These are plastics whose molecules contain sulfone groups ( SO2 ) to the main chain, as well as a variety of aromatic or aliphatic constituents such as ether or isopropylidene groups. Aliphatic polysulfones are less stable, for example, to hydrolysis, but are of interest for some biomedical applications. Four major types of polysulfones have been of commercial interest: polysulfone, polyarylsulfone , polyethersulfone and polyphenylsulfone.