Poly(oxy-1,4-phenylene-carbonyl-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1,4-phenylene-carbonyl-1,4-phenylene) (PEKEKK) was heated in the presence of potassium carbonate at 340°C to yield another aromatic polyetherketone, which contained not only the original EKEKK repeating unit but also the EK and EKK repeating units. This indicates that trans-etherification reaction occurs between molecules of PEKEKK. Poly(oxy-1,4-phenylene-carbonyl-1,4-phenylene) (PEK) reacted with poly(oxy-1,4-phenylene-sulfonyl-1,4-phenylene) (PES) in the presence of potassium carbonate at 300°C to yield a trans-etherification product, i.e., a block copolymer of P(EK)/(ES). A further reaction decreased the crystallinity of the block copolymer to yield a random copolymer.