Effects of reaction temperature on the formation of polyurethane prepolymer structures

For the first time, the molecular weight distribution and extent of side reactions have been quantified in polyurethane prepolymers. A combination of NMR, size exclusion chromatography, and mass spectrometry has been used to measure the molecular weight distribution. The values obtained are consistent with Flory's step polymerization theory. The type and amount of side reactions as a function of reaction temperature have also been investigated. At low temperature of reaction, the amount of branches arising from allophanate linkages is negligible. With only a relatively modest change in reaction temperature, this type of side reaction increases dramatically, so that at 145 °C as much as 10% of the nitrogens participate in allophanate linkages. The effect of this reaction on the molecular weight distribution was also measured. Upon formation of these side products, the molecular weight distribution broadens and the average increases. In this prepolymer system, the reactivity is independent of molecular weight.