Transition metal catalysts VI. Bis‐(cyclopentadienyl) transition metal polymerization catalysts

In the reaction of aluminum alkyls or aryls with (C5H5)2TiCl2 or (C5H5)2Ti(C6H5)2, there is rapid exchange of groups between the aluminum and titanium centers. When mixed alkyl and phenyl aluminum compounds react competitively with (C5H5)2TiCl2 to form ethylene polymerization catalysts, alkyl groups are more efficient than phenyl groups in initiating polymer chains. Since there is both selectivity in the initiation step and rapid exchange of groups, the presence or absence of phenyl endgroups in the polymer formed from different phenyl‐containing catalysts cannot be used to identify the active catalyst site, as has been proposed. Additional evidence is presented which suggests that the π‐bonding of the cyclopentadienyl compounds is disrupted either wholly or in part under the polymerization conditions, and at least part of the catalytic activity is due to the decomposition products.