In equilibrium reactions, it is therefore necessary to remove the condensation product continuously and efficiently from the reaction medium in order to drive the equilibrium towards polymer. The condensation product is therefore removed at reduced pressure and high temperatures (150–320 °C, depending on the monomers) to prevent the back reaction. With the progress of the reaction, the concentration of active chain ends is decreasing and the viscosity of the melt or solution increasing. For an increase of the reaction rate, the reaction is carried out at high end group concentration (preferably in the bulk), promoted by the elevated temperatures.