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.