Propene polymerization with MgCl₂ supported Ziegler catalysts: Activation by hydrogen and ethylene

Abstract

Complementary experiments confirm the recent findings that the rate of propene polymerization is temporary enhanced if a small amount of ethylene is introduced during the polymerization, normalizing after consumption of ethylene by copolymerization. In addition, the study of the sequence distribution of propene monomeric units in the copolymerization carried out under various conditions revealed that the activation by ethylene does not involve the propagation steps, but the re-initiation after deactivation. A similar temporary activation effect was observed after the addition of small amounts of hydrogen. In this case, however, hydrogen is not consumed. Thus, the deactivation observed after the initial rate enhancement is due to another reason. The deactivation could be shown to be reversible after removal of the adsorbed hydrogen when either ethylene or hydrogen was added again to allow re-initiation of the polymerization. One of the important reasons of the continuous decrease of the propene homopolymerization rate may be due to the fact, that, after a transfer step, the re-initiation of polymerization does not take place spontaneously. Small amounts of ethylene or hydrogen allow the reinitiation to take place, at least for some kind of active sites.

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.…”

Propylene polymerization with catalysts containing divalent titanium

“…In a ization was investigated in the presence and in the absence of hydrogen. The results of polymersubsequent aricle, 28 the activating effects of hydrogen and ethylene on propylene polymerization were izations at 70ЊC in hexane slurry, at an ethylene pressure of 7 bar, are illustrated in Figure 2. It is compared; when 5% ethylene was present, only a limited hydrogen activation effect was observed, apparent that in the case of ethylene polymerization the Ti-arene catalyst has a higher activity prompting the suggestion that hydrogen and ethylene activation of propylene polymerization involve than MgCl 2 /TiCl 4 and that in both cases an acceleration rather than a decay profile is apparent a similar mechanism.…”

Hydrogen Effects for Propylene Polymerization with Ultra Low TiCl3 Loading MgCl2‐Supported Catalyst

“…In order to overcome this problem, more detailed understanding of the hydrogen acting mechanism has been regarded as the most important subject. [5,6] In a series of our studies, the hydrogen effects were investigated with the stoppedflow technique, by which the chain transfer reactions by Al-alkyl cocatalyst or monomer could be negligible and quasi-living polymerization can be realized within an extremely short polymerization period (ca. 0.2 s).…”