A polymer is a substance made up of very large molecules called macromolecules, which are made up of many small repeating units that are chemically linked one after another.
The word polymer comes from the Greek and literally means “many parts.” Each of these “parts” that make up the polymer is called a monomer (a part). For this reason, we can imagine a polymer as a molecule made up of many monomers linked together as if they were little legos or building blocks.
The monomers that make up a polymer can be of different types and, moreover, they can all be equal to each other or a combination of different monomers. This results in the existence of several different types of polymers, as we will see later.
Today, we live surrounded by all kinds of polymers. All plastics, from those that make up grocery bags to the acrylic panels of a fish tank, are polymers. Proteins, flours, and most of the carbohydrates we eat are too. Even the DNA molecule that contains all of our genetic information is a polymer.
Polymers can be manufactured with very different properties, but even so, there are some properties that are common to most of them:
- They generally form amorphous solids rather than crystalline solids.
- Most are thermoplastic , which means they can be molded like clay at high temperatures.
- Some are elastic while others are very stiff.
- Some, like nylon, are very resistant to stress.
- They are almost always inert or not very reactive so they can last a long time without decomposing.
- Most synthetic polymers are not biodegradable.
- Some are recyclable.
- Some are transparent like glass (or more, even) while others are completely opaque.
- Most are insulators, that is, they are very poor electrical conductors.
- Most are poor thermal conductors.
Polymers can be classified according to different criteria:
According to its origin:
- Synthetics: they are those that are produced artificially in chemical plants such as plastics.
- Natural or biopolymers: are those produced by living beings such as proteins and nucleic acids.
- Semisynthetics: are those produced by transformation or modification of natural polymers.
According to the monomers used
- Homopolymers: are those formed by a single type of monomer. All subunits are equal to each other, such as polyethylene which is made up solely of ethylene molecules linked together.
- Copolymers: consist of polymers of 2 different monomers. These can be distributed in the chain at random, alternating or in blocks of each monomer.
- Heteropolymers: are those formed by more than two different monomers. Proteins are an example of polymers made up of 21 different monomers that are amino acids.
According to its physical properties
- Thermoplastics: they are those that can be heated until melted and cooled again while maintaining their properties.
- Thermosetting: they are those that, when heated, are transformed into harder and more resistant polymers by the formation of new bonds between the chains. Bakelite is a good example of this kind of polymer.
- Elastomers: they are elastic polymers such as rubber.
According to its structure
- Linear: linear polymers consist of long chains of monomers joined one after another without the appearance of branches.
- Branched: unlike linear ones, these polymers have long central chains, but they also have other shorter side chains that form branches.
- Cross-linked: these polymers consist of linear or branched polymers in which the main chains are linked to each other by means of other shorter chains.
- Cross- linked : Instead of monomer chains, cross-linked polymers consist of extensive two-dimensional or three-dimensional networks as in the case of graphite, diamond, and concrete.
According to the type of bond between the monomers
- Polyolefins: are formed by the union of unsaturated hydrocarbons such as ethylene.
- Polyesters: are those in which the monomers are linked by means of the condensation between an acid and an alcohol.
- Polyethers: in these cases the monomers are linked by means of an oxygen atom.
- Polyamides: monomers are linked by condensation between an acid and an amine.
- Cellulose polymers : they consist of modified biopolymers in which sugar monomers are linked by means of a glycosidic bond.
- Acrylic polymers: they are a family of polymers derived from acrylic acid and its esters.
- Vinyl polymers: they are similar to polyolefins but with substituents such as chlorine or fluorine.
- Polyurethanes: they are polymers derived from isocyanates.
Examples of polymers
1. Polyethylene (PE)
It is by far the most produced and used polymer in the world. They consist of a linear polymer made up of ethylene units one after the other. It is the plastic used in the manufacture of balls.
2. Polyethylene glycol (PEG)
It is a homopolymer that belongs to the polyether family. This polymer is used as a laxative, as a base for making creams and as lubricants for the skin.
3. The DNA molecule
Deoxyribonucleic acid is a biopolymer made up of four monomers called nucleotides, which are thymine, guanine, cytosine, and adenine. It contains all the genetic information encoded in the form of the sequence of these nucleotides.
This biopolymer is similar to DNA with the difference that the nucleotides contain ribose instead of deoxyribose. They participate in the transcription and translation of the genetic code.
It was the first synthetic polymer that existed. It is a very hard and temperature resistant material that is used for knife handles and many more applications.
Another biopolymer, but in this case the monomers are glucose molecules, which is a sugar. The plants produce to store energy.
7. Teflon (PTFE)
Teflon is the trade name for polytetrafluoroethylene or PTFE. It is found on the non-stick surfaces of many pots and pans, as well as pipe joints to make the joint watertight.
It is the most abundant biopolymer (and polymer) on earth. It is part of the cell walls of all plant cells and is an almost inexhaustible source of raw material for the synthesis of biodegradable semi-synthetic polymers.
They are heteropolymers made up of 21 different monomers. They are also biopolymers, since they come from living things.
10. Polystyrene (PS)
Polystyrene is an aromatic polymer that is widely used to produce elastic foams.
11. Polyvinyl chloride (PVC)
This thermoplastic is known throughout the world for its use in water pipes.
12. Polyurethane (PU)
They can be used for the manufacture of foams, as well as for the manufacture of gloss finish resins for wood and other materials.
This is an example of an inorganic polymer in which the chain is not carbon atoms but silicon atoms interspersed with oxygen. It is used among other things for the manufacture of flexible and heat resistant parts such as baking molds.
14. Polyethylene terephthalate (PET)
This is a clear, colorless plastic that is widely used today for bottling water.
15. Polymethylmethacrylate (PMMA)
It is a very hard and almost completely transparent solid polymer that is used as a substitute for glass.