Determination of the concentration of active centres and the rate constants of the separate stages in polymerization of ethylene on “deposited” catalysts based on TiCl 4 and Al(C 2H 5) 3

Abstract

Determination of the concentration of active centres on the surface of catalysts prepared by depositing TiCl 4 on carriers such as magnesium oxide of different porosity or aluminium silicate, by termination of the polymerization of ethylene by ethanol labelled with tritium in the OH group, has shown that in a catalyst on MgO containing 0·6% of Ti the fraction of active centres is about 20% of the total quantity of titanium deposited and the propagation rate constant is 2380 1./mole·sec, i.e. both these quantities are considerably higher than the corresponding values given in the literature for “typical” Ziegler-Natta catalyst systems. This makes it understandable why the yield of polyethylene with 1 g of Ti in the system TiCl 4/MgO (0·6% Ti)−Al(C 2H 5) 3 (I) is 30 to 40 times greater than the yields with the typical TiCl 4−Al(C 2H 5) 3 and TiCl 3−Al(C 2H 5) 3 systems. Study of the effect of the polymerization conditions on the molecular weight and molecular weight distribution of the polyethylene permitted determination of the numerical values of the rate constants of the main reactions involved in chain termination of chain transfer to monomer and to triethylaluminium, and of spontaneous disproportion. In polymerization of ethylene on (I) at 70° the values of these constants were 6·64×10 −2 l./mole·sec, 2·30×10 −2 l. 1 2 / mole 1 2 / sec and 4·4×10 −4 sec −1 respectively.