Control of the molecular structure of ethylene-1-hexene copolymer by surface functionalization of SBA-15 with different compositions of amine groups

Abstract

The surface of SBA-15 was functionalized with mixtures of two different silanes, N-[(3-trimethoxysilyl)propyl]ethylenediamine (NS) and 4-(triethoxysilyl)butyronitrile (NCy), after which (n-BuCp)2ZrCl2 and methylaluminoxane (MAO) were grafted onto the amine-functionalized SBA-15 to examine its effect on ethylene homopolymerization and ethylene-1-hexene copolymerization. As the NCy/NS molar ratio in the feed decreased, the Zr content in the supported catalyst and ethylene homopolymerization activity increased owing to the strong interaction between NS and (n-BuCp)2ZrCl2, whereas dual-silane functionalized catalysts showed little difference as regards the activity of ethylene-1-hexene copolymerization. The molecular weight (MW), polydispersity index (PDI) and chemical compositional distribution (CCD) were significantly influenced by the composition of the two silanes in feed. The MW of the homo- and copolymers increased as the molar ratio of NCy/NS in the supported catalyst increased. The presence of NCy in the supported catalyst made the PDI and MW narrower and larger, respectively. NCy was found to have a tendency to form more homogeneous active sites during the grafting process compared to the NS. Therefore, a change in the NCy/NS molar ratio can control the MW and PDI of the resulting polyethylene. From the results of a TREF analysis, it was also found that the nature of active species grafted onto the amine groups can be controlled by the type of amine group and by the NCy/NS molar ratio on the surface of SBA-15. Conclusively, the controllability of PDI, CCD and MW during ethylene-1-hexene polymerization was demonstrated using dual-silane functionalized catalysts.