The mixed associated species {xsBuLi, (y - x)sBuMe(2)SiOLi} formed by reaction of sec-Butyllithium (sBuLi) with octamethylcyclotetrasiloxane (MeaSiO)(4) in toluene have been used to initiate the anionic polymerization of methyl methacrylate (MMA) in that solvent at 0 degrees C. The active species generated by this new type of initiator have been characterized by Li-7 NMR at 0 degrees C. The Li-7 NMR spectra of sBuLi and sBuMe(2)SiOLi mixtures in toluene show that only one single type of {sBuLi, 5sBuMe(2)SiOLi} mixed species prevails at [sBuMe(2)SiOLi]/[sBuLi] molar ratios higher than ca. 21, as result of a large excess of {6sBuMe(2)SiOLi} species. The Li-7 NMR has also concluded to the formation of {PMMALi, 5sBuMe(2)SiOLi} species upon the addition of MMA to that initiator solution, supporting the theory that only one type of ionic species is active during the MMA conversion. These active centers are stable for at least 1 h at 0 degrees C as confirmed by Li-7 NMR.