Homoleptic and Heteroleptic Barium Benzyl Complexes: Synthesis and Reactivity as Initiators for Anionic Styrene Polymerizations

Dibenzylbarium is prepared in yields of 80−90% by reaction of the tetramethyl ethylenediamine adduct of benzyllithium with the bis-THF adduct of barium bis(bis-trimethylsilylamide) or with barium bis(2,4,6-tri-tert-butyl phenolate). It is converted, by reaction with
diphenylmethane in THF, to bis(diphenylmethyl)barium in 90% yield and, by reaction with
1,1-diphenylethene in THF, to bis(1,1,3-triphenylpropyl)barium in essentially quantitative
yield. The latter is soluble also in hydrocarbon solvents, while dibenzylbarium and
bis(diphenylmethyl)barium are soluble only in THF. Reaction of the bis-chelate complex
(C5Me4SiMe2C6H5)2Ba, the phenyl residues of which are coordinated to the Ba center, with
each of the bis(arylalkyl)barium species generates heteroleptic barium complexes with one
chelate and one reactive arylalkyl ligand. Homoleptic and heteroleptic triphenylpropylbarium
complexes both induce, in cyclohexane solution, living polymerization of styrene to atactic
polystyrene.