Study of Some Kinetic Peculiarities of Ethylene Polymerization with Supported ZN Catalysts Based on Data on Number of Active Centers
It is shown the highly active ZN catalysts of propylene polymerization have very low ethylene polymerization rate (100 g PE/(g cat.h.bar) at 700C).
Three effects affecting the ethylene polymerization rate for supported ZN catalysts are studied: i) morphology of catalyst particles; ii) prepolymerization; iii) -olefin introduction. The decrease of the size and density of catalyst particles increases the ethylene polymerization rate. Last can be increased also by prepolymerization under mild conditions. If prepolymerization with ethylene has insignificant effect, prepolymerization with propylene or with ethylene/propylene mixture essentially increase the ethylene polymerization rate (up to 5000 g PE/(g cat.h.bar)). It is shown by 14CO the number of active sites does not depend practically on prepolymerization and the increase of polymerization rate is related to the increase of ethylene concentration near the active sites inside PE particle. Prepolymerization conditions providing the maximum rate of ethylene polymerization affect the propylene polymerization rate to a small extent.
It is found the prepolymer influences the morphology of polymer particles. If the prepolymer is PP the morphology of PE particles like to that of PP (respectively, catalyst) particles. If the prepolymer is ethylene/propylene copolymer the spherical particles of final polymer can be obtained.
The introduction of a-olefin into PE in small amounts (less than 5%) sharply increases the ethylene polymerization rate. Number of active sites and propagation rate constant are determined by 14CO. Reasons of -olefin effect on the rate are discussed.