Coordination polymerization at very low amounts of methylaluminoxane as cocatalyst

The effect of very low amounts of methylaluminoxane as an activating cocatalyst in the coordination polymerization has been investigated in the syndiospecific polymerization of styrene with a half-sandwich metallocene catalyst in the presence of triisobutylaluminum at molar ratios of methylaluminoxane/transition metal from 0/1 to about 20/1 in comparison to the polymerization behavior at high molar methylaluminoxane (MAO)/metal ratios. As a result, there cannot be observed any polymerization reaction below a true molar ratio MAO/Ti of 6:1. At higher molar ratios until about 20, the polymerization conversion is increasing significantly with the MAO/Ti molar ratio. These observations and the results of the determination of the kinetic reaction order can be explained with Barron’s tert-butyl aluminoxane based model of MAO as a cage of six monomeric MAO units (AlOMe)6 in contrast to Sinn’s MAO model of a cage of twelve monomeric units (AlOMe)12 and are discussed with the results received at usually applied much higher MAO/transition metal ratios leading to a first-order dependence of the polymerization rate on the MAO concentration. From the thermal behavior of the syndiotactic polystyrenes synthesized it can be concluded, that the stereospecificity of the polymerization reaction is not affected by MAO at low MAO concentrations.