摘要：

Arelevant fraction of the polyethylene produced in the world (about 30%) is obtained with the Phillips process. Many efforts in the last 30years have been devoted to establish the valence state and the structure of the catalytically active species formed by reduction with ethylene. However, no certain conclusions have been obtained so far, even using aCO-prereduced simplified system. In this review it will be shown that the CO -reduced system, although highly homogeneous from the point of view of the valence state (definitelyII) and nuclearity, is heterogeneous as far the local structure of the sites is concerned. Only Cr(II) ions with the lowest coordination (which unfortunately are only aminor fraction of the total) are responsible for the catalytic activity, while the overwhelming majority of surface sites play the role of spectator under normal reaction conditions. In the second part of the review the proposed initiation/polymerization mechanisms are fully reported. Apeculiarity of the Cr/SiO 2 system, which makes it unique among the polymerization catalysts (Ziegler--Natta, metallocenes, etc.), lies in the fact that it does not requires any activator (such as aluminium alkyls etc.) because ethylene itself is able to create the catalytic center from the surface chromate precursor. It will be shown that aunifying picture has not yet been achieved, even in this case. The aim of this review is to illustrate, on one side, how much progress has been made recently in the understanding of the site's structure and, on the other side, the strategies and the techniques which can be adopted to study the catalyst under working conditions. It will be shown that the methods adopted for the Cr/SiO 2 system have paradigmatic character and can be extended to other catalytic systems.

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