Siloxane-modified polyurethanes were prepared through isophorone diisocyanates (IPDI) poly(tetramethylene oxide) (PTMO), and siloxanes. IPDI served as the hard segment in the structure. Both PTMO and siloxanes were diols and served as the soft segments. In addition, different chemical structures of siloxanes were used, in which siloxane chains would remain in the main chain of polyurethanes (PU) or become the side chain of PU. First, the reactivities of PTMO and siloxanes to react with IPDI in bulk system were studied through DSC, in which the reaction heat was related to their reactivities. Copolymerization of IPDI, PTMO, and siloxanes in bulk were also studied. The results showed that hydrophobicity and steric hindrance of siloxane diols led to their low reactivities. Next, a series of siloxane-modified PU in toluene solvent were synthesized, and the conversion of NCO groups was determined by the method of chemical titration. In the synthesis of PU copolymers in a solution polymerization, because of low reactivity of siloxanes, a two-step procedure was adopted. The siloxane diol was first reacted with IPDI in toluene to form NCO-terminated prepolymer. Then PTMO was added to form final PU copolymers. The addition of side-chain siloxanes resulted in PU copolymers with higher molecular weight than main-chain siloxanes. Both main-chain and side-chain siloxanes increased the elongation at break and tensile strength of final PU copolymers. The microphase-separation of siloxane segments was observed by SEM, which was the main cause for the improved mechanical properties.