Reductive Dehalogenation vs Substitution in the Polyetherification of Bis(aryl chloride)s Activated by Carbonyl Groups with Hydroquinones: A Potential Competition between SET and Polar Pathways

Abstract

Aromatic poly(ether ketone)s are frequently synthesized by nucleophilic substitution of the halide of a bis(aryl halide) by a bisphenolate. The bis(aryl halide) is activated toward nucleophilic attack by a carbonyl group. When the bisphenol is hydroquinone, bis(aryl fluoride)s afford high molecular weight polymers, whereas bis(aryl chloride)s often yield low molecular weight polymers. This paper demonstrates that bis(aryl chloride)s and hydroquinones can be condensed to produce high molecular weight polymers. The ability to obtain high molecular weight polymers from bis(aryl chloride)s is dependent on the structure of the bisphenolate and the reaction conditions. The molecular weight that can be achieved in the polyetherification of bis(aryl chloride)s with hydroquinone or substituted hydroquinones can be limited by the occurrence of reductive dehalogenation. Reductive dehalogenation has not been observed in the condensation of bisphenolates which are weaker electron donors (i.e., have less negative oxidation potentials) than the dianion of hydroquinone, such as the bisphenolates of 4,4'-isopropylidenediphenol (Bisphenol A or BPA), 4,4'-oxydiphenol (ODP), 1,1-bis(2-methyl-4-hydroxy-5-tert-butylphenyl) ethane (MHBPE), etc., with bis(aryl chloride)s under identical conditions to those used in the polyetherifications involving the hydroquinones. These results strongly suggest that reductive dehalogenation is due to single electron transfer (SET) from the dianion of hydroquinone (or substituted hydroquinones) to the 4-chlorobenzophenone moiety. In many cases, the molecular weights of the polymers synthesized from bis(aryl chloride)s and hydroquinones are not determined by the reactivity of the monomers but by their selectivity. The factors that govern the selectivity between aromatic nucleophilic substitution and reductive dehalogenation are discussed