Polysulfones
These materials are an important group of engineering plastics. Aliphatic polysulfones were first synthesized at the end of the nineteenth century [109]. That synthesis was based on reactions of SO2 with olefins:
Aliphatic sulfones, however, lack good thermal stability and are not commercially important. Aromatic sulfones, on the other hand, have many desirable physical properties. They are clear, rigid, tough materials, with a high Tg. Several aromatic sulfones are prepared commercially.
The original preparation of aromatic polysulfones was described in 1958 [110]. This was followed by investigations of many different structures of polysulfones. One current commercial material is a condensation product of 2,2′ bis(hydroxyphenyl) propane with 4,4′-bis(chlorophenyl) sulfone. It forms by a Williamson synthesis, because the reactivity of the halogens is enhanced by the sulfone groups [47]:
The condensation takes place at 160°C in an inert atmosphere and in some suitable solvents, like chlorobenzene. Commercially, polymers are available in molecular weight ranges from 20,000 to 40,000. Much higher molecular weight materials, however, form readily. Aromatic polysulfones also form by the Friedel-Craft reaction [111], as for instance:
The reaction takes place at 80-250°C in a solution or in the melt. Lewis acid catalysts are used in concentrations of 0.1-1.0 mole percent. Cross-linking through polysubstitution does not appear to be a problem. Some chain branching, however, does occur because ortho substitution is possible:
It is estimated, however, that the ortho substitution amounts to not more than 5-10%. Removal of the catalyst after the reaction is tedious. Table 7.6 presents the Tg values of some aromatic polysulfones.
