Cycloheptyl-fused N,N,N′-chromium catalysts with selectivity for vinyl-terminated polyethylene waxes: thermal optimization and polymer functionalization

Abstract

Five chromium(III) chloride complexes, [2-{(Ar)NCMe}-9-{N(Ar)}C₁₀H₁₀N]CrCl₃ (Ar = 2,6-Me₂C₆H₃Cr1, 2,6-Et₂C₆H₃Cr2, 2,6-i-Pr₂C₆H₃Cr3, 2,4,6-Me₃C₆H₂Cr4, 2,6-Et₂-4-MeC₆H₂Cr5), each chelated by a sterically and electronically different cycloheptyl-fused N,N,N′-bis(imino)pyridine, have been synthesized by the reactions of CrCl₃(THF)₃ with the corresponding ligand (L1/L1′–L5/L5′). The molecular structure of Cr2 highlights both the steric properties exerted by the inequivalent N-2,6-ethylphenyl groups and the puckering of the fused cycloheptyl ring; a distorted octahedral geometry is conferred about the metal center. On activation with methylaluminoxane (MAO) or modified MAO (MMAO), Cr1–Cr5 displayed their optimal activity for ethylene polymerization at temperatures between 70 and 80 °C with the least sterically demanding Cr1 proving the most productive (1.44 × 10⁷ g (PE) per mol (Cr) per h). The polyethylenes formed are of low molecular weight (M_w range: 0.66–3.56 kg mol⁻¹) with narrow molecular weight distributions and display high levels of end-group unsaturation. Furthermore, the amenability of these vinyl-terminated polyethylenes to undergo functionalization via epoxidation has been demonstrated.