Another Look at Site Heterogeneity in Ziegler-Natta Catalysts for Polyolefin Production

摘要

Ziegler Natta (ZN) catalysts are used to produce a wide range of polyolefins on an industrial scale. It has long been known that polymer produced on these catalysts have a relatively wide distribution in chain lengths; values for the Poly-Dispersity Index (PDI) are generally well above 2, the value that would be expected if a single catalyst site type were active. These catalysts are conceptualised as containing a number of different active sites, which produce polymer with different average molecular weights. No physical explanation for the existence of different site types has been offered. In fact it has been shown that there is a close link between activity and oxidation state, and that only a single type of polymerising site is present. In this work, we propose that sites differ not in propagation rate but in the rate at which chain growth is terminated by a range of terminating agents, including hydrogen, monomer and co-monomer. Each terminating agent is considered to act at a particular ‘‘pseudo-site’’. The ability of this interpretation to simulate a wide range of polymer properties is demonstrated. The pseudo-sites model is then used to investigate experimentally-determined chain length distribution data for ethylene-1-butene copolymer produced in lab reactors. It is shown that this new view of the heterogeneity of ZN active sites can successfully reproduce key polymer properties, including PDI and mean chain lengths. Based on these results, it is proposed that the pseudo-sites model can be used to consolidate experimental data into a single set of parameters to describe the behaviour of a Ziegler-Natta catalyst for polyolefin production.