Copolymerization of ethylene and 1-hexene using (n-BuCp)2ZrCl2 catalyst activated by the cross-linked MAO supported cocatalyst

Abstract

Metallocene/methylaluminoxane (MAO) catalytic systems have grown in importance since their discovery in the 1980s. Many studies concerning the effect of metallocene structures both on catalyst copolymerization reactivity and on copolymer properties have been carried out. Copolymers of ethylene with α-olefins such as 1-butene, 1-hexene and 1-octene are very important commercial products classified as linear low density polyethylene (LLDPE). There have been many efforts towards finding more efficient catalysts to produce copolymers having desired physical properties. Ziegler-Natta catalysts produce copolymers with wide molecular weight distribution (MWD) and short chain branching distribution (SCBD) because of multiple active sites. In contrast, homogeneous metallocenes are single-site catalysts and produce very uniform copolymers with a narrow MWD and SCBD. It is very useful to understand the origin of MWD and SCBD of copolymers made with different catalysts, especially how to manipulate catalyst structure to modify polymer microstructure, since these distributions determine the polymer applications. The MWD of polymers can easily be determined by gel permeation chromatography (GPC). However, the analysis of SCBD of copolymers is more elaborate. The most common technique for this purpose is to use temperature rising elution fractionation (TREF). TREF is now a common technique in the polyolefin industry to analyze the SCBD of copolymers. The subject of this work is to study the copolymerizations of ethylene and 1-hexene using (n-BuCp)2ZrCl2 activated with the cross-linked methylaluminoxane (MAO) supported cocatalyst. The effects of 1-hexene concentrations in the feed on catalyst activity, copolymer composition, and polymer properties were investigated. Comparison of the catalyst properties with (n-BuCp)2ZrCl2 was carried out with the different cocatalyst system of the cross-linked MAO supported cocatalyst, the known MAO supported cocatalyst (SMAO) system, and soluble MAO. In addition, the microstructure, molecular weight distribution and morphology of the resulting copolymers were explored.