The anionic polymerization of methyl methacrylate (MMA) was carried out in the presence of ethyl alpha-lithioisobutyrate (alpha-LiEtlB)/quaternary ammonium salts (QAS) in toluene and tetrahydrofuran/toluene (vol. ratio 75/25) at -60 degrees C. It was found that the tacticity of PMMA strongly depends on size and shape of QAS. A highly isotactic polymerization in toluene was observed, when using the system alpha-LiEtlB/trimethylhexadecylammonium bromide. The initiator system alpha-LiEtIB/tetrahexylammonium chloride produces polymers with a high (> 50%) syndiotactic content and relatively low polydispersity. The influence of QAS on the mode of polymerization is due to specific salt effect resulting from the cation exchange reaction, from the participation of the balky substituent in the primary solvation shell, and ionic aggregation between the growing anion, Q(+) and LiX.