The pressure effect on the reaction kinetics, molecular weight, and the microphase separation of the soft and hard segment of the polyurethane, synthesized under high pressure, was studied. The polymerization rate increased with increasing synthesis pressure and the activation energy of polymerization decreased with increasing synthesis pressure from 22.4 kcal/mol at 1 kg/cm2 to 19.4 kcal/mol at 3000 kg/cm2. The volume of activation of the polyurethane polymerization was −19.8 cm3/mol at 60°C. Microphase separation between soft and hard segment decreased with increasing synthesis pressure and increased with increasing synthesis temperature. Molecular weight and molecular weight distribution were not influenced by the synthesis pressure.