Patent ID: 7317121

Claim:
A process for preparing oligocarbonate polyols, comprising A) reacting dimethyl carbonate with a polyol component, in the presence of an ytterbium compound transesterification catalyst, at pressures of atmospheric pressure to 6 bar and temperatures of between 100 to 170° C., wherein step A) comprises n≧2 successive substeps A i ) in which i takes values of 1 to n, and in each substep A i ), a pressure 1 p i of between 3 to 6 bar is established in the reactor, a portion x i of the total amount of dimethyl carbonate is added to the liquid phase of the reaction mixture and, after a reaction time t i at a temperature T i , the methanol formed in the reaction is removed from the reaction mixture by distillation in the form of a gaseous methanol/dimethyl carbonate mixture, in the course of which the pressure is reduced, at constant temperature T i , from 1 p i to a pressure 2 p i , and in the last substep A n ), the portion x n still absent from the total amount is added and the pressure 2 p n at the end of the substep is atmospheric pressure; and B) reducing the pressure further until a pressure of ≦100 mbar has been attained, passing an inert gas stream into the reaction mixture, such that further methanol/dimethyl carbonate mixture is distilled off in parallel, and such that the temperature of the reaction mixture does not exceed 195° C.