Organomanganate reagents [R3Mn]Li--(+) (R = Bu, Me) were found to polymerize methyl methacrylate in the presence of potassium tert-butylate. A conversion of the tacticity of the resulting poly(methyl methacrylate)s from heterotactic (mr = 54%) to isotactic (mm = 58%) was observed upon changing the R group of the initiator from Bu to Me. The addition of triisobutylaluminium was found to efficiently control (M) over bar (w) and (M) over bar (w)/(M) over bar (n) of the resulting polymers.