Immobilization and activation of 2,6-bis(imino)pyridyl Fe, Cr and V precatalysts using a MgCl2/AlRn(OEt)3−n support: Effects on polyethylene molecular weight and molecular weight distribution

Ethylene polymerizations carried out with various bis(imino)pyridyl iron, chromium and vanadium complexes immobilized on a MgCl2/AlRn(OEt)3−n support gave relatively broad polyethylene molecular weight distributions in the case of iron, but high molecular weight and a very narrow molecular weight distribution with vanadium, indicative of a single active species. The narrow MWD was confirmed by melt rheometry. Similar results were obtained after reaction of the bis(imino)pyridyl complex LVCl3 (6) with MeLi or AlEt3, where alkylation of the pyridine ring gives a complex L′VCl2 (7). In the case of chromium, a bimodal distribution was obtained, with evidence of incomplete catalyst immobilization. The polyethylene molecular weights obtained with the iron complexes were strongly dependent on the substituents in the bis(imino)pyridyl ligand, and were somewhat higher than have been obtained in homogeneous polymerization. In contrast, the molecular weights obtained with the bis(imino)pyridyl chromium and vanadium complexes were much higher that those previously obtained under homogeneous conditions. In all cases, the activities of the immobilized catalysts were higher than those found in homogeneous polymerization.

In contrast to the relatively broad molecular weight distribution of polyethylene prepared with various bis(imino)pyridyl iron complexes immobilized on a MgCl2/AlRn(OEt)3−n support, immobilization of a bis(imino)pyridyl vanadium(III) complex on the same support gave very narrow molecular weight distribution, indicative of a single active species in the case of vanadium but not with iron. The vanadium-based system also gave significantly higher molecular weight.Shear frequency dependence of storage modulus (G  ′) at constant strain of (♦ and ▴) polyethylene (M¯w/M¯n=1.8–2.1) prepared using an immobilized V catalyst, compared to (■) a reference polyethylene (M¯w/M¯n=5.6) prepared using an immobilized iron catalyst. Figure optionsDownload as PowerPoint slide