Polyurethane Foam (also known as poly-foam) is used in mattresses and mattress toppers. People are often familiar with polyurethane foam as the type of foam used in sofa cushions or cheap "egg crate" mattress toppers. Polyurethane foam is made up primarily of compounds derived from petrochemicals.
Many mattress manufacturers use polyurethane foam as the top layer of spring mattresses. This is done primarily because polyurethane is very inexpensive, not because it is high quality. More recently, many online mattress retailers have begun using polyurethane foam as a base layer in their mattresses instead of using spring coils.
There are several different varieties and densities of polyurethane foam. Generally, higher density polyurethane foam is a bit more durable while low-density polyurethane foam can degrade very quickly (sometimes within a year). Even high density polyurethane foam is less dense than latex foam or memory foam. All polyurethane foam, no matter what density, will degrade faster than any latex foam of decent quality.
Polyurethane foam can add some softness and support, but it will not provide the same pinpointed support associated with memory foam or latex foam