The role of dormant sites in propene polymerization using methylalumoxane activated metallocene catalysts

Propene polymerization of methylalumoxane (MAO) activated rac-Me₂Si(Benz[e]Indenyl)₂ZrCl₂ ( BI ) and rac-Me₂Si(2-Me-Benz[e]Indenyl)₂ZrCl₂ ( MBI ) was studied to investigate the influence of the ligand substitution pattern and the role of dormant sites. Poly(propene) end group composition as well as regio- and stereoirregularities were examined by means of ¹H- and ¹³C-NMR spectroscopy. Dormant sites, resulting from 2, 1-propene insertion, were reactivated either by β-hydrogen transfer to propene, yielding 2-butenyl end groups, or by 1, 2-insertion of propene, yielding regioirregularities. Propene polymerization in the presence of hydrogen gave n-butyl end groups and less regioirregularities as expected for hydrogenolysis of such dormant sites. Methyl substitution in 2-position of the benz[e]indenyl ligand suppressed β-hydrogen transfer to propene, and increased molecular weight with increasing propene concentration. Also, activation energy increased from 30 kJ/mol ( BI /MAO) to 59 kJ/mol ( MBI /MAO). For both catalysts activity depended on propene concentration. The order of reaction relative to propene was 1.7.