TPU are formed by the reaction of diisocyanates with short-chain diols (chain extender) and long-chain diols.

The practically unlimited amount of possible combinations producible by varying the structure and/or molecular weight of the three reaction compounds allows for an enormous variety of different TPU. This allows urethane chemists to fine-tune the polymers structure to the desired final properties, such as hardness, chemical resistance, and yellowing resistance.