Silylium ions (R3Si+) are found to catalyze both 1,4-hydrosilylation of methyl methacrylate (MMA) with R3SiH to generate the silyl ketene acetal initiator in situ and subsequent living polymerization of MMA. The living characteristics of the MMA polymerization initiated by R3SiH (Et3SiH or Me2PhSiH) and catalyzed by [Et3Si(L)](+)[B(C6F5)(4)](-) (L=toluene), which have been revealed by four sets of experiments, enabled the synthesis of the polymers with well-controlled M-n values (identical or nearly identical to the calculated ones), narrow molecular weight distributions (D=1.05-1.09), and well defined chain structures {H[MMA](n)H}. The polymerization is highly efficient too, with quantitative or near quantitative initiation efficiencies (I*=96-100%). Monitoring of the reaction of MMA+Me2PhSiH+[Et3Si(L)](+)[B(C6F5)(4)](-) (0.5 mol%) by H-1 NMR provided clear evidence for in situ generation of the corresponding SKA, Me2CC(OMe)OSiMe2Ph, via the proposed Et3Si+-catalyzed 1,4-hydrosilylation of monomer through frustrated Lewis pair type activation of the hydrosilane in the form of the isolable silylium-silane complex, [Et3SiHSiR3](+)[B(C6F5)(4)](-). (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1895-1903