A novel macrodiol based on mixed silicone and carbonate chemistries was synthesized and used as a soft segment precursor in the synthesis of two series of segmented polyurethane (PU) copolymers varying in hard segment content and soft segment molecular weight. The hard segments in these copolymers were derived from 4,4-methylene diphenyl diisocyanate and 1,4-butane diol. The phase transitions, microphase separation behavior, and mechanical properties of the copolymers were investigated using a variety of experimental methods. When compared with segmented PU copolymers having predominately poly(dimethyl siloxane) soft segments, these siloxane-carbonate soft segment copolymers exhibit enhanced intersegment mixing, and consequently relatively low mechanical modulus. With relatively low modulus and siloxane units in the soft phase, the siloxane-carbonate PUs have potential for use in cardiac and orthopedic biomedical applications. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: PolymPhys 49: 865-872, 2011