represents the base of its chain does not have all the atoms that are in the monomer to which it could be degraded.

On the other hand, there are methods that have been developed more recently, among which plasma polymerization stands out, whose characteristics do not agree perfectly with any of the types of polymerization explained above.

In this way, polymerization reactions of synthetic origin, both those of addition and condensation, can occur in the absence or in the presence of a catalyst species.

Condensation polymerization is widely used in the manufacture of many compounds commonly found in everyday life, such as dacron (better known as polyester) or nylon.

Other forms of polymerization

In addition to these artificial polymer synthesis methods, there is also biological synthesis, which is defined as the area of ​​study that is responsible for the research of biopolymers, which are divided into three main categories: polynucleotides, polypeptides and polysaccharides.

In living organisms, synthesis can be carried out naturally, through processes that involve the presence of catalysts such as the polymerase enzyme in the production of polymers such as deoxyribonucleic acid (DNA).

In other cases, most of the enzymes used in biochemical polymerization are proteins, which are polymers formed on the basis of amino acids and which are essential in the vast majority of biological processes.

In addition to the biopolymeric substances obtained by these methods, there are others of great commercial relevance, such as vulcanized rubber that is produced by heating rubber of natural origin in the presence of sulfur.

Thus, among the techniques used for polymer synthesis through chemical modification of polymers of natural origin are finishing, crosslinking and oxidation.

Types of polymers

The types of polymers can be classified according to different characteristics; for example, they are classified into thermoplastics, thermosets, or elastomers according to their physical response to heating.

Furthermore, depending on the type of monomers from which they are formed, they can be homopolymers or copolymers.

Similarly, according to the kind of polymerization by which they are produced, they can be addition or condensation polymers.

Likewise, natural or synthetic polymers can be obtained depending on their origin; or organic or inorganic depending on its chemical composition.

Properties

- Its most notable characteristic is the repetitive identity of its monomers as the basis of its structure.

- Its electrical properties vary according to its purpose.

- They present mechanical properties such as elasticity or resistance to traction, which define their macroscopic behavior.

- Some polymers exhibit important optical properties.

- The microstructure they have directly affects their other properties.

- The chemical characteristics of polymers are determined by the attractive interactions between the chains that form them.

- Its transport properties are relative to the speed of intermolecular movement.

- The behavior of its states of aggregation is related to its morphology.

Examples of polymers

Among the large number of polymers that exist are the following:

Polystyrene

Used in containers of different types, as well as in containers that are used as thermal insulators (to cool water or store ice) and even in toys.

Polytetrafluoroethylene

Better known as Teflon, it is used as an electrical insulator, also in the manufacture of rollers and to coat kitchen utensils.

Polyvinyl chloride

Used in the production of wall channels, tiles, toys and pipes, this polymer is commercially known as PVC.

References

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