In situreal‐time environmental TEM of gas phase Ziegler–Natta catalytic polymerization of propylene

Abstract

In situ gas phase polypropylene (PP) formation over heterogeneous ballmilled TiCl4‐MgCl2 Ziegler–Natta (Z–N) catalysts (containing no electron donors or selectivity control agents) has been observed for the first time in real time using environmental transmission electron microscopy (ETEM). The MgCl2 support and associated Z–N catalysts were characterized with electron diffraction and high‐resolution electron microscopy. The support was found to be composed of highly defective β‐MgCl2 nanocrystallites, 3 nm in size. Ball milling these supports in TiCl4 to create the pro‐catalyst increased the degree of disorder and resulted in a higher fraction of amorphous material. The surfaces of the catalysts consist of amorphous material, various terminations of β‐MgCl2, and other contaminant phases. The ETEM observations revealed initial nanoscale PP growth in the form of globules and continuous non‐uniform polymer layers. The growth rate was measured to be 0.2 nm s−1 at 1 Torr of C3H6 pressure. The continuous growth implies that the active Ti sites are located on both the crystalline and non‐crystalline regions of the MgCl2 surface. These active Ti sites appear to be located at the polymer/MgCl2 buried interface and there does not appear to be any significant diffusion limitation for polymer layers up to 150 nm in thickness. Our data and calculations show that only 1 in 105 collisions with the active site results in monomer insertion into the polymer chain.