A process for the production of polymers in the presence of a polymerization catalyst in a reactor assembly including a fluidized bed reactor comprising a bottom zone, a middle zone and an upper zone, an inlet for the fluidization gas located in the bottom zone, an outlet for the fluidization gas located in the upper zone; the outlet for the fluidization gas being coupled with the fluidized bed reactor via inlet; the upper, middle and bottom zones differentiate with respect to their equivalent diameters; the equivalent cross-sectional diameter of the bottom zone being strictly monotonically increasing with respect to the flow direction of the fluidization gas through the fluidized bed reactor; the middle zone 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 is strictly monotonically decreasing with respect to the flow direction of the fluidization gas through the fluidized bed reactor; whereby strictly monotonically increasing equivalent cross-sectional diameter means that the average diameter increases with respect to the flow direction of the fluidization gas through the fluidized bed reactor; whereby strictly monotonically decreasing equivalent cross-sectional diameter means that the equivalent cross-sectional diameter will decrease 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; wherein the fluidized bed reactor is characterized in that no distribution grid is present; 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 to the upper zone, wherein the unobstructed passageway is characterized by a ratio of a free cross-section / total cross-section with respect to the partition between the bottom zone and the middle zone of at least 0.95, whereby the free cross-section is the area allowing interchange of gases and whereby the total cross-section is the area of the inner reactor cross section limited by the walls of the fluidized bed reactor, the process comprising feeding a gas mixture comprising at least one monomer into the bottom zone of said fluidized bed reactor; withdrawing a stream of gas optionally containing solids from the upper zone of said fluidized bed reactor so as to produce an upwards flowing gas stream within said fluidized bed reactor; feeding polymerization catalyst into said fluidized bed reactor; polymerizing said at least one monomer in the presence of said polymerization catalyst; whereby the superficial gas velocity within the middle zone of said fluidized bed reactor is lower than the terminal velocity for the majority of polymer particles contained in the fluidized bed. whereby the increase of the reciprocal of the square root of the superficial velocity of the fluidization gas expressed in m/s, 1 v, per one meter is from 0.66 to 2.5 within said bottom zone in the direction of flow of the fluidization gas.