The mechanism of aromatic nucleophilic substitution is closely related to carbonyl addition elimination.
Mostly, bisphenols are used as the nucleophillic components. There are examples using diamines, dithiols, difunctional aliphatic alcohols, and other double nucleophiles, but these are rare. The chemistry invariably begins when a base like NaOH or K2CO3 is used to deprotonate the bisphenol, as in this example for Bisphenol A:
To form PEEK, the bisphenolate is reacted with difluorobenzophenone.
In this example, fluorine is the leaving group of choice because it is much more reactive than other halogens like chloride. Unfortunately, fluorinated compunds are much more expensive than chlorinated ones.
The most common form of PEEK is the one shown, derived from Bisphenol A. This polymer is a remarkable material, highly crystalline, thermally stable, resistant to many chemicals, very tough. It can be melt-processed at very high temperatures (>300 °C), and is useful for special applications like pipes in oil refineries and chemical plants, and parts for aerospace, where high price is not a limitation.
Similar chemistry can be used, substituting a sulfonyl group for the ketone, above.
Sulfone is a better electron withdrawing group than ketone, so chloride can be used here in place of fluoride as for PEEK, above. (Of course, fluoride would work well for poly(ethersulfone), but at higher cost.)
This polymer is often called simply "Polysulfone," but this is slightly misleading, especially when compared to structurally similar PEEK.
Unlike PEEK, poly(etherethersulfone) is completely amorphous, probably a result of the relatively large size of the sulfonyl group, and the kink in the polymer backbone caused by the narrow C-S-C bond angle (usually close to 100°). Therefore, it can be processed at lower temperature than PEEK, but the material is not as resistant to heat and chemicals.
Recently, more complicated electron withdrawing groups have been employed, leading to a family of polyethers that have heterocyclic rings in the backbone. The idea here is the same as in the cases above, except that an electron deficient heterocycle takes the place of a ketone or sulfone.
One example among many by the IBM group led by J. Hedrick and J. Labadie is:
The mechanism involves accomodation of the negative charge in the intermediate by the heterocycle, as shown in the partial structure below.
Polymers in this family have very high Tg, are thermally stable, and often are soluble in ordinary solvents.
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