Kinetic Modeling of the Effect of MAO/Zr Ratio and Chain Transfer to Aluminum in Zirconocene Catalyzed Propylene Polymerization

Abstract

A kinetic model based on the coordination−insertion mechanism was developed to characterize metallocene-catalyzed propylene polymerization using two different catalyst systems: rac-Et(Ind)2ZrCl2/MAO (I/MAO) and rac-Et(4,7-Me2-1-Ind)2ZrCl2/MAO (II/MAO). Slurry propylene polymerizations were performed in a semibatch reactor at 40 °C to investigate the effects of propylene partial pressure and MAO/Zr ratio. The kinetic model accounts for the formation of regioirregularities, the occurrence of chain transfer to trimethylaluminum (TMA), and β-hydride chain transfer to both monomer and metal to predict the effects of propylene partial pressure and the MAO concentration on polymer molecular weight and the formation of isobutyl end groups. A systematic optimization strategy was applied to estimate the kinetic parameters from on-line measurements of the reaction rate and end-of-batch measurements of the molecular weights and percentages of end groups. The formation of 2,1-insertions was more frequent for catalyst II/MAO (k s = 97.4 L mol-1 s-1 vs k s = 49.4 L mol-1 s-1 for I/MAO). The M w of polymer produced with I/MAO decreased at low pressures due to the high rate of monomolecular β-hydride transfer to the metal (k H = 26.5 s-1). Chain transfer to TMA was more significant with the catalyst II/MAO (k Al = 5.46 × 10+3 L mol-1 s-1 vs k Al = 1.97 × 10+3 L mol-1 s-1 for I/MAO).

Modeling of Slurry Polymerization of Ethylene Using a Soluble Cp2ZrCl2/MAO Catalytic System

“…Because each active center has its own rate constants and gives polymer chains with special structures, the bulk polymer made with heterogeneous Ziegler‐Natta catalysts is in fact a mixture at molecular level. Therefore investigation of ethylene polymerization with soluble metallocene catalysts has many advantages such as: (1) considering only one type of active center being sufficient for quantitative study of polymerization kinetics, while even in polymerization using supported metallocene catalysts more than one type is involved;2–4 (2) non‐encapsulation of catalyst particles by polymer molecules which usually occurs in supported catalysts, making mass and heat transfers difficult;5, 6 (3) simple kinetic mechanism, that becomes complex in polymerization of asymmetrical monomers;7, 8 and, (4) formation of unique polymer structure that facilitates its characterization and interpretation.…”

“…In some works, estimates of kinetic constants are based on time averaged values of reaction rates and final molecular weights 12–16. In more recent models, the kinetic parameters have been estimated using instantaneous reaction rates and end of batch molecular weights 4, 7, 8, 17–32…”

“…No general methodology has been developed that can guarantee good parameter estimation in such non‐linear models. There are vast numbers of optimization methods that can be applied to parameter estimation, like Sobol's systematic search method, Levenberg‐Marquardt algorithm,33 and orthogonal collocation on finite elements 7, 8. These methods make use of algebraic objective functions or are coupled with non‐linear differential equations.…”

New Approach in Modeling of Metallocene‐Catalyzed Olefin Polymerization Using Artificial Neural Networks

“…In this scenario, the network will modify the weight of interconnections between neurons to reproduce the given rate constants. Limits of the rate constants were selected according to the published data [7][8][9][10] and the information about the kinetics of our target catalyst. It should be mentioned that the steps of initiation and deactivation rate constant were selected logarithmic to emphasize the effect of low values as well as high values.…”

Ethylene–Propylene–Silsesquioxane Thermoplastic Elastomers

“…[25,26] Prior to polymerization, the reactor was heated to 70 8C, purged with nitrogen gas for 90 min, and cooled to room temperature. In an inert atmosphere drybox, a solution of POSS in toluene was prepared; also added was 1 mL of MAO to scavenge any impurities.…”