Polyurethane

A method for preparing a polyurethane, comprising:

• reacting a polybutadiene having a hydroxyl group at a terminal end thereof with an ε-caprolactone to obtain a polymer polyol of formula (I);
• reacting the polymer polyol and an unsymmetric diisocyanate to obtain a urethane prepolymer having an isocyanate group at a terminal end thereof, wherein an amount of the unsymmetric diisocyanate is determined so that a molar ratio of a mole number of isocyanate groups derived from the unsymmetric diisocyanate to a mole number of hydroxyl groups derived from the polymer polyol is within a range of 2.1 to 3.8;
• and then
• reacting a chain extender with the urethane prepolymer to obtain the polyurethane, wherein an amount of the chain extender is determined so that a molar ratio of the mole number of the isocyanate groups derived from the unsymmetric diisocyanate with reference to a total mole number of hvdroxyl groups derived from the polymer polyol and the chain extender is within a range of 0.9 to 1.1,
• HO—X1—Y—X2—OH (I)
• wherein:
• X1 and X2 each independently represents a polyester component, and
• Y represents a polybutadiene component consisting of:
• a 1,4-structure consisting of a repeating unit represented by formula (IVb);
• and
• a 1,2-structure consisting of a repeating unit represented by formula (Vb):
• wherein a molar ratio of the 1,2-structure/the 1,4-structure is within a range from 90.4/9.6 to 95/5.