Predicting the Molecular Weight Distribution of Polyethylene for Mixed Systems with a Constrained-Geometry Metallocene Catalyst in a Semibatch Reactor

Abstract

Bimodal molecular weight distributions (MWDs) were obtained in a semibatch reactor (Beigzadeh, D.; Soares, J. B. P.; Duever, T. A. Macromol. Rapid Commun. 1999, 20, 541−545) with a mixed metallocene catalyst system containing a constrained-geometry catalyst (CGC−Ti), generating long branches through insertion of chains with terminal double bonds (TDBs). A Galerkin finite element model (FEM) has been constructed in PREDICI describing the MWD and branching as a function of the catalyst ratio. Several options of this model were tested, one of them being based on single-catalyst data. The best agreement between model and experiments could be achieved when the model allowed for transfer of macromonomers formed at the linear catalyst and subsequent insertion at CGC−Ti to form long branches. This option explains the high-MW mode of the MWD as resulting from branching rather than from a higher propagation/termination ratio at CGC−Ti as compared to a linear catalyst. The parameter estimation procedure based on fits of measured MWDs and branching densities as varying with the catalyst ratio produced a set of kinetic parameters. Rate coefficients associated with growth termination at the linear catalyst (kβ,lin) were associated with the position of the low-MW peak in the MWD. Further crucial kinetic parameters turned out to be concerned with β-hydride elimination and macromonomer insertion at CGC−Ti. The model was employed to investigate the deactivation of the catalyst, the effect of growth stoppage by hydrogen, and the dynamics of the semibatch reactor. We compared our kinetic coefficients to those reported in previous work.