Harder, Sjoerd; Feil, Florian:

Dimeric Benzylcalcium Complexes: Influence of THF in Stereoselective Styrene Polymerization.

In: Organometallics, Jg. 21 (2002) ; Nr. 11, S. 2268-2274
ISSN: 0276-7333
Zeitschriftenaufsatz / Fach: Chemie
Abstract:
Two dimeric benzylcalcium complexes were prepd. 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 sym. 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 addn. of THF results in cleavage of the dimer in THF-solvated monomers. The THF-free dimeric benzylcalcium complex 6 initiates the living polymn. of styrene. Polymers enriched in syndiotactic sequences are obtained (ca. 85% r-diads). Addn. of equimolar amts. of THF (Ca/THF ratio is 1:1) does not disturb the stereocontrol in the polymn. 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.