Synthesis Of Ethylene/1-Octene Copolymers With Ultrahigh Molecular Weights By Zr And Hf Complexes Bearing Bidentate Nn Ligands With The Camphyl Linker

Ultrahigh molecular weight polyethylene (UHMWPE) is a class of high-performance engineering plastics, exhibiting a unique set of properties and applications. Although many advances have been achieved in recent years, the synthesis of UHMWPE is still a great challenge. In this contribution, a series of zirconium and hafnium complexes, [2,6-(R-1)(2)-4-R-2-C6H2-N-C(camphyl)=C(camphyl)-N-2,6-(R-1)(2)-4-R-2-C6H2]MMe2(THF) (1-Zr: R-1 = Me, R-2 = H, M = Zr; 2-Zr: R-1 = Me, R-2 = Me, M = Zr; 1-Hf: R-1 = Me, R-2 = H, M = Hf; 2-Hf: R-1 = Me, R-2 = Me, M = Hf), bearing bidentate NN ligands with the bulky camphyl backbone were synthesized by the stoichiometric reactions of alpha-diimine ligands with MMe4 (M = Hf or Zr). All Zr and Hf metal complexes were analyzed using H-1 and C-13 NMR spectroscopy, and the molecular structures of complexes 1-Zr and 1-Hf were determined by single-crystal X-ray diffraction, revealing that the original alpha-diimine ligand was selectively reduced into the ene-diamido form and generated an 1,3-diaza-2-metallocyclopentene ring in the metal complexes. Zr complexes 1-Zr and 2-Zr showed moderate activity (up to 388 kg(PE)center dot mol(-1)(M)center dot h(-1)), poor copolymerization ability, but unprecedented molecular weight capability toward ethylene/1-octene copolymerization. Therefore, copolymers with ultrahigh molecular weights (>600 or 337 x 10(4) g center dot mol(-1)) were successfully synthesized by 1-Zr or 2-Zr, respectively, with the borate cocatalyst [Ph3C][B(C6F5)(4)]. Surprisingly, Hf complexes 1-Hf and 2-Hf showed negligible activity under otherwise identical conditions, revealing the great influence of metal centers on catalytic performances.