Developing different synthetic approaches to realize controlled or living polymerization is of great interest to polymer chemists to obtain polymers with defined molecular weight, narrow molecular weight distribution and unambiguous structures. Herein, we report an effective and efficient Lewis pair for methyl methacrylate (MMA) polymerization under ambient conditions by utilization of a new type of Lewis base, pyridinylidenaminophosphines (PyAPs). Among the currently investigated Lewis pair catalyzed MMA polymerization systems, Lewis pair (LP) catalysts based on PyAPs as Lewis bases (LBs) and organoaluminum compounds as Lewis acids (LAs), PyAPs possessing di-tertbutylphosphine (PyAP-^tBu) LB and a moderately Lewis acidic and sterically hindered methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) [MeAl(BHT)₂] (BHT: 2,6-di-tert-butyl-4-methylphenol) LA have been demonstrated as the best LP catalysts in MMA Lewis pair polymerization (LPP). Moreover, the solid-state structure of the enolaluminate intermediate PyAP-^tBu-CH₂C(Me)C(OMe)OAl(BHT)₂Me (INT1) was characterized by X-ray diffraction analysis. The polymerization of MMA exhibited activity up to 6400 h⁻¹ TOF and a relatively high initiation efficiency, producing polymers with an ultrahigh molecular weight (M_n up to 790 kg mol⁻¹) and narrow molecular weight distributions (Đ = 1.15). This polymerization with a controlled characteristic has been verified by several pieces of evidence, including chain-end analysis, a linear increase of M_nvs. monomer conversion and the monomer-to-initiator ratio and the synthesis of a diblock copolymer. In combination with MeAl(BHT)₂ as the Lewis acid, PyAP-Cy and PyAP-ⁱPr could polymerize MMA in the range of 800 equiv., to 1600 equiv., producing polymers with M_n up to 230 kg mol⁻¹ and 240 kg mol⁻¹, respectively. The electronic matching of the relative strength of LAs and LBs, as well as the steric interplays of Lewis pairs might account for the realization of controlled MMA polymerization.