A series of poly(cyclohexylethylene-b-ethylene-co-ethylethylene) (C-E/E-E) diblock copolymers containing approximately 50% by volume glassy C blocks and varying fraction (x) of E-E repeat units, 0.07 <= x <= 0.90, was synthesized by anionic polymerization and catalytic hydrogenation. The effects of ethyl branch content on the melt state segment-segment (;() interaction parameter and soft (E/E-E) block crystallinity were studied. The percent crystallinity ranged from approximately 30% at x = 0.07 to 0% at about x >= 0.30, while the melting temperature changed from 101 degrees C at x = 0.07 to 44 degrees C at x = 0.28. Dynamic mechanical spectroscopy was employed to determine the order-disorder transition (ODT) temperatures, from which chi was calculated assuming the mean-field prediction (chi N-n)ODT 10.5. Previously published results for the temperature dependent binary interaction parameters for C-E (x = 0.07), C-E-E (x = 0.90), and E-E-E (x = 0.07 and x = 0.90) fail to account for the quantitative x dependence of chi, based on a simple binary interaction model. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 566-574, 2010