Hypersilyl-substituted complexes of group 1 and 2 metals: Syntheses, structures and use in styrene polymerisation.
Alkali and alk.-earth metal complexes with sterically demanding hypersilyl-substituted cyclopentadienyl or fluorenyl ligands [hSi-Cp or hSi-Flu; hSi = (Me3Si)3Si] were prepd. The crystal structures of [Li(hSi-Flu)(THF)2], [K(hSi-Flu)(THF)], [Ca(hSi-Flu)2(THF)2], [Sr(hSi-Flu)2(THF)2] and [Ca(hSi-Cp)2(THF)] were detd. All structures show the enormous steric bulk of the hypersilyl substituent, which either results in a redn. in the no. of solvent mols. in the coordination sphere of the metal or in a distortion of the ligand-metal interaction. Heteroleptic calcium and strontium complexes with a benzyl ligand (2-Me2N-a-Me3Si-benzyl) and a sterically demanding hSi-Flu ligand were prepd. and used as initiators in the polymn. of styrene. The polymers produced with the heteroleptic benzylcalcium initiator are enriched in syndiotactic sequences, although they show the same tacticities as polymers obtained with benzylcalcium initiators contg. the less bulky Me3Si-Flu ligand. Introduction of the bulky hypersilyl group in heteroleptic benzylstrontium initiators resulted in a slight increase of the syndiotacticity of the polymers.
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