Our preferred polymers which are generally thermally stable at very high temperatures, even above their melting points consist of repeating units having the structure ##STR11## where Z is an oxygen atom or a sulphur atom, and R1, R2, R3 and R4 are each selected from the group consisting of halogen atoms, alkyl groups containing from 1 to 4 carbon atoms and alkoxy groups containing 1 to 4 carbon at