The new catalytic system comprising a diester as an internal Lewis base and a silane (triethoxyphenylsilane) as an external Lewis base presents important differences to the classical third generation catalyst with two esters in the system, i. e. a high isotacticity is obtained with a low ratio Al/Lewis base, the stereospecificity is stable with temperature and time, the sensitivity to hydrogen concerning the average molecular weight is high and the activity, which remains low without hydrogen, is greatly enhanced by the transfer agent. The selectivity of the system is controlled in a complex manner by the concentration and ratio of the cocatalytic system.

As in the case of the third generation MgCl,-supported stereospecific catalysts, characterized by the use of aromatic esters as internal and external Lewis bases, the new improved family of catalysts, containing an aromatic diester as internal Lewis base and a trialkoxyphenylsilane as external Lewis base, was found to be sensitive to hydrogen. A very strong activation effect, followed by deactivation, was observed upon introduction of small amounts of hydrogen. In the present case the effect is reversible, unless rather high hydrogen pressure (2 bar) is used. The activation effect was observed with any of the cocatalysts and even in the absence of an external Lewis base. The effect is reduced or even suppressed if enough ethylene is used in the copolymerization with propene. Owing to the competition between ethylene and hydrogen for the activation, it is assumed that the step sensitive to hydrogen is that of initiation rather than propagation.

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.

No abstract

No abstract