Abstract：A concept of critical number-average molecular weight(Mn c) was proposed to guide solvent-free synthesis of polyurethane. The waterborne polyurethane dispersion(WPU) was prepared in low-solvent and solvent-free manner from the raw material isophorone isocyanate(IPDI), various polyether diols, and dimethylolbutanoic acid(DMBA) or dimethylolpropanoic acid(DMPA). The results showed that a low viscosity and easily dispersible pre-polymer could be prepared by controlling the molecular weight of the dispersion below Mn c and introducing moderate amount of hydrophilic monomer. The obtained value of Mn c was in the range of 1 800-2 400 g/mol, depending upon the loading of hydrophilic monomer, the structure of soft segment and the presence of a small amount of solvent. The value of Mn c increased with the increase of hydrophilic monomer loading or the presence of some solvent. 4% monomer loading was appropriate for getting a stable dispersion, because high monomer loading led to high water absorption on the PU film. Compared to low-solvent synthesis, solvent-free synthesis was characterized as an environmental benign technology with 100% modulus yield and low water absorption as well as high tensile strength of the PU film.
