Transition metal catalyst active in absence of alkylmetals

2 Transition metal catalysts active in the absence of alkylmetals 

Many transition metals and their compounds with organic ligands initiate the polymerization of alkenes and/or dienes. Some of them do not need any special treatment to this end while others require the presence of some organic or mineral compound or a special physical modification. In contrast to ZN catalysts, they are active without an organometal of Groups I—III. They are commonly known as metal alkyl free (MAF) catalysts. Many of their features are, of course, in common with ZN catalysts. MAF catalysts initiate stereoselectively controlled polymerization. Even less is known of their operating mechanism than that of ZN catalysts. It is assumed that propagation also occurs on the transition metal-carbon bond. 

The Philips catalyst [290] is probably the most important. It is still used for producing over 60% of the world s polyethylene. The most frequently used kind is prepared by impregnating a support (silica, silicoalumina, etc. with a surface area 200 m2 g-1) with Cr compounds, drying and activating with a stream of dry air (with addition of CO) at 700-1100 K. Chromates are assumed to be formed, but only a small part forms active centres 

The catalyst contains Cr in several oxidation states. The mechanism of the Cr—C bond formation has not been safely explained so far these bonds in particular, together with their immediate vicinity, are assumed to be the active centres. 

There exists a large number of MAF catalysts and only their general features can be briefly mentioned here. When the cocatalyst (alkylmetal) is omitted in a typical ZN system, the remaining transition metal salt is rarely active. Some transition metals, especially Ti, V, Cr, Co, Ni, Zr, Nb, Mo, W, Pd, Rh, and Ru, are however, active, often after special treatment. Generally alkenes are more readily polymerized by transition metals from the left-hand side of the periodic table and dienes by metals from the right-hand side. 

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Transition metal catalyst active in absence of alkylmetals. 

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