Kinetics of ethylene polymerization over titaniummagnesium catalysts: The reasons for the observedsecond order of polymerization rate with respect toethylene

The data on the effect of ethylene concentration on polymerization rate for several modifcations of modern highly active titanium–magnesium catalysts TiCl4/MgCl2 are presented. These catalysts differ in titanium content and conditions of supportpreparation, activities, and the shape of kinetic curves. It is found that the observed order of polymerization rate with respect toethylene in the range of ethylene pressures of 0.5–6 bar is 1.8-2.1 for all catalysts used (polymerization at 80°C, AlEt3 used as acocatalyst). When AlEt3 was replaced with Al(i-Bu)3, the reaction order decreased to 1.3-1.4. In order to elucidate the possiblereasons for the observed high order with respect to ethylene, we analyzed the data on the effect of monomer concentration onthe molecular weight of polyethylene. The results gave grounds for suggesting that the observed order with respect to monomeris attributable to the effect of ethylene concentration on the number of active sites. The possible reaction scheme explaining thenonlinear dependence of the polymerization rate on monomer concentration was proposed based on these data. Polyolefns J(2015) 2: 27-38

Keywords:

polyethylene (PE) , polymerization kinetics , Ziegler-Natta polymerization , molecular weight distribution / molarmass distribution