The invention relates to a reactor assembly for the production of polymers including a fluidized bed reactor (1) comprising a bottom zone (5), a middle zone (6) and an upper zone (7), an inlet (8) for the fluidization gas located in the bottom zone (5), an outlet (9) for the fluidization gas located in the upper zone (7); the outlet (9) for the fluidization gas being coupled with the fluidized bed reactor (1) via inlet (8); the equivalent cross-sectional diameter of the bottom zone (5) being monotonically increasing with respect to the flow direction of the fluidization gas through the fluidized bed reactor; the middle zone (6) having an essentially constant equivalent cross-sectional diameter with respect to the flow direction of the fluidization gas through the fluidized bed reactor; the equivalent cross-sectional diameter of the upper zone (7) being monotonically decreasing with respect to the flow direction of the fluidization gas through the fluidized bed reactor; wherein that the ratio of the height of the fluidized bed reactor to the equivalent cross- sectional diameter of the middle zone of the fluidized bed reactor is from 2 to 10; and wherein there is an unobstructed passageway in the direction of flow of the fluidization gas through the fluidized bed reactor from the bottom zone (5) to the upper zone (7).