A Ziegler-Natta-catalyst was used in ultra low-yield slurry prepolymerization followed by liquid propylene (main) polymerizations. Complete catalyst disintegration down to 1.5-2 mu m particle size is observed at prepolymerization yields of 10 g per g cat. The initial (main) polymerization rate increased up to 55% and the final average particle diameter can be controlled between 50 and 1500 mu m at main polymerization yields of 20 kg PP per g cat center dot hr(-1). Tension generation within the particle and the absence of a polymer layer explains these results.