Stereospecific anionic polymerization of styrene initiated by R 2Mg/ROMt ‘ate’ complexes

Abstract

Various ‘ate’ complexes formed by reaction of alkali metal (Li, Na, K) derivatives and alkyl metals (Mg, Al, Zn) were used as initiator for the styrene anionic polymerization in hydrocarbon media and their influence on polystyrene microstructure has been investigated. A strong dependence of the polymer tacticity on both the nature of the alkali metal and of the associated metal alkyl used is observed. Binary systems like potassium derivatives/dialkylmagnesium yield isotactic-rich polystyrene. Eighty-five percent isotactic polystyrene (mm triad) is obtained in methylcyclohexane at −40 °C with the potassium tert-butoxide/ n, s-dibutylmagnesium system. The characteristics of solubility of the stereoregular polystyrenes are in agreement with isotactic polystyrene chains containing some stereostructural defects and suggest that one single type of isospecific propagating centres is operating during the polymerization. When lithium or sodium derivatives are used in association to dialkylmagnesium instead of a potassium derivative, a strong decrease of the isotacticity is observed. The influence, on the extent of stereoregulation, of other parameters of these initiating systems as well as that of the solvent has also been investigated. The stereoregulation mechanisms involved in these styrene anionic polymerizations may be related to styrene insertion into carbon–metal bonds of the ‘ate’ complexes with a covalent character.