Structure–performance correlation for cyclohexylalkyldimethoxysilanes as external donors in a Ziegler–Natta catalyst: a combined experimental and computational study

Abstract

Herein, cyclohexylalkyl (methyl-ED1, propyl-ED2, isobutyl-ED3, and n-octyl-ED4) dimethoxysilanes are synthesized and used as external donors (EDs) in a Ziegler–Natta (ZN) catalyst for propylene polymerization. The role of hydrocarbon groups on the structure of alkoxysilane, as an external donor, is examined. In this study, the roles of size and nature of hydrocarbon groups (methyl, propyl, isobutyl, and n-octyl), which play an important role in regulating the activity of the ZN catalyst, are investigated. The experimental results suggest that the productivity of the ZN catalyst can be increased by replacing the less sterically hindered methyl group with propyl/isobutyl/n-octyl hydrocarbon groups. It is observed that the productivity of propylene polymerization is 6.5 kg g_cat⁻¹ for the commercial donor cyclohexylmethyldimethoxysilane ED1 (known as C-donor) and 7.0–8.0 kg g_cat⁻¹ in the case of ED2–ED4. Therefore, the sterically hindered hydrocarbon group present on the external donor molecule plays an important role in increasing the productivity of propylene polymerization compared with the C-donor (ED1). Density functional theory calculations are utilized to gain deeper insights into the structure and electronic behavior of external donors. During propylene polymerization, external donors are added in combination with triethylaluminum (TEAL), and the ED–TEAL interaction is examined to elucidate the correlation between donor structure and ZN catalyst performance. The structure–activity correlation is derived from the calculated binding energy between the external donor and TEAL, which regulates the productivity of polypropylene. The results of polymerization data and DFT studies reveal that as the binding energy in the TEAL-ED complexes decreases, the productivity of the ZN catalyst increases, and alkoxysilanes with bulkier groups enhance the catalyst activity, isotacticity and molecular weight of polypropylene compared with the methyl group present in the commercial external donor ED1.