Abstract: Particulate olefin polymers are prepared by catalytic polymerization of monoolefins in an auxiliary liquid containing the monomers to be polymerized in dissolved form and the particulate polymer product in suspended form. The reaction mixture is recycled in a loop to which the starting materials are fed and from which the particulate polymer formed is removed. The essential features are (a) that a side stream is branched off from the loop, (b) that the side stream is caused to pass through a separating zone in which particulate olefin polymer is separated by centrifugal acceleration at temperatures and pressures in the same range as present in the loop and (c) that the separated particulate olefin polymer is discharged through flashing means.