This reaction is analogous to carbonyl addition-elimination, in that it is a two step process where the negative charge is accomodated by an electron withdrawing group. To emphasize the simularity, this example uses a ketone:
The negative charge on the intermediate is spread out into the benzene ring and onto the ketone by resonance. The resonance form with the charge on oxygen looks very much like the intermediate in carbonyl addition elimination, except that there is a benzene ring between the nucleophile and the carbonyl.
The initial attack is usually the rate limiting step. Fluorine works better than other halogens as a leaving group in this case because it is small (low steric hindrance), and very electronegative (causing a strong partial positive charge on the carbon bearing F, making it more attractive to the incoming nucleophile). This is contrary to SN2 reactions, which are single step, and for which fluorine is a poor choice compared to the larger halogens Cl, Br, and I.
The comparison of aromatic nucleophilic substitution to carbonyl addition-elimination is an example of "vinylogy," which means the action of a functional group is transmitted through a conjugated pi-bonded framework to a remote location.
Reference:
Bisphenols are most often used as the nucleophillic components. The chemistry begins when a base like NaOH or K2CO3 deprotonatea the bisphenol, as in this example for Bisphenol A:
The other monomer has two groups like this:
where EWG is an electron withdrawing group such as a ketone, ester, sulfone, etc., and X is a leaving group such as Cl or F.
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.
The polymerization chemistry is very much like that of PEEK, except substituting a sulfonyl group for the ketone.
Sulfone is a stronger 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 polysulfone, but at higher cost.)
Like polycarbonate, many other polysulfones could be synthesized, but the particular one shown here is by far the most common commercially, so that the general term "polysulfone" usually refers to this particular one. Worse, it is seldom called "poly(etherethersulfone)," despite its close structural similarity to 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 (close to 100°). Therefore, it can be processed at lower temperature than PEEK, but the material is not as resistant to heat and chemicals.
Back to Step Polymerization
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