A method for controlling the injection of catalyst slurry in a polymerization reactor is described herein. The method includes the preparation of polyethylene with a solid catalyst and a hydrocarbon diluents. The method includes providing solid catalyst and a hydrocarbon diluent in one or more storage vessels such that a catalyst slurry is obtained in said vessel, transferring said catalyst slurry from said storage vessel to a mixing vessel wherein said catalyst slurry is diluted for obtaining a suitable concentration for use in a polymerization reaction, pumping said diluted catalyst slurry at a controlled flow rate from said mixing vessel to said polymerization reactor through one or more conduits, by means of a membrane pump provided in each of said conduits, and the membrane pump is controllable as a function of reactant concentration within the polymerization reactor.