Monomers and Polymers in Chemistry

Models of Polythene and Polyamide
Polymers, such as Polythene and Polyamide are built from subunits called Monomers.

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A monomer is a type of molecule that has the ability to chemically bond with other molecules in a long chain; a polymer is a chain of an unspecified number of monomers. Essentially, monomers are the building blocks of polymers, which are more complex type of molecules. Monomers—repeating molecular units—are connected into polymers by covalent bonds.

Monomers

The word monomer comes from mono- (one) and -mer (part). Monomers are small molecules which may be joined together in a repeating fashion to form more complex molecules called polymers. Monomers form polymers by forming chemical bonds or binding supramolecularly through a process called polymerization.

Sometimes polymers are made from bound groups of monomer subunits (up to a few dozen monomers) called oligomers. To qualify as an oligomer, the properties of the molecule need to change significantly if one or a few subunits are added or removed. Examples of oligomers include collagen and liquid paraffin.

A related term is "monomeric protein," which is a protein that bonds to make a multiprotein complex. Monomers are not just building blocks of polymers, but are important molecules in their own right, which do not necessarily form polymers unless the conditions are right.

Examples of Monomers

Examples of monomers include vinyl chloride (which polymerizes into polyvinyl chloride or PVC), glucose (which polymerizes into starch, cellulose, laminarin, and glucans), and amino acids (which polymerize into peptides, polypeptides, and proteins). Glucose is the most abundant natural monomer, which polymerizes by forming glycosidic bonds.

Polymers

The word polymer comes from poly- (many) and -mer (part). A polymer may be a natural or synthetic macromolecule comprised of repeating units of a smaller molecule (monomers). While many people use the term 'polymer' and 'plastic' interchangeably, polymers are a much larger class of molecules which includes plastics, plus many other materials, such as cellulose, amber, and natural rubber.

Lower molecular weight compounds may be distinguished by the number of monomeric subunits they contain. The terms dimer, trimer, tetramer, pentamer, hexamer, heptamer, octamer, nonamer, decamer, dodecamer, eicosamer reflects molecules containing 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and 20 monomer units.

Examples of Polymers

Examples of polymers include plastics such as polyethylene, silicones such as silly putty, biopolymers such as cellulose and DNA, natural polymers such as rubber and shellac, and many other important macromolecules.

Groups of Monomers and Polymers

The classes of biological molecules may be grouped into the types of polymers they form and the monomers that act as subunits:

  • Lipids - polymers called diglycerides, triglycerides; monomers are glycerol and fatty acids
  • Proteins - polymers are known as polypeptides; monomers are amino acids
  • Nucleic Acids - polymers are DNA and RNA; monomers are nucleotides, which are in turn consist of a nitrogenous base, pentose sugar, and phosphate group
  • Carbohydrates - polymers are polysaccharides and disaccharides*; monomers are monosaccharides (simple sugars)

*Technically, diglycerides, and triglycerides are not true polymers because they form via dehydration synthesis of smaller molecules, not from the end-to-end linkage of monomers that characterizes true polymerization.

How Polymers Form

Polymerization is the process of covalently bonding the smaller monomers into the polymer. During polymerization, chemical groups are lost from the monomers so that they may join together. In the case of biopolymers of carbohydrates, this is a dehydration reaction in which water is formed.

Resources and Further Reading

  • Cowie, J.M.G. and Valeria Arrighi. "Polymers: Chemistry and Physics of Modern Materials," 3rd ed. Boca Taton: CRC Press, 2007. 
  • Sperling, Leslie H. "Introduction to Physical Polymer Science," 4th ed. Hoboken, NJ: John Wiley & Sons, 2006.  
  • Young, Robert J., and Peter A. Lovell. "Introduction to Polymers," 3rd ed. Boca Raton, LA: CRC Press, Taylor & Francis Group, 2011.