Transition metal alkyl compounds

D. G. H. Ballard Polymerisation by homogeneous transition metal alkyl compounds, pp. 213-264 (25). 

II. Soluble Transition Metal Alkyl Compounds as Polymerization Catalysts. 266... 

III. Ligand Replacement in Transition Metal Alkyl Compounds and... 

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

The results of polymerizing ethylene using varying sigma-bonded transition metal alkyl compounds are summarized in Table VII. It is evident that none of the catalysts are very active and are comparable with the simple allyl compounds listed in Table I. 

The above substitution effects appear to be independent of the nature of the ligand (16) since the benzyl compounds behave similarly, Table XI. It would appear from these observations that the introduction of anionic ligand would be sufficient to increase activity of transition metal alkyl compounds for polymerization. This, however, is probably an oversimplifica-... 

IV. Heterogeneous Polymerization Catalysts Derived from Transition Metal Alkyl Compounds... 

Transition metal alkyl compounds react with the -OH groups on the surface of silica in a manner similar to that described for the silanol [reaction (13)] and as with the latter more than one type of bonding is possible. Silica dried at 200°C reacts with Zr(allyl)4 to give two molecules of propene per metal atom and utilizing in the course of this process two -OH groups per metal atom. The chemistry of the process is accurately described by the equation... 

Polymerization of Ethylene with Transition Metal Alkyl Compounds in Toluene at 80°C... 

Polymerization of Propylene by Transition Metal Alkyl Compounds Toluene as Solvent, Temperature 65°C. Ethylene Pressure 10 atm (IS, 16)... 

Ballard et al. (15) have found that transition metal alkyl compounds of... 

It has been shown (p. 266) that transition metal alkyl compounds containing Cpd and C6H6 groups, ir-bonded to the metal inactivate the metal center for polymerization. It has also been shown by Nyholm and Aresta (45), in the platinum series, that five- or six-membered rings containing only sigma and ir-carbon-to-metal bonds are very stable compounds. These observations add chemical plausibility to reaction (29). 

Comparison of the homogeneous polymerizations of transition metal alkyl compounds with their heterogeneous equivalents shows that the higher activity of the latter is due to ... 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

Two notable points from the aforegoing discussion are as follows. (1) By far the majority of the known monohalogenoalkyl compounds are of the group VTII transition elements there are very few early transition metal halogenoalkyl compounds known. (2) Very few monofluoroalkyl metal complexes have been prepared. The lack of early transition metal halogenoalkyl compounds may be in part due to the high electropositivity of these metals, which facilitates a- and / -elimination reactions. Related hydride elimination reactions almost certainly occur more easily for early transition metal alkyl compounds than for later transition metal compounds. In this regard it is particularly noteworthy that one of the only early transition metal haloalkyl compounds mentioned is the fluoroethyl scandium com-... 

Decrease in MW is also a consequence of labilization of the metalorganic bond brought about by the nature of the alkyl group, an equilibrium that leads to polymers having a vinyl endgroup (c). The resulting hydride reacts in the same manner as the transition metal alkyl compound. 

Ballard, D. G. H., Jones, E., Wyatt, R. J., Murray, R. T., and Robinson, P. A. 1974. Highly active polymerization catalysts of long life derived from a- and Jt-bonded transition metal alkyl compounds. Polymer 15 169-174. 

Transition-Metal Alkji and Related Catalysts There continues to be a lively interest in the use of transition-metal alkyl compounds as polymerization catalysts both in the absence and in the presence of alkylaluminium cocatalysts. Hie more active catalysts of this type are invariably supported sterns and indeed it is of some interest that many non-supported transition metal alkyls do not polymerize propylene at all. Sinc this field has been extensively reviewed in recent years by Ballard and also features in an article by Yermakov only the most recent publications will be mentioned in this Report. 

The lability of many transition metal—alkyl compounds toward reduction is a further feature in this type of catalysis. Formally, the reaction may be written ... 

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