M.C. White, Chem 153 EMPolymerization-303- Week of november 18. 2002 orsional isomers in stereoselective propylene polymerization this catalyst led to the formation of highly isotactic, high molecular weight polypropylene, with purely enantiomorphic site control at low temperatu in contrast, polymerizations with MAO s metallocene catalyst were"much less selective Erker JACS1993(115)4590 ICNMR and HNMr studies at -50%C showed that in solution the neoisomenthyI-substituted metallocenes exist primarily as a single, C2-symmetric species. In contrast, the neomenthyl-substituted catalysts exist as 4: 1 ratio of C. Cr-symmetric metallocene species. The Asymmetric Asymmetric authors speculate that with the neomenthyl-substituted symmetrIc metallocene species may have given rise to the formation of alternating isotactic and nearly atactic sequences along the growing polymer chain. IsotacticM.C. White, Chem 153 EM Polymerization -303- Week of November 18, 2002 R* *R Me Zr Cl R* = R* = neoisomenthyl neomenthyl this catalyst led to the formation of highly isotactic, high molecular weight polypropylene, with purely enantiomorphic site control at low temperature. in contrast, polymerizations with the neomenthyl-substituted metallocene catalyst were "much less selective" MAO Cl Torsional isomers in stereoselective propylene polymerization 13C NMR and 1H NMR studies at -50 oC showed that in solution the neoisomenthyl-substituted metallocenes exist primarily as a single, C2-symmetric species. In contrast, the neomenthyl-substituted catalysts exist as a 4:1 ratio of C2:C1-symmetric metallocene species. The authors speculate that with the neomenthyl-substituted catalysts the switching between C2 and C1-symmetric metallocene species may have given rise to the formation of alternating isotactic and nearly atactic sequences along the growing polymer chain. R* R* R* R* Zr R Me Zr R Me C2-symmetric C1-symmetric R* = Isotactic Atactic neomenthyl Erker JACS 1993 (115) 4590