The invention relates to a device for producing polymers, preferably for processing and polycondensation of polyester, having a substantially cylindrical reactor (1), which has an inlet opening (2) on one side and an exit opening (4) on the other side and an outlet (20) for discharging gases. According to the invention, said device is developed such that the device has a compact design, and such that the device allows the use of higher-viscosity initial substances, even when generating smaller volumes, which is optimally adjustable in respect of the process conditions thereof and which represents a variable, cost-effective solution. According to the invention, the reactor (1) has a continuous, central shaft (6), on which agitating elements or agitating elements and conveying elements are arranged, the inlet opening (2) is connected to an entry extruder (3), the shaft (6) of the reactor and the shaft (6) of the entry extruder (3) form a common shaft (6), and a drive (7) for the common shaft (6) is allocated to the entry extruder (3).