Effect of monomer diffusion in the polymerization of olefins over Ziegler‐Natta catalysts

Abstract

By using an extremely high stereospecific catalyst, three kinds of copolymerizations were carried out with combinations of monomers which differ in thecrystallinity of the homopolymers.Addition of a small quantity of comonomer to the homopolymerization systems producing very high crystalline polymers was found to cause a remarkable increase in the apparent polymerization rate as well as a marked decrease in the polymer crystallinity. Such an irregular increase in the polymerization activity was, however, not observed when a small amount of comonomer was added to the homopolymerization systems producing low-crystalline or amorphous polymers. These observations have led to the conclusion that resistance of monomer diffusion through the polymer film should be one of the significant parameters for the apparent polymerization rate when very high crystalline polymers, like high density polyethylene, isotactic polypropylene etc., are produced. ~ a) Revised manuscript from July 29, 1988.

Polymerization of ethylene with (C5Me5)2Zr(NMe2)2 cocatalyzed by common alkyl aluminums

“…The increase in the activity is explained by an easier ionic dissociation of Zr-R-Al bond, which increases the population of the cationic active species and to the low-coordinating power of the methylene chloride molecule. Similar solvent effects were reported for the olefin polymerizations by various metallocene catalysts combined with MAO [11][12][13][14][15].…”

Effects of molecular dispersion state of surface Ti species on ethylene-propylene copolymerization with TiCl3-based Ziegler-Natta model catalyst

“…These phenomena are well known as the activation effect in copolymerization, where two kinds of monomers synergistically enhance the activity, and have widely observed for copolymerization both of ethylene with α-olefins and of propylene with higher α-olefins, being irrespective of employed catalysts (ZN or metallocene catalysts). [12][13][14][15][16][17][18][19][20][21][22] Though it has not been completely clarified yet, various mechanisms for the activation effect have been proposed; i) decrease of crystallinity of formed copolymers due to the incorporation of comonomer to accelerate the diffusion of monomers or alkylaluminum, 12,13 ii) reactivation of dormant sites by comonomer, [14][15][16] iii) increase of the active site concentration in copolymerization with the assumption that active sites for ethylene and propylene polymerization might not be identical and that both the active sites work for copolymerization, [17][18][19] iv) acceleration of catalyst particles disintegration during copolymerization, 18 and so on. Considering that Cat-A gave poorly crystalline polypropylene (X c~8 wt%), the mechanism i) was not likely relevant to the observed activation.…”

Condensed Mode Cooling for Ethylene Polymerization: Part II. The Effect of Different Condensable Comonomers and Hydrogen on Polymerization Rate

“…In their review of the relevant literature of the time, Karol et al noted that this “comonomer effect” seemed to be wide spread and observed for many different types of catalysts, including titanium and vanadium‐based Ziegler–Natta catalysts, chromium catalysts, as well as metallocenes. Several explanations have been offered for these observations, including disintegration of the particles, increased monomer diffusion, displacement of absorbed or complexed molecules, activation of dormant sites by comonomer, an increase in the propagation rate, or formation of new active sites by comonomer …”