Poly fluorine displacement reaction

Scheme 2.17 Synthesis of hb poly(arylene ether)s from monomers containing nitro group (a) via selective and sequential S vjAr (preferential fluorine displacement followed by nitro displacement) reaction of As-and BC-type monomers (b) via nitro displacement reaction of A2C-type monomer. Taken from Ref. [76],...

It is interesting to note that all the new aromatic systems, as described, undergo displacement polymerizations in DMAC solvent by the K2CO3 method, except perfluoroalkylene [10] and amide activated polymerization [9], which were performed in NMP solvent. The displacement polymerization in DMAC solvent was carried out at 155-164°C. poly(aryl ether ketones) require less reaction time (3-6 h) than other aromatic systems for synthesis of polyethers [15]. Synthesis of the fluorinated polyether as reported by Irvin et al. [16] was carried out at room temperature for 16 h (Mw = 75,000), whereas the same polymer by Mercer et al. [17] was synthesized at 120°C for 17 h (Mw = 78,970). 

Diazotization in the presence of boron trifluoride enables diazonium tetrafluoroborates to be isolated from the reaction mixture and purified. Subsequent controlled decomposition produces the required fluoroaromatic. Although explosion hazards and the toxicity of the isolated salts are significant concerns with this process, known as the Balz-Schiemann process, 4,4 -di-fluorobenzophenone (BDF. 6) has been prepared by this route as a monomer for the production of the engineering plastic poly(ether ether ketone) , or PEEK , by condensation with 1,4-dihydroxybenzene in the presence of potassium carbonate. BDF 6 is superior to its chlorine analog because in aromatic systems the nucleophilic displacement of fluorine is more facile than that of chlorine, leading to a shorter polymerization time and a better quality product containing less degradation impurities. 

The order of nucleophilic substitution for the displacement of fluorine atoms in pentafluoropyridine, as outlined above, is generally in the order 4 > 2 > 3, but this reactivity can be altered by reaction with bifunctional nucleophiles. Substitution of the 4-position may be followed by attack at the adjacent 3-position due to the geometric constraints of the system as outlined in Fig. 8.9. Similarly, tetrafluor-opyridine derivatives bearing substituents at the 4-position react with appropriate difunctional nucleophiles to give poly functional annelated sy stems. 

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