Metallocene-Catalyzed Semi-Batch and Continuous Polymerization of Ethylene

Abstract

<p>An integrated methodology has been utilized to analyze the factors that affect the activity of metallocene catalysts and the microstructure of the polymers produced under various reaction conditions for both semi-batch and continuous solution polymerization of ethylene. The highly active metallocene catalyst system Cp₂ZrCl₂/MAO was focused on in order to determine the fundamental kinetic rate constants, and a thorough characterization of the polymer microstructure was carried out using high temperature size exclusion chromatography (SEC), carbon-13 nuclear magnetic resonance spectroscopy (¹³C NMR), differential scanning calorimetry (DSC) and shear thinning behaviour as determined by I₁₀/I₂. In the semi-batch polymerization of ethylene in toluene utilizing Cp₂ZrCl₂/MAO, the shape of the polymerization rate-time curves were found to depend on monomer diffusion rates with low ethylene concentrations and poor sparging/mixing impellers giving steady-states while high ethylene concentrations and superior impellers led to decay-type curves/ Increasing temperature increased the catalyst activity with an activation energy for propagation (Ep) of 28.5 kJ/mol while strongly decreasing PE MWs. Several types of commercial and developmental aluminoxanes were studied which indicated that the structure and type of aluminoxane co-catalyst did not influence the MWD of PE although significantly influenced the catalyst activity. Mixing additional TMA with MAO was found not to alter the MWD. For all conditions studied at temperatures between 20-90 ∘C in semi-batch, the PE produced was found to approximate Flory's most probable distribution for a single site-type catalyst. The rate constants for propagation, β-scission and chain transfer to ethylene at 70∘C were found to be kp = 2.7 x 10⁴(M∙s)⁻¹, ktr.β = 0.27 s⁻¹ and ktr.M= 3.8 (M∙s)⁻¹. Long chain branches per 1000 carbons (λN) up to 1.2 were found and the ratio of melt flow indexes indicating shear thinning, I₁₀/I₂, were found up to 15. A high pressure, high temperature CSTR was utilized for the solution polymerization of ethylene at 1500 psig in toluene using the metallocene catalyst system Cp₂ZrCl₂/MAO/TMA. Increasing Cp₂ZrCl₂ concentration led to a decrease in PE MWs while the catalyst activity increased. With increasing temperature between 140-200∘C, the MWs of PE decreased and polydispersities increased while the catalyst activity decreased with an apparent activation energy of -93 kJ/mol. The deactivation of the catalyst is first-order with rate constant kd = 2.1 x 10⁻³ s⁻¹ while kp= 5 x 10³ (M∙s)⁻¹ and ktr.β 3 s⁻¹ at 140∘C. λN's up to 0.6 were found and I₁₀/I₂ were found up to 6.</p>