Randomly copolymerized poly(carbonate) glycols were employed as starting materials for the synthesis of polyurethane elastomers (PUEs). The poly(carbonate) glycols had hexamethylene (C-6) and tetramethylene (C-4) units between carbonate groups in various composition ratios (C-4/C-6 = 0/100, 50/50, 70/30, and 90/10), and the number-average molecular weights of these poly(carbonate) glycols were 1000 and 2000. The PUEs were synthesized with these poly(carbonate) glycols, 4,4'-diphenylmethane diisocyanate, and 1,4-butanediol by a prepolymer method. Differential scanning calorimetry measurements revealed that the difference between the glass-transition temperature of the soft segment in the PUEs and the glass-transition temperature of the original glycol polymer decreased and the melting point of the hard-segment domain increased with an increasing C-4 composition ratio. The microphase separation of the poly(carbonate) glycol-based PUEs likely became stronger with an increasing C-4 composition ratio. Young's modulus of these PUEs increased with an increasing C-4 composition ratio. This was due to increases in the degree of microphase separation and stiffness of the soft segment with an increase in the C-4 composition ratio. The molecular weight of poly(carbonate) glycol also influenced the microphase-separated structure and mechanical properties of the PUEs. The addition of different methylene chain units to poly(carbonate) glycol was quite effective in controlling the microphase-separated structure and mechanical properties of the PUEs. (C) 2004 Wiley Periodicals, Inc.