In nature and in the world of synthetics, there are many substances with molecular masses in the thousands and hundred thousands containing hundreds or thousands of atoms. These molecules are called macromolecules.

Macromolecular substances are almost everywhere in nature and in synthetics. Synthetic macromolecules are examples of how chemists take ordinary substances from nature, like coal, oil, air, and water, and make new materials with useful applications.

A polymer is a macromolecule in which all of the molecules have a small characteristic structural feature that repeats itself again and again. For example, here is a molecule of polyethylene:

etc.-CH2CH2-CH2CH2-CH2CH2-CH2CH2-CH2CH2-etc.

The structure has one structural unit, or monomer, that occurs repeatedly--ethylene (ethane), CH2CH2. The structure of the polymer can therefore be represented by:
-(CH2CH2)n- Where n can be several thousands.

The reaction that makes a polymer out of a monomer is called polymerization. If we let a to be a repeating unit, then its polymer can be:
Etc.-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-etc.

But sometimes during polymerization, a long chain may develop branches that themselves are long.
a etc.
|
a a a-a-a-a-etc.
| | |
etc.-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-etc.
| | | |
a a a a
|
etc. a-a-etc. a

Copolymers

Sometimes two monomers are mixed together and polymerized. For example, saran is a copolymer of CH2=CCl2 and CH2=CHCl and its molecules have the following units in their structure:

-CH2-CCl2-CH2-CHCl-

When copolymers form though, there are several possibilities when they are polymerizing:

alternate copolymer
etc.-a-b-a-b-a-b-a-b-a-b-a-b-a-b-a-b-a-b-a-b-a-b-a-b-a-b-a-b-etc.

block copolymer
etc.-a-a-a-b-b-a-a-a-a-a-b-b-b-a-a-b-b-b-b-b-a-a-a-b-b-b-a-a-a-a-a-etc.

random copolymer
etc.-a-a-b-a-b-a-a-a-b-b-a-b-b-a-a-a-b-b-a-b-b-a-a-a-b-a-b-a-a-etc.

graft copolymer
etc.-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-etc.
| | | | | | |
b b b b b b b
| | | | | | |
b b b b b b b
| | | | | | |
etc. etc. etc. etc. etc. etc. etc.

cross-linked copolymer
a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a
| | | |
b b b b
| | | |
b b a-a-a-a-a-a-a-a-a-a-a-a-a-a
| | |
a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a-a b
| |
b b