Dimeric benzylcalcium complexes: Influence of THF in stereoselective styrene polymerization

Two dimeric benzylcalcium complexes were prepared and structurally characterized by single-crystal X-ray diffraction: {(Me3Si-fluorenyl)[o-(dimethylamino)benzyl]Ca}(2) (6) and {(EtMe4Cp)[o-(dimethylamino)benzyl]Ca}(2) (8). Both complexes show a core of Ca2+ ions that are symmetrically bridged by benzyl anions. Fluorenyl or cyclopentadienyl ligands are bonded in terminal positions: The Me2N groups coordinate each on a different Ca2+ ion. Their dimeric nature is preserved in apolar solvents (benzene, toluene), and addition of THF results in cleavage of the diner in THF-solvated monomers. The THF-free dimeric benzylcalcium complex 6 initiates the living polymerization of styrene. Polymers enriched in syndiotactic sequences are obtained (ca. 85% r-diads). Addition of equimolar amounts of THF (Ca/THF ratio is 1:1) does not disturb the stereocontrol in the polymerization reaction. A Ca/THF ratio of 1:4, however, significantly impairs the stereocontrol. We propose an insertion step that. proceeds with a high degree,of syndiotactic selectivity and an inversion mechanism that racemizes the chiral chain-ends. Excess THF increases the rates of inversion of the chiral carbanionic chain-ends, thus destroying the stereoregularity in the polymer chain.