Meta-Activated Nucleophilic Substitution - use in Preparation of Polymers

Polyphenylene ethers are important high performance engineering plastics and their synthesis depends on efficient aryl ether linkage formation, usually by S_nAr reactions. This reaction usually requires the aromatic ring to have activating groups such as sulphone, ketone and imide and polymers containing these functional groups have been successfully commercialised using this reaction. Usually the activating group is in the ortho or para position, although there are one or two examples of meta activation; these reactions are, however, not suitable for polymer preparation. Perfluoralky groups activate fluoro or nitro leaving groups for displacement by phenoxy anions but only ortho or para positions have been reported. A recent paper (Chung I S et al, J Amer Chem Soc, 2001, 123, 11071) shows that displacement of nitro groups by phenoxy anions occurs when CF3 in a meta position activates, but fluoride is less easily displaced. Reactions were complete in 12h at 175^o in NMP without any side reactions in the case of NO₂. The reaction was used to make polymers with weight-average molecular weights of over 50,000 and high thermal stability.