Stereoregulation in the anionic polymerization of styrene initiated by superbases

Polystyrenes with different stereoregularities were obtained by initiation of the styrene anionic polymerization via alkyllithium/alkali metal alkoxide systems in hydrocarbon media. Depending on the nature of the metal alkoxide associated to alkyl lithium and more particularly of the alkali metal counter-ion (lithium, sodium, potassium), the synthesis of polystyrene exhibiting predominantly isotactic or syndiotactic triads was achieved. The use of soluble metal alkoxide improves the stereospecificity as well as the initiation efficiency with respect to initial alkyl lithium. The contribution of propagating active species yielding a single type of polymer chains with stereochemical irregularities is supported by the solubility characteristics of the isotactic and syndiotactic rich polystyrenes. A reaction mechanism involving the formation of bimetallic complexes between the alkyllithium and the alkali metal alkoxide and ligand-metal exchanges is discussed.