Fully organic Lewis pairs, combining a phosphine such as tri-n-butylphosphine ((PBu3)-Bu-n), tritert-butylphosphine ((PBu3)-Bu-t) or tris(2,4,6-trimethoxyphenyl) phosphine (TTMPP) as a Lewis base, and N-(trimethylsily)bis(trifluoromethane sulfonyl)imide (Me3SiNTf2) as a Lewis acid, are shown to directly initiate the polymerization of methyl methacrylate (MMA) at room temperature in toluene. A dual reaction mechanism involving an optimal TTMPP:Me3SiNTf2 ratio of 1:2 accounts for the control of the polymerization. Molar masses of poly(methyl methacrylate)'s (PMMA's) can be varied by the initial [MMA](0)/[TTMPP](0) molar ratio. Chain extension experiments confirm that a majority of chains of a TTMPP/Me3SiNTf2-derived PMMA can be reactivated. Both density functional theory (DFT) calculations and stoichiometric studies reveal that TTMPP and Me3SiNTf2 form a P-silyl phosphonium intermediate that is in equilibrium with the corresponding frustrated Lewis pair (FLP). This FLP could correspond to the active form of the initiation step. Both computational and experimental data support the existence of a cooperative mechanism during the TTMPP/Me3SiNTf2 Lewis pair-mediated polymerization of MMA.