Monomers and Polymers Chemistry

Introduction to Monomers and Polymers

Polymers, such as polythene and polyamide (shown here) are built from subunits called monomers.
Polymers, such as polythene and polyamide (shown here) are built from subunits called monomers. SSPL, Getty Images

Monomers are the building blocks of more complex molecules, called polymers. Polymers consist of repeating molecular units which usually are joined by covalent bonds. Here is a closer look at the chemistry of monomers and polymers.

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 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 which 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 (polymerizes into polyvinyl chloride or PVC), glucose (polymerizes into starch, cellulose, laminarin, and glucans), and amino acids (which polymerize into peptides, polypeptides, and proteins).

Glucose 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 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 are diglycerides, triglycerides; monomers are glycerol and fatty acids
  • Proteins - polymers are 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)

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.

*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.