If the isocyanate index (NCO / OH) is greater than 2, there will be excess isocyanate monomer in the prepolymer. If NCO / Oh ≫ 2, the product is actually a mixture of isocyanate terminated prepolymer and isocyanate monomer. This mixture is called a semi prepolymer.

The semi prepolymer method is often used in the production of MDI type CPU elastomer products. This is because the viscosity of MDI prepolymer is high, the molecular weight of commonly used chain extender such as 1,4-butanediol is small, the coordination amount is small, the measurement and proportion are difficult to be accurate, and it is not easy to mix evenly. In order to overcome these disadvantages, in the synthesis of prepolymer, a part of oligomer polyol is added, and the semi prepolymer with low viscosity is synthesized first. Then, in the second step of chain extension reaction, the oligomer polyol (or oligomer polyol) and small molecule glycol chain extender are reacted with the semi prepolymer. In this way, the volume of the two components is relatively close, and the viscosity of the material is relatively low, which can greatly improve the accuracy of measurement and proportioning, and improve the mixing effect. On the other hand, MDI has low vapor pressure and is not easy to volatilize, so it will not cause much harm to the environment if it exists in the semi prepolymer. This is also the reason why the semi prepolymer method is often used in MDI system. However, in terms of the physical properties of the products, the semi prepolymer method is usually lower than the prepolymer method. The vapor pressure of TDI is high and volatile, which is harmful to the environment and human health. Therefore, it is not suitable to make semi prepolymer. TDI type prepolymers have low viscosity and are often used with chain extenders such as MOCA. Their molecular weight and complex weight are much larger than 1,4-butanediol, and the metering and mixing effects are better.