Repeat unit

A repeat unit (mer) is not to be confused with the term monomer, which refers to the small molecule from which a polymer is synthesized.

In copolymers there are two or more types of repeat unit, which may be arranged in alternation, or at random, or in other more complex patterns.

Chemists tend to consider the repeat unit as -[CH2-CH2]- since this polymer is made from the monomer ethylene (CH2=CH2).

Polystyrene has a chain where the substituent R is a phenyl group (C6H5), corresponding to a benzene ring minus one hydrogen: -[CH2-CH(C6H5)]n-, so the repeat unit is -[CH2-CH(C6H5)]-.

The polymer is formed by the condensation reaction of the two monomers terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HO-CH2-CH2-OH), or their chemical derivatives.