Poly(p-phenylene) derivatives with pendant-type side chains of various lengths were synthesized in order to investigate the effect of side chain structure on properties such as sulfonation, thermal stability, mechanical strength, water uptake, dimensional stability, proton conductivity, and polymer electrolyte fuel cell (PEFC) properties. The sulfonation reactions were affected by the electron density of side chains. All sulfonated polymers showed thermal stabilities up to 240 degrees C, which is sufficient for polymer electrolyte membranes used in PEFC. Although the glass transition temperatures were about 200 degrees C regardless of the side chain structure, the mechanical strength was influenced by the side chain length. The number of adsorbed water molecules per sulfonic acid group (X) decreased with increasing side chain length, resulting in a lower swelling ratio. The proton conductivity increased with an increase in ion exchange capacity and lambda, and S-P1(2.79), Which has the shortest side chain, showed the highest proton conductivity. All the sulfonated polymer membranes exhibited sufficient cell performance, but S-P2(2.27) showed the highest performance, revealing a different order of performance among the membranes from that of proton conductivity. It is suggested that the higher water uptake of S-P1 provides not only higher proton conductivity but also lower carrier density, resulting in increased membrane resistance in fuel cell tests.