Copolymerization of ethylene with 1-hexene and 1-octene: correlation between type of catalyst and comonomer incorporated

The behaviour of catalytic systems based on zirconium compounds for the copolymerization of ethylene with 1-hexene and 1-octene is reported. The metallocenes (CH₃)₂SiCp₂ZrCl₂, Cp₂ZrCl₂ (Cp = η⁵-cyclopentadienyl), C₂H₄[Ind]₂ZrCl₂ and (Ind = η⁵-indenyl) were chosen for this study. The bridged catalysts, (CH₃)₂SiCp₂ZrCl₂ and C₂H₄[Ind]₂ZrCl₂, and the metallocene Cp₂ZrCl₂ showed similar catalytic activities for home- and copolymerization of ethylene with 1-hexene. ¹³C NMR analysis showed that the composition of copolymerization products depends on the catalytic system, in other words, on the ligand structure of the transition metal. Copolymers obtained using the bridged catalysts have greater incorporation of comonomer. Thermal analysis and viscosity measurements demonstrated that an increase of the comonomer concentration reduces the melting point, the crystallinity and the molecular weight of the copolymer. Results from infrared spectroscopy showed that β-elimination is one of the possible termination reactions. The monomer reactivity ratios r were determined for all catalytic systems using Fineman-Ross and ¹³C NMR methods. The values of r₁ (M₁ = ethylene) and r₂ (M₂ = α-olefin) showed an effect of the type of metallocene and of α-olefin on the structure of the copolymer obtained.