Effects of propylene prepolymerization on ethylene/1‐hexene and ethylene/1‐octene copolymerization with an immobilized metallocene catalyst

Abstract

It has been known for some time that the activity of a heterogeneous catalyst for ethylene polymerization can be increased if a prepolymerization is carried out first in the presence of propylene or another a-olefin. Recently, it has been demonstrated that the increased polymerization activity can be attributed to a significant lowering of the monomer diffusion barrier in ethylene homopolymerization. The aforementioned studies were based on the use of MgCl2-supported Ziegler–Natta catalysts. However, monomer diffusion limitation in ethylene polymerization is also a frequently encountered phenomenon in ethylene polymerization with immobilized single-center catalysts and provides the most likely explanation for the increase in activity when ethylene polymerization is carried out in the presence of an a-olefin comonomer. The presence of the comonomer reduces the crystallinity of the polymer, leading to increased monomer sorption and easier monomer mass transport through the growing polymer particle, and facilitates catalyst fragmentation. However, compositional heterogeneity in the resulting copolymer may still occur, even in the case of a single-center catalyst. Fink et al. ascribed the compositional distribution of ethylene/1hexene copolymers prepared with a SiO2/methylaluminoxane (MAO)/zirconocene system to the formation, during the initial stages of polymerization, of a copolymer envelope around the catalyst particle. Easier diffusion of the smaller monomer, ethylene, with respect to 1-hexene was proposed to lead to a polymer particle comprising an ethylene-rich center surrounded by an outer layer of the copolymer, thus giving a broad overall chemical composition distribution (CCD). In recent ethylene/1-hexene copolymerization studies carried out with a catalyst comprising a zirconocene and MAO immobilized on a silica support, we found that the CCD of the copolymers became broader as the polymerization time was increased as a result of the gradual formation of a relatively high molecular weight, ethylene-rich fraction. Again, this is a strong indication that significant monomer diffusion effects can play a role not only in ethylene homopolymerization but also in copolymerization. Taking into account that monomer diffusion effects may be reduced if a prepolymerization of the catalyst is carried out first with a Correspondence to: J. C. Chadwick (E-mail: j.c.chadwick@ polymers.nl)