Polyurethanes are compounds that are formed by the polymerization of carbamates. A carbamate (or urethane) is characterized by the following group in its chemical structure:
   R   R
    \ /
     R     (R is a hydrocarbon group or H)

Difunctional urethanes (i.e. compounds that have two carbamate groups) can be polymerized with a difunctional alcohol. For instance:

           H H                 _/
           | |                / \
  x  H-O-C-C-OH  +    x O=C=N- O -CH3  →
           | |                \_/
           H H

       1,2-ethanediol    a dicarbamate

      _                  _
     |                    |
     |                O   |
     |              N-C-O-|-
     | H H        _/      |
     | | |   O H / \      |
    -|-C-C-O-C-N- O -CH3  |  a polyurethane
     | | |       \_/      |
     | H H                |
     |_                  _|x

More than 95% of the polyurethanes in commercial use are generated from three monomers: toluene diisocyanate (TDI), methylene diisocyanate (MDI) and 4,4-methylene diphenyl diisocyanate.

Polyurethanes are resins with a very high versatility, especially when they are combined with other resins. The physical, chemical and electrical properties of these polymers allow these materials to be used as insulation foam, as foam for furniture and carpet cushioning, for clothing liners, packaging, rocket fuel, elastomers and adhesives.

The most well known application of polyurethane is as a foam. The foaming effect is generated by the addition of a blowing agent or a foaming agent during the polymerization. this could be a low-boiling liquid such as dichloromethane, or a compound that gives off carbon dioxide or nitrogen. For instance, azodicarbonamide gives off carbon monoxide, nitrogen and ammonia upon decomposition. Using this method, foams with densities ranging from 1.5-1000 kg/m3 can be made.

Typical elastomers that contain polyurethanes are Spandex fibers, such as Lycra, Glospan and Numa. These fibers contain approximately 85% polyurethane.

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