M.C. White, Chem 153 EMPolymerization-298- Week of november 18. 2002 Cossee mechanism for Ziegler Natta polymerizations According to the Cossee mechanism, propagation of the polymer occurs exclusively at the Ti center. The role of the alkyl aluminum pecies is thought to be that of initiator by alkylating the TiCI olefin coordination CHT cHI Representation of a TiCl lattice with an cis-carbometalation via a concerted 4-membered TS open coordination site on the surface Cossee190(17)12 Cossee stereochemical model for isotactic polypropylene formation Polvmer Polymer CHT si-face re-face Representation of a stereogenic Ti center on the edge of a chiral TiCl crystal. The growing polymer occupies the open quadrant. The olefin preferentially binds via its si-face placing its methyl substituent trans to the bulky polymer chain. Modem MgCl2-supported Ziegler Natta Cossee tz1960(17)17 catalysts are highly stereoselective resulting in formation of essentially Brintzinger ACIEE 1995(34)1143. only the isotactic polymerM.C. White, Chem 153 EM Polymerization -298- Week of November 18, 2002 Cossee mechanism for Ziegler Natta polymerizations Cl Cl Ti Cl Cl Ti Ti Cl Cl Cl Ti Cl Cl Ti Ti Cl Cl Cl Ti Cl Cl Ti Ti Cl Cl Cl Ti Cl Cl Ti Ti Cl Representation of a TiCl3 lattice with an open coordination site on the surface According to the Cossee mechanism, propagation of the polymer occurs exclusively at the Ti center. The role of the alkyl aluminum species is thought to be that of initiator by alkylating the TiCl3. olefin coordination cis-carbometalation via a concerted 4-membered TS. Cossee TL 1960 (17) 12. Cossee mechanism for Ziegler Natta heterogeneous polymerization. Cossee stereochemical model for isotactic polypropylene formation: Cl Cl Ti Cl Cl Ti Ti Cl Polymer si-face favored Cl Cl Ti Cl Cl Ti Ti Cl Polymer re-face disfavored Representation of a stereogenic Ti center on the edge of a chiral TiCl3 crystal. The growing polymer occupies the open quadrant. The olefin preferentially binds via its si-face placing its methyl substituent trans to the bulky polymer chain. Modern MgCl2-supported Ziegler Natta catalysts are highly stereoselective resulting in formation of essentially only the isotactic polymer. Cossee TL 1960 (17) 17. Brintzinger ACIEE 1995 (34) 1143