M.C. White, Chem 153 EMPolymerization-304 Week of november 18. 2002 Torsional isomers in stereoselective propylene polymerization Recall Often up to 100 isotactic pentad 6.25 isotactic pentad ( fraction of stereosequences (fraction of stereosequence containing 5 adjacent isotactic containing 5 adjacent chiral- hiral-me The bridge between the indenyl ligands is removed to allow rotation about the Observation of both the metal ligand bond axis. Bulky phenyl substituents are incorporated into the acemic-like and meso-like indene ligand to inhibit the rate of compounds in the crystal unit cell indicates that the torsional isomers ligand rotation such that it is slower han monomer insertion but faster than are energetically similar Chiral racemic-like propagation/termination. The result is production of an isotactic-atactic ereoblock copolymer M note:another way Block copolymer is produced with alternating isotactic-atactic domains described is by the stereochemical relationship between stereogenic centers: "m" for meso andr" for racemic Isotactic pentad Atactic block Isotactic pentad Isotactic pentad content=63-28.1 pentad would be[mmmm] Waymouth Science 1995 267217-219M.C. White, Chem 153 EM Polymerization -304- Week of November 18, 2002 Recall: Zr P Zr P chiral- racemic achiral-meso Atactic polymer 6.25 % isotactic pentad (fraction of stereosequences containing 5 adjacent isotactic centers) Isotactic polymer Often up to 100 % isotactic pentad (fraction of stereosequences containing 5 adjacent isotactic centers) The bridge between the indenyl ligands is removed to allow rotation about the metal ligand bond axis. Bulky phenyl substituents are incorporated into the indene ligand to inhibit the rate of ligand rotation such that it is slower than monomer insertion but faster than propagation/termination. The result is production of an isotactic-atactic stereoblock copolymer. Observation of both the racemic-like and meso-like compounds in the crystal unit cell indicates that the torsional isomers are energetically similar. note: another way in which polymer tacticity is often described is by the stereochemical relationship between adjacent stereogenic centers: "m" for meso and "r" for racemic. For example, an isotactic pentad would be [mmmm] Waymouth Science 1995 267 217-219. Torsional isomers in stereoselective propylene polymerization Zr Zr P P ki k-i Me Me Chiral racemic-like Achiral meso-like kpi kpa Isotactic pentad Atactic block Isotactic pentad Isotactic pentad content = 6.3-28.1 % Block copolymer is produced with alternating isotactic-atactic domains