Polymers
Ever wondered how grocery bags, tires, paints, non-stick pans, face shields and water pipes are related to each other?
They are all made from polymers!
Polymer-paperclip analogy: Each clip can be thought of as one monomer and the collection of these paperclips is like a polymer. Image source: https://www.futurelearn.com/info/courses/everyday-chemistry/0/steps/22343
What are polymers?
Polymers are chemical compounds consisting of large molecules. These large molecules are made up of smaller molecules called monomers which are linked together via chemical bonds. The word polymer is derived from Greek words ‘poly’, - which means many, and ‘mer’ which means parts, referring to each of the monomers (which in Greek mean “one part”) that make up a polymer. [1-3] The simplest way to think of a polymer molecule is by imagining a collection of paperclips linked to each other. [4] Each individual paperclip can be thought of as a monomer while the collection of those clips can be thought of as a polymer chain. Like paperclips, the monomers in a polymer chain can be linked to each other in a linear or branched manner leading to either linear/branched architecture of the polymer chain. Depending on the number of monomers in a polymer chain, the chemical identity of those monomers and the polymer chain architecture, polymers of varying structure and properties can be obtained. This is what makes polymers promising materials for various industrial and household applications such as grocery bags, adhesives, face shields etc.
DNA is a natural polymer made up of nucleotides. Image source: https://commons.wikimedia.org/wiki/File:ADN_animation.gif
Are all polymers man-made?
The examples in the beginning of this article may have motivated you to think that all polymers are man-made. On the contrary, many polymers are found in nature. As an example, our DNA is a polymer, a polynucleotide made up of monomers called nucleotides which contain a five carbon sugar molecule attached to a phosphate group and a nitrogenous base. Other natural polymers include polypeptides that make up silk and hair, polysaccharides like starch in grains, cereals, and potatoes and cellulose that makes up the cell wall in plants.
The first man-made or synthetic polymer, Bakelite, was invented in 1907 by Leo Baekeland as a substitute to shellac, a natural polymer made from the shells of Asian lac beetles which is used as insulation for electrical wiring. [5, 6] Bakelite was marketed as the “material of a thousand uses” [7] since it was not only a good insulator but also durable, heat resistant and could be molded into any desired shape. Since then, numerous polymers have been synthesized including polyethylene (a common plastic) and polyesters. One thing to note is that although the terms polymers and plastics are often used interchangeably, they are not the same. Plastics are a subset of polymers that have the property of plasticity, or the ability to be molded into desired shapes.
Addition polymerization reaction for creating Polyethylene. Image source: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Book%3A_Introductory_Chemistry_(CK-12)/25%3A_Organic_Chemistry/25.19%3A_Polymerization_-_Addition_Polymers
How are polymers made?
A polymer is synthesized by joining small molecules into a long chain of molecules by a chemical process called polymerization. [8] One way of doing this is by “addition polymerization”, whereby monomers are added together in a repeating pattern resulting in no other byproducts. In addition polymerization, a double bond connecting two carbon atoms of a monomer is broken using a free radical molecule (in the figure above, this is denoted by X*). This leads to opening of the double bond, allowing the monomer to link with the next monomer molecule. A polymer chain is formed by repeated additions of monomer molecules along this basic unit.
The other process by which polymers are synthesized is called “condensation polymerization”, where monomers react with each other to form larger structural units while releasing smaller molecules (such as water or methanol) as a byproduct.
Demonstration of a branched polymer architecture using paperclips. Image source: https://www.futurelearn.com/info/courses/everyday-chemistry/0/steps/22343
How does the molecular arrangement within a polymer affect its properties?
The structure and properties of a polymer are dictated by its molecular arrangement. As an example, polyethylene consists of long ‘spaghetti-like’ polymer chains that, when heated, easily slide past each other, but become entangled upon cooling. This allows melting and successive reformation of polyethylene. Such polymers that can be melted and reshaped are called thermoplastics [9, 10]. Polystyrene and propylene are other thermoplastics.
Unlike the thermoplastics, thermosetting polymers like epoxy, rubber and polyurethanes contain crosslinked chains in a network that prevents them from softening when heated. [9] These remain in shape once set and are commonly used in applications such as car tires, electrical components, and glues. Additionally, the length of the polymer chain also affects its properties. As a polymer gets longer the total binding forces that hold the molecule are greater making the polymer stronger. [4]
The architecture (linear/branched) of the polymer also affects its density and in turn strength. Recalling our paperclip analogy, if paperclips are connected in a manner such that some clips branch off a main line it will be difficult to achieve a compact packing of branched paperclips. [4] Along similar lines, multiple polymer chains with side branches cannot pack in a compact fashion leading to a polymer with a low density. An example is the low-density polyethylene (LDPE) which we use for grocery bags and wrapping materials. On the contrary, linear polymer chains without side branches can line up into a stronger crystalline structure forming a polymer with a higher density. Plastic bottles, plumbing pipes and food containers are all made from high-density polyethylene (HDPE).
Where else are polymers used in our daily lives?
Other commonly used synthetic polymers and their applications include [11]:
● Polypropylene (PP): Carpet, upholstery
● Polyethylene low density (LDPE): Grocery bags
● Polyethylene high density (HDPE): Detergent bottles, toys, food containers
● Poly(vinyl chloride) (PVC): Piping, decking
● Polystyrene (PS): Toys, foam
● Polytetrafluoroethylene (PTFE, Teflon): Non-stick pans, electrical insulation
● Poly(methyl methacrylate) (PMMA, Lucite, Plexiglas): Face shields, skylights
● Poly(vinyl acetate) (PVAc): Paints, adhesives
● Polychloroprene (Neoprene): Wetsuits
Other interesting links to learn more about polymers
Polymers: from DNA to rubber ducks
Bakelite: First Synthetic Plastic
References:
1. Polymer. https://www.thefreedictionary.com/polymer (acessed 08/17/2021).
2. Polymer. In Britannica, T. Editors of Encyclopaedia, 2021, August 11 ed.
3. The Basics: Polymer Definition and Properties. https://plastics.americanchemistry.com/plastics/The-Basics/(accessed 08/19/2021).
4. Polymers: from DNA to rubber ducks. https://www.science.org.au/curious/everything-else/polymers (accessed 08/19/2021).
5. The Story of Bakelite, the First Synthetic Plastic. https://www.thoughtco.com/story-of-synthetic-plastic-1991672(accessed 08/19/2021).
6. Bakelite: The greatest discovery of the late 19th century. https://plastics-themag.com/Bakelite-The-greatest-discovery-of-the-late-19th-century (accessed 08/19/2021).
7. Leo Hendrick Baekeland and the Invention of Bakelite. https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/bakelite.html (accessed 08/19/2021).
8. Polymers and Polymerization Reactions. https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_General_Chemistry_(Petrucci_et_al.)/27%3A_Reactions_of_Organic_Compounds/27.08%3A_Polymers_and_Polymerization_Reactions(accessed 08/19/2021).
9. Thermoplastics vs. thermosets: What’s the difference? https://www.fastradius.com/resources/thermoplastics-thermosets/ (accessed 08/19/2021).
10. Thoughts About Thermoplastics. https://www.pslc.ws/macrog/plastic.htm (accessed 08/19/2021).
11. What Is a Polymer? https://www.cewheelsinc.com/what-is-a-polymer/ (accessed 08/19/2021).
