Generally Poly Urethane is used in manufacturing machine drive belts so it can withstand the pull under stress.
Comparing with silicone rubber strings, Poly Urethane has a smoother surface. This is a real benefit so players can play it smoothly without talc powder,or any stress on finger tips.
Since Poly Urethane is harder than silicone rubber, the strings last a long time… yet can play the same mellow sound as with silicone bass strings.