A Ziegler‐Natta/metallocene hybrid catalyst was produced and utilized in the polymerization of ethylene with the aim of producing bimodal polyethylene. The MgCl2 adduct was prepared by a melt quenching method and Cp2ZrCl2 and TiCl4 catalysts were loaded, respectively, after treating the surface with TiBAl. The polymerization kinetics involved an induction period, followed by fragmentation and expansion of particles. SEM micrographs revealed that the spherical morphology was retained through the initial mild reaction conditions of induction period. The polymers produced showed bimodal molecular weight distribution patterns, suggesting that both components of the hybrid catalyst were active over the support.
A series of supports – differing in their textural properties, the nature of their surface sites, and in their crystallinities – were investigated in the sequential grafting reaction of Cp2ZrCl2 and (nBuCp)2ZrCl2 mixtures. The catalyst systems were analyzed by Rutherford backscattering spectrometry, AFM, EXAFS, and nitrogen adsorption. All systems were shown to be active in ethylene polymerization reactions when methylaluminoxane was used as the cocatalyst. Experimental results are discussed in terms of the relationships between the grafted metal content, the catalyst activity, the surface roughness and polarity, the interatomic Zr–C and Zr–O distances of the grafted species and the molecular weights of the resulting polyethylenes.
A kinetic model is developed for the heterogeneous free‐radical copolymerization of vinylidene fluoride and hexafluoropropylene in supercritical CO2. The model accounts for polymerization in both the dispersed (polymer‐rich) phase and in the continuous (polymer‐free) supercritical phase, for radical interphase transport, diffusion limitations, and chain‐length‐dependent termination in the polymer‐rich phase. A parameter evaluation strategy is developed and detailed to estimate most of the kinetic parameters a priori while minimizing their evaluation by direct fitting. The resulting model predictions compare favorably with the experimental results of conversion and MWD at varying monomer feed composition, monomer concentration, interphase area, and pressure of the system.
