Origin of broad molecular weight distribution of polyethylene produced by Phillips-type silica-supported chromium catalyst

The origin of the broad molecular weight distribution of polyethylene produced by Phillips-type silica-supported chromium (Cr/SiO2) catalysts was studied by density functional calculations using active site models with various coordination environments. Difference of the coordination environment of chromium showed remarkable variations for both of the insertion and the chain transfer energies, resulting in a broad range of molecular weight from 102 to 1010 g/mol at 350 K. The results clarified that the special catalytic property of Cr/SiO2 for broad molecular weight distribution is attributed to the heterogeneity of the coordination environment of the chromium species.

Graphical abstractActive site structures for broadening the molecular weight distribution of polyethylene by Phillips-type silica-supported chromium (Cr/SiO2) catalysts were disclosed by density functional calculations using active site models with various coordination environments.Figure optionsDownload full-size imageDownload high-quality image (147 K)Download as PowerPoint slideHighlights► Properties of Cr/SiO2 were sensitive to the coordination environment. ► A ligand at an equatorial position led to a lower molecular weight. ► A ligand at an axial position led to a higher molecular weight.