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The conventional poly(ether ether ketone) (PEEK) and two other PEEK copolymers (PEEEKK-PEK and PEEEKK-PEEEK) were successfully synthesized and investigated on their thermal stabilities, mechanical properties, and rheological behaviors. Both of the PEEEKK-PEK and PEEEKK-PEEEK were composed of the same chemical components with conventional PEEK (phenyl–ether–ketone equals 3:2:1), but the sequences of their components (phenyl, ether, and ketone) were different. Differential scanning calorimetry analysis and dynamic mechanical analysis indicate that PEEEKK-PEK and PEEEKK-PEEEK have extremely close glass transition temperatures and melting points with conventional PEEK, which suggests their similar operating temperatures. They also process similarly high mechanical properties based on their stress–strain tests. However, PEEEKK-PEK, PEEEKK-PEEEK, and PEEK exhibit significant differences in their rheological behaviors; PEEEKK-PEK even shows an excessively low melt viscosity (51% of PEEK). These results reveal the effects of sequence distribution on polymer properties and thereby demonstrate the processing viscosity of PEEK could be decreased without sacrificing its operating temperature or material performances.