A polymer is a chemical substance composed of macromolecules.
So, what is a macromolecule?
A macromolecule is a very large molecule, 10 to 1000 times larger in size than any ordinary molecule. For instance, a molecule of water has a diameter of 0.2 nm, that is, 2 x 10-7 mm, whereas the diameter of a macromolecule may range from 1000 to 100.000 nanometers (10−5 to 10−3 mm). This size range can be achieved because any macromolecule contains thousands of atoms.
**But what is the most important feature of macromolecules? **
In macromolecules we can identify simple chemical units linked together and repeated many times as in a long chain. These chemical units are called repeat units.