Method for producing prepolymers with an isocyanate termination for producing polyurethanes

Method for preparing an isocyanate-terminated polyurethane prepolymer having a free diisocyanate monomers content of less than or equal to 0.1% by mass and having an NCO value of 2 to 6, said method comprising the following steps:

• a) reacting at a temperature of 20 to 70° C. with degassing:
• i. diisocyanate monomers selected from diisocyanates having two NCO groups having a difference in reactivity higher than 6, said difference in reactivity resulting from the dissymmetry of the monomers and/or from a substitution effect;
• ii. polyols with a functionality of 2 having an average molar mass of 150 to 3000 g/mol;
• the diisocyanate monomers and the polyols being in such a quantity that the ratio of “number of NCO functions” to “number of OH functions” varies from 1.4 to 2;
• b) degassing and stabilizing the reaction medium obtained in step a) at a temperature of 50° C. to 110° C.;
• c) recovering said isocyanate-terminated polyurethane prepolymer.