In polymer chemistry, a structural unit is a building block of a polymer chain. It is the result of a monomer residue which has been polymerized. Contrast this with repeating unit, which is the shortest sequence that can be found repeatedly in a polymer. This distinction is necessary in order to estimate the molecular weight of polymers correctly.
Consider this example: Take polyethylene terephthalate. The monomers which could be used to create this polymer are ethylene glycol and terephthalic acid:
HO-CH2-CH2-OH and HOOC-Ph-COOH
In the polymer, there are two structural units, which look like -O-CH2-CH2-O- and -CO-PH-CO-
The repeating unit looks like -CH2-CH2-O-CO-Ph-CO-O-