Hydrogen effect in the formation of active center on the catalyst
Upon addition of MAO, the chromium pre-catalyst is believed to undergo methylation and reduction to form an active center capable to coordinate at least two ethylene in order to form the key intermediate metallacyclopentane[17]. A proposed mechanism is used based on previous works by Sasol[18], in which two ethylene molecules both coordinate onto the metal center axially to achieve minimal steric around the active center. Figure 5 shows the Gibbs free energy profiles for the activation process, all figures relative to the starting molecule A0 where two ethylene molecules are just coordinated onto the metal center.