We report a numerical technique to investigate the branching process of linear Ziegler-Natta polypropylenes. Using an iterative procedure, linear chains are created based on the molecular weight distribution (MWD) of linear Ziegler-Natta polypropylenes as determined by GPC. By varying one parameter, MWDs of polymers with various levels of branching are simulated, and the simulated MWDs agree very well with experimental GPC data of branched polymers. From the simulation, the average branching parameters as well as the branching distributions of the polymers can be obtained. The branching information is related to the moments of the MWD, and a criterion of the onset of gelation is proposed. The melt flow rate (MFR) is correlated with the weight-average molecular weight. These relationships make it possible to design a polymer having a prescribed MFR. (C) 2003 Elsevier Ltd. All rights reserved.