Lewis-acidic organometallic compounds of the lanthanides and the alkaline earth metals as catalysts for the activation of carbonyl groups

1,3-dialkyl-4,5-dimethylimidazol-2-ylidenes form adducts with various bis(a lkyl-cyclopentadienyl) complexes of magnesium, calcium, strontium, barium, samarium, and ytterbium. The X-ray structures of selected metallocenes and carbene adducts are discussed. Liquid-crystalline homo- and block copolymer s with a narrow molecular mass distribution index were obtained in high yie lds by living polymerization of mesogenic methacrylates with organosamarium complexes. Depending on the structure of the catalyst and the polarity of the solvent used, different, partially competitive reaction mechanisms are observed.