Copolymers of telechelic poly(2,6-dimethyl-1,4-phenylene ether) (PPE) segments, uniform crystallisable tetra-amide units (T6T6T, 6–15 wt%) and different diols (C2–C36, polytetramethylene oxide) as an extender were synthesised. The telechelic PPE segment was end-functionalised with terephthalic ester groups and had a molecular weight of 3100 g/mol. The coupling between the PPE segment and the T6T6T unit was made with diols. The influence of the length and flexibility of the diol-extender and the concentration of the T6T6T units were studied on the thermal (DSC) and thermal–mechanical (DMA) properties of the copolymers. A crystalline T6T6T phase in the copolymers was evident from 9 wt% onwards. The length of diol extender had an effect on the glass transition temperature of the PPE phase, the crystallinity of the T6T6T segments and modulus above the glass transition temperature. With ethylene glycol the Tg of the copolymer was high but the crystallinity of the T6T6T rather low. With dodecanediol or hexanediol as an extender the Tgs of the PPE phase were somewhat lower, but the crystallinities of the T6T6T segments higher. With C36 and polytetramethylene oxide diols, the Tg were strongly decreased and broad and the modulus above the glass transition temperature not so high.