Impact of the method of Mg2+ cation introducing into the titanium post-metallocene catalytic system on their activity

Abstract

This work shows for the first time the influence of the method of introducing Mg compounds into a post-metallocene-type catalytic system on its catalytical performance and the properties of the resulting polymer, and it is concluded that the real catalytic center is heterometallic Ti/Mg complexes. Three different approaches were compared to introducing Mg compounds into catalytic systems containing a titanium(IV) dichloride complex with an (O^O)²⁻ type ligand and an organoaluminium compound—the use of Al/Mg co-catalysts, the immobilization of a Ti complex on the surface of anhydrous MgCl₂ and in situ preparation of heterometallic Ti/Mg complexes. All three approaches lead to the production of ultra-high molecular weight polyethylene, but systems with externally introduced magnesium chloride showed the lowest productivity. On the contrary, heterometallic Ti/Mg precatalysts are successfully activated by conventional organoaluminium compounds (Et₃Al, ⁱBu₃Al, MAO) and are superior to the reference catalytic system both in activity and in polymer molecular weights. Their use makes it possible to reduce the Ti/Mg molar ratio in catalytic systems from 1:300 to 1:1–2. The resulting UHMWPE samples are suitable for solvent-free solid-phase processing into high-strength, high-modulus film threads.