Correlation of Arrhenius parameters for UHMWPE synthesis with ethylene solubility characteristics in different polymerization media

Abstract

ABSTRACT:The effect of different polymerization media like n-hexane, cyclohexane, isooctane, n-decane, toluene, varsol, and light normal paraffin (LNP) on the kinetics of the slurry polymerization of ethylene using a highly active Ziegler Natta (ZN) catalyst for synthesizing UHMWPE was studied. Attempts have been made to determine the solubility of ethylene in the above polymerization media in a very basic manner and to correlate same with the process activation energy based on the Arrhenius plots. The ethylene solubility seemed to depend on the number of carbon atoms in the media, besides other parameters like geometry, dipole moment, etc. It is obvious and well understood that the monomer (ethylene) concentration has a direct bearing on the polymerization kinetics, which influenced the activation energy (Ea) besides other parameters like catalyst/cocatalyst concentration, temperature, etc which were kept constant during the study. The role of the catalyst system in controlling the activation energy was also further exemplified by employing a different ZN catalyst system wherein higher activation energy was observed. This was ascribed to restricted activation pathways for the catalyst under the comparable experimental conditions employed. As soon as better activation pathways for the catalyst were enabled the activation energy dropped down remarkably. The Ea for the synthesis of ultra-high molecular weight polyethylene (UHMWPE) using traditional MgCl 2 supported Ti catalyst was found to be 5-12 kcal/mol which compared well with the values obtained by other researchers using other similar catalyst systems for different ethylene polymerization processes.

Systematic Comparison of Slurry and Gas‐Phase Polymerization of Ethylene: Part I Thermodynamic Effects

“…[ 22 ] Nonetheless, aromatic solvents, such as, toluene, might interfere with catalyst activation due to their distinct electronic environment. [ 18 ] The methodology we are proposing cannot be used to slurry polymerizations with diluents that may affect site activity. Moreover, we assumed that the diluent does not change the molar ratio of cocatalyst to pre‐catalyst at the polymerization active sites.…”

“…Focusing on the chemical nature of diluents, Kishor et al. polymerized ethylene in n ‐hexane, cyclohexane, iso‐octane, n ‐decane, and toluene [ 18 ] using an MgCl 2 supported Ti catalyst activated with a mixture of TIBA and IPRA. They found that the catalyst activities in saturated alkanes and cycloalkane were similar, but decreased when toluene was used.…”

“…Laboratory-scale polymerizations are mostly conducted in semi-batch slurry reactors, because they are easier to operate than gas-phase reactors, it is simple to design the reactor system and agitator, and the diluent helps control reactor temperature and particle overheating. [8][9][10][11][12][13][14][15][16][17][18][19] Nevertheless, one needs to avoid undesired phenomena that may affect the measured polymerization kinetics such as catalyst leaching-particularly important for supported metallocene catalysts-dissolution of polymer chains with low molecular weight and/or high comonomer content, and reactor fouling. [20] In bench-scale stirred-bed gas-phase reactors, on the other hand, one needs to deal with different types of technical issues to generate reliable results.…”

“…[20] In bench-scale stirred-bed gas-phase reactors, on the other hand, one needs to deal with different types of technical issues to generate reliable results. [8][9][10][11][12][13][14][15][16][17][18][19] In this case, an inert seedbed-NaCl or polymer powder-is needed to disperse the supported catalyst particles and to prevent them from sticking to each other and to the reactor walls. Seedbed materials have high surface areas and can adsorb impurities.…”

A study of molecular weight regulation in polyethylene by titanium diolate catalysts

“…17 The observed difference in activity can further be rationalized considering that the interaction between the polar solvent and the catalytic species makes the latter more stable and separates the ion pairs between the cationic complex and the co-catalyst counter-ion forming a coordinative unsaturated cationic complex, which does not strongly interact with the solvent, and makes the separated cationic species more accessible for the incoming ethylene thereby enhancing the catalyst productivity. 18 The hydrocarbon solvent cyclohexane shows poor activity due to the low solubility of the catalyst in this solvent.…”