M.C. White, Chem 153 EM Polymerization -299- Week of november 18. 2002 Metallocenes as homogeneous polymerization catalysts No reaction is observed in the absence of Et2 AICl or Et AL. Both EtAlCl and Et Al alone produce only oligomers. Unlike the heterogeneous Ziegler-Natta polymerization catalysts, these catalysts are ineffective at polymerizing a-olefins( propylene) EtAICI polyethylene Natta JACS 1957(79)2975 Breslow JACS 1957(79)5072 Breslow's proposed mechanism cr 8. CI EtAICl T代 Insertion Polarization of the Ti-CI bond by the Lewis acidic Al center promotes H Breslow JACS 1959(81)81M.C. White, Chem 153 EM Polymerization -299- Week of November 18, 2002 Metallocenes as homogeneous polymerization catalysts TiIV Cl Cl Et2AlCl n polyethylene No reaction is observed in the absence of Et2AlCl or Et3Al. Both Et2AlCl and Et3Al alone produce only oligomers. Unlike the heterogeneous Ziegler-Natta polymerization catalysts, these catalysts are ineffective at polymerizing α-olefins (propylene). Natta JACS 1957 (79) 2975. Breslow JACS 1957 (79) 5072. Breslow's proposed mechanism: Breslow JACS 1959 (81) 81. TiIV Cl Al Cl δ+ δ- TiIV Cl Cl Et2AlCl σ-bond metathesis? Cl TiIV Cl Al Cl Cl TiIV Cl Al Cl δ+ δ- Cl cis- migratory insertion propagation TiIV Cl Al Cl Cl P β-hydride H elimination P TiIV H Cl Al Cl Cl TiIV H Cl Al Cl δ+ Cl (termination) Polarization of the Ti-Cl bond by the Lewis acidic Al center promotes ethylene coordination/insertion. P