Why are proteins considered polymers but lipids not?
Polymers are several smaller, similar units (monomers) that together in a chain create a larger molecule with new properties.
Macromolecules are giant molecules that are produced by the bonding of smaller molecules. Carbohydrates, lipids, proteins and nucleic acids are all considered to be macromolecules.
However lipids are not considered to be polymers, because lipids do not contain monomers and polymers are made up out of monomers. Moreover, the basic units of lipids are fatty acids and glycerol molecules, which do not form repetitive chains (thus lipids contain non-similar units).
A property of monomers is also that the bonds dissolve in water. This is the case for proteins and carbohydrates, but this is not the case for lipids (which does not dissolve in water).