We prepared polyurethane block copolymers with both 50 and 70% soft segment concentrations, using 4,4'-diphenylmethane diisocyanate-polypropylene glycol) prepolymer and 1,4-butanediol, cis-2-butene-1,4-diol, and 2-butyne-1,4-diol as chain extenders. The effects of the different chain extenders were observed during synthesis and in the final products. A comparison of spectroscopic, mechanical, and thermal data reveals that polymer properties can be significantly altered by differences in chemical bonding within the chain extender backbone. Although all data support the expected differences in phase morphology between the two series of samples, they also suggest that increasing chain extender unsaturation reduced reactivity with isocyanate, adversely affected hydrogen bonding, lowered the degree of crystallinity of the hard segments, and decreased phase separation. The tensile strength, elongation, modulus, and elastic recovery decreased and the electrical conductivity of iodine-doped samples increased with increasing chain extender unsaturation. The thermal stability of the urethane group was also lower in samples with increased unsaturation.