PEO-based polymer electrolytes usually show a high degree of crystallinity, leading to a drop of ionic conductivity, at temperatures below their melting point [1]. However, the incorporation of foreign units in the polymer chains, forming a copolymer, can reduce the crystallinity while keeping the conductivity high if the units are properly chosen. In this study, a series of poly(styrene-co-styrene trifluoromethanesulfonylimide of potassium)-b-poly(ethylene oxide)-b-poly(styrene-co-styrene trifluoromethanesulfonylimide of potassium) (P(S-co-KSTFSI)-b-PEO-b-P(S-co-KSTFSI)) BAB triblock copolymers was synthesized by reversible addition-fragmentation chain transfer polymerization (RAFT) in a dispersed medium. It was found that the copolymers, with or without styrene, containing 59% (wt%) or more PEO, are semi-crystalline while, below this value, they are amorphous. However a low PEO content can be compensated by a low KSTFSI/PEO ratio for the BAB copolymers with styrene since materials containing only between 53 and 58% (wt%) PEO can still crystallize for a KSTFSI/PEO ratio of about 40/100. For the copolymers that crystallize, with or without styrene, the degree of crystallinity decreases with a reduction of the PEO content. In the case of a constant PEO content, the degree of crystallinity increases with the styrene content (and with the decrease of the KSTFSI/PEO ratio). In all cases, only one T g is observed, the only difference is the width, which is larger for the copolymers than the corresponding homopolymers.