Abstract: A crystallizable thermoplastic aromatic polyetherketone, the polymeric chain of which contains the repeat unit--Ar.sub.1 --CO--Ar.sub.2 --O--where Ar.sub.1 and Ar.sub.2 are aromatic radicals which may be the same or different and comprise at least one aromatic ring, the polymeric chains having terminal ionic groups --A--X, where A is an anion and X is a metal cation. The anion is preferably selected from sulphonate, carboxylate, sulphinate, phosphonate, phosphate, phenate and thiophenate and the cation is preferably alkali metal or alkaline earth metal. The products may be used in blends with other polyetherketones as a nucleant for the other polyetherketones.