Mechanism of olefin polymerization on supported Ziegler-Natta catalysts based on data on the number of active centers and propagation rate constants

The number of active centers (C-g) and propagation rate constants (k(g)) for the polymerization of propylene and ethylene on highly active titanium-magnesium catalysts (TMCs) of different compositions at 70° C were determined using the method of (CO)-C-14 inhibition of polymerization. In the polymerization of propylene on the TiCl4/D-1/MgCl2-AlEt3/D-2 system (D-1 is dibutyl phthalate or 2,2'-diisobutyl-1,3-dimethoxypropane; D-2 is a silane), the effects of D-1 and D-2 donors on the values of C-g and k(g) were studied. It was found that the donors decreased the values of k(g) for nonstereospecific centers, had no effect on the values of kg for stereospecific centers, and increased the fraction of stereospecific centers, as well as the fraction of sleeping centers regardless of their stereospecificity. The rate constants of isotactic-chain transfer with C3H6, AlEt3, H-2, and ZnEt2 were determined. In the polymerization of ethylene, a number of TMCs exhibited strong diffusion limitations, which manifested themselves in a dramatic decrease in the determined values of k(g). It was demonstrated that diffusion limitations can be removed by decreasing the particle size and the concentration of active centers and by performing prepolymerization with propylene. The values of k(g) in ethylene polymerization were similar for stereospecific and nonstereospecific centers.