Polymerisation control process

1. A (co-)polymerization process which comprises:

(i) continuously (co-)polymerizing a principal monomer in a polymerization zone of a fluidized bed reactor in the presence of a metallocene polymerization catalyst under reactive conditions to form polymer powder particles;

(ii) withdrawing polymer powder particles from the reactor;

(iii) withdrawing from the continuous (co-)polymerization process a hot recycle stream containing the principal monomer and at least another unreacted reactant;

(iv) cooling part or all of said recycle stream withdrawn from said reactor to a temperature sufficient to form a liquid and a gas; and

(v) recycling said cooled recycle stream containing the principal monomer and the at least another unreacted reactant through the polymerisation zone in said reactor, wherein a density SPAN of the polymer powder particles formed is at or below 0.0025, the density SPAN of the polymer powder particles formed being the absolute value of the density difference in g/cm 3 between the average density of polymer powder particles with a particle size above 90% cumulative volume of the volume/particle size distribution of the (aggregate) polymer particles withdrawn from the reactor and the average density of polymer powder particles with a particle size below 10% cumulative volume of the volume/particle size distribution of the (aggregate) polymer particles withdrawn from the reactor.

2. The process according to claim 1 , wherein the principal monomer is ethylene or propylene.

3. The process according to claim 2 , wherein the principal monomer is ethylene.

4. The process according to claim 1 , wherein the density SPAN of the polymer powder particles is at or below 0.0023.