Polymerization of olefins through heterogeneous catalysis. XVIII. A kinetic explanation for unusual effects

Abstract

ABSTRACT:The development of a detailed kinetic model describing some of the unusual effects observed in catalyzed olefin polymerization is presented. Based on the method of moments, the model describes the rate effects of hydrogen and comonomers, as well as the ability of certain systems to incorporate long chain branches via internal and/ or terminal double bond polymerization. Examples are provided demonstrating the model's ability to predict rates and degrees of polymerization with ethylene, propylene, and 1-hexene monomers. In the case of propylene, multiple insertion mechanisms are modeled and compared with experimental sequence length and end group data. In other examples the model is used to simulate an oscillating metallocene catalyst and a metallocene catalyst capable of branch addition via terminal double bond polymerization.

Ethylene Polymerization Kinetics with a Heterogeneous Metallocene Catalyst – Comparison of Gas and Slurry Phases

“…Reversible complex formation from an active site with one monomer molecule is generally accepted. [20][21][22] Marques et al [23] proposed the double coordination mechanism for Ziegler-Natta catalyzed polymerizations based on the Ystenes [24] trigger mechanism. In the trigger mechanism, a monomer molecule forms a complex with the active site and the insertion takes place after the approach of a second monomer molecule.…”

A Multi‐Scale Modeling Approach for the Prediction of Molecular and Morphological Properties in Multi‐Site Catalyst, Olefin Polymerization Reactors

“…In addition to this MWD decrease, the polymerization rate can decrease as well due to the slow reinitiation of the active centers, following chain transfer to hydrogen. [20,21] In Figure 6, the dynamic evolution of the cumulative MWD in the bed is depicted for a hydrogen concentration C Hydrogen ¼ 3 mol-%. It is important to point out that during the dynamic operation of the FBR, the MWD moves to higher chain lengths as the system approaches its final steady state operation.…”

Particle morphology for polyolefins synthesized with supported metallocene catalysts

“…A general kinetic scheme for olefin polymerization has been discussed by Chen 28 and Shaffer and Ray. 29 This kinetic scheme (summarized in Table I) accounts for multiple active sites, with each active site having its own kinetic scheme involving activation, deactivation, site transformation, chain transfer, propagation, as well as internal and terminal double bond reactions. In the present study, only site activation, propagation, and site deactivation are considered for a single-site metallocene catalyst.…”

“…Initial condition: t ϭ 0 C pot ϭ 0.2xE cat (29) For the single site metallocene catalyst, the fractional active site concentration (moles active sites/ mole metal) is found from the values obtained from eqs. (28) and (29) for the dead site and potential site concentrations, respectively, as…”