There are some puzzles in the polymerization of propylene on heterogeneous Ziegler-Natta catalysts. The broad molecular weight distribution (MWD) of polypropylene might be caused by two reasons. The first reason is the diffusional resistance by polymer around the active sites. The second reason is the presence of more than one kind of active sites on Ziegler-Natta catalyst. Both of the two reasons have been tested experimentally

but no general explanation has been generally accepted. A new kinetic model

namely Uniformly-distributed Multigrain Model (UMGM) is proposed to explain the broad MWD and reaction rate during the growth of a single catalyst-polymer particle. The model is based on experimental data and actual plant operation results. At the beginning stage of polymerization

a catalyst macroparticle breaks into thousands of uniformly-distributed primary particles. These microparticles are all identical in radius and uniformly-distributed in the growing macroparticle throughout the whole reaction. Monomers diffuse through polymer blank and react on the microparticles surface to form new polymer. Discussions are concentrated on diffusion effect and it is supposed that only one type of active site is distributed on catalyst surface. The model can account for multiple-active sites quite easily. Some simulation results are given to compare the physical and chemical reasons for broad MWD. The influences of catalysts particle and subparticle size

catalysts activity and monomer diffusion coefficient on polymerization are analyzed.The results show that the degree of polymerization (DP) and polydispersity index Q have strongly antisymmetric distribution under different diffusion coefficient D. D iffusion effects are more prominent with highly active catalyst. When catalyst radius and primary particle radius are small

diffusion resistance is not important. UMGM can be extended to multiple active sites and more complex polymerization kinetics. It lays a foundation for the modeling and optimization of actual industrial reactors.pposedthatonlyonetypeofac