The difference between structural unit and repeating unit?

[royzizzle]

Homework Statement

The difference between structural unit and repeating unit

Homework Equations

The Attempt at a Solution

http://en.wikipedia.org/wiki/Structural_unit

wikipedia gives an example with polyethylene terephthalate but I can't figure it out based on the slight difference

 

[Kalirren]

Polyethylene teraphthalate has two structural units but only has one repeating unit.

Structural units are substructures that are derived directly from the inputs.

If you call the repeating unit [-R-], then you can write a polymer as -[-R-]-_n.

So while polyethylene teraphthalate has the repeating unit [-O-CH₂-CH₂-O-CO-Ph-CO-], it's built from two distinct inputs and each of those is a different structural unit.

To take another example, polyethylene, which once was also called polymethylene, -[-CH₂-]-_n, has the repeating unit [-CH₂-] but has the structural unit [-CH₂-CH₂-] because it is derived from ethylene, not methyl radical.